Is the Diminuto/Dwarf1 Gene Involved in Physiologic Adrenocortical Size Regulation and Tumor Formation?

Christian A. Koch and George P. Chrousos

National Institutes of Health Pediatric and Reproductive Endocrinology Branch National Institute of Child Health and Human Development Bethesda, Maryland 20892

Address all correspondence and requests for reprints to: George P. Chrousos, M.D., FACP, FAAP, MACE, National Institutes of Health, Pediatric and Reproductive Endocrinology Branch, National Institute of Child Health and Human Development, Building 10, Room 9D42, Bethesda, Maryland 20892. E-mail: chrousog@mail.nih.gov and kochc{at}exchange.nih.gov

In this issue of JCEM, Sarkar et al. (1) report very interesting data on the diminuto/dwarf1 gene in benign cortisol-producing adrenocortical adenomas. Adrenal nodules are now frequently discovered by contemporary imaging modalities, with an overall prevalence of about 4% (2, 3). These are in accordance with earlier autopsy series, which reported adrenal adenomas in up to 8% of cases (4, 5). The majority of these nodules are hormonally inactive, but when they produce hormones, these are mostly cortisol and/or aldosterone. The molecular pathogenesis of most adrenocortical adenomas and carcinomas remains unknown, but the majority of these lesions are of monoclonal origin (6, 7, 8).

An early and proven approach to elucidate tumorigenesis has been by analyzing hereditary neoplasms, since these already have an identifiable "first hit," the inherited gene defect. Indeed, subsequent hits may be easier identified in familial than in sporadic tumors (9, 10). Signal transduction systems, including G protein-coupled receptors and G proteins, when activated chronically/constitutively in endocrine tissues, may cause an increase in intracellular levels of cAMP and hormone hypersecretion and/or cell proliferation. This is the case for at least 40% of somatotropinomas and toxic thyroid adenomas, in which the GHRH and TSH signaling systems are pathogenetically involved. However, analysis of components of the ACTH receptor (ACTHR) signal transduction system in adrenocortical adenomas or carcinomas has suggested that these are not involved in a major fashion in the pathogenesis of these tumors (7, 11, 12), yet aberrant expression of ectopic G-coupled receptors, such as those of GIP and the catecholamines, have been determined to be of pathophysiologic significance in some adrenal adenomas (13). On the other hand, investigations focused on the involvement of growth factors, such as IGF-II, and components of apoptotic pathways, such as p53, have yielded useful information about the pathogenesis of malignant adrenal neoplasms (7, 11, 12, 14).

Further analysis of apoptotic pathways may shed new light on the pathogenesis of adrenocortical adenomas (15, 16, 17). Inhibitors of apoptosis (IAP), such as the Bcl-2 family of proteins and survivin, are conserved evolutionarily and define a "new" category of "oncogenes," the antiapoptotic genes. Under normal conditions, IAP may regulate homeostasis between the cytoplasm and key intracellular organelles. The critical apoptosis-controlling factors have been defined in the nematode Caenorhabditis elegans, and one such factor is the product of the diminuto/dwarf1 gene. The sequences of diminuto/dwarf1 homologues are highly conserved among Caenorhabditis elegans, plants, and mammals. The genomic sequence of Diminuto spans approximately 37,000 bp encoded by nine exons. The diminuto/dwarf1 protein is 561 amino acids long with a calculated molecular mass of 65 kDa (18, 19). In Arabidopsis plants, diminuto is important in regulating cell elongation (18). Mutant diminuto genes lead to reduced expression of a ß-tubulin gene, TUB1, resulting in smaller plant cells and shorter plants (dwarfs). Plants use campesterol as a precursor for brassinosteroid biosynthesis. Diminuto/dwarf1 is located on the cytoplasmic side of microsomal membranes and is important for the conversion of isofucosterol to sitosterol and of 24-methylenecholesterol to campesterol (18) (Fig. 1Go). Interestingly, ß-sitosterol from pumpkin seed extracts has been successfully used to treat benign prostatic hyperplasia in men (20), suggesting that this plant steroid might lead to reduced expression of diminuto/dwarf1 in the prostate gland and subsequent cell shrinkage.



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Figure 1. Plant steroid biosynthesis. (Modified from Ref. 18.)

