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From: Contemporary Endocrinology: Androgen Excess Disorders in Women:
Polycystic Ovary Syndrome and Other Disorders, Second Edition Edited by: R. Azziz et al. © Humana Press Inc., Totowa, NJ
26
Molecular Mechanisms of Insulin Resistance in the Polycystic Ovary Syndrome
Theodore P. Ciaraldi
SUMMARY
Whole body resistance to the glycemic actions of insulin is a frequent occurrence in individuals with poly- cystic ovary syndrome (PCOS). Reductions in insulin action on glucose metabolism have been demonstrated in a number of tissues or cells from PCOS subjects. These involve changes in either, or both, the sensitivity and maximal responsiveness to insulin. Meanwhile, the mitogenic and ovarian steriodogenic actions of insulin are intact. Altered phosphorylation of key insulin-signaling intermediates are prime candidates for impaired insulin signal transduction that persists after correction of the in vivo metabolic and hormonal environment.
Key Words: Glucose transport; insulin receptor; insulin receptor substrates-1/2; serine phosphorylation;
skeletal muscle; adipose tissue; polycystic ovary syndrome; insulin resistance.
1. INTRODUCTION
The high frequency of impaired glucose tolerance and insulin resistance for control of glucose metabolism in polycystic ovary syndrome (PCOS) has long been recognized, and further understand- ing of the nature of this aspect of PCOS depends on the answers to a number of questions:
1. Which tissues and responses display insulin resistance? More specifically, does insulin resistance extend to the ovary and steroidogenesis?
2. Does insulin resistance in PCOS differ from that in type 2 diabetes?
3. Which aspects of insulin resistance in PCOS are acquired from the hyperinsulinemic, hyperandrogenic, and hyperlipidemic environment characteristic of the syndrome, and which might be intrinsic or genetic in cause?
4. What are the specific molecular mechanisms of this insulin resistance; what events in insulin action might be altered?
Because an earlier chapter (see Chapter 24) dealt with the confluence of insulin resistance and hyperinsulinemia, this chapter will focus on studies in tissues from subjects with PCOS, which reveal both acquired and intrinsic aspects, and in cells cultured from similar subjects, where the environ- ment can be controlled and intrinsic features revealed. Findings from these in vitro investigations will also be related, when possible, to behaviors in the intact individual. One point that will soon become apparent is that insulin action in PCOS is highly heterogeneous, and care must be taken in generalizing from results in specific cohorts of subjects.
2. BACKGROUND
2.1. Insulin Sensitivity and Responsiveness
In the consideration of insulin action at either the whole body, tissue, or cellular level, attention
must be paid to two concepts: sensitivity and responsiveness. Insulin sensitivity refers to the shape of
the dose–response curve for a particular action and describes the effect that is attained at a certain hormone concentration, regardless of the final capacity of that system. Impairments in this parameter have been described as “receptor defects.” Responsiveness describes the absolute change from baseline attained at a maximal hormone level; differences here are ascribed to “postreceptor” or
“effector” defects. Such defects can be present independently or in concert.
Although insulin is a highly pleotrophic hormone, responses can be categorized in two general classes. Metabolic responses include control of glucose, fatty acid, and protein metabolism, often occuring after acute hormone exposure and involving changes in protein activity. Mitogenic responses include the control of proliferation, differentiation, cell survival, and modulation of gene expression.
