University of Technology, Sydney. Faculty of Science.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 gen der disparity in atherosclerotic calcification we
hypothesised that androgens promote differentiation of VSMC into mineralising
osteoblast-like cells. Th erefore, the aims of this st udy wer e 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 Tstimulated
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