Insulin resistance in polycystic ovarian disease.

Abstract: The classic polycystic ovarian syndrome (PCOS) was originally described by Stein and Leventhal as the association of amenorrhea with polycystic ovaries and, variably, hirsutism and/or obesity. It is estimated that 5 to 10% of women of reproductive age have PCOS. Although insulin resistance is not part of the diagnostic criteria for PCOS, its importance in the pathogenesis of PCOS cannot be denied. PCOS is associated with insulin resistance, independent of total or fat-free body mass. Postreceptor defects in the action of insulin have been described in PCOS that are similar to those found in obesity and type 2 diabetes. Treatment with insulin sensitizers, metformin, and thiazolidinediones (TZDs) improve both metabolic and hormonal patterns and also improve ovulation in PCOS. Recent studies have shown that women who have PCOS have higher circulating levels of inflammatory mediators such as C-reactive protein, tumor necrosis factor, tissue plasminogen activator, and plasminogen activator inhibitor-1 (PAI-1). It is possible that the beneficial effect of insulin sensitizers in PCOS may be partly due to a decrease in inflammation.

Key Words: polycystic ovarian syndrome, insulin, resistance, sensitizers, thiazolidinediones

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Polycystic ovarian syndrome (PCOS) is characterized by chronic anovulation and hyperandrogenism (hyperandrogenism can exist in the absence of hyperandrogenemia, eg, enhanced tissue sensitivity to androgens) in premenopausal women. According to revised guidelines of the PCOS Consensus Workshop Group, (1) two of the following three manifestations must be present for a diagnosis of PCOS: (1) irregular or absent ovulation, (2) elevated levels of androgenic hormones, and/or (3) enlarged ovaries containing at least 12 follicles each. (1) Polycystic ovaries are defined as those found on ultrasound to contain 12 or more follicles measuring 2 to 9 mm in diameter and/or have an increased volume of 10 mL or greater. The presence of one ovary fulfilling these criteria is enough to meet the definition of polycystic ovaries. Other hyperandrogenic disorders such as nonclassic congenital adrenal hyperplasia and androgen-secreting tumors must be excluded before a diagnosis of PCOS can be made. It should be stressed that polycystic ovaries are not a necessary feature of PCOS and that many women with polycystic ovaries do not have PCOS. Women found to have polycystic ovaries on an incidental ultrasound should not be diagnosed with PCOS unless there is corroborating clinical evidence of the syndrome. Androgen excess may present with or without skin manifestations. It is estimated that 5 to 10% of women of reproductive age have PCOS. (2) Around 50% of women with PCOS are obese and tend to have an android pattern of obesity. (3) Chronic anovulation may present as irregular menstrual periods or amenorrhea. It is not necessary to document anovulation by ultrasonography or progesterone measurements in the presence of a clear clinical history. In fact, PCOS occurs in 85 to 90% of women with oligomenorrhea and in 30 to 40% of women with amenorrhea. (4) Anovulation in PCOS is associated with steady levels of gonadotropins and ovarian steroids. Women with PCOS are thus in a "chronic estrous state." Constant estrogen exposure leads to proliferation and hyperplasia of the endometrium, which can lead to unpredictable bleeding episodes. Unopposed estrogen exposure can be confirmed by a progesterone withdrawal test (medroxyprogesterone acetate 10 mg/d for 10 days), done after a negative urine pregnancy test.