2.2. The Basics of Insulin Signaling
Work over the last two decades has revealed much about the processes by which insulin signaling
occurs to these ends (reviewed in ref. 1). Figure 1 presents a simplified schematic illustration of the
most studied pathways. A number of other pathways and intermediates have been identified, such as
the APS/CAP/cbl pathway, or atypical protein kinase Cs, for the stimulation of glucose transport, but
because there are no data regarding these alternative pathways in PCOS, they will not be considered
here. There are several crucial things to understand about insulin signal transduction as it relates to
insulin resistance. The initial event in the process is binding of insulin to its cell surface receptor. The
receptor is a heterotetramer, consisting of two extracellular D-subunits, containing the hormone rec-
ognition sites, and two membrane-spanning E-subunits. Binding of the hormone causes a conforma-
tional change in the D-subunits that is transmitted to the E-subunits, resulting in activation of the
Fig. 1. Schematic for major pathways and events involved in insulin signaling for metabolic and mitogenic responses. IRS, insulin receptor substrate; PI3K, phosphotidylinositol 3-kinase; Akt, protein kinase B; GS, glycogen synthase; GSK3, glycogen synthase kinase 3; GLUT4, insulin-dependent glucose transporter; mTOR, mammalian target of rapomycin; p70S6K, ribosomal protein S6 kinase; SOS, son of sevenless; MAPK, mito- gen-activated protein kinase.tyrosine kinase activity intrinsic to the E subunit. The first substrate phosphorylated is the E-subunit itself (autophosphorylation), further activating the kinase activity. This autophosphorylation creates recognition sites for domains on other molecules, recruiting them to the receptor. Key among these are the insulin receptor substrate (IRS) proteins. There are four different IRS proteins with only partial overlap in function. IRS proteins can then be tyrosine phosphorylated by the receptor kinase, permitting them to dock with other proteins. Most important of these is the lipid kinase phosphati- dylinositol 3-kinase (PI3-K), whose activity is increased when associated with IRS proteins. The lipid product of PI3-K, phosphatidylinositol-3,4,5-trisphosphate, is able to activate a further down- stream lipid-dependent serine kinase, and this phosphorylation cascade continues on to endpoints such as activation of the rate-limiting enzyme for glycogen storage, glycogen synthase. Other kinases are involved in stimulating the translocation of a specific form of glucose transports protein from intracellular pools to the cell surface, where they can mediate increased glucose entry into the cell.
The IRS proteins are also involved in signaling to mitogenic responses, although other docking or scaffolding proteins function there as well. Phosphorylation of those docking proteins, IRS-1/2 and Shc, activates a separate phosphorylation cascade. These signaling pathways are not mutually exclusive—
crosstalk also occurs. Much less is known about the behavior of the mitogernic pathway in PCOS.
An important general principle in insulin signal transduction is that these serine phosphorylations can be either activating or deactivating, depending on the target. Indeed, an important regulatory mechanism is serine phosphorylation of IRS-1 and IRS-2: both protein kinase C (PKC) ]/O and GSK3 can accomplish this feat (2). Serine phosphorylation of IRS-1 reduces its ability to be tyrosine phos- phorylated by the insulin receptor and to associate with PI3-K. This represents an important feedback mechanism for limiting insulin action. Serine phosphorylation of IRS-1 and IRS-2 is also elevated in a number of insulin-resistant conditions (e.g., diet-induced obesity, stress, hyperinsulinemia [2]), implicating it as an important control point.
2.3. Insulin Signaling in PCOS
In the vast majority of studies of PCOS, insulin action has been characterized from the homeo- static model assessment for insulin resistance (HOMA-IR), the 75-g oral glucose tolerance test (OGTT), or the modified frequently sampled intravenous glucose tolerance test. There is still a de- bate about the reliability of such measures (3) in different conditions, but one obvious shortcoming is that these approaches do not permit assessment of the role of different tissues in glucose homeostasis, as is possible with the glucose clamp procedure. In the most complete study of in vivo insulin action to date, Dunaif and colleagues performed multiple-dose hyperinsulinemic-euglycemic clamps in non- obese and obese subjects with PCOS, along with body mass index (BMI)-matched normal cycling controls (4). With regard to insulin-mediated whole body glucose disposal (IMGD), individuals with PCOS displayed a reduced IMGD at the highest insulin infusion tested as well as rightward shifts in their dose–response curves. Thus, those with PCOS had impairments in both insulin responsiveness and sensitivity for IMGD.
