In vivo regulation of the steroidogenic activity of rat luteal cells by insulin

The role of endogenous hormones in the etiology and prevention of breast cancer: the epidemiological evidence

Breast cancer is the most common cause of cancer death in women worldwide. Rates vary about fivefold around the world, but they are increasing in regions that until recently had low rates of disease. Despite the numerous uncertainties surrounding the etiology of breast cancer, intensive epidemiological, clinical, and genetic studies have identified a number of biological and social traits as risk factors associated with breast cancer. Principal among them is the evidence of BRCA1 and BRCA2 susceptibility genes, familial history of breast cancer, age, higher socioeconomic status, ionizing radiation, tallness in adult life, alcohol consumption, and a variety of hormone and metabolic factors. Among the hormonal influences, a relevant etiological function has been ascribed to unopposed exposure to elevated levels of estrogens and androgens. In addition, new epidemiologic evidence has indicated that among the metabolic factors, glucose metabolism, hyperinsulinemic insulin resistance, and insulin-like growth factor bioavailability may also play a role in breast cancer. These endocrine and metabolic factors may represent future targets for breast cancer prevention.

[Plasma insulin, IGF-I and breast cancer]

Several recent epidemiological studies have shown an increase in breast cancer risk among women who have elevated plasma levels of testosterone, reduced levels of sex hormone-binding globulin (SHBG), and hence elevated levels of bioavailable androgens and estrogens not bound to SHBG. This endocrine profile is generally associated with obesity and chronic hyperinsulinemia, of which it is most likely a result. Lack of physical activity, obesity, and a diet rich in rapidly digestible carbohydrates and poor in fibre favour the development of insulin resistance and hyperinsulinemia. The elevated insulin levels, in turn are related to decreases in plasma and tissue levels of IGFBP-1 and IGFBP-2 (insulin-like growth factor-binding proteins), and this may increase the availability of insulin-like growth factor-I (IGF-I) to its receptors. Like insulin, IGF-I also inhibits the hepatic synthesis of SHBG, whereas both hormones stimulate the ovarian synthesis of sex steroids. Moreover, insulin and IGF-I can both enhance the development of breast tumours, through their cognate receptors within the mammary tissue. Taken together, these observations lead to the hypothesis that breast cancer risk may be increased in women with elevated plasma insulin levels, and/or with elevated levels of bioactive IGF-I. Hyperinsulinemia and an increased IGF-I bioactivity could thus be an important physiological link between a western lifestyle, overnutrition, a hyperandrogenic sex steroid profile, and increased breast cancer risk. Prospective cohort studies will be needed to test this hypothesis, and to study in greater detail the possible relationships of breast cancer risk with plasma levels of IGF-I and IGFBPs. Confirmation of a relationship of breast cancer risk with plasma insulin levels, on the one hand, or with total plasma IGF-I, on the other hand, could open up new perspectives for breast cancer prevention, either by changes in dietary intake patterns and physical activity, or by the use of certain chemopreventive drugs.

Energy balance and cancer: the role of insulin and insulin-like growth factor-I

Recent theories propose that a Western lifestyle may increase cancer risk through alterations in the metabolism of insulin and insulin-like growth factors (IGF: McKeown-Eyssen, 1994; Giovannucci, 1995; Kaaks, 19%; Werner & LeRoith, 1996). Insulin regulates energy metabolism, and increases the bioactivity of IGF-I, by enhancing its synthesis. and by decreasing several of its binding proteins (IGFBP; IGFBP-1 and -2). Insulin and IGF-I both stimulate anabolic processes as a function of available energy and elementary substrates (e.g. amino acids). The anabolic signals by insulin or IGF-I can promote tumour development by inhibiting apoptosis, and by stimulating cell proliferation. Furthermore, both insulin and IGF-I stimulate the synthesis of sex steroids, and inhibit the synthesis of sex hormone-binding globulin (SFIBG), a binding protein that regulates the bioavailability of circulating sex steroids to tissues. The present paper reviews epidemiological findings relating the risk of cancers of the colo-rectum, pancreas, breast, endometrium and prostate to body size (obesity, height) and physical activity, and discusses the relationships between obesity and physical activity and plasma levels of insulin, IGF-I and IGFBP. Subsequent sections review epidemiological findings relating cancer risk to indices of chronic hyperinsulinaemia, and to plasma levels of IGF-I and IGFBP. Conclusions are that chronic hyperinsulinaemia may be a cause of cancers of the colon, pancreas and endometrium, and also possibly of the breast. On the other hand, elevated plasma IGF-I, as total concentrations or relative to levels of IGFBP-3, appears to be related to an increased risk of prostate cancer, breast cancer in young women, and possibly cob-rectal cancer. For cancers of the endometrium, breast and prostate, these findings are discussed in the context of relationships between insulin and IGF-I and levels of bioavailable sex steroids.

Regulation of follicular luteinization by a gonadotropin-releasing hormone agonist: relationship between steroidogenesis and apoptosis

The purpose of this study was to evaluate the effects of GnRH-analog (Leuprolide acetate, LA) administration on follicular luteinization in equine chorionic gonadotropin plus human chorionic gonadotropin (eCG + hCG)-superovulated prepubertal treated rats. Results indicate that LA treatment decreases circulating levels of progesterone (P) and P accumulation in collagenase-dispersed ovarian cell cultures, though estradiol (E2) production is increased. These data suggest that cells from the LA group may be less luteinized following gonadotropin treatment. Studies performed on histological ovarian sections after different times of eCG administration showed that LA injections produce lower amounts of corpora lutea and antral follicles, and a greater number of atretic and preantral follicles. The basal and LH-stimulated P and progestagen accumulations are decreased in incubations of corpora lutea isolated from the LA group. In addition, the mitochondrial cholesterol side-chain cleavage (P450SCC) levels in corpora lutea from LA-treated rats are reduced, indicating that the decrease in P production observed is due in part to an alteration in the steroidogenic luteal capability. Immunocytochemical localization of nuclei exhibiting DNA fragmentation by the technique of terminal deoxynucleotidyl transferase end-labeling showed that LA treatment causes an increase in the number of apoptotic cells in preantral and antral follicles at all times studied (1, 2, 4, or 7 days of LA administration). A similar effect, though less pronounced, was observed in corpora lutea. It is concluded that LA treatment produces a failure in the steroidogenic luteal capability and an increase of apoptotic mechanisms in the ovary, producing as a consequence an interference in the follicular recruitment, growth, and luteinization induced by gonadotropins.