The Importance of ß-Cell Failure in the Development and Progression of Type 2 Diabetes

Steven E. Kahn

Division of Metabolism, Endocrinology and Nutrition, Department of Medicine, University of Washington and Veterans Affairs Puget Sound Health Care System, Seattle, Washington 98108

Address all correspondence and requests for reprints to: Steven E. Kahn M.B., Ch.B., Veterans Affairs Puget Sound Health Care System (151), 1660 South Columbian Way, Seattle, Washington 98108. E-mail: skahn{at}u.washington.edu

The pathogenesis of type 2 diabetes is complex and in most instances clearly requires defects in both ß-cell function and insulin sensitivity (1). Together, these abnormalities result in increased rates of glucose release by the liver and kidney as well as decreased clearance from the circulation (2, 3). For the last decade, a great deal of attention has been directed at further understanding the role of insulin resistance as an important contributor to the development and maintenance of the hyperglycemia of type 2 diabetes. During this same period, the well described vital role of the pancreatic islet, and specifically the ß-cell, in this process has been largely neglected.

Perhaps one of the most striking and sobering findings of the United Kingdom Prospective Diabetes Study (UKPDS) was the reaffirmation of the clinically recognized progressive nature of type 2 diabetes (4). Every day clinicians all over the world find themselves struggling to maintain good glycemic control in subjects with type 2 diabetes, and the results of this study clearly confirm that, even with the use of algorithmic approaches aimed at maintaining superb glucose control, it is very difficult to maintain individuals at the desired levels of glycemia. In fact, in the UKPDS after 9 yr only 25% of the subjects in the intensive treatment arm were achieving a HbA1c less than 7% with monotherapy alone (5). When one examines the outcome in the different groups based on their initial assignment, this goal was attained in 8% of subjects given dietary therapy, 13% receiving metformin, 24% taking sulfonylureas, and 42% of individuals using insulin. The reason(s) for the progressive deterioration in glycemic control observed in the UKPDS have been addressed using the Homeostasis Model Assessment (HOMA). This model provides a simple approach for estimating insulin sensitivity and ß-cell function and lends itself to use in large studies such as the UKPDS. With this approach, the UKPDS has clearly demonstrated that the progressive nature of diabetes in this cohort of individuals with recently diagnosed type 2 diabetes is an ongoing decline in ß-cell function without a change in insulin sensitivity (6, 7). It is of interest that a similar observation was made in the Belfast diet intervention study in which the progressive deterioration of glycemic control was associated with a progressive deterioration of ß-cell function without a change in insulin sensitivity (8). However, when using simple approaches such as HOMA, one does not necessarily gain insights into what may be the characteristics and underlying pathology responsible for the observed changes.

This Clinical Review focuses on the role of the ß-cell in the pathogenesis of type 2 diabetes. The recent evidence documenting support for the existence of this defect well before the diagnostic criteria for diabetes are attained will be discussed. Finally, while as yet we do not understand all the possible mechanisms responsible for these functional alterations, a synopsis of the information that is being gathered and will likely bear on our success at treating this relentless metabolic disorder will be provided.

The nature of ß-cell dysfunction in type 2 diabetes

It is well accepted that for hyperglycemia to exist in type 2 diabetes, ß-cell dysfunction has to be present. This alteration is manifest in a number of different ways including reductions in insulin release in response to glucose (9, 10, 11) and nonglucose secretagogues (12, 13, 14, 15), changes in pulsatile (16) and oscillatory insulin secretion (17), an abnormality in the efficiency of proinsulin to insulin conversion (18, 19, 20, 21, 22, 23, 24), and reduced release of islet amyloid polypeptide (IAPP), also known as amylin (25, 26, 27).

Reductions in insulin release can be demonstrated in individuals with type 2 diabetes following oral glucose loading (Refs. 9, 10, 27 and 28 ; Fig. 1AGo). In these individuals, the absolute responses occurring early (typically within 30 min) after administration of glucose are reduced whereas those that are observed later in the test may be greater due to the fact that lack of early insulin secretion leads to hyperglycemia later in the test (9, 10). The nature of the relationship between this early phase insulin response and glucose tolerance has been demonstrated to be nonlinear in nature (Ref. 27 ; Fig. 1BGo). Thus, small decreases in this early response can have dramatic effects on the later glucose excursion in subjects with diabetes whereas larger changes may have a smaller effect in individuals with normal glucose tolerance. Although it can be demonstrated that type 2 diabetes is associated with a reduction in early insulin release as a measure of ß-cell function, the different secretory functions involved in this process cannot be discerned from this simple test. Thus, delineation of the different components of ß-cell function has been addressed primarily using iv testing.



