Online Issues

ILAR Journal Vol 45(2)

Role of the Nonhuman Primate for Research Related to Women's Health

David F. Archer

David F. Archer, M.D., is Professor of Obstetrics and Gynecology and Director of the CONRAD Clinical Research Center, Norfolk, Virginia.

Abstract

This overview of the current status of medical problems that affect women is related to current studies on pathophysiology and therapeutic interventions using nonhuman primates to demonstrate the utility of the primate model for the study of disease processes in women. The current medical literature on women's health is compared with the literature on nonhuman primate research. The findings reviewed in the articles of ILAR Journal Volume 45 Issue 2 of 2004 are evaluated in the context of the scope and problems associated with disease entities in women. Nonhuman primate research with known information regarding women's disease is discussed, and the utility of the animal model for the study of human disease is highlighted, based on its significant relevance due to similarities of nonhuman primate and human subjects' physiology, metabolism, and responses to therapeutic interventions. Additional advantages of the animal model include the ability to control the experimental environment and the capacity to perform chronic study procedures. These findings allow us to utilize the nonhuman primate as the most relevant model in the animal world for the study of human disease processes.

Key Words: breast cancer; cardiovascular disease; diabetes mellitus; infertility; nonhuman primate; pathophysiology; social status; therapueutic interventions

A variety of animal models have been utilized to investigate the physiological mechanisms, reactions to drugs, and the overall toxicological exposure to high doses of potential drug products. Although animal models are useful, they have inherent differences that reduce their applicability to human physiology. These differences can be found in intestinal absorption, metabolism, distribution of a product in the circulation and tissues, and ultimately elimination or excretion from the body. Within mammalian species, rodents are different from primates, and the nonhuman primate, although close to the human subject, is not exactly the same. It is noteworthy that these subtle differences have become more and more apparent in recent studies with the continuing and increasing need for surrogate animal models to aid in elucidating normal and abnormal function in the human subject.

One of the chief drawbacks to investigations in human subjects is the obvious significant limitation on invasive procedures and the need for caution and monitoring during the Phase I or first use of a compound. The nonhuman primate and other animal models have been useful in that they can be used in a repetitive fashion to evaluate the progress of an induced disorder or they have outcomes in chronic or dose-escalating experiments that are similar, if not identical to, those found in the human subject (Sibal and Samson 2001).

The purpose of this issue of ILAR Journal is to present essays from investigators with experience in the applicability of primate models to human disease entities. This forum focuses on what is known and what can be learned from the nonhuman primate relative to disease processes in human subjects. Because of the nature of these essays, I have arbitrarily divided them into two sections: those that deal with the intact animal comparable to a premenopausal woman, and those that address conditions associated with the perimenopausal transition and ultimately the postmenopausal status in women.

Premenopausal Women

Anovulatory Infertility

One of the basic human rights is the right to procreate. Fecundity is manifest throughout our culture by a variety of symbols associated with marriage and/or puberty. These symbols, such as rice thrown at a wedding, are designed to represent fecundity for the couple. It has been estimated that infertility affects between 37 million and 70 million married couples in the world (DHHS 2003). In the United States, it has been estimated based on the 1988 National Center for Health Statistics that there are approximately 4.9 million infertile women, equivalent to 8.4% of the population between the ages of 15 and 44 yr, who had impaired fertility (Jones and Toner 1993). Of these women, 2.2 million reported primary infertility or no pregnancies in their medical history. It is also well established that for a significant number of these couples, infertility is attributed to the male factor. Recent advances in assisted reproductive technologies (ARTs¹) have allowed for an enhanced capability of the infertile male to fertilize an oocyte with a resultant pregnancy (Oehninger et al. 1995). The overall treatment outcome (successful delivery of an offspring) for male infertility has improved and is now estimated to be approximately 30% (Windt et al. 2002). Pregnancy rates for a simple case of anovulation or anovulatory infertility in women are about 80% (Jones and Toner 1993).