Gonadotropins, androgens, and ovarian steroids in PCOS

Women with PCOS have higher mean concentrations of luteinizing hormone (LH), increased bioactivity of LH, and low to low-normal levels of follicle-stimulating hormone (FSH). (5,6) The precise mechanism(s) responsible for enhanced LH secretion in PCOS are not completely understood, although past studies have demonstrated the potential influence of hypothalamic GnRH activity and ovarian steroid feedback. (7-10) Insulin has also been implicated as a potential regulator of LH secretion in PCOS. In vitro studies have shown that cultured rat anterior pituitary cells exposed to insulin exhibited increased basal and GnRH-stimulated LH and FSH release in a dose-dependent manner. (11-13) By comparison, in vivo studies involving indirect manipulation of serum insulin levels through administration of insulin-lowering drugs or dietary caloric restriction have not yielded consistent results as to the effect of insulin on gonadotropin secretion. (14-18) It was observed recently by Mehta et al (19) that increased LH secretion in women with PCOS as well as in women without PCOS was unaltered by prolonged insulin infusion. Pulsatile LH release and gonadotropin responses to multidose GnRH were similar before and during a 12-hour hyperinsulinemic, euglycemic clamp. It was thought that lack of insulin effect may have been the result of insulin resistance, which is a common feature of PCOS. (20,21) Later, it was demonstrated that even after improvement of insulin sensitivity with pioglitazone treatment, there was no difference in baseline LH values, LH pulsatility, or maximally stimulated percent LH increment after GnRH with or without insulin infusion in women with PCOS. (22) Previously, it was believed that an LH/FSH ratio of greater than 2 was part of the diagnostic criteria of PCOS. Obese women with PCOS, however, do not have elevated LH levels; therefore a normal LH level or normal LH/FSH ratio does not rule out PCOS. In fact, the LH/FSH ratio is no longer included in the diagnostic criteria for PCOS. (23) Under the influence of low but constant levels of FSH, multiple follicles of the ovary are stimulated but do not achieve maturation. The lifespan of the follicles may extend over several months, leading to multiple follicular cysts. Luteinized in response to constant and relatively high LH levels, these "arrested" follicles provide a constant supply of steroids. The atretic follicle becomes an androgenic follicle by default because atretic follicles are deficient in aromatase activity. Cultured follicular cells from the small follicles of polycystic ovaries produce small amounts of estradiol but show a dramatic increase in estrogen production when stimulated by FSH or insulin-like growth factor (IGF)-1. (24) FSH therapy induces a larger cohort of follicles to develop in women with PCOS when compared with other infertile women. (25,26) A deficient in vivo ovarian response to FSH, possibly due to impaired interaction between signaling pathways associated with FSH and IGF-1, may be a key event in the pathogenesis of anovulation in PCOS. Hyperandrogenism is usually suggested by the presence of hirsutism (approximately 80% of women with PCOS) and can be documented by measuring androgen levels in the blood. Free testosterone is the most frequently elevated steroid in the blood in PCOS. Circulating levels of total testosterone, androstenedione, and dehydroepiandrosterone (DHEA) are also elevated. In obese women with PCOS, sex hormone binding globulin (SHBG) levels are decreased (a well-known effect of obesity), which leads to an increase in free testosterone levels. Furthermore, insulin is a negative regulator of SHBG production by the liver, (27) and SHBG levels are decreased in hyperinsulinemic conditions such as metabolic syndrome and visceral obesity. (28,29) Interestingly, concentrations of sulfated DHEA (DHEAS) are also increased in the blood. DHEAS is secreted exclusively by the adrenal glands. The mechanism of increased DHEAS production by the adrenals is not yet known, although insulin and IGF-1 have been shown to upregulate adrenal 17-hydroxylase and 17,20-lyase activity. (30)

PCOS, inflammation, and cardiovascular disease

Insulin resistance has been associated with an increased incidence of cardiovascular disease, and atherosclerosis is now considered to be an inflammatory disorder. (31,32) Insulin resistance has recently been associated with increased levels of inflammatory mediators in the blood. (33,34) Studies have therefore been conducted to look at inflammation in PCOS Gonzalez et al (35) noted increased levels of tumor necrosis factor (TNF)-[alpha] (the cytokine that causes insulin resistance and is secreted by the adipose tissue) in women with PCOS as compared with control subjects. Interestingly, lean women with PCOS had higher TNF-[alpha] levels than normal lean women, whereas the levels were similar in obese women with PCOS and obese control subjects. Kelly et al (36,37) noted increased C-reactive protein levels and tissue plasminogen activator (t-PA) levels in women with PCOS as compared with healthy weight-matched control subjects. However, when adjusted for insulin sensitivity, C-reactive protein was no longer significantly different between groups, but t-PA levels remained significantly different. Women with PCOS also have higher PAI-1 activity and higher fibrinogen levels than control subjects. (38) However, in another study, PAI-1 levels were not significantly different from control subjects when adjusted for body mass index (BMI). (39) Glueck et al (40) demonstrated that PAI-1 activity was an independent risk factor for miscarriages in PCOS. Although the above studies suggest that PCOS is associated with a state of increased inflammation, clinical studies have yet to definitively demonstrate an increased rate of cardiovascular disease in PCOS. (41) Thiazolidinediones have been shown to decrease inflammation in obese and diabetic subjects. (42-44) Thiazolidinediones (TZDs) have also been shown to reduce carotid intimal medial thickness, normalize vascular endothelial function, and improve fibrinolytic and coagulation parameters. (45) Rosiglitazone therapy for 26 weeks reduced MMP-9 (a matrix metalloproteinase, implicated in atherosclerotic plaque rupture) and C-reactive protein levels in type 2 diabetics. (46) In studies in PCOS women, troglitazone reduced PAI-1 levels (47) and improved endothelium-dependent vasodilation. (48) It is possible that the beneficial effect of TZDs in PCOS may be partly due to the decrease in inflammation. Metformin has also been shown to decrease PAI-1 and C-reactive protein levels in women with PCOS. (49,50)