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Figure 1. A, Ratio of the incremental responses of immunoreactive insulin (IRI) and glucose (G) over the first 30 min following oral glucose ingestion in 94 Japanese-American subjects with varying glucose tolerance. Of the 94 subjects, 56 had normal glucose tolerance, 10 had impaired glucose tolerance, and 28 had type 2 diabetes. Significant decreases in the {Delta}IRI/{Delta}G (P < 0.0001) occurred with decreasing glucose tolerance. B, Relationship between the ratio of the incremental immunoreactive insulin and glucose responses ({Delta}IRI/{Delta}G) over the first 30 min following oral glucose ingestion and glucose tolerance, the latter determined as the incremental glucose area (AUC Glucose) during the oral glucose tolerance test. Subjects with normal glucose tolerance ({blacksquare}), impaired glucose tolerance ({diamond}), and type 2 diabetes ({blacktriangleup}) are indicated. The relationship between these variables is nonlinear with an r2 value of 0.38 (P < 0.0001). [Reproduced with permission from S. E. Kahn et al.: Diabetes 47:640–645, 1998 (27 ).]

 
When glucose is administered iv, the dynamics of insulin release comprise two phases: first and second. The first-phase response peaks within 2–5 min following glucose administration and lasts approximately 10 min. This response is thought to represent the release of a pool of secretory granules that is present in close proximity to the ß-cell plasma membrane (29, 30, 31). The second-phase response commences shortly after glucose administration and is more prolonged being maintained for the period that the glucose level is elevated. This second-phase response is believed to represent in large part the release of secretory granules that are being mobilized within the ß-cell for release (29, 30, 31) and includes many granules that contain newly synthesized insulin (32, 33). In individuals with type 2 diabetes both these responses are diminished. In fact, the lack of a first-phase response is a sine quo non for diabetes and is absent in all individuals who are hyperglycemic (9, 11, 34). Of interest, but of unknown cause, is the observation that the first-phase response may in fact be negative in subjects with marked elevations of fasting glucose (35). The second-phase response may seem relatively normal in subjects with type 2 diabetes early on. However, this apparently normal response occurs at the expense of hyperglycemia so that when subjects are matched for the degree of glucose elevation, it is clear that the second-phase insulin response is also reduced in subjects with type 2 diabetes (15, 34, 36).

Besides its ability to directly stimulate insulin release, glucose also modulates the ß-cell’s response to other secretagogues. The iv administration of a nonglucose secretagogue such as the amino acid arginine (15), peptides such as secretin (37), the ß-adrenergic agonist isoproterenol (37), and sulfonylureas such as tolbutamide (38) is associated with an acute insulin response. When the same quantity of the nonglucose secretagogue is injected in the presence of an elevated glucose level, the magnitude of the response is increased (14, 15). This effect of glucose to enhance the insulin response to these other secretagogues is termed glucose potentiation (14, 39). The magnitude of these responses is "normal" in age and obesity-matched healthy and type 2 diabetic subjects. However, when the glucose levels are matched, these responses are clearly diminished in type 2 diabetes (14). When a full dose-effect curve from 100–600 mg/dl glucose is performed, it has been demonstrated that the maximum secretory capacity of the ß cell is diminished whereas its half-maximum or sensitivity to glucose is unchanged (15). The severity of this reduction in ß-cell secretory capacity is related to the fasting glucose level in an inverse, nonlinear manner (Ref. 40 ; Fig. 2Go). The nature of this relationship suggests that, as with many other endocrine organs, a significant proportion (50–75%) of the secretory capacity of the cell is lost by the time fasting hyperglycemia develops. However, because fasting hyperglycemia is a relatively late event in the pathogenesis of diabetes and ß-cell dysfunction is progressive, lesser reductions in ß-cell secretory capacity will be associated with reduced glucose tolerance before fasting hyperglycemia is manifest. Such a postulate is supported by the UKPDS in which ß-cell function was reduced some 50% at the time of diagnosis of fasting hyperglycemia (6, 7). The finding of a reduction in ß-cell secretory capacity has implications for possible mechanisms responsible for the loss of ß-cell function in type 2 diabetes as a reduction in secretory capacity has been associated with experimental, generalized reductions in islet mass (41, 42).



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Figure 2. Relationship between ß-cell secretory capacity (AIRmax) and fasting plasma glucose in 9 subjects with type 2 diabetes (•) and 10 subjects with normal glucose tolerance ({circ}). There is a broad range of ß-cell secretory capacity in the healthy subjects due to large differences in insulin sensitivity, whereas in the subjects with type 2 diabetes the range is narrower as a manifestation of the impaired islet function. The nonlinear relationship between these two parameters (r = -0.76; P < 0.0001) demonstrates that the degree of ß-cell function is a determinant of the fasting glucose level. In subjects with type 2 diabetes, ß-cell secretory capacity is reduced by approximately 75% when the fasting glucose level increases. [Adapted from Ref. 40 .]