Animals, in general, do not experience infertility. It is interesting that the contribution of Dr. Abbot and colleagues (Abbott et al. 2004) regarding the unique understanding of anovulatory infertility in women comes at a time when the ART programs are faced with an increasing concern over what is known as the "poor responder" to stimulatory protocols (Surrey and Schoolcraft 2000; Tarlatzis et al. 2003; Weissman et al. 2003). These women, although still actively menstruating and with follicles (oocytes) remaining in the ovaries, do not respond with follicular development to standard ovulation induction protocols. The exact reason or reasons for the poor response are currently unknown (Tarlatzis et al. 2003).

Fertilization is an issue as women age. Based on assisted reproductive outcomes, there is evidence that poor outcomes with aging oocytes can be overcome with the use of a donor oocyte from a younger female (Faber et al. 1997). These changes in oocytes with aging would be important to understand, and the primate model would lend itself to an active investigation in terms of the aging of the oocyte (Keefe et al. 2003).

The nature of follicular development and ovulation as well as the hypothalamic pituitary ovarian axis are very similar between the nonhuman primate and the human subject (Knobil 1988, 1990). As reported in this issue, the findings related to hypothalamic pituitary ovarian function have resulted in an increase in the awareness and the application of clinical treatments for women who have a form of hypothalamic dysfunction manifest by low gonadotropic levels and no follicular development.

In terms of increasing reproductive outcomes in women, and to overcome the problem of the poor responder, it would be very valuable to follow investigations of follicular responses to gonadotropin stimulation in the aging cynomolgus macaque. Significant information could also be obtained from the cynomolgus macaque regarding new therapeutic interventions for women with polycystic ovarian disease.

Although not addressed directly in this journal issue, one of the most compelling problems for the physician treating the infertile couple is not the fact that fertilization does not occur (successful fertilization has been reported with ARTs to be ~ 80%), but that successful pregnancy outcome (delivery of a viable infant) continues to be approximately 30% (Wright et al. 2003). This difference between fertilization and delivery appears to occur principally in the luteal phase of the cycle related to implantation, genetic abnormalities of the embryo, or an early pregnancy loss. The nonhuman primate model could be a useful surrogate to investigate implantation, although this model, to date, has not been extensively utilized in this regard.

Endometriosis

Pelvic endometriosis is one of the major medical problems for women in the United States and is associated with a significance incidence of infertility (Elsheikh et al. 2003). Endometriosis has been reported to be present in 40 to 70% of infertile women who have no other apparent cause of infertility. Endometriosis is perhaps an even greater problem because of attendant morbidity associated with chronic abdominal or pelvic pain (Murphy 2002). The general incidence of endometriosis is between 5 and 15% in reproductive age women. This percentage is difficult to calculate because of the relative asymptomatic status of endometriosis in some women. Endometriosis has been found to be a chronic disease with significant impairment of function due to pelvic pain, dysmenorrhea, and dyspareunia. A fact of even greater importance is that many women have undergone repeated surgical procedures, aimed at the reduction or eradication of endometriosis because of pelvic pain. To elicit a "cure," endometriosis requires total hysterectomy with bilateral salpingo oophorectomy in many instances, which leaves the woman sterile (ACOG 1999).

The pathophysiology of endometriosis is poorly understood. Sampson's theory of tubal regurgitation of viable endometrial tissue with implantation and growth of these endometrial fragments in the peritoneal cavity has persisted despite considerable controversy (Ramey and Archer 1993; Redwine 2002). Other alternative theories, particularly the theory of coelomic metaplasia, must continue to be considered. Understanding not only the development but also the mechanism of inheritance of endometriosis in a given family would be a significant advance in the field (Treloar et al. 2002). Based on the studies presented in this review by Drs. Story and Kennedy (2004), the nonhuman primate model could serve as a successful means of evaluating not only the inheritance patterns of endometriosis, but also the potential mechanisms of the initiation of endometriosis. The endometriosis model developed in the nonhuman primate would be useful for investigation of new therapeutic options and their ability to suppress or eradicate endometriosis. Because endometriosis is associated with a significant dysfunction in women, the use of the animal model as a surrogate would be a most attractive means of advancing our knowledge of the factor(s) involved in the initiation, growth, and ultimately regression of endometriosis.

