Agonist-induced tension determines vascular reactivity during anoxia and reoxygenation

Life Sciences, Vol. 50, pp. 1737-1744 Printed in the USA

Pergamon P r e s s

MINIREVIEW OSTEOARTHRITIS IN WOMEN: ITS RELATIONSHIP TO ESTROGEN AND CURRENT TRENDS Ching-Lin Tsai, and Tang-Kue Liu Department of Orthopaedic Surgery, School of Medicine, National Taiwan University, Taipei, Taiwan, ROC (Received in final form March 27, 1992)

SUMMARY With our expanding knowledge of osteoarthritis (OA) over the years, our concept of this "aging" disease has been re-evaluated to that which is the opposite of traditional views. To clinicians and scientists, OA is no longer the inevitable disease of aging, as one conceptualizes gray hair. Epidemiological studies show a higher incidence of OA affecting polyarticular joints in women than age-matched men, particularly those over the age of 55. This discrepancy in sex difference in the OA incidence highlights the significance of sex hormones and their alterations in menopause. Evidence indicates that this alteration possibly occurs early in adult life and may well persist into menopause. As well, this hormonal perturbation is thought to be consequent to obesity in these women. Both in vivo and in vitro studies suggest that estrogen is chondrodestructive via the receptor-mediated mechanism. The finding of estrogen receptor in canine, rabbit, and human articular cartilage further confirms this hypothesis. Recent findings of elevated synovial estradiol level and higher estrogen receptor bindings in human osteoarthritic cartilage strongly suggest the importance of local uptake of estradiol (E2) and the possible up-regulation of estrogen receptors. Estrogen, like other hypothesized etiologies, is important in the development of OA in women. Osteoarthritis (OA) is a major aging disease of the elderly, particularly those over the age of 55 (1-3), and affects the non-weight bearing interphalangeal and other small joints, and the weight-bearing knee and hip joints (4). This disease is characterized by the progressive degeneration of articular cartilage. As the complex and multifactorial nature of this "aging" disease is realized, the etiology and the pathogenesis of OA remain the advertent topic of cartilage degeneration. Several models have been developed in an attempt to investigate and establish the etiology and pathogenesis of OA from the observations of osteoarthritic changes in animal models. Evolving with the animal models are the hypotheses of cartilage degeneration, such as stiffened subchondral bone and the deteriorating response and repair ability of chondrocyte to external factors such as aging,inflammation, and genetic and metabolic diseases (5-7). Regardless of the etiology, the common pathway of cell injury or death with release of 0024-3205/92 $5.00 + .00 Copyright © 1992 Pergamon Press Ltd All rights reserved.

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degradative enyzmes is suggested in the destruction of cartilage (8,9), which follows the initial events. The findings of clinically non-symptomatic, non-progressive erosions in the non-weight bearing sites of knee and hips (10) indicate that simple wear alone is not the initiating factor in the production of OA changes in normal cartilage, and that OA can not be regarded as the inevitable consequence of aging. Rather, the interplays of multiple factors are responsible for the heterogeneity of OA, while the importance of specific hypothesis and model to individual joints must be emphasized (1,11,12). Among the risk factors associated with OA incidence are obesity, trauma, genetic factors, and hormonal and metabolic imbalance (11, 13-15). The importance of obesity in the development of OA is well documented. Studies of two English population samples of different socioeconomical status reveal that the incidence of primary OA of Grades 3 and 4, based on the system of the Atlas of Standard Radiographs of Arthritis, rises "exponentially" after the age of 50 (2,16), and is more prevalent in women than men between the age of 45 and 55 (2,17). The same trend is demonstrated in other studies of white population; the age-related increment of OA incidence applies to multi-articular involvement in women aged over 55 (4,14,16,18-20). In 1925, the gender difference in the afflicted OA patients was first related to menopause ; the term "menopausal arthritis" was used to describe a defined subset of obese perimenopausal women with polyarticular arthritis (21). A preponderance of generalized OA involving fingers, spine, and knee was found in women aged between 50 and 52 (19). It is suggested that the metabolic changes occurring perimenopausally and postmenopausally, such as the changes of sex hormone levels and the increased ratio of free estrogen and progesterone, are involved in the development of polyarticular joint degeneration in women, particularly in obese postmenopausal women (22). OA AND SEX HORMONES Background Since 1925, clinical observations of various populations have suggested a relationship between the decreasing level of circulating sex hormones and the development of OA involving digits, polyarticular joints, spine, and knees (14 , 21,23)° In addition to the description of menopausal arthritis by Cecil and Archer (21), in a small group of women, Hall observed the development of joint symptoms following the menopause or hysterectomy (24). The suggestion of the causative role of menopause and estrogen loss in generalized OA leads to the early studies of the therapeutic effects of estrogens given to small trials of women with OA ; the replacement therapies were uneventful (19,21), indicating that the decrease of sex hormones in menopause is not directly responsible for the development of OA. Thus, further evidence of an increased level of endogenous estrogen has been suggested in the case-controlled studies. Women in polyarticular OA group have a higher rate of hysterectomy (24%) and oophorectomy (50%), resulting from dysfunctional uterine bleeding and fibroids, as well as more gynaecological problems than the rheumatoid arthritis and the control group (25). While the number of hysterectomies in these women with OA is double that in the age-matched control (25), an excess of estrogens (25, 26) is attributed to the higher rate of surgeries in these OA women. Although it is argued that loss of estrogens, consequent to hysterectomy and oophorectomy, may predispose these women to the early development of OA (27), there is no indication of the duration between the time of surgery and the onset of OA to prove such a hypothesis. In menopause, estrone (E,) is continuously and perhaps principally produced via peripheral aromatization of the adrenal androstenedione in the adipose tissue and muscle, and later converted to its potent form, estradiol (E2) (28-33). The production of estrogen is conceivably more highly elevated in obese postmenopasual women in whom the excessive fat mass and muscle tissue are the main sources of estrogen production.