 
Using frequent sampling over prolonged periods of time, it has been clearly demonstrated that insulin release can occur in both a pulsatile and an oscillatory manner. In healthy subjects, pulses in insulin release occur spontaneously every 8–10 min (43). The origin of these pulses seems to be intrinsic to the islet as isolated islets in culture still demonstrate the presence of these pulses despite the fact that they lack their neural connection (44). In subjects with type 2 diabetes, this pulsatile pattern of insulin release is disrupted (45). The loss of these pulses may also play an important role in the development of a portion of the insulin resistance of the disease as delivery of insulin as a continuous infusion is associated with impaired insulin action, that does not occur when insulin is administered in a pulsatile pattern (46, 47). It is also of interest that these rapid pulses are superimposed on larger oscillations in insulin release. These prolonged oscillations have a phase of approximately 120 min and can also be made to oscillate in concert with a variable rate iv infusion of glucose administered with a similar periodicity (48, 49). It is uncertain what regulates these longer oscillations but it is possible that these are related to a signal arising outside the islet. These natural oscillations and the ability of glucose to entrain them have both been demonstrated to be defective in subjects with type 2 diabetes (17, 49).

Two other components related to ß-cell function are worthy of mention as they are both disturbed in type 2 diabetes. The first relates to the insulin biosynthetic process. Insulin production requires the cleavage of insulin out of its larger precursor peptide proinsulin, resulting in the formation of insulin and C-peptide. This process occurs within the secretory granule while it transits the ß-cell and matures and involves the action of two endoproteases, PC1/3 and PC2 (50, 51, 52). When the contents of the granule are acutely released in response to ß-cell stimulation, in healthy subjects about 2% of all insulin-like immunoreactivity is composed of intact proinsulin and its cleavage intermediate des-31,32-split proinsulin (19, 24), suggesting that under normal conditions proinsulin to insulin processing is incomplete. In type 2 diabetes, the efficiency by which the cell processes proinsulin is reduced. Thus, in hyperglycemic individuals, following acute stimulation the proportion of proinsulin-like molecules is increased to between 5% and 8% (19, 24). In contrast to the findings examining acutely released products, in the fasting state, the proportion of circulating proinsulin-like molecules is approximately 15% in healthy subjects and is increased 2- to 3-fold in subjects with type 2 diabetes (18, 19, 20, 21, 22, 23, 24). These increased proportions in the basal state are due to the slower metabolic clearance rate of proinsulin-related peptides (53). The degree of elevation in proinsulin-like molecules is linearly related to the degree of hyperglycemia suggesting that the proportion of proinsulin is a marker of the magnitude of ß-cell dysfunction (22, 40). The basis for this disproportionate increase in the release of proinsulin-like molecules in type 2 diabetes is still not understood. Two different hypotheses have been proposed. One suggests that this inefficiency represents a primary defect in ß-cell function (54), whereas the other proposes that increased secretory demand results in the release of a less mature ß-cell granule at a time when proinsulin to insulin conversion is incomplete (55).

The second interesting component relates to the description in 1987 of a new ß-cell peptide. This 37 amino acid peptide, designated IAPP, was isolated from the amyloid deposits that are commonly found in the islets of subjects with type 2 diabetes (56, 57). IAPP has been colocalized with insulin in the ß-cell secretory granule (58) and, therefore, is secreted along with insulin in response to glucose and other stimuli (59). In keeping with its cosecretion with insulin, release of IAPP is diminished in individuals with type 2 diabetes (25, 26, 27). Studies examining whether IAPP can induce insulin resistance or impair ß-cell function have produced variable and nonconclusive results (60, 61, 62). The native peptide has however been demonstrated to slow gastric emptying and, thus, delay glucose absorption (63). Whether deficient IAPP release contributes to the pathophysiology observed in the diabetes disease process is still unclear.

Thus, it is clear that ß-cell dysfunction exists in individuals with type 2 diabetes. This dysfunction is global involving a number of different measures of the functional integrity of the ß-cell. Furthermore, the degree of ß-cell dysfunction is related to degree of hyperglycemia, suggesting that if hyperglycemia and ß-cell dysfunction are present at the time of diagnosis, dysfunction must also be present before the fasting and/or 2-h glucose levels reaching the diagnostic cutpoints for diabetes. Supporting evidence for this is presented next.

ß-cell dysfunction is present before the development of type 2 diabetes

Data from the UKPDS suggests that the onset of the ß-cell dysfunction associated with diabetes occurs well before the development of hyperglycemia, and may commence many years before diagnosis of the disease (7). However, this suggestion is based on an extrapolation of findings in subjects with established type 2 diabetes. While this concept is gaining support, it has not been a universally accepted idea. Thus, although there is no doubt that defects in ß-cell function exist in all subjects with hyperglycemia, when this abnormality begins and what factors may be responsible for producing this change has been a subject of great debate. Part of the failure to recognize the existence of defects in ß-cell function early in the course of the development of diabetes has been related to the failure to consider that the systems involved in glucose regulation cannot always be assessed in isolation. Recent advances in our understanding of the modulating effect of insulin sensitivity on ß-cell function have brought a new understanding and therefore a new interpretation to the assessments of insulin release in individuals at risk of developing type 2 diabetes.