A different set of results were obtained for insulin suppression of hepatic glucose production
(HGP): maximal suppression was attained in all subject groups, whereas sensitivity was altered only
in the obese PCOS group (4), suggesting that the nature of insulin resistance may vary between
tissues. It is interesting to note that subjects with type 2 diabetes, when compared with nondiabetic
subjects matched for obesity, display insulin-responsiveness defects for both IMGD and HGP (re-
viewed in ref. 5) but essentially normal insulin sensitivity. Thus, an added feature of PCOS is the
impact on insulin sensitivity. Studies performed in isolated cells have provided additional insight
into the nature of insulin action in PCOS. The literature will be discussed first from the perspective of
how it relates to the issues of insulin sensitivity and responsiveness and potential differences between
tissues. After that the impact of PCOS on individual steps in insulin signaling will be considered.
2.4. Insulin in Individual Tissues in PCOS 2.4.1. Studies in T Lymphocytes
One of the first such studies employed phytohemagglutinin-activated T lymphocytes (6). While not a classic insulin target tissue, T lymphocytes do respond to insulin by increasing glucose utiliza- tion, including stimulation of pyruvate dehydrogenase (PDH). T lymphocytes were isolated from normal cycling controls, subjects with PCOS, and those with both PCOS and type 2 diabetes (PCOS- T2D): all subjects were obese, and samples were cultured for 3 days before acute exposure to varying concentrations of insulin. Stimulation of PDH in PCOS-T2D T lymphocytes was lower than control and PCOS groups at all insulin concentrations. In PCOS cells, stimulation of PDH was reduced at a submaximal hormone level but normal at the maximal insulin level (6). Thus, T lymphocytes from PCOS subjects display reduced insulin sensitivity and normal responsiveness, whereas the combina- tion of PCOS and diabetes introduced impaired responsiveness.
2.4.2. Studies in Adipocytes
Several reports in a more relevant insulin target, adipocytes isolated from the subcutaneous abdominal depot, followed soon after. One report studied obese PCOS and control subjects (7), another used non-obese subjects (8), and a third included both non-obese and obese women with PCOS (4). Taking stimulation of glucose transport as the insulin response, all three laboratories found the dose–response curves for PCOS adipocytes to be right-shifted compared to the weight-matched controls. The reduction in insulin sensitivity, calculated from the half-maximally effective insulin concentration (ED
50), ranged from 50% (4) to eightfold (Fig. 2) (7). The work of Dunaif et al. (5) was able to separate the PCOS-related reduction in sensitivity from that resulting from obesity. The impact on insulin responsiveness was more variable, with no difference from controls in one report (7) and a reduction in others (4,8). Resistance was also present for the ability of insulin to suppress catechola- mine-stimulated lipolysis, with reduced sensitivity seen by several groups (8,9). Again there was heterogeneity regarding insulin responsiveness: normal in one report (9) and impaired in another (8).
Thus, impaired insulin sensitivity is a common feature of PCOS, both at the whole body level (4) and in isolated adipocytes (4,7,8).
One possible source of variability in results on insulin responsiveness is that the PCOS subjects for these reports were classified on the basis of reproductive criteria and not segregated or stratified according to glucose tolerance. Although none of the PCOS subjects in the report of Ciaraldi et al. (7) were diabetic, the presence of impaired glucose tolerance was not ruled out, while a number of the obese PCOS subjects studied by Dunaif et al. (4) had abnormal responses to an OGTT, including several with type 2 diabetes. Although the findings in isolated adipocytes are generally reflective of insulin action on whole body glucose metabolism, it will be interesting to see if the behavior of adipocytes can distinguish between PCOS subjects with normal or abnormal glucose tolerance.
2.4.3. Studies in Fibroblasts
Because isolated adipocytes are studied within several hours after tissue collection, their behavior is greatly influenced by the in vivo metabolic environment: levels of substrates, metabolites, and hormones. Thus, the results from the above studies cannot distinguish between effects on insulin action intrinsic to PCOS, possibly genetic differences, and those acquired from the environment.
Studies addressing this question employed skin fibroblasts cultured from PCOS and control subjects.
Glucose metabolism in fibroblasts is responsive to insulin at the level of glycogen synthesis. Differ- ences in this action are seen between fibroblasts from nondiabetic and type 2 diabetic subjects (10), indicating that insulin resistance is retained in culture and supporting this cell type as a useful model.