Insulin resistance and PCOS

The association between hyperinsulinemia and PCOS was first noted by Burghen et al (51) in 1980, when they discovered a significant positive correlation between insulin, androstenedione and testosterone levels among women with PCOS. Subsequent studies confirmed insulin resistance as the cause of hyperinsulinemia. It is estimated that 20 to 40% of women with PCOS have impaired glucose tolerance, a number approximately 7-fold higher than the rates in age and weight-matched women. (21,52) Prevalence of type 2 diabetes mellitus is also increased in women with PCOS (15 vs 2.3% in women without PCOS). (53) Lean women with PCOS have lower rates of carbohydrate intolerance than obese women with PCOS, but even lean women with PCOS have higher rates than age and weight-matched control subjects. Thus, PCOS is associated with insulin resistance independent of total or fat-free body mass. Obese women with PCOS are more insulin-resistant than obese non-PCOS or nonobese women with PCOS. (21,54) Ehrmann et al (55) demonstrated pancreatic beta cell secretory dysfunction in a subset of women with PCOS, and this subset probably has the highest risk of developing carbohydrate intolerance and type 2 diabetes. (56) The Rotterdam consensus panel recommends oral glucose tolerance tests for obese patients with PCOS. (1) Conversely, in a small study, Peppard et al (57) found PCOS in 8 of 30 premenopausal women with type 2 diabetes. Insulin resistance is characterized by postreceptor defect in the action of insulin. The cause of this defect is still being elucidated. The first step in insulin action involves binding to the cell-surface receptor. (58) Abnormalities in both insulin receptor tyrosine kinase (IRTK) activity and in mediators distal to the receptor are present in insulin resistance states. (59) Serine phosphorylation of insulin receptor decreases IRTK activity. (60,61) Studies in adipocytes from women with PCOS reveal adipocyte insensitivity to inhibition of lipolysis by insulin as well as a decrease in maximal rates of adipocyte glucose uptake. (62,63) Although these defects are also present in obesity and type 2 diabetes, they can occur in PCOS in the absence of obesity. Dunaif et al (64) reported decreased insulin receptor autophosphorylation in 50% of fibroblasts removed from women with PCOS, and this was due to increased receptor serine phosphorylation. Serine phosphorylation, as noted above, has been associated with decreased insulin receptor tyrosine autophosphorylation. In fact, this is the probable mechanism of TNF-[alpha]--induced insulin resistance. (65) Since serine phosphorylation of P450c17 (the key regulatory enzyme of androgen biosynthesis) increases enzyme activity leading to androgen biosynthesis, (66) it is possible that a single defect (serine phosphorylation) can produce both insulin resistance and hyperandrogenism in a subgroup of PCOS patients. (67) Lin et al (68) showed that reduced insulin stimulated lactate production in granulosa-lutein cells obtained from women with PCOS, whereas the same cells obtained from normal ovulatory subjects responded with increased lactate production after insulin exposure. In vitro human theca cell studies have shown that insulin has direct stimulatory effects on ovarian steroidogenesis. (69-71) Nestler et al (69) showed that insulin produced a greater increase in androgen production by theca cells isolated from women with PCOS than in cells obtained from subjects without PCOS and that this effect is mediated specifically through insulin receptors rather than through IGF (insulin-like growth factor) receptors "cross-talk." There are some data to suggest that insulin enhances the effect of LH on preovulatory ovarian follicles, causing premature activation and subsequent follicle arrest. (72) It is possible that hyperinsulinemia (due to insulin resistance) drives the LH effect on ovarian theca cells to cause androgen excesses, which are intrinsically programmed to produce more androgen. (73) Excess androgens are known to interfere with the process of follicular maturation, (74) thus inhibiting ovulation and producing more arrested follicles. It has been postulated that the PCOS ovaries are more resistant to the metabolic effects of insulin than to the steroidogenic effects of insulin. (2) Further studies are needed to clarify the "selective insulin resistance" phenomenon.