Insulin sensitivity has long been recognized to be an important factor determining the magnitude of the insulin response to ß-cell stimulation (64, 65). Thus, when ß-cell function is assessed, obese individuals who are insulin resistant manifest greater responses than age-matched lean subjects (10, 12, 66, 67, 68). The concept that a feedback loop between the insulin-sensitive tissues and the ß cell exists and determines this adaptive response was first advanced by Bergman et al. (69) and confirmed by us in humans (65). The nature of this relationship is such that insulin sensitivity and ß-cell function are inversely and proportionally related so that the product of these two parameters is always a constant (Fig. 3Go). This constant is referred to as the disposition index (70). Our understanding of the nature of this relationship has also highlighted the fact that if two individuals have identical absolute insulin responses, the only way their ß-cell function can be considered to be similar is if they have identical insulin sensitivity. Conversely, if these same individuals differ in terms of insulin sensitivity, it has to be concluded that their ß-cell function differs.



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Figure 3. Relationship between insulin sensitivity and ß-cell function quantified as the first-phase insulin response (AIRglucose) in 93 (55 males and 38 females) apparently healthy, nondiabetic subjects under the age of 45 yr. The cohort demonstrates a broad range of insulin sensitivity and ß-cell function. The solid line depicts the best-fit relationship (50th percentile) whereas the broken lines represent the 5th, 25th, 75th, and 95th percentiles. The relationship is best described by a hyperbolic function so that any change in insulin sensitivity is balanced by a reciprocal and proportionate change in ß-cell function. [Reproduced with permission from S. E. Kahn et al.: Diabetes 42:1663–1672, 1993 (65 ).]

 
Based on this concept of a feedback loop and the need to consider the magnitude of insulin responses in the face of the prevailing degree of insulin sensitivity, it has become evident that subjects who are at high risk of developing type 2 diabetes have diminished ß-cell function at a time when many of them still have normal glucose tolerance. Thus, first- degree relatives of individuals with type 2 diabetes (71, 72, 73), women with a history of either gestational diabetes (74, 75, 76) or polycystic ovarian disease (71, 77), older subjects (78, 79, 80), and individuals with impaired glucose tolerance (81) can all be demonstrated to have a reduced first-phase insulin response to iv glucose administration (Figs. 4Go and 5Go). Using this approach, we have found that the vast majority of individuals who have two first-degree relatives with type 2 diabetes fall below the mean (50th percentile) for the relationship between insulin sensitivity and insulin secretion, in keeping with their high-risk status (unpublished observation). In keeping with the observations in first-degree relatives and the fact that type 2 diabetes is an inherited, polygenic syndrome, it is interesting to note that a recent study examining the heritability of ß-cell function, assessed in relation to insulin sensitivity (SI x AIRglucose), demonstrated a heritability of 67% in 120 subjects who had either impaired or normal glucose tolerance and 70% when only the 94 normal glucose tolerant individuals were examined (82). Interestingly, in this same study, when AIRglucose was considered alone, it could not be shown to be an inherited phenotype (82). Thus, it would seem that attempts to identify ß-cell defects contributing to typical type 2 diabetes are likely to be more successful if they take into account the modulating effect of insulin sensitivity on ß-cell function.



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Figure 4. Percentile lines for the relationship between insulin sensitivity (SI) and the first-phase insulin response (AIRglucose) based on data from 93 normal subjects (65 ). Mean data from six other studies are plotted. The 10 subjects with type 2 diabetes are insulin resistant and have markedly impaired insulin secretion (162 ). Thirteen healthy older subjects demonstrate that aging is associated with insulin resistance and a reduction in ß-cell function (79 ). Reduced ß-cell function is also manifest in 8 women with a history of gestational diabetes (GDM) (74 ), 11 women with polycystic ovarian disease (PCO), and a family history of type 2 diabetes (71 ), 21 subjects with impaired glucose tolerance (IGT) (81 ), and in 14 subjects with a first-degree relative with type 2 diabetes mellitus (72 ). The reduction in ß-cell function in these latter three groups is compatible with their high risk of subsequently developing type 2 diabetes. [Reproduced with permission from J. Vidal and S. E. Kahn: Genetics of Diabetes Mellitus (edited by W. L. Lowe, Jr.), Kluwer, Stamford, CT, 2001, pp 109–131 (163 ).]

 


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Figure 5. Individual measurements of insulin sensitivity and ß-cell function in a group of first-degree relatives of subjects with type 2 diabetes from three different families indicated by different symbols. These individuals were studied when their fasting glucose levels were normal and exhibit broad ranges of both SI and AIRglucose. When these two parameters are assessed together, it is apparent that some individuals have well-preserved ß-cell function while others have markedly deficient responses and, thus, would be predicted to be at very high risk of progressing on to develop hyperglycemia. [Reproduced with permission from S. E. Kahn: Diabetes Rev 4:372–389, 1996 (72 ).]