Two reports agreed that the mitogenic actions of insulin, measured as thymidine incorporation into
DNA, were normal in PCOS fibroblasts (10,11). Variability was again present for the metabolic
action of insulin, measured as glucose incorporation into glycogen. One group showed reduced sen-
sitivity and responsiveness to insulin (11), and in the other the dose–response curves were
superimposable for control and PCOS fibroblasts, although differences were present in adipocytes from the same subjects (10). Reasons for the differences in results are not readily apparent, but they are indicative of the heterogeneity seen both in vivo and in vitro.
2.4.4. Studies in Muscle
More relevant data about insulin action can be obtained using human skeletal muscle cells grown in culture. We and others have shown that satellite cells from human skeletal muscle tissue can be propagated in culture and differentiated so that they reflect the morphological and biochemical prop- erties of mature skeletal muscle (12). Perhaps most important, such cells from individuals with type 2 diabetes retain defects in glucose metabolism and insulin action similar to those observed in vivo (12). Corbould et al. employed a modification of this system to investigate glucose metabolism in obese subjects with PCOS (13). Surprisingly, they found that glucose uptake was elevated at all insulin levels in PCOS muscle cells, even as there were impairments in insulin signaling (see Subsection 2.5.). This contrasts with our initial work (currently available only in abstract form [14]), where we found defects in skeletal muscle cell glucose uptake that mirrored those seen for whole body glucose disposal. One possible reason for this discrepancy could be that the expression of sev- eral key markers of the myotube phenotype, creatine kinase activity, and a-myosin heavy-chain pro- tein were threefold greater in cells from their subjects with PCOS than in controls (13), suggesting a
Fig. 2. Dose–response curves for insulin receptor occupancy (top), receptor autophosphorylation (middle), and glucose transport stimulation (bottom) in adipocytes and isolated receptors (middle) from age and body mass index (BMI)-matched normal cycling (q) and polycystic ovary syndrome (PCOS) (䊉) subjects. Results are normalized against the maximal activity for each function for each individual, presented as average values.Vertical dashed lines indicate a physiological insulin concentration (0.42 nM). (From ref.8.)
greater degree of differentiation. Because insulin responsiveness for glucose metabolism develops during myotube differentiation, this could complicate comparisons between populations at different stages of differentiation.
2.4.5. Studies in the Ovary
Studies on insulin action in the ovary show greater agreement, perhaps because of less method- ological variation. Most of these studies have been performed in cultured granulosa cells. Wills and Franks showed that insulin stimulation of steroidogenesis, estradiol (E2), and progesterone (P) syn- thesis was mediated through the insulin receptor, not the insulin-like growth factor-1 receptor, and that the insulin receptor was also functioning in granulosa cells from PCOS ovaries (15). Extending those observations, Franks and colleagues found that the insulin sensitivity and responsiveness of stimulation of E2 and P production was normal in PCOS granulosa cells (16,17). In addition, the mitogenic actions of insulin (thymidine incorporation) are also normal in PCOS (18). However, PCOS granulosa cells displayed insulin resistance for several aspects of glucose metabolism. Insulin stimu- lation of lactate production, a measure of glycolysis, was attenuated (17,19), as was stimulation of glucose incorporation into glycogen (18). Insulin responsiveness of glucose metabolism in PCOS granulose cells is so low that it is difficult to ascertain if sensitivity is also altered.
2.4.6. Summary
Even with the variability in data, contributed to by subject heterogeneity, as well as the differing methodologies and experimental systems, several conclusions can be drawn. One is that nonmetabolic actions of insulin, such as mitogenesis (fibroblasts and granulose cells) and steroidogenesis (granu- losa cells), are normal in PCOS. The other is that insulin’s effects on glucose (glucose transport, lactate production, glycogen synthesis) and lipid (antilipolysis) metabolism are impaired in multiple tissues, adipocytes, and granulosa cells, whereas the evidence is mixed in fibroblasts and skeletal muscle cells. This selective insulin resistance would mean that compensatory hyperinsulinemia induced to control circulating glucose levels would result in augmented stimulation of ovarian steroidogenesis. The next key questions are: What are the mechanisms for this selectivity? Where does insulin signaling diverge for metabolic and mitogenic/steroidogenic responses? For this reason, elucidating the mechanism(s) of insulin resistance in PCOS has the potential to greatly expand our understanding of insulin signal transduction.