Perimenopausal Women

Much has been written recently regarding the perimenopausal transition in women. Perhaps the hallmark of this transition is menstrual irregularity (Burger et al. 2002; Soules et al. 2001; Treloar 1981). It is apparent from several of the articles in this issue that in the aged nonhuman primate (> 24 yr of age), there is indeed menstrual irregularity and ultimately a menopausal state similar to that occurring in women. The life expectancy of the nonhuman primate after this natural menopause is only 1 to 2 yr, making it less than ideal for many postmenopausal studies. The perimenopausal woman has an increasing occurrence of anovulatory cycles thought to be secondary to ovarian follicular depletion and other factors affecting normal ovarian physiology as a woman ages (Burger et al. 2002). Changes in the menstrual cycle with infrequent ovulation are a hallmark of the perimenopausal transition. How the endocrine changes in the perimenopausal transition relate to ultimate disease outcomes in the postmenopausal woman is not well investigated. In other words, we do not yet know whether the duration of the irregular menstrual cycles with fluctuating hormone levels of 4 to 5 yr have differential long-term outcomes compared with the woman who has regular cycles up to the age of 52, when menstruation ceases.

The most interesting aspect of Drs. Kaplan and Manuck's (2004) article on ovarian dysfunction with social subordination and disease is how social status could affect ovarian function and/or other diseases of humans. Certainly we know that changes in the premenopause such as polycystic ovarian disease can have significant impact in the postmenopausal individual for cardiovascular disease (Talbott et al. 1995, 1998). Socioeconomic status in women has now been linked to significant increases in cardiovascular risk (Brizinka and Padmos 1994; Huff and Gray 2001). Significant alterations in menstrual cyclicity (ovarian function) appear to result in enhanced occurrence of coronary artery disease in postmenopausal women (Bairey Merz et al. 2003). This aspect of human physiology and resultant dysfunction have been minimally addressed in the medical literature.

The data from the primate model suggest that hypoestrogenism associated with changes in the social hierarchy in the primate results in significant atherosclerosis and associated bone loss. This model demonstrates not only a reduction in serum estrogen levels but also an increase in serum cortisol levels as part of the stress response situation. There is evidence of comparable changes in humans with ovarian dysfunction and low plasma estradiol levels associated with extensive coronary artery atherosclerosis (Bairey Merz et al. 2003). Both elevated estradiol and androgens in the chronic anovulation associated with polycystic ovary disease and elevated insulin levels have been related to early coronary artery disease (Talbott et al. 1995, 1998). It is possible that anovulation from any stress-related cause, or even the chronic use of corticosteroids for a variety of diseases, could result in significant dysfunction in the premenopausal woman that ultimately translates into disease in the postmenopausal woman.

Postmenopausal Women

Cardiovascular Disease

The global population totaled six billion people in 1999 (Diczfalusy 2001). The population is estimated to be nine billion in 2054. Not only is the population growing, but it is aging. Sixteen percent of the world population and 27.6% of the European population are projected to be older than 65 yr in 2050. Individuals older than 80 yr will represent more than 10% of the total population in 14 countries, including nine in Europe. The future trends are for an increased healthcare cost associated with the aged. At present and in the future, the leading cause of disease and death is and will be cardiovascular.

There may well be an upper limit of how long the body can remain free of a significant life-threatening or chronic disease. The challenge of this new century is to develop and implement preventive health aspects that can reduce the incidence of cardiovascular disease, and maintain health.

Perhaps the most contentious issue in the last 4 yr in preventive healthcare for the postmenopausal woman has been the finding that estrogen does not reduce the incidence of coronary heart disease. This information has come from three randomized clinical trials: the Heart and Estrogen Progestin Replacement Study (HERS¹), the Estrogen Replacement and Atherosclerosis Study (ERA¹), and the Women's Health Initiative (WHI¹) (Herrington et al. 2000; Hulley et al. 1998; Manson et al. 2003; Rossouw et al. 2002). Before 1998, observational trials and other indirect parameters of cardiovascular function, principally lipids, strongly supported the concept that the use of estrogen or estrogen plus progestin reduced the occurrence of coronary heart disease in postmenopausal women (Barrett-Connor and Grady 1998). The prospective, randomized clinical trials (HERS and WHI) have contradicted this prevailing view.