 
A recent longitudinal study in Pima Indians did examine both these variables taking into account the known hyperbolic relationship between insulin sensitivity and ß-cell function (83). In the study in this very high-risk population, 48 subjects with normal glucose tolerance were followed for an average of 5 yr with multiple measures being performed over this period (Fig. 6Go). Seventeen of these individuals progressed from normal glucose tolerance through impaired glucose tolerance to diabetes. In the subjects who progressed to hyperglycemia, insulin secretion declined progressively by 78% whereas insulin sensitivity declined by 14%. In the 31 individuals who did not develop diabetes, a similar 11% decrease in insulin sensitivity was associated with a 30% increase, rather than a decrease, in insulin secretion. Examination of the data obtained in these two groups at baseline provides another interesting observation. At their initial assessment, when all subjects had normal glucose tolerance, those subjects who comprised the group that subsequently progressed to diabetes had ß-cell function that was markedly decreased for their degree of insulin resistance compared with those subjects who did not progress over time. In fact, at the time of this baseline assessment, these individuals fell below the 95th confidence interval for the relationship between insulin sensitivity and insulin secretion, in keeping with the existence of a severe impairment of ß-cell function (Fig. 6Go). With the concept of a hyperbolic relationship between insulin sensitivity and ß-cell function in mind, reexamination of data from other studies of high-risk subjects with normal glucose tolerance would suggest that the conclusions in those papers regarding ß-cell function may, in fact, be erroneous given the lack of a difference in insulin levels despite the presence of insulin resistance in high-risk subjects (84, 85, 86, 87, 88, 89, 90).



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Figure 6. Changes in ß-cell function measured as the acute insulin response to glucose (AIR) relative to changes in insulin sensitivity measured by the clamp technique at a low insulin concentration (M-low). These measurements were made in 11 Pima Indians in whom glucose tolerance deteriorated from normal glucose tolerance (NGT) to impaired glucose tolerance (IGT) to diabetes (DIA) (progressors), and in 23 subjects who maintained normal glucose tolerance (NGT) throughout (nonprogressors). The lines represent the prediction line and the lower and upper limits of the 95% confidence interval of the regression between the AIR and M-low as determined from a population of 277 Pima Indians with normal glucose tolerance. [Reproduced with permission from C. Weyer et al.: J Clin Invest 104:787–794, 1999 (83 ).]

 
In addition to the first-phase insulin response, reductions in ß-cell function have also been demonstrated in other studies that have used a variety of different approaches (16, 27, 49, 71, 75, 91, 92, 93, 94). However, in the absence of measurements of insulin sensitivity, not all the studies have come to the same conclusion. Nevertheless, when one tallies all the studies, it does seem that reductions in the early phase insulin response to oral glucose (27, 91, 92), second-phase insulin secretion in response to iv glucose (93), glucose potentiation of ß-cell function (94), pulsatile insulin secretion (16), oscillatory insulin release (49, 71, 75), and the ability of glucose to entrain insulin secretion (49, 71, 75) are all present in subjects who would be predicted to be at very high risk of subsequently developing type 2 diabetes. The subject groups from whom these conclusions have been drawn include those typically thought to be at high risk such as first-degree relatives, women with a history of gestational diabetes or polycystic ovarian disease and individuals with impaired glucose tolerance.

If progressive ß-cell dysfunction is likely to be an important component in the pathogenesis of type 2 diabetes, it would be anticipated that other changes in ß-cell function may also be present before the development of fasting hyperglycemia. A disproportionate elevation in proinsulin levels has been demonstrated in individuals who typically have reductions in glucose-stimulated insulin secretion such as individuals with impaired glucose tolerance (34, 95), and such an alteration in proinsulin levels can be found at baseline in subjects who progress to type 2 diabetes over a 5-yr follow-up period (Ref. 96 ; S. Haffner, personal communication). As would be anticipated for scenarios that represent milder alterations in ß-cell function, the magnitude of these changes is intermediate between those in healthy subjects and those with diabetes. These data suggest that alterations in the efficiency of proinsulin to insulin processing probably occur before the time of clinical diagnosis.

In keeping with the changes in insulin secretion, the release of IAPP is also diminished in groups of subjects with impaired glucose tolerance (26, 27). The known colocalization of IAPP with insulin in the same secretory granules (58) and the corelease of these two peptides (27, 59, 97, 98) suggests that insulin sensitivity is likely to also be an important modulator of IAPP secretion and plasma levels. We have shown this to be the case (99), and using this information, we have recently been able to demonstrate that IAPP release is diminished in first-degree relatives of individuals with type 2 diabetes, when considered in light of the prevailing degree of insulin sensitivity (100). However, while IAPP release is reduced, it does not seem to provide any additional information beyond that obtained with insulin as a marker of ß-cell function.

In summary, examination of a number of different parameters of ß-cell function highlight that it is reduced well before the onset of hyperglycemia and that this seems to be a generalized event. The effect of this progressive decline in ß-cell function is a transition from normal glucose tolerance through impaired glucose tolerance to diabetes (Fig. 7Go). The mechanism(s) that underlies this change and the progressive decline in this measure is the subject of a great deal of research.



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Figure 7. Model for the relationship between progressive ß-cell dysfunction and deterioration of glucose tolerance. As ß-cell function declines, glucose tolerance deteriorates so that the criteria for impaired fasting glucose and/or impaired glucose tolerance are reached. Subsequently, with a further loss of ß-cell function, diabetes develops based on either or both the fasting and 2-h glucose levels.

