Hypogonadal bone loss: sex steroids or gonadotropins?

For many years, postmenopausal osteoporosis has been attributed to estrogen deficiency (1). This established view has, nevertheless, been challenged by the recent proposal that FSH is required for hypogonadal bone loss and that high levels of FSH cause the condition (2). The conclusion arose from studies of FSHR knockout (FORKO) and FSH knockout (FSH / ) mice, which were found to be resistant to bone loss despite severe estrogen deficiency. Surprisingly, hypogonadal FSH / mice even exhibited increased bone mass at the femur, whereas eugonadal FSH / heterozygotes displayed a generalized increase in bone mass with evidence of reduced bone resorption (2). FSHR protein was detected in osteoclasts, and FSH increased osteoclastogenesis and dentine resorption in vitro but did not influence bone formation or osteoblast activity directly. The novel idea that high circulating FSH causes hypogonadal bone loss sparked a series of editorials and correspondence debating the issue (3–7). Important criticisms emerged and several key questions have now been investigated experimentally. The findings are reported by Gao et al. in this issue of Endocrinology (8). It has been argued that women with hypothalamic or hypergonadotrophic hypogonadism have similar rates of trabecular bone loss even though hypothalamic hypogonadism results in low FSH and estrogens, whereas hypogonadism after oophorectomy or the menopause results in high FSH and low estrogens. Thus, bone loss in women occurs equally in response to estrogen deficiency in the presence of either low or high FSH levels (4). Furthermore, hypogonadal hpg mice harbor a partial deletion of the GnRH gene (9) and develop severe FSH, LH, and sex hormone deficiencies (10), thus providing a model of hypothalamic hypogonadism. Both male and female hpg mice exhibit severe bone loss even though FSH is undetectable (11, 12), and this is reversed after testosterone or estrogen replacement. Nevertheless, bone loss in hpg mice is accompanied by predominant effects on osteoblastic bone formation (11), whereas FSH acts directly in osteoclasts (2) and may not necessarily be expected to influence defects of osteoblast function (7). Despite this, in hpg mice, FSH is not required for the low bone mass evident in hypothalamic hypogonadism and a lack of FSH does not prevent amelioration of the phenotype in response to sex steroids. How might these observations be reconciled with the findings in FORKO and FSH / mice, which were interpreted to indicate an estrogen-independent and direct effect of FSH on the skeleton resulting in bone loss and a requirement for FSH in the manifestation of hypogonadal bone loss (2)? Unfortunately, circulating levels of estrogen, testosterone and LH in FORKO and FSH / mice were not reported (2). Nevertheless, in previous studies homozygous FORKO mice were shown to have elevated LH concentrations and a 10fold increase in testosterone (13–15), whereas early ovarian failure and age-related elevations of FSH, LH, and testosterone were reported in heterozygotes (16). Raised LH has also been documented in FSH / mice (15). In addition to high levels of androgens, and in contrast to the findings of Sun et al. (2), FORKO mice displayed an age-related kyphosis due to vertebral compression associated with radiographic and histological evidence of bone loss (13). Thus, it is argued that the FORKO mouse per se is a poor model of hypogonadism because of elevated androgens, and that FSH / mice do not appropriately represent the perimenopause (4, 17). Are elevated levels of LH-dependent testosterone actually responsible for the lack of bone loss observed in FORKO mice (2), as suggested by others (5, 6)? Estrogens are undoubtedly important in females. Estrogen replacement alone is sufficient to reverse or prevent low bone mass after acute estrogen withdrawal (1). Moreover, female estrogen receptor (ER) and double knockout mice exhibit bone loss that cannot be prevented by estrogen replacement (18) despite normal levels of FSH (19). Estrogens and the peripheral conversion of androgens to estrogens by the aromatase enzyme are also important in males because ER deletion (20) or aromatase deficiency (21) in men both result in bone loss. Furthermore, high levels of testosterone in aromatase knockout mice do not prevent bone loss accompanied by high bone turnover, whereas the phenotype is reversed by estrogen replacement in both sexes (22, 23), suggesting that aromatization of androgens is important for maintenance of bone. On the other hand, the increased osteoclastic bone resorption and high levels of FSH observed in aromatase knockout mice (24) have been proposed to indicate that FSH increases bone resorption despite high levels of testosterone (7). It was suggested further that conversion of testosterone to estrogen in FORKO and FSH / hypogonadal mice is unlikely to be important because of low levels of aromatase expression and negligible serum concentrations of estrogen in these animals (7). In this issue of Endocrinology, Gao et al. address this debate experimentally for the first time using female FORKO mice (8). In these studies (8), FORKO mice were hypogonadal with reduced levels of estradiol but elevated LH accompanied by a marked increase in testosterone. After ovarian transplantation, circulating testosterone levels in FORKO mice were restored to normal, estrogen levels increased and LH levels fell. In contrast to observations reported by Sun et al. (2), FORKO mice exhibited age-related bone loss accompanied by reduced osteoblastic bone formation, together with evidence of increased osteoclastic bone resorption. Ovarian transplantation of FORKO mice resulted in amelioration of the bone mineral density and trabecular bone losses obAbbreviations: ER, Estrogen receptor; FSHR, FSH receptor; FSH / , FSH -subunit knockout; FORKO, FSH receptor knockout.