Pharmacotherapy for PCOS

Spironolactone in PCOS. Spironolactone, an antiandrogen, has been in use for the treatment of hyperandrogenism for nearly two decades. Its main benefit stems from blocking androgen receptors with a minor contribution from a decrease in androgen synthesis. Although experience with the drug in PCOS is limited, it has a good safety record at doses of 50 to 100 mg, both on a short- and a long-term basis. (75-79) Ammini et al (80) did a randomized, open-labeled study comparing the efficacy and safety of metformin (1,000 mg daily) and spironolactone (50 mg daily) in 69 subjects with PCOS. There was no significant effect on BMI, waist-to-hip ratio, blood pressure, oral glucose tolerance test parameters, and insulin sensitivity, although a significant fall was observed in 1- and 2-hour insulin levels. Both drugs showed significant improvement in menstrual cycle pattern, hirsutism score, and androgen levels, suggesting their efficacy in the treatment of PCOS. Spironolactone appears to be a better choice than metformin in view of better efficacy on hair growth and patient acceptance; however, metformin was superior in improving glucose tolerance and insulin sensitivity. Superior positive effects of metformin on insulin sensitivity, however, did not translate into proportionate clinical benefit in these PCOS subjects. This raises doubts about insulin resistance as the sole underlying factor.

Metformin in PCOS. Metformin is a biguanide that reduces plasma glucose concentrations in patients with type 2 diabetes. Metformin in type 2 diabetics does not lead to weight gain and can induce weight loss in some patients. Metformin predominantly works by reducing hepatic glucose production and inhibiting gluconeogenesis both directly and indirectly (by decreasing free fatty acid concentrations). (81,82) There are some data to suggest that it may slightly improve peripheral insulin sensitivity. (83,84) Studies with metformin in PCOS revealed reductions in androgen levels and improvements in ovulation when metformin was given for a duration of 10 to 24 weeks (in various studies). However, only some of these studies revealed an effect independent of the weight loss induced by metformin. (85-88) Metformin has also been found to reduce the high rates of gestational diabetes in PCOS. (89)