 
Potential mechanisms for the progressive ß-cell failure in the pathogenesis and progression of type 2 diabetes

A number of different hypotheses have been advanced as explanations for the development of ß-cell dysfunction in type 2 diabetes. These include ß-cell exhaustion due to the increased secretory demand arising from insulin resistance (101), desensitization of the ß-cell due to the elevations in glucose (102, 103), lipotoxicity (104), and a reduction in ß-cell mass, the latter possibly due to amyloid deposition (105, 106). The argument made for the existence of a ß-cell defect in high-risk subjects and the progression thereof would suggest that these alterations should also be present before the clinical diagnosis of diabetes. What evidence then supports one or more of these possibilities?

It is well accepted that under normal circumstances insulin resistance increases the secretory function of the ß-cell (64, 65). This increase in the need for insulin biosynthesis and release has led to the suggestion that over a prolonged period of time, the increasing demand associated with increasing resistance will result in "exhaustion" of the ß cell so that it will ultimately fail (101). There are a number of observations that would tend to argue against this as a primary mechanism involved in the pathogenesis of type 2 diabetes. First, insulin resistance is common, occurring in nearly all obese subjects (65, 68, 107). However, even though diabetes is more prevalent in obese subjects, only a small proportion of obese individuals ultimately develop diabetes (108). Second, the recent longitudinal data from the Pima Indians highlights the fact that ß-cell function is enhanced in apparently healthy subjects as insulin resistance progresses (83). Third, induction of short-term experimental insulin resistance with nicotinic acid is associated with an adaptive increase in ß-cell function manifest as increased insulin release and a decrease in the proportion of proinsulin in plasma (109, 110). Therefore, it would seem that a failure to adequately adapt to insulin resistance may be due to a genetically programmed ß-cell abnormality associated with an inability of the normal ß-cell to adapt to insulin resistance and increased secretory demand thus uncovering a defect in ß-cell function. On the other hand, the ß-cells in those without such a genetic lesion would adapt and prevent the development of hyperglycemia.

Glucose has been suggested to not only be a ß-cell stimulant but to also potentially modify ß-cell function in a deleterious manner. This concept is known as "glucose toxicity" or "glucose desensitization" (102, 103). It has been demonstrated in vitro when islets or ß-cell lines have been exposed to increased glucose concentrations (103) and in vivo in animal models in which ß-cell mass has been surgically reduced (111) or glucose levels have been dramatically increased by administration of a continuous glucose infusion (112). All these instances have been associated with reductions in insulin secretion in response to typical secretagogues including glucose itself. In vitro, the increased glucose levels have been associated with a reduction in the expression of the insulin and PDX-1 genes (113), the latter the gene responsible for regulation of ß-cell replication (114). Balancing these findings are observations in humans that would suggest that glucose toxicity may not be a primary factor in the loss of ß-cell function observed in individuals as they progress from a state of high risk to fasting hyperglycemia or possibly even in the early stages of diabetes. Thus, in apparently healthy human subjects, the continuous infusion of glucose for periods of up to 42 h is not associated with a decrease in insulin release but rather has been shown to increase the first- and second-phase insulin responses to iv glucose (115, 116) and to enhance the potentiating effect of glucose on insulin secretion (117). The same intervention in individuals with type 2 diabetes does not result in these changes suggesting that prior prolonged exposure to hyperglycemia may deleteriously impact this adaptive response (118). It is of interest that the glucose levels attained in these studies in healthy subjects were equivalent to or exceeded those that have been shown to be associated with the loss of the first-phase response in subjects at high risk of developing the disease (11). In addition to the findings regarding stimulated secretion, infusion of glucose has been demonstrated to produce changes that suggest that the efficiency of proinsulin processing is enhanced (119). This latter finding is of interest as it has been demonstrated that the enzymes responsible for proinsulin to insulin conversion within the ß-cell are up-regulated by exposure to glucose (120, 121). Whether this functional change does not occur in the presence of diabetes is unknown. Finally, the findings in the UKPDS would suggest that in the early stages of diabetes, glucose is unlikely to be a critical factor in determining the progression of ß-cell dysfunction. This suggestion comes from the observations made in the group who received intensive therapy. In these individuals, glucose control was normalized the first year after commencement of the intervention, based on the reduction of the hemoglobin A1c level into the normal range. Despite this "normalization" of glucose levels and continuation of the therapy that had achieved this level of control, the disease progressed so that over time hyperglycemia returned and worsened (4). Based then on these series of observations, it would seem that glucose may be a factor in reducing ß-cell dysfunction in type 2 diabetes but that this effect is likely to occur later rather than earlier, and may well contribute to ß-cell dysfunction once this secretory abnormality is present.

