The role of Gla proteins in vascular calcification.
Arterial calcification occurs with increasing age and in association with a diverse range of diseases, including atherosclerosis, diabetes, and uremia. It occurs at two sites in the vessel wall--in the media where it is known as Monckeberg's sclerosis and in the intima where it is invariably associated with atherosclerosis. Although there are similarities between them, the molecular mechanisms underlying these two forms of calcification may be distinct. Evidence is accumulating that vascular calcification is an active process that has many similarities with ossification, including local expression of bone-associated collagenous and noncollagenous proteins. The recent generation of a matrix gamma-carboxyglutamic acid (Gla) protein (MGP) knockout mouse, which exhibits extensive and lethal calcification and cartilaginous metaplasia of the media of all elastic arteries, has refocused attention on the role of Gla-containing proteins in vascular calcification. Gla-containing proteins have glutamic acid residues that must by gamma-carboxylated by vitamin-K-dependent gamma-carboxylase to enable them to bind calcium and function normally. Therefore, there is considerable scope for both transcriptional and posttranslational modifications of Gla protein function. Recent studies in humans have shown that although MGP mRNA is constitutively expressed by normal vascular smooth muscle cells (VSMCs), it is substantially upregulated in cells adjacent to both medial and intimal calcification. Studies in rats and on cultured human VSMCs showing that inhibition of MGP function by warfarin can accelerate spontaneous calcification have emphasized the potential importance of posttranslational processing in determining MGP function. It is therefore plausible that environmental influences such as diet and medication may have significant effects on vascular calcification. Furthermore, recent studies have shown that several other Gla-containing proteins with the potential to regulate or perhaps contribute to vascular calcification are present in the human vasculature. Future studies on the role of Gla-containing proteins combined with advances in noninvasive imaging techniques to quantify vascular calcification may lead to identification of individuals at particular risk of vascular calcification and the evaluation of novel therapies aimed at regulating its development or progression.