2.5. Impact of PCOS on Individual Steps in Insulin Signaling
To understand the nature of selective changes in insulin signaling characteristic of PCOS, it will be useful to review step by step what is known about insulin signaling in tissues and cells from PCOS subjects. Starting at the initial event at the target cells, the recognition of insulin at the cell surface, the majority of investigators have found both the number and affinity of insulin receptors to be nor- mal in PCOS. This result was observed in freshly isolated adipocytes (4,7), cultured fibroblasts (10,11), and granulosa cells (18). Expression of the insulin receptor protein was also normal in skel- etal muscle (20) and cultured muscle cells (13). There are exceptions to this behavior, as reduced insulin binding has been reported in adipocytes (8) and T lymphocytes (7). In general, however, the general consensus is that the hormone-binding function of the insulin receptor is intact in PCOS.
The second function of the insulin receptor is the tyrosine kinase activity of the E subunit, both for
autophosphorylation and toward other substrates. Insulin-stimulated autophosphorylation in PCOS
adipocytes displayed a normal sensitivity (Fig. 2), with a modestly reduced absolute maximal
response (7). Kinase activity of partially purified adipocyte insulin receptors against an artificial
exogenous substrate was normal (7), suggesting that this function was intact. That is in contrast to
type 2 diabetes, where receptor kinase activity is greatly impaired (Table 1). Consistent with normal
receptor function, a number of investigators have probed the insulin receptor gene in PCOS subjects
and found either no missense or nonsense mutations (21), or, if single-nucleotide polymorphisms
were identified, they were not associated with insulin resistance (22).
However, Dunaif and colleagues identified a subpopulation (6 of 12 subjects studied) of individu- als with PCOS who displayed reduced insulin-stimulated autophosphorylation in part as a result of augmented basal phosphorylation (23). Fibroblasts cultured from these individuals also showed reduced insulin stimulation of both autophosphorylation and receptor kinase activity against an exogenous substrate (23); receptor phosphorylation was normal in the remainder of the study sub- jects. Interestingly, if fibroblast insulin receptors from those first individuals were purified before assay, insulin action on phosphorylation was normal, suggesting that a factor in fibroblasts extrinsic to the insulin receptor was responsible (24). A defining feature of the receptors from these insulin- resistant subjects was that they had elevated serine phosphorylation in the basal state (23); treatment of their fibroblasts with PKC inhibitors normalized insulin signaling (24). Although the principle of increased serine phosphorylation of insulin signaling molecules (2) as a means of damping insulin signaling was noted earlier, the physiological impact of the increased receptor serine phosphoryla- tion in these specific individuals is uncertain, as there were no correlations between in vitro measures of receptor phosphorylation and in vivo insulin action (23): studies of greater numbers of subjects with this interesting behavior may be needed to clarify the causes and consequences of elevated receptor serine phosphorylation.
Additional data regarding insulin receptor phosphorylation in PCOS are limited. In contrast, receptor autophosphorylation was normal in cultured skeletal muscle cells (13), as was insulin action. Thus, it appears that heterogeneity in PCOS extends to the molecular level, where it is pos- sible to have altered insulin action on metabolic responses in the presence of either normal or impaired receptor kinase activity.
Analyzing the concentration dependence of the initial steps in insulin signaling, receptor binding, and autophosphorylation in comparison to a final response can highlight differences between PCOS and control subjects. Figure 2 shows that at a physiological insulin level (0.43 nM), 30–40% of insulin receptors are occupied and autophosphorylated in adipocytes from both control and PCOS subjects (7). That same concentration resulted in approximately 90% of the maximal stimulation of glucose transport in control adipocytes, suggesting that an amplification of the insulin signal occurred distal to the receptor kinase. However, such amplification does not occur in PCOS adipocytes; the dose–response curves are superimposable for all three parameters. This comparison indicates that insulin resistance in PCOS involves impairment in the efficiency of insulin signal transduction.
