Department of Obstetrics and Gynecology, Cedars-Sinai Medical Center, Los Angeles, California 90048
Address all correspondence and requests for reprints to: Ricardo Azziz, M.D., M.P.H., M.B.A., Department of Obstetrics and Gynecology, Cedars-Sinai Medical Center, Suite 160W, 8635 West Third Street, Los Angeles, California 90048. E-mail: . azzizr{at}cshs.org
The polycystic ovary syndrome (PCOS) is one of the most common endocrine disorders, affecting 46% of unselected women of reproductive age. More recently, it has been recognized that in addition to endocrine abnormalities many patients with PCOS demonstrate metabolic aberrations. Most significant among these is the presence of insulin resistance. Overall, approximately 5060% of PCOS patients suffer from insulin resistance (1, 2), compared with a prevalence of insulin resistance in the general population of 1025%, depending on definition and mean population body weight (3, 4, 5).
Insulin resistance in PCOS leads to the development of compensatory hyperinsulinemia, and this hyperinsulinemia seems to play a major role in the pathogenesis of the hyperandrogenism of PCOS. Hyperinsulinemia stimulates androgen secretion by the ovarian theca, excess growth of the basal cells of the skin resulting in acanthosis nigricans, and abnormal hepatic and peripheral lipid metabolism. That insulin resistance and hyperinsulinemia underlie many of the features of PCOS is supported by the fact that the administration of various insulin-reducing or sensitizing agents, including diazoxide, metformin, troglitazone, and d-chiro-inositol, has been found to improve clinical features in many of these patients.
The resistance to the action of insulin in PCOS generally refers to the impaired action of this hormone on glucose transport and lipolysis, principally in adipocytes, in the presence of relatively normal insulin binding (6, 7, 8, 9). Nonetheless, the mechanism(s) underlying the abnormal insulin signaling observed in women with PCOS remains unknown. In this issue of JCEM, Li et al. (10) study the role of increased serine kinase activity in the previously reported subnormal tyrosine autophosphorylation activity of the insulin receptor (IR). The investigators are extending the previous investigations of Dunaif et al. (8), when it was observed that 7 of 14 nondiabetic PCOS women studied demonstrated a significantly lower insulin-induced tyrosine phosphorylation of the IR in cultured skin fibroblasts, compared with controls. Alternatively, in these same women insulin-independent 32Phosphate incorporation into the serine residues of the ß-subunit of the IR was decreased. The remaining PCOS women had basal- and insulin-stimulated IR autophosphorylation similar to controls. This previous report had suggested that increased IR serine phosphorylation possibly decreased the tyrosine kinase activity of the receptor and was one mechanism determining the post-binding defect in insulin action characteristic of PCOS.
In the present study, the investigators cleverly used novel kinase inhibitors and activators, determining their effect on the degree of IR autophosphorylation in cultures of skin fibroblasts from seven women with PCOS and four controls. Presumably, the seven PCOS women included were those who had previously demonstrated increased serine and decreased tyrosine phosphorylation of the IR in cultured skin fibroblasts. The investigators first confirmed that the fibroblasts of the PCOS patients selected had a subnormal insulin-stimulated IR autophosphorylation activity. H7, a nonselective protein kinase inhibitor, completely reversed the defect in IR autophosphorylation, whereas H89, a selective inhibitor of protein kinase A, effected a partial reversal. An inhibitor of protein kinase C, Gö6983, did not have any effect on the decreased autophosphorylation. Small molecule activators of IR tyrosine kinase activity, TLK16998 and Merck L7, enhanced insulin-stimulated IR autophosphorylation. Finally, the degree of autophosphorylation was no different between PCOS and controls after isolation of the IR by immunocapture.
The results of these experiments clearly extend our understanding of insulin action in PCOS. Overall, the data suggest that a factor extrinsic to the IR is responsible for the decreased autophosphorylation previously observed in the PCOS women studied, and that this may be a protein kinase, although not necessarily a serine kinase. Furthermore, the study indicates that activators of tyrosine phosphorylation can reverse the reported defect in IR autophosphorylation, suggesting a novel avenue for therapy. However, there are various caveats that must be considered when assessing these and any studies of insulin signaling in PCOS.