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Circulating Levels of Sex Hormones While in young women, both E1 and E~ are produced in considerable quantity (E1:224, E~: 293 pmol/L), the levels of E2 and E, drop in normal non-obese postmenopausal women, with a lower level of E2 than E, (34). Obese postmenopausal women are suggested to have higher free E~ levels (35). In a study of 155 women with spontaneous ovarian failure, aged from 34 to 83 years, the plasma levels of testosterone, progestins, and estrogens did not change with advancing age; however, the levels of E2 and E, are increased and significantly correlated with the percent ideal weight of the subjects (36). In a small trial, the E ,/androstenedione ratio, E,, and E2 are found to significantly correlate with the degree of fat mass or obesity (33). The detrimental effects of obesity on humans have long been documented in relation to insulin receptor dysfunction and increase of cancer of the reproductive tract in both young (37) and postmenopausal women (32, 38), suggesting the involvement of the obesity-related perturbation of sex hormones in these diseases. Epidemiological studies of various populations have indicated a causative role of obesity in bilateral knee OA as well as other non-weight bearing joints with significant sex difference (22, 39-42). Obesity is a known cause of hyperestrogenism in women, and is associated positively with the level of total and free E 2 as well as an increased conversion rate of androstenedione into E, (32). The level of sex hormone binding globulin (SHBG) is also negatively associated with the degree of obesity; the lower the level of SHBG, the higher the level of free estrogen will be available to the target tissue in obese subjects (35, 43). The association between obesity-related sex hormone changes and OA is implicated in that the hormonal changes occuring early in life may be the pathophysiology in the development and manifestation of OA postmenopausally. CURRENT OA MODELS RELATING TO ESTROGEN AND ITS ANTAGONIST, TAMOXIFEN Various investigations focusing on the effects of estrogen on the development of OA have been rigorously attempted and studied in various animal models. However, inconsistent results are reported and the interpretations are often complicated by the use of different species and sex, or even the means of OA induction. Certainly, the observations in the spontaneous OA model cannot be extrapolated or equated to those manifested in the surgically induced models. Using C57BL Jax 6 male mice susceptible to spontaneous OA, Silberberg and Silberberg demonstrated that weekly subcutaneous injections of estradiol benzoate for 5 months reduce the incidence of OA in the growing (1-6 months) and young (6-11 months) mice, but not in the old mice (44). The inhibiting effects of estrogen on OA development are possibly achieved by acting directly on cartilage (44,45). In a series of experiments using a partial meniscectomy OA model, Rosner et al. demonstrated sex difference in the worsening effects of estrogen and an association of E 2 with the pathogenesis of OA. E2, while suppressing the sulfate incorporation in non-oophorectomized rabbits with induced OA, as well as testosterone at a pharmacological dosage, is ineffective in worsening or alleviating the lesions (46). However, E2 administration in adult oophorectomized mice accentuates the OA incidence in these mice with genetic OA (44,45); this discrepancy in the effects of estrogen in these studies probably arises from the use of different species and OA models. In another investigation, the weekly intramuscular injections of estradiol valerate (0.4 mg/kg) 1 week before and 12 weeks after partial meniscectomy significantly suppressed the sulfate incorporation in female virgin rabbits; however, the total proteoglycan (PGs) concentration remains unchanged (47). The aggravating effect of E~ on the OA lesions was not demonstrated, in part due to the long interval between the administration and the inconsistent circulating concentration of E~ before and during the course of OA in these non-oophorectomized rabbits (47). The effect of estrogen, given to the oophorectomized rabbits at a pharmacological dosage above that required for estrogen therapy, is assessed in these animals after partial meniscectomy; the supplemental E2 treatment worsens the osteoarthritic changes with