Regardless of the magnitude of the final response, to attain a set relative insulin response requires more occupied and activated insulin receptors in PCOS adipocytes. Hyperinsulinemia would be the expected response to compensate for this difference.
Except in the subset of individuals with elevated serine phosphorylation of their insulin receptors (23), the presence of normal receptor function in PCOS places altered signaling further downstream.
The existing data in PCOS extend only to the next two, related, steps; phosphorylation of IRS-1/2 and
Table 1Comparison of Insulin Action and Signaling in Subcutaneous Adipocytes From Obese Subjects With PCOS and Those With Type 2 Diabetes
Activity PCOS Type 2 diabetesa
Glucose transport: maximal response (7) (4,8)
Glucose transport: sensitivity (4,7,8)
Insulin receptor: binding (4,7)
Insulin receptor: autophosphorylation (8)
Insulin receptor: kinase activity (8)
IRS-1: tyrosine phosphorylation
Results presented as relative to activity in age and BMI-matched normal cycling individuals.
aFrom ref.28.
PCOS, polycystic ovary syndrome; IRS-1, insulin receptor substrate-1.
association with/activation of PI3-K. IRS-1-associated PI3-K activity was reduced in skeletal muscle from PCOS women when compared to age and BMI-matched control subjects (20). The reduction was seen at both submaximal (100 mU/mL) and maximal (2000 mU/mL) circulating insulin levels.
This occurred in the presence of normal tissue content of IRS-1 and the p85 regulatory subunit of PI 3-K. The PCOS subjects included in this study included those with both normal and impaired glucose tolerance.
What is unknown is the relationship between in vivo glucose tolerance and insulin action and differences in PI3-K activity. This information may be important because the same investigators reported that insulin-stimulated IRS-1-associated PI3-K activity (normalized to IRS-1 expression) was reduced in skeletal muscle cells cultured from PCOS subjects, even as insulin action on glucose metabolism was normal (13). The reverse situation was observed in cultured fibroblasts; insulin- stimulated IRS-1-associated PI3-K activity was normal while there was insulin resistance for stimu- lation of glycogen synthesis (11). Further studies will be needed to firmly establish the relationship between PI3-K activity and insulin action, if the relationship varies between tissues, and if any such relationship is acquired by the in vivo environment or intrinsic to PCOS and retained during culture.
Although the story regarding IRS-1/2-associated PI3-K activity in PCOS is incomplete, there is interesting information regarding IRS-1/2. Several investigators have reported that the incidence of the Gly972Arg polymorphism in IRS-1 is elevated two- to threefold in PCOS subjects compared to weight-matched controls (25). Individuals homozygous for the Arg972 allele have higher insulin levels and HOMA-IR values, suggesting that altered IRS-1 function resulting from this amino acid substitution could contribute to insulin resistance. However, other investigators have found normal frequency of the Gly972Arg in PCOS (26). Both results have been seen in populations of different ethnicities, so the differences may depend more on other variables, such as the specific definition of PCOS.
Several other lines of evidence suggest that the IRS proteins may play an important role in dis- criminating between metabolic and mitogenic responses and modulating insulin action in PCOS. One is that in peripheral tissues such as skeletal muscle and adipose tissue, the major portion of insulin signaling to metabolic responses is mediated through IRS-1, whereas IRS-2 plays a greater role in the ovary, especially with regard to reproductive function. Immunohistochemical staining showed a decreased expression of IRS-1 in granulosa cells from PCOS subjects compared to follicles from the same stage of development in ovulatory ovaries (27). This would be consistent with a diminution of insulin action on metabolism and a preservation of mitogenic and steroidogenic responses. Cul- tured muscle cells from PCOS subjects display elevated phosphorylation of IRS-1 on Ser
312(13).