While we routinely use glucose as the substrate we evaluate when managing diabetes clinically, the disease is a global metabolic disorder that is also characterized by changes in fat and protein metabolism. Thus, it is of interest that recent data obtained in animal models of diabetes have suggested the possibility that changes in lipid metabolism may contribute to the development of ß-cell dysfunction (104). In fact, morphological studies of pancreas samples from rodent models of diabetes have demonstrated the accumulation of triglyceride within islets (122). Whereas it has been demonstrated that lipid accumulates in ß-cells in humans and that this accumulation is increased in older subjects (123), studies of this morphological change have not been reported in humans with type 2 diabetes. Because it is unclear whether it is a causative factor in the development of ß-cell dysfunction, this morphological change should be evaluated to determine whether it occurs in human diabetes and, if so, how frequently. In contrast to conditions that likely lead to the development of glucose toxicity, Westernization and the accompanying increase in dietary fat intake may contribute to alterations in ß-cell function (124). Although human studies have not examined this effect of fat in a systematic way, studies examining the effect of differences in carbohydrate intake provide indirect support as they involved a reciprocal alteration in the proportion of calories derived from fat. In these studies, the increase in dietary carbohydrate (and decrease in dietary fat) resulted in improved glucose tolerance as a result of an increase in insulin secretion and an improvement in insulin sensitivity in older subjects (125, 126) and individuals with type 2 diabetes (127). More rigorous examination of the effects of increased dietary fat intake and/or altered fat metabolism on ß-cell function has been performed in mice (128) and dogs (129, 130). In both species, ingestion of a high-fat diet was associated with reductions in insulin release determined in vitro (128) and in vivo (129, 130). In the canine studies, insulin sensitivity declines as the dogs become obese but when ß-cell function declines, glucose tolerance deteriorates (129, 130). What mechanism(s) underlies this effect of dietary fat has not been established. As the development of obesity commonly results in an accumulation of intra-abdominal fat that appears to be a metabolically active fat depot (131), it is possible that factors emanating from fat may be the critical mediator. One candidate is free fatty acids, the fluctuations of which are known to be critical for the maintenance of ß-cell function. However, chronic increases of this nutrient may have a deleterious effect on the ß-cell (132, 133, 134). This adverse effect seems not only to result in a decline in insulin release but may also have an effect to reduce the efficiency of proinsulin to insulin conversion within the ß-cell (133, 134). Recent data would suggest that for fatty acids to have a deleterious effect on ß-cell function and for esterification to occur so that neutral lipids can accumulate in the islet, hyperglycemia may also need to be present (135). These data are somewhat in contrast with those from Boden et al. (136) who failed to observe a deleterious effect of fatty acids on insulin secretion during a 48-h infusion of lipids along with heparin in humans. Whether or not it turns out that fatty acids are critical, other candidate molecules derived from adipose tissue may also play a role. These would include leptin that is released largely from sc fat and the cytokine TNF-{alpha}, both of which have been suggested to decrease ß-cell function in vitro (137, 138, 139, 140, 141).

The potential importance of reduced ß-cell mass to explain impaired maximal secretory capacity for insulin secretion has also been raised by a number of studies that have shown that this measure is reduced in individuals with type 2 diabetes (142, 143, 144). However, this reduction in mass cannot explain the entire pattern of functional changes observed in type 2 diabetes. The etiology of this mass reduction may be multifactorial. It is possible that an increase in programmed cell death, known as apoptosis, may occur as a result of the deranged metabolic state such as elevation in glucose and free fatty acids (145, 146). The observation of amyloid deposits a century ago provides another plausible mechanism to explain a portion of the reduced ß-cell mass (106, 147). The relationship between these amyloid deposits and glucose metabolism has been difficult to examine in humans, but has been studied in animal models. In nonhuman primates, it has been shown that the accumulation of islet amyloid is associated with a progressive reduction in both insulin secretion and glucose tolerance (148). In this study, the development of fasting hyperglycemia was a late event and only occurred after there was marked amyloid deposition. This finding suggests that if amyloid related ß-cell mass reduction is the only abnormality contributing to a disturbance in glucose metabolism, a marked degree of mass loss must exist for fasting hyperglycemia to occur. However, because of the multifactorial nature of type 2 diabetes, such a large degree of ß-cell mass reduction is not likely to be necessary in type 2 diabetes. To better study this phenomenon of islet mass reduction by amyloid, transgenic mice bearing the amyloidogenic human IAPP gene have been produced (149, 150, 151). Using these models, we and others have been able to demonstrate that islet amyloidogenesis occurs in mice fed a diet containing increased dietary fat (149), is increased in female mice that are estrogen deficient following oophorectomy (152), is not necessarily increased by hyperglycemia (153), but is associated with reduced ß-cell function and the development of hyperglycemia as amyloid deposition increases (150, 151). These findings are applicable to the clinical syndrome of type 2 diabetes in that the prevalence of the disease is increased in populations consuming diets containing increased quantities of fat (124, 154) and may be reduced by postmenopausal estrogen therapy (155). Finally, in addition to being the amyloidogenic precursor for the large, light microscopy visible amyloid deposits associated with islet mass reduction, IAPP has been shown to form amyloid fibrils that are not identifiable by light microscopy (156). These fibrils have been demonstrated to be toxic to these cells in vitro, thereby resulting in death by apoptosis (157, 158). Thus, the formation of the fibrils themselves may have deleterious effects on ß-cell function before their coalescing to form amyloid deposits visible by light microscopy.