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no significant suppression of PGs synthesis (48,49). The use of tamoxifen, at a dosage similar to that for clinical treatment of breast cancer, improves the OA lesions, possibly by competitive bindings of cytoplasmic estrogen receptors (49,50). Further studies demonstrate the therapeutic effect of tamoxifen on experimentally induced OA in non-oophorectomized rabbits (51) and the osteoarthritic-inhibiting effects of intraarticular injected tamoxifen, possibly via a mechanism achieved by blocking the estrogen receptor in the cartilage (49,52). Although the possibility of a receptor interaction between estrogen and cartilage is strongly implicated, these results only demonstrate the worsening effects of E2 on pre-existing OA lesions. The role of estrogen, thus, is shown in these models to accentuate the OA development, in which the etiological role of estrogen is not demonstrated as suggested in other reports (22,26,50). In order to eliminate the influence of other possible etiologies of OA and side effects of systemic injections of E2 such as weight loss and increased risks to other target tissues (50,53), a novel animal OA model was developed, using adult oophorectomized rabbits subjected to 12 weeks of intraarticular injections of estradiol benzoate at a dosage equivalent to 0.3 mg/kg/day, to demonstrate the direct /n vivo effect of E, and the cartilage degeneration (54). Typical pathological features of human OA such as fibrillation, cell clonings, and loss of cartilage layers are observed in this novel knee OA model. E2 induces OA development even at a lower dosage (0.06 mg/kg/day), but with a slower pace. Minor pathological changes of cartilage were only observed after 12 weeks of low dose estradiol benzoate treatment. These observations favor the notion that the persistence of low estrogen levels or short episodes of high estrogen concentration in the target tissue over a long period of time may induce cartilage degeneration, such as those of human OA. It is speculated that intraarticular injections of estradiol benzoate possibly result in metabolic changes of chondrocytes, such as the decreases in staining for lactate dehydrogenase activity, which might precede the matrix disintegration of PGs loss and collagen disruption (55). Compensatory synthesis of PGs in the early stage of experimentally induced OA (56, 57) may be attributed to the slight loss of PGs in the E 2-treated animals with early OA changes. Recent studies on PGs and OA suggest that postsynthetic modification of PGs and the release of degradative enzymes are the initial cellular changes in the early osteoarthritic process (56), and the toss of PGs may precede any apparent cartilage surface fibrillation or roughing in human osteoarthritic cartilage (58). ESTROGEN EFFECTS ON CARTILAGE METABOLISM The effects of estrogens on cartilage and OA are controversial due to the methodology, types and concentrations of estrogen used, species, age of the animals, and even the duration of experimental observations (Table 1). Since articular cartilage is a hormonally sensitive tissue, its PGs synthesis and cell proliferation can be greatly affected by the systemic administration of estrogen or incubation with estrogen in monolayer chondrocyte culture. The inhibitory effects of high concentrations of estrogen ( > 10 nM) on cartilage metabolism are evidenced by the decreased uptakes of 3~S-sulfate in cultured chondrocytes (61,62). In v/vo study of rat costal cartilage after subcutaneous injection of E~ also revealed suppressed sulfate metabolism (63). This hypothesis of direct action of estrogen is shared by other investigators. E~ (435-1219 pg/ml) is shown to suppress the DNA synthesis in cultured human chondrocytes and osteosarcoma cell lines, but the degree of suppression is concentration-dependent (67). A direct in vitro action of E~ was also demonstrated during skeletal growth; while cartilage cells, from rabbit fetuses and those aged from 2-80 days, show age-related increases of sulfate incorporation in the presence of various concentrations of E 2 (10 '° - 10"BM), the response in female-derived culture is twice that in male-derived culture (59). Prostaglandins production in chondrocytes culture is stimulated in the presence of high concentrations of E~ (68), which in turn can suppress PGs synthesis in chondrocytes, as demonstrated in bovine chondrocyte culture (66). ESTROGEN RECEPTORS IN ARTICULAR CARTILAGE In other hormonally sensitive tissues, estrogen diffuses across the cell membrane and

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to its specific cytosolic receptor (30,69,70). The hormone-receptor complex is translocated into