HERS was a multicenter prospective randomized placebo controlled trial of conjugated equine estrogen and medroxyprogesterone acetate versus placebo in 2763 postmenopausal women with established coronary heart disease (CHD¹) whose average age was 67 yr at enrollment. The main outcome was recurrent coronary heart disease events. The use of hormone replacement therapy (HT¹) was associated with an increased number of events in the first year, but the overall outcome was null during the 4.1 yr of the trial and the associated 2.6 yr of follow-up in a smaller cohort of women. These data have been interpreted as indicating that HT has no effect on established atherosclerosis. WHI was reported as a primary prevention trial. It was a multicentered prospective randomized trial of conjugated equine estrogens and medroxyprogesterone acetate versus placebo similar to HERS. WHI enrolled 16,608 postmenopausal women aged 50 to 79 yr with an intact uterus. The initial report indicated a statistically significant increase in the incidence of myocardial infarction in the women in this trial, with a hazard ratio (HR¹) of 1.29 (95% confidence interval [CI¹) 1.02-1.63; Rossouw et al. 2002). This finding was subsequently reduced to a nonsignificant HR of 1.24 (95% CI 1.00-1.54) in the most recent paper (Manson 2003). Both studies have found that HT--estrogen plus progestin--neither increases nor decreases the occurrence of CHD in older postmenopausal women. Both studies found an increased occurrence of CHD events in the first year after initiation of HT (Hulley et al. 1998; Manson et al. 2003). Neither study found an overall significant increase or decrease in coronary heart disease outcomes in these older women. Although these studies were prospective and randomized with large numbers of women, the discussion has centered around the age and lack of HT use since menopause in the participants, which is further discussed below. Due to the media attention in the United States, both studies have served to increase physician and public awareness of the potential risk:benefit ratio of hormone therapy in postmenopausal women. Current guidelines stress that younger women with menopausal symptoms can use HT for symptomatic relief, but that each patient should have an individual risk:benefit assessment.

The cynomolgus macaque has provided a significant model in terms of the development of atherosclerosis. Studies over the past 15 yr from the Wake Forest University School of Medicine have indicated that hormones--estradiol, conjugated equine estrogen, progesterone, medroxyprogesterone acetate, or tibolone--have significant favorable effects on the development of atherosclerosis in this model. Changes in lipids and lipoproteins in this menopausal model with these steroids are similar to those found in the human in almost all instances.

A discrepancy exists between the primate model and the human observational studies regarding cardiovascular disease and hormone therapy compared with the current randomized clinical trials, WHI and HERS. Both WHI and HERS are null outcomes. There was no significant increase in the relative hazard (RH¹) of CHD in the treated versus the placebo groups in WHI (RH 1.24, 95% confidence interval [CI¹] 1.00-1.54), whereas for myocardial infarction and/or CHD deaths, the RH was 0.99 (95% CI 0.80-1.22) in HERS (Hulley et al. 1998; Manson et al. 2003). These data are in contradistinction to an overall reduction in the incidence of cardiovascular disease of approximately 30% in women using HT in the observational studies (Barrett-Connor and Grady 1998). Although myocardial infarction is not an end point in the primate model, reduction in the extent of atherosclerotic plaque in coronary arteries or iliac vessels has been found with the use of hormonal treatments compared with no therapy.

These findings have resulted in a hypothesis that early use of hormone therapy without an extended time of hypoestrogenism can reduce or retard the development of atherosclerotic plaque (Grodstein et al. 2003). This interesting hypothesis based on the nonhuman primate data clearly needs to be studied further in women (Karas and Clarkson 2003). The older aged postmenpausal women studied in the WHI and HERS trials had HT initiated after a period of hypoestrogenism (Grady et al. 2002; Manson et al. 2003). These women were found to have an increased incidence of coronary heart disease events only in the first year of HT. The presence of existing atherosclerosis in the ERA trial found that HT had little to no effect on reducing plaque size (Herrington et al. 2000). Younger women using estradiol versus placebo in a prospective randomized clinical trial were found to have a reduction in the progression of carotid intimal medial thickness, a surrogate for coronary artery atherosclerosis (Hodis et al. 2001). Thus, there exists compelling information in younger women that an early active intervention may result in a different outcome from that found in the randomized clinical trials of older women.