Phosphorylation at this site is strongly associated with impaired IRS-1 function and insulin resistance (2). However, it is difficult to reconcile the increased Ser
312-IRS-1 phosphorylation and decreased insulin-stimulated IRS-1-associated PI3-K activity, both features of insulin-resistant states, with the normal insulin action on glucose metabolism seen in the same cells. Further studies will be needed to determine the relationships between signaling pathways and final responses.
One of the key questions in this field of study is if insulin resistance in PCOS differs from that in
type 2 diabetes. The most complete information regarding such comparisons has been gathered in
isolated adipocytes and is summarized in Table 1. In making such comparisons, it is crucial to match
for other confounding variables such as age, obesity, and, of course, gender, also taking into account
the considerable heterogeneity present in the syndrome. When this is done two conclusions become
apparent. One is that insulin receptor function, binding and kinase activity, is essentially normal in
PCOS whereas it is impaired in diabetes. Second, type 2 diabetes is primarily a state of impaired
insulin responsiveness, whereasa major feature of PCOS is defects in insulin sensitivity. Further
comparisons of signaling pathways between these two groups have the potential to reveal much about
the roles of specific signal-transduction events.
3. CONCLUSIONS
Insulin resistance is a common, although not absolute, feature of PCOS, occurring in as much as 50% of subjects. The insulin resistance is present in both lean and obese individuals with PCOS and is exacerbated by obesity. At the level of the whole body, insulin resistance is displayed as a reduced sensitivity to the ability of insulin to stimulate glucose disposal, manifested primarily in skeletal muscle. Studies in freshly isolated adipocytes have shown reduced sensitivity for stimulation of glu- cose transport and suppression of lipolysis, with variable effects on insulin responsiveness. Resis- tance to the metabolic actions of insulin is seen in a number of other tissues, including skin fibroblasts and granulosa cells, with variable findings in skeletal muscle. Nonmetabolic actions of insulin, including stimulation of mitogenesis and E2 and P synthesis, appear to be normal. Insulin resis- tance in PCOS differs in some ways from that present in type 2 diabetes, being primarily a reduction in the efficiency of insulin signal transduction. Potential sites for defective insulin signaling impli- cated in PCOS include elevated serine phosphorylation of the insulin receptor and IRS-1/2, post- translational modifications that impair the function of these proteins. Many populations of PCOS subjects also have an increased frequency of a polymorphism in IRS-1 that is associated with hyperinsulinemia and insulin resistance. Selective defects in insulin signaling to metabolic responses would lead to compensatory hyperinsulinemia, overstimulating steroid production in the normally insulin-sensitive ovary.
4. FUTURE DIRECTIONS
Although several key questions concerning insulin resistance in PCOS have been resolved to some extent—that resistance is present in multiple tissues and is selective for metabolic responses—other questions remain open. Among these are the role of different tissues in whole body insulin action, what aspects of resistance are acquired and which are intrinsic, and conclusive identification of the specific signaling events altered in PCOS. Future studies will need to investigate further downstream events as well as pathways other than the prototypical PI3-K pathway. Given the mounting evidence for improvement of the metabolic and reproductive features of PCOS by treatment with antidiabetic agents, it will be informative to determine the impacts of these treatments of insulin signaling. It will also be important to discriminate between PCOS subjects with normal insulin action and the approxi- mately 50% who are insulin resistant: Is it possible to distinguish at the cellular and molecular level between resistant and sensitive states? In our laboratory we are employing isolated adipocytes and cultured muscle cells obtained from insulin-resistant and -sensitive subjects to address these open questions.
KEY POINTS
• Insulin resistance for metabolic responses (glucose transport, glycogen synthesis) is present in multiple tissues with PCOS.
• Insulin action for stimulation of mitogenesis and steroidogenesis is normal in multiple tissues, including the ovary.
• Insulin resistance for metabolic responses is, in part, an intrinsic property of PCOS tissues.
• Impaired insulin action involves posttranslational modification of signaling molecules, including altered phosphorylation of the insulin receptor and IRS-1/2.
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