Thiazolidinediones. The peroxisome-proliferator--activated receptors (PPARs) are a subfamily of the 48-member nuclear receptor superfamily (90) and regulate gene expression in response to ligand binding. (91,92) Three PPARs, designated PPAR-[alpha], PPAR-[delta], (also known as PPAR-[beta]), and PPAR-[gamma], have been identified to date. PPAR-[alpha] is expressed predominantly in the liver, heart, and muscle and in the vascular wall. Fibrates such as fenofibrate, bezafibrate, ciprofibrate, and gemfibrozil act as full or partial PPAR-[alpha] agonists. In general, PPAR activation enhances free fatty acid oxidation, controls expression of multiple genes regulating lipoprotein concentrations, and has antiinflammatory effects. PPAR-[alpha] agonists prevent or retard atherosclerosis in mice and human beings. (93-95) PPAR-[delta] is expressed in many tissues, with the highest expression in the skin, brain, and adipose tissue. PPAR-[gamma] is expressed most abundantly in adipose tissue but is also found in pancreatic [beta]-cells, vascular endothelium, and macrophages. (96,97) In January 1997, the first thiazolidinedione, troglitazone, was approved as a glucose-lowering therapy for patients in the United States with type 2 diabetes. Troglitazone was subsequently withdrawn from the market in March 2000 because of hepatotoxicity. The two currently available PPAR agonists, rosiglitazone and pioglitazone, were approved in the United States in 1999. Thiazolidinediones consistently lower fasting and postprandial glucose concentrations as well as free fatty acid concentrations in clinical studies. (98-100) Insulin concentrations also decrease in most studies. (98-100) Such changes indicate that thiazolidinediones act as insulin sensitizers, which has been confirmed by direct measurements in in vivo studies in human beings. For example, treatment of nondiabetic subjects or those with type 2 diabetes for 3 to 6 months with troglitazone, rosiglitazone, or pioglitazone increases insulin-stimulated glucose uptake in peripheral tissues. (98,100-103) In similar studies, thiazolidinediones increase hepatic insulin sensitivity (the ability of insulin to suppress endogenous glucose production) and insulin sensitivity in adipose tissue (measured from the ability of insulin to suppress free fatty acid concentrations). (100) Studies with TZD in subjects with PCOS have shown an improvement of the androgen levels and ovulation rate and enhanced insulin sensitivity without any reduction in the weight of subjects. (16,17,47) Studies have now been done with rosiglitazone showing a decrease in testosterone, androstenedione, and DHEA levels and an increase in SHBG (thereby causing a decrease in free testosterone levels), along with an improvement in insulin sensitivity. (104,105) Troglitazone has recently been shown to have independent effects on ovarian steroidogenesis (106) and thus a direct effect of TZD apart from improvement of insulin resistance cannot be ruled out. In a recent study done by Ortega-Gonzalez et al (107) involving head-to-head comparison of pioglitazone, 52 women with PCOS were randomly allocated to receive either pioglitazone (30 mg a day, n = 25) or metformin (850 mg 3 times daily, n = 27) and were assessed before and after 6 months. This study showed that a 6-month administration of pioglitazone in obese women with PCOS and severe insulin resistance was as effective as metformin in decreasing fasting blood serum insulin concentration and the insulin levels during a 2-hour oral glucose tolerance test without significantly changing fasting blood glucose concentration. Similarly, pioglitazone and metformin caused significant decreases in hirsutism and serum concentrations of free testosterone and androstenedione. This study selected only markedly obese women with PCOS with acanthosis nigricans and the most advanced degree of insulin resistance; hence results may not apply strictly to all women with PCOS. However, pioglitazone seemed to be more effective in improving insulin sensitivity, because fasting serum insulin concentrations were significantly lower after pioglitazone than after metformin treatment. These favorable effects of pioglitazone occurred despite a significant increase in body weight, BMI, and the waist-to-hip ratio associated with the use of pioglitazone but not with metformin. These paradoxical results can be explained by the beneficial shift from abdominal to subcutaneous fat simultaneous with the improvement in insulin sensitivity induced by TZD. (108-110) The same group of investigators recently reported an increase in metoclopramide-stimulated prolactin release in obese women with PCOS after pioglitazone administration for 24 weeks. (111) Therefore, it was suggested that long-term pioglitazone administration may lead to increased hypothalamic dopaminergic tone, and this can be the mechanism of amelioration of insulin resistance in obese insulin-resistant patients. Rosiglitazone therapy has also been shown to improve ovulation rates in PCOS with clomiphene citrate therapy. (112) Pioglitazone may improve hyperandrogenism through a mechanism similar to troglitazone. The putative ligand-mediated activation of PPAR-[gamma]2 by troglitazone impairs androgen and stimulates progesterone biosynthesis in primary cultures of porcine theca cells (113) by blocking the expression of the cytochrome P450-17-[alpha] hydroxylase/C17-20 lyase gene and CYP protein phosphorylation, which decreases the LH insulin-driven theca cell androgen production. (114)

Conclusion

Although insulin resistance is not a part of the diagnostic criteria for PCOS, its importance in the pathogenesis of PCOS cannot be denied. The treatment of PCOS in the past has largely centered on antiandrogen therapy for symptomatic control, cyclic hormones for regular menses, and ovulation induction for infertility. Although weight loss is helpful in the therapy of PCOS, it may be difficult to achieve. Furthermore, a significant percentage of women with PCOS are lean but insulin-resistant. Insulin sensitizers are unique in PCOS because they offer both metabolic and gynecologic benefit. Although the use of insulin sensitizers in PCOS has not been approved by the Food and Drug Administration, it is probable that PCOS will be a recognized indication for TZDs and metformin in future.

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There is no security on this earth, there is only opportunity.
--General Douglas MacArthur

Vishal Bhatia, MBBS, MD

From the Department of Internal Medicine, Mercy Hospital of Buffalo, State University of New York, Buffalo, New York.

Dr. Bhatia has no disclosures to declare.

Reprint requests to Dr. Vishal Bhatia, Department of Internal Medicine, Mercy Hospital of Buffalo, State University of New York, 565, Abbott Road, Buffalo, NY 14220. Email: [e-mail]

Accepted May 9, 2005.

RELATED ARTICLE: Key Points

* Polycystic ovarian syndrome involves anovulation and hyperandrogenism.

* Polycystic ovarian syndrome is associated with insulin resistance.

* Insulin sensitizers are promising agents for polycystic ovarian syndrome.

* Insulin sensitizers have not been approved by the Food and Drug Administration for polycystic ovarian syndrome.