 
Recently, the human diminuto homologue termed "selective Alzheimer’s disease indicator 1" (seladin-1) was shown to confer resistance to Alzheimer’s disease-associated neurodegeneration and oxidative stress (19). Seladin-1 is ubiquitously expressed in human tissues. In brains obtained from nondemented subjects, transcripts of seladin-1 were detected in all areas tested with highest levels in the hippocampus and amygdala. In brains of Alzheimer’s patients, seladin-1 expression was reduced in the inferior temporal lobe, amygdala, and hippocampus, compared with neurons in the frontal cortex from the same patients and to those of frontal and temporal cortices from control nondemented brains. Seladin-1 exerts antiapoptotic effects by inhibiting caspase 3 activation. Also, because it contains a domain characteristic for oxidoreductases, it may be required for flavin adenine dinucleotide-dependent oxidation of a specific metabolic intermediate necessary for cell growth and differentiation. Memory loss, hippocampal, and brain cortical atrophy are well recognized complications of long-term glucocorticoid excess (21, 22, 23, 24) and might be related to reduced expression of an "antiapoptotic" diminuto type protein.

In peripheral human tissues, high diminuto transcript levels are found in the adrenal gland, prostate, lung, fetal liver and other tissues and organs (19). It is, therefore, not surprising that Sarkar et al. (1) found diminuto expression throughout the adrenal cortex zones, with the highest transcript levels in the zona fasciculata. Whether cortisol production by these adrenocortical adenomas was caused by diminuto and/or ACTHR overexpression remains unknown. The latter was suggested by a study showing persistent expression of ACTHR in adenomatous tissue, implicating a role in the autonomous production of cortisol via intracellular cAMP elevations (11). Lack of ACTH, a trophic factor for the adrenal cortex, leads to increased apoptosis and adrenal atrophy. Indeed, a decrease of ACTH secretion by the CRH antagonist antalarmin leads to marked increases in adrenocortical apoptosis and diminution of adrenal size, mostly as a result of zona fasciculata atrophy, whereas stress has the opposite effect (i.e. a marked decrease in adrenocortical apoptosis and adrenal hypertrophy/hyperplasia) (16). How crucial diminuto expression is in the physiologic regulation of adrenal size is unknown but testable (Fig. 2Go). On the other hand, because of the antiapoptotic properties of seladin-1, diminuto overexpression in adrenocortical adenomas may participate in their pathogenesis, as a result of the increased growth of the clones that overexpress it (1).



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Figure 2. Diminuto expression and the hypothalamic-pituitary-adrenal axis. In a patient with a cortisol-producing adrenal adenoma, the adrenal cortex adjacent to the adenoma and the contralateral adrenal cortex show reduced diminuto expression, increased apoptosis and atrophy, whereas the adenoma itself has increased diminuto expression and diminished apoptosis. Increased cortisol levels in this patient cause reduced CRH and ACTH secretion, and might lead to reduced diminuto expression and increased apoptosis in the hypothalamic and hippocampal brain regions with subsequent atrophy in these areas. In a patient with normal functioning adrenal glands, diminuto expression and apoptosis are regulated by physiological needs and exposure to different radicals and toxins, with variable cortisol production rates.

 
Although it remains to be determined whether diminuto overexpression in adrenocortical adenomas plays a role in tumor formation, other IAP such as survivin and bcl-2 have been shown to serve as prognostic markers in other endocrine tumors. For instance, overexpression of survivin and/or bcl-2 is a highly significant independent risk factor for disease progression in neuroblastomas (25, 26). However, such overexpression might be related to gene amplification by genomic instability (27): in neuroblastoma, there is gain of 17q25 containing the survivin locus. In adrenocortical neoplasms, previous studies only examined selected chromosomal regions for genetic aberrations (7). It remains unknown whether amplification of 1p32 containing the diminuto locus occurs in adrenocortical adenomas, possibly explaining the overexpression of diminuto in these tumors, another testable hypothesis.

Footnotes

Abbreviations: ACTHR, ACTH receptor; IAP, inhibitors of apoptosis; seladin-1, selective Alzheimer’s disease indicator 1.

Received September 13, 2001.

Accepted September 13, 2001.

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