Thus, whereas a large body of work has been performed to try and better understand the pathogenesis of the impairments of ß-cell function in type 2 diabetes, the exact mechanism(s) responsible is still not clearly defined. Rather, it would seem that the deterioration in ß-cell function is multifactorial and multiplicative with the contribution possibly varying from individual to individual. As an example, a model for an interaction of dietary fat, glucose and islet amyloid is illustrated in Fig. 8Go. In this model, in individuals who are genetically determined to be at risk of developing type 2 diabetes, a prolonged increase in dietary fat intake induces ß-cell dysfunction. This reduction in function results in reduced insulin secretion that in turn results in the development of hyperglycemia. This alteration in ß-cell function also involves changes in the manner in which the ß-cell handles the amyloidogenic precursor IAPP and allows islet amyloidogenesis to occur. As these deposits progressively increase, they replace ß-cell mass further aggravating the ability of the islet to produce and secrete insulin. The existence of sustained hyperglycemia in these individuals with impaired ß-cell function further aggravates ß-cell function as a result of "glucose toxicity." Because the disease is progressive, it is likely that these effects feed forward aggravating the clinical syndrome and in most individuals requiring increases in therapy aimed at reducing hyperglycemia.



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Figure 8. Model of the interaction of genetics and the environment to produce ß-cell dysfunction, amyloid, and hyperglycemia. For diabetes to develop, a genetic ß-cell lesion(s) must exist. Consumption of a diet containing increased amounts of fat in conjunction with this genetic defect results in the development of ß-cell dysfunction. This dysfunction is associated with an alteration in insulin secretion so that hyperglycemia develops. In addition, ß-cell dysfunction is associated with the development of amyloid fibrils that contain islet amyloid polypeptide (IAPP). Progressive amyloid deposition results in a loss of ß cells, further reducing insulin release and aggravating hyperglycemia. Formation of amyloid fibrils is also associated with an impairment of ß-cell function. In addition, the presence of hyperglycemia may further aggravate ß-cell function by desensitizing the ß cell.

 
The future for preventing the progressive ß-cell failure of type 2 diabetes

Because it is now becoming apparent that the relentless decline in ß-cell function commences well before the clinical diagnosis of diabetes is made, future approaches to the therapy of the disease have to include attempts at prevention. While we may be fortunate to find a means for doing so before the pathogenesis of this process is fully unraveled, our chances of achieving this goal will be enhanced by gaining a better understanding of the genetic alterations and the metabolic process(es) that underlies this progressive ß-cell dysfunction.

As discussed, there are a number of possible mechanisms responsible for the development of the ß-cell dysfunction of type 2 diabetes. The concept that hyperglycemia and elevated free fatty acids contribute to ß-cell dysfunction would imply that aggressive control of these parameters should result in improved insulin release and could prevent progression. However, based on the UKPDS in which the intensive policy group underwent aggressive glucose control, ß-cell function continued to decline. Whether control of lipids would produce similar or different results is subject to determination. As the deposition of islet amyloid would be predicted to result in an ongoing loss of ß-cell mass, it is possible that a small nidus of amyloid could be sufficient to explain the early progressive failure of ß-cell function observed in type 2 diabetes. Therefore, inhibition of the amyloidogenic process may well require the development of inhibitors that prevent the binding of secreted IAPP to formed fibrils, well before large amounts of amyloid are visible by light microscopy.

Finally, a few recent observations related to peroxisome proliferator-activated receptor-{gamma} raise some interesting possibilities. The discovery of resistin, a peptide that is produced and secreted by adipocytes and is capable of inducing insulin resistance in rodents (159), opens additional avenues for research. It is possible that differences in the release of this peptide may mediate changes in ß-cell function. If so, whether these will result in an increase or a decrease in insulin output remains to be determined. In addition, recent reports of a potential effect of thiazolidinediones to preserve ß-cell function in animal models of diabetes (160, 161) provides the impetus for clinical testing of the interesting possibility that the use of these agents may slow the progressive decline in ß-cell function observed in type 2 diabetes.

Conclusions

Hyperglycemia has conclusively been demonstrated to be an important contributing factor in the development of the ravaging complications of type 2 diabetes. The challenge to attain and maintain normoglycemia is compounded by the progressive nature of the disease that in large part seems to be due to a continuous decline in ß-cell function that starts many years before diagnosis. Whereas a greater number of therapeutic options are available for lowering plasma glucose, none have been shown to reliably slow the progressive loss of ß-cell function. Thus, the future is filled with many challenges that will surely involve genetic, physiological, and pharmacological approaches that likely will have to focus early on the ß-cell to be beneficial.

Acknowledgments

I thank the faculty, collaborators, fellows, and technicians who have helped direct my thinking and thus contributed in no small measure to this manuscript.

Footnotes

This work was supported by NIH Grants DK-02654, DK-17047, DK-50703, and RR-37; the Medical Research Service of the Department of Veterans Affairs; and the American Diabetes Association.

Abbreviations: HOMA, Homeostasis Model Assessment; IAPP, islet amyloid polypeptide; UKPDS, United Kingdom Prospective Diabetes Study.

Received February 5, 2001.

Accepted April 12, 2001.

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