The investigators have chosen to use cultured skin fibroblasts in an effort to study insulin action independent from the in vivo environment. However, the study of tissues that are not primary insulin targets, and consequently contain relatively low levels of IRs, may yield unusual findings. Because skin fibroblasts are not a principal target of insulin action (11), the study of insulin action in these cells generally requires large amounts of study material to obtain a measurable amount of IR. For example, a study of insulin action in the ovary required more than 3 g of tissue to obtain the necessary amount of IR (12). Although not stated, it is also likely that Li et al. (10) required large numbers of confluent fibroblast cultures to complete the phosphorylation experiments. This could then have resulted in artifactual abnormalities in insulin action caused by the concentration of an interfering factor that is normally present only in low or background levels [i.e. the extrinsic inhibiting kinase observed by Li et al. (10) in this issue of the journal]. Nonetheless, the fact that Ciaraldi et al. (6) also observed in adipocytes a 30% decrease in the maximum insulin-induced phosphorylation of the IR is reassuring (6).
Second, in most instances, conclusions regarding insulin signaling in PCOS have been derived from single studies, with little external corroboration. For example, even the present study is based on tissue obtained from the same women included in the original report by Dunaif et al. (8), which presumably had demonstrated an exaggerated insulin-induced serine phosphorylation of the IR. This raises the possibility that the defect observed might be peculiar to the population being studied and may not represent a generalized abnormality in PCOS. In fact, other investigators did not observe impairment in insulin action in the cultured fibroblasts of their PCOS patients (13).
We should also note that the phenotype of PCOS is heterogeneous, and that insulin resistance is not a universal phenomenon in PCOS. Overall, between 25% and 60% of PCOS patients can be considered to have insulin resistance, as assessed by the euglycemic clamp or the frequently sampled iv glucose tolerance test, when body mass-matched controls are used (1, 2). Hence, further corroborative studies are required in different patient populations and tissues, and by different laboratories.
Third, how does the finding of subnormal IR tyrosine phosphorylation result in the cellular phenotype? Intrinsic defects in IR tyrosine autophosphorylation should be expected to have widespread cellular implications. In fact, adipocytes from patients with PCOS appear to have defects in catecholamine-induced lipolysis (14) and in the maximum response or sensitivity of glucose transport to insulin (6, 7, 9, 15, 16). The maximum inhibition (7) or the sensitivity of lipolysis to inhibition (9) by insulin was also found to be lower in sc abdominal adipocytes of PCOS women, although it was normal in omental (visceral) adipocytes (17), compared with controls. Alternatively, and in contrast to the defects in glucose transport and lipolysis, no defects in the mitogenic action of insulin, at least in fibroblasts, has been observed (13, 18).
These data would suggest that the PCOS patients studied have abnormal signaling along the phosphatidylinositol-3 (PI-3) kinase/Akt/GLUT-4 cascade and not the mitogen-activated protein kinase pathway. This makes it unlikely that the defect is at the level of the IR. Consistent with this concept, Cusi et al. (19) studied type 2 diabetes mellitus patients, obese nondiabetics, and lean controls, and reported that insulin resistance was associated with a defect in the PI-3 kinase pathway, but not the mitogen-activated protein kinase cascade. However, few distal components of the insulin signaling pathways in PCOS have been studied to allow for such a determination. Consistent with the observed cellular behavior, in adipocytes the amount of GLUT-4 was found to be lower in PCOS (on either a membrane protein or cell surface basis) than controls (16). Nonetheless, in fibroblasts no difference in basal and insulin-stimulated IR substrate (IRS)-associated PI-3-kinase activity was observed (18). In contrast, in myocytes of PCOS patients a lower IRS-1-associated PI-3 kinase activity compared with controls was observed, with higher IRS-2 but similar IRS-1 and PI-3 kinase p85 subunit content (20). More distal aspects of insulin signaling, beyond IR action and phosphorylation, will need to be studied to more accurately determine the location of the defect in these patients.
The study of insulin signaling in PCOS is fraught with many difficulties, and Li et al. (10) should be congratulated for continuing to probe and determine the mechanisms underlying their initial observations. We also look forward to seeing other studies, hopefully from different centers, corroborating their findings in different patient populations and with other tissue models, and including the study of more distal aspects of the insulin signaling.
Acknowledgments
Footnotes
Supported in part by NIH Grants 2R01-HD29364 and 1-K24-HD01346-01.
Abbreviations: IR, Insulin receptor; IRS, IR substrate; PI-3, phosphatidylinositol-3; PCOS, polycystic ovary syndrome.
Received July 22, 2002.
Accepted July 22, 2002.
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