Data from the Framingham study suggest that hypoestrogenism plays a role in cardiovascular disease in women. There was a greater incidence of cardiovascular disease in women who had undergone a surgical or premature menopause compared with age-matched menstruating women.

A healthy user bias has been hypothesized to explain the reduced cardiovascular disease outcome in women who use hormone therapy (Barrett-Connor and Grady 1998). Exercise for 30 min three times a week in the observational stage of the WHI was associated with a 30% reduction in the incidence of CHD (Manson et al. 2002). Women who elect to use hormone therapy have been found to be more concerned over diet, exercise, and general health compared with nonusers.

Animal models continue to support a positive effect of estrogen on a variety of cardiovascular parameters. Reduction in atherosclerotic plaque size, reactivity of the coronary artery wall, and/or plasticity of the coronary artery wall (alone or in combination) have been associated with estrogen and estrogen plus progestin therapy in the cynomolgus macaque. This association is clearly evident in the review by Drs. Williams and Suparto (2004). These findings support the contention that an interval of low estrogen levels associated with an atherosclerotic diet result in larger plaque size compared with the same diet, but with the early intervention of hormone replacement.

The problems of an increased incidence of breast cancer and no effect on CHD have had a significant effect on the use of HT in women. Many women who have concerns that were raised by the WHI in 2002 have sought alternative therapies. Dr. Appt (2004) points out that alternative natural therapies such as soy and soy with or without isoflavones added have not been shown either in the primate or human to have a significant impact on coronary heart disease, bone loss, or menopausal symptoms (specifically hot flushes). A significant number of women (26%) who had stopped HT have opted to restart HT due to their menopausal symptoms (Grady et al. 2003). One of the unanswered questions is whether the use of a complementary therapy such as soy could have a synergistic effect with HT, as suggested by Dr. Appt.

Stroke and Venous Thrombosis

One of the more recent but consistent findings in postmenopausal women using HT has been an increase in the occurrence of venous thrombosis and/or pulmonary emboli (Rossouw et al. 2002). These findings have occurred both in observational studies and in randomized controlled clinical trials (Daly et al. 1996; Grodstein et al. 1996; Hulley et al. 1998; Jick et al. 1996). Thrombosis in the venous and arterial system may well be different or have different etiologies. When thrombosis occurs in the arterial system it causes myocardial infarction or stroke. This process is important, as Dr. Murphy and colleagues (2004) point out herein, because stroke in humans can be the result of multiple factors; it is evident from observations of both thrombotic and hemorrhagic stroke in humans. We know that there are risk factors other than age for stroke--most notably diabetes mellitus, hypertension, and cigarette smoking (Wassertheil-Smoller et al. 2003). Because these diseases have different basic underlying pathophysiological changes, it is important to understand what we can do to prevent or to improve the outcomes in stroke.

To date, the primate model and other animal species have been investigated only in terms of their ability to show a positive outcome with the use of hormone therapy in terms of reducing or minimizing the degree of central nervous system damage due to an artificial thrombotic event. What we require from the primate model in the future is a means of developing a stroke model in which active interventions can be used to show improvement in terms of the extent of tissue necrosis or subsequent neurological impairment, and to have these improvements applied directly to the human. There is no compelling evidence that in individuals who have a stroke, the use of estrogen resulted in less immediate or delayed neurological impairment. Data from the Women's Estrogen for Stroke Trial study of estradiol in women who had experienced a stroke do not demonstrate an increase or a decrease in the recurrence rate of thrombotic stroke (Viscoli et al. 2001). The most tantalizing aspect of the study of postvascular occlusion in animal stroke models to date is that estrogens appear to reduce the extent and the severity of the central nervous system damage after an arterial occlusive event (Wise and Dubal 2000; Wise et al. 2000). How to extend this observation to humans is difficult to imagine. Residual dysfunction after stroke is highly variable and is compounded by a multitude of other factors such as age and other chronic diseases.

Cognition and Mood

Cognition is an important aspect of aging now that women have the capacity to live up to 35 yr or longer after their menopause. The increasing occurrence of degenerative disease of the central nervous system, with the most prominent one known as Alzheimer's disease, is a significant public and personal health problem. Alzheimer's disease has a female:male ratio of 3:1 (Zandi et al. 2002).