TABLE 1: THE EFFECTS OF ESTRADIOL ON CARTILAGE METABOLISM Sources

3sS incorporation 3H incorporation

References

Rabbit Articular cartilage

$ "-" 1

46 47 53 48

Epiphyseal cartilage Chondrocyte culture

t t

Chondrocyte culture

~

61

Costal cartilage Chondrocyte culture Epiphyseal growth plate

~ 1

63 62 64

~

59 60

Mouse Rat

Bovine Chondrocyte culture

65 66

Human Chondrocyte culture t, increase;

|

67

l, decrease; ,,.,, no change

3sS incorporation: 3SS-sulfate incorporation represents synthesis of proteoglycans ~H incorporation: 3H-thymidine incorporation indicates DNA synthesis in chondrocytes

the nucleus to activate certain genomes and the subsequent RNA and protein synthesis. The findings of high affinity estrogen receptors in rabbit and canine articular cartilage, chondrocyte culture, baboon temporomandibular cartilage, and human osteoarthritic cartilage (49,71-75), as well as the the induction of OA by intraarticular injection of E2 (54), all suggest a possible uptake of E2 into chondrocytes as a risk factor of OA. The affinity of estrogen receptors is influenced by age; the affinity is higher in chondrocytes derived from the pubertal rabbit than the prepubertal rabbit and the difference is suggested to be inherent and independent of its surrounding estrogenic environment (72). However, the correlation of estrogen receptor and its inhibitory effects on PGs synthesis in chondrocytes appears to be species-specific. PGs synthesis in bovine articular cartilage culture is significantly suppressed after incubation with E2, and with negative findings of estrogen receptors (65), an observation which is not shared in other species. In addition to the possible genetic variations in the receptor affinity, estrogenic environment may also be of significance in regulating the chondrocyte metabolism in adult animals. A possible up-regulation of estrogen receptor binding sites by estrogen was demonstrated in

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oophorectomized rabbits subjected to 3-week intraarticular injections of estradiol benzoate (0.3 mg/kg/day), with a preferential distribution of estrogen receptors in the femoral condyle of control and experimental groups (77). This reflects the anatomical difference in its receptor level. In human osteoarthritic cartilage of both sexes obtained at the time of total knee replacements, higher estrogen receptor binding sites were found in the medial compartment (Grade 3 to 4 of Kellgren grading system) than in the lateral compartment (Grade 0 to 1) of both femoral condyle and tibial plateau (75). In these OA patients , the synovial E2 levels were found elevated, compared with the normal value in normal postmenopausal women (75). It is suggested that synovial E2 level and E ~ receptor bindings are correlated to the severity of OA changes. These increases of receptor numbers in both the E2-induced osteoarthritic rabbits and human osteoarthritic cartilage, as well as the finding of high synovial E2 concentration, highly suggest that endogenous or local estrogen level may be another etiological factor in OA. Since a correlation of estrogen to the pathogenesis of OA is implicated, particularly in obese postmenopausal women, it is of interest to know if any potential risk is associated with estrogen use in oophorectomized women or postmenopausal women and OA. However, little data are available regarding the estrogen use and OA in humans. Theoretically, estrogen use should increase the stiffness of subchondral bone and therefore increase the risk of OA, as bone stiffness is suggested to be an etiological factor in OA (5). In a Framingham Osteoarthritis study conducted in 831 females, long-term estrogen use was not shown to be associated with the radiographic knee OA (27). However, due to the use of various forms of conjugated estrogen in this study and deficient knowledge of plasma estrogen levels in these estrogen users, the possible role of endogenous estrogen in OA cannot be totally excluded. The importance of free or unopposed estrogen in the development of OA has been suggested (22,54,55,75). CONCLUSIONS Undoubtedly, aging has substantial effects on this multifactorial disorder (78); however, its heterogeneous subsets with variation in the disease pattern and affected joints call for different models for separate studies of OA in defined groups. The induction of OA by estradiol and the findings of estrogen receptors both in animal and in human OA cartilage further suggest a causative role of estrogen among the numerous hypothesized factors. It is therefore hypothesized that the elevated, sustained levels of free E2 resultant from obesity or other known sources over a long time in young or perimenopausal women may be one of the etiologies of OA which frequently occurs in postmenopausal women. The detrimental effect of high levels of unopposed E2 is not manifested in younger women, which in part may be due to the "hypothesized" slow effect of E~ on cartilage, the protective effects of progesterone, and the "active" repairing ability of young cartilage resisting the episodes of microdamages from hormonal imbalance. Cartilage lesions presumably occur with long-term endogenous local E, uptake, and progress into osteoarthritis, as the aging and other factors intervene in the development of the disease.

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