Role of Androgens in Vascular Smooth Muscle Cell Calcification

Abstract

Calcification is a common feature of advanced atherosclerotic lesions and is a clinically significant predictor of cardiovascular events. Coronary calcification is more prevalent in men than age-matched women. However, atherosclerotic calcification increases in postmenopausal women, who present with lower levels of estrogen, suggesting that sex hormones play a critical role in its pathogenesis and progression. This has implications for hormone therapy treatment that is used to treat age-related conditions such as osteoporosis and menopause Extensive observational studies into estrogen replacement therapy have revealed that postmenopausal women treated with estrogen exhibit less extensive atherosclerotic calcification. The effects of androgens on atherosclerotic calcification have, however, received little attention and consequently its mechanisms remain poorly understood. This study therefore explored the effects of androgens on atherosclerotic calcification.

In vitro studies postulate vascular smooth muscle cell (VSMC) differentiation into mineralising osteoblast-like cells as a key mediator of atherosclerotic calcification. Given the gender disparity in atherosclerotic calcification we hypothesised that androgens promote differentiation of VSMC into mineralising osteoblast-like cells. Therefore, the aims of this study were to 1) examine the effects of androgens in vascular smooth muscle cell differentiation and calcification and 2) elucidate the molecular mechanisms of androgen action in this process, using phosphate-induced bovine and murine in vitro models of calcification.

This study demonstrated that co-treatment of bovine coronary artery smooth muscle cells (BCASMC) with phosphate and testosterone (T) and dihydrotestosterone (DHT) promoted calcification. Investigation of the molecular mechanisms underlying calcification in the bovine model revealed T-stimulated calcification was estrogen receptor (ER) driven. DHT, however, mediated its effects via the androgen receptor (AR). Further investigation of molecular mechanisms showed DHT regulated ALP activity whereas T did not. T, therefore, promoted calcification in an ER-driven, ALP independent pathway in contrast to DHT, which mediated its effects via an AR-driven, ALP dependent pathway.

A primary mouse cell-based calcification model was also established. In contrast to the bovine model, it was found that T and DHT treatment did not promote calcification in the murine model. The lack of androgen promotion of calcification in this model was associated with the absence of ALP activity. The conclusion drawn from the bovine model, of a mechanistic role for ALP in the DHT /AR driven mineralisation but not for T-driven mineralisation, suggested that in the murine cells an ER pathway is not functioning.

In conclusion, the studies presented in this thesis demonstrate that T and DHT promote differentiation of vascular smooth muscle cells into osteoblast-like cells capable of mineralisation. T and DHT mediate calcification via alternative pathways that can involve AR and ERs. A potential mechanistic role for ALP in DHT /AR-driven mineralisation has been established.