Alzheimer's disease is one form of senile dementia, which occurs principally in individuals over the age of 75. The use of hormones and their effect on cognition and mood are only now being fully appreciated as part of the quality of life of the aging individual. The ability to carry out tasks, to recall, to understand and comprehend, and to not be chronically depressed are significant issues for the population. Women have a greater predilection for depression than men (Archer 1999). Changes in cognition and dementia are related to alteration in the central nervous system. Morphological changes in terms of interaction between cells (dendritic outgrowth), or the deposit of beta amyloid in the hippocampus, are easily discernable in animal models or in vitro. Despite these obvious morphological and biochemical changes, the lack of social support during pregnancy has been shown to contribute to the occurrence of depression in women (Archer 1999).

Concentrations of neurotransmitter substances (serotonin, norepinephrine, or dopamine), which can be significantly altered by the use of hormonal manipulation, are difficult to study directly in the human (Archer 1999). These neurohormonal substances, particularly serotonin and dopamine, need to be investigated further, and nowhere would it be more important than using a primate model. Animal models allow us to minimize extraneous factors that might alter important outcomes, such as cognition and mood. Drs. Shively and Bethea (2004) indicate that there are sufficient preliminary data in the primate that would allow us to begin to investigate hormonal interventions systematically in terms of central nervous system effects that could mimic mood changes or depression.

Osteoporosis

Osteoporosis-related fractures, particularly those of the hip, result in significant mortality and morbidity. Of the individuals who experience an osteoporotic-related hip fracture, 30% do not survive more than 6 mo. Prevention of osteoporosis is now possible with several agents capable of reducing bone turnover. Hormone therapy, selective estrogen receptor modulators, bisphosphonates, calcitonin, and parathyroid hormone analogs all have been shown to have a significant impact on improving bone density and possibly bone quality (Body et al. 2002; Ettinger 2003; Fedelsova et al. 2000; Lindsay et al. 2002). Bone density is measured objectively with dual energy x-ray absortiometry. Architectural quality of bone is only now being actively investigated, and it appears to play a significant role in terms of bone strength and resistance to fracture. HT has been shown in the WHI study to reduce the statistical incidence of fractures of the hip, vertebral body, and other nonvertebral sites (Cauley et al. 2003). These data were based on known low bone mineral density in a group of women not at specific risk for fracture. The findings suggest that there may be a way for new therapeutic interventions to improve bone quality.

The primate model appears to be similar to the human in terms of bone loss in low estrogen states or after oophorectomy. Dr. Jerome (2004) makes a compelling case that osteoporosis, or low bone mass, and bone loss occur in the nonhuman primate in a fashion similar if not identical to that in the human patient. This model would be useful for the investigation of new antiresorptive compounds. The interaction between estrogen and androgen on reducing bone resorption, and how these hormones interact in preventing bone loss, could be studied in this animal model.

Neoplasia

One of the most controversial aspects of postmenopausal hormone replacement therapy is whether estrogen or estrogen plus progestin is an initiator of neoplasia in the breast or a promoter of breast cancer growth (Clemons and Goss 2001). Many of the studies that have been carried out using hormones have shown a growth promoter effect rather than an induction of cellular neoplasia. The occurrence of breast cancer is more prevalent in the aged individual associated with low estrogen levels. Age is the most important factor in the increased incidence of breast cancer (CGHFBC 1997). Eighty percent of breast cancers occur in postmenopausal women who have never received hormone therapy. The relation between estrogen and progestin and the growth and development of the mammary glands in all mammalian species has resulted in this relation being seen as a significant link in the development of breast cancer. A direct effect of hormones as causing neoplastic changes is tenuous at this time. The identification or diagnosis of breast cancer in women using hormone therapy appears to be increased (Chlebowski et al. 2003). Women who have previously used hormone therapy for 5, 10, or more years are not at increased risk of breast cancer within 1 yr of discontinuing the hormonal treatment (Beral 2003; Beral et al. 1999). These latter findings would support a hypothesis of a promoter effect of hormones, rather than an inducer of breast cancer.

The primate breast is similar to the human, but breast cancer does not appear to occur as frequently as in the human. It is possible that the duration of exogenous hormonal exposure has been too short (i.e., <3 yr). This factor could result in the failure to find an association between hormonal therapy and endometrial or breast cancer in the current studies. Endometrial hyperplasia in primates does not appear to occur at the same rate as hyperplasia of the endometrium in the human. Dr. Cline (2004) indicates that studies in the primate published to date strongly support a mitogenic role for estrogen, both in the breast and in the endometrium. The effect of estrogen plus progestin on increased mitogenic activity compared with estrogen in the breast was described earlier by this group (Cline et al. 1996). Despite this increased mitogenic activity, transition to frank carcinoma has yet to be demonstrated. It is possible that the primate model, which does not have a high incidence of neoplastic transformation in the breast and endometrium, reflects genetic or environmental factors that are different from factors affecting humans, rather than a specific hormonal cause of neoplasia.

Glucose and Insulin Metabolism and Diabetes Mellitus

Obesity, which currently characterizes more than 20% of the population in every one of the 50 United States, is a major risk factor for diabetes mellitus. Body mass index is positively correlated with the occurrence of type 2 diabetes mellitus. Central obesity, which is the deposition of visceral fat, is known to be associated with diabetes mellitus; and the clinical finding of a waist:hip ratio approaching 1.0 is a clinically useful tool to identify women at risk for diabetes mellitus.

Increased body weight, centripetal obesity, and insulin resistance are known to occur in postmenopausal women. Drs. Bruns and Kemnitz (2004) make this point in their manuscript in this issue of ILAR Journal. Weight gain is less in postmenopausal women using HT compared with placebo in the randomized Postmenopausal Estrogen Progestin Intervention (PEPI¹) trial (Writing Group for the PEPI Trial 1995). The effect of HT on the incidence of type 2 diabetes mellitus is not as clear. The HERS trial found an incidence of diabetes mellitus in women with existing coronary heart disease of 26.5% (734/2763 participants) (Kanaya et al. 2003). Fasting glucose increased in the women in the placebo group, while remaining unchanged in the HT group. The HR for diabetes mellitus was 0.65 (95% CI 0.48-0.89) in the group using HT (p = 0.006). The overall rate of diabetes mellitus was 6.2% in the HT group versus 9.5% in the placebo group.

Women using HT have lower fasting glucose values compared with nonusers of HT (Fineberg 2000). The implications are that use of HT may decrease the risk of developing type 2 diabetes mellitus. Of even greater interest is an assessment by the Northern California Kaiser Permanente Group of 24,420 diabetic women who had not had a myocardial infarction. Findings included an HR of 0.81 (95% CI 0.66-1.00) for myocardial infarction in the diabetic women who had used 0.625 mg of conjugated equine estrogen for more than 1 yr.

Obviously, more research is needed in postmenopausal women to clarify the effects of HT on glucose and insulin metabolism. In a consensus opinion, the North American Menopause Society was unable to arrive at any statement regarding the effect of HT on glucose homeostasis (NAMS 2000). The use of the nonhuman primate offers advantages of manipulation of the environment with reduction in confounding variables. The conclusion of Drs. Bruns and Kemnitz, that the nonhuman primate has similarities to human subjects in body composition, insulin sensitivity, and type 2 diabetes mellitus, supports this contention.

Summary

The usefulness of the nonhuman primate and other animal models for the study of women's health is significant. Probably the most important aspect is the fact that these models allow us to control the environment and manipulate the therapeutic interventions in a manner that can never be carried out to the same extent in humans. In this way, piece by piece and brick by brick, the understanding of the pathophysiology of disease and interventions that can alter disease can be addressed. The expectation is that these findings in primates will be of great help in understanding human disease processes.

¹Abbreviations used in this article: ART, assisted reproductive technology; CHD, coronary heart disease; CI, confidence interval; ERA, Estrogen Replacement and Atherosclerosis Study; HERS, Heart and Estrogen Progestin Replacement Study; HR, hazard ratio; HT, hormone replacement therapy; PEPI, Postmenopausal Estrogen Progestin Intervention; RH, relative hazard; WHI, Women's Health Initiative.

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