Identification of α2‐HS glycoprotein as a human serum carrier of matrix Gla protein (MGP) and evidence for vitamin K4 as a modulator of gene expression in proliferating vascular smooth muscle cells

MGP is a 15 kDa vitamin K‐dependent matrix protein containing 4 Ca ++ binding γ‐carboxyglutamic acid (Gla) residues formed post translationally in the endoplasmic reticulum. The protein is synthesized in the aortic wall where it serves as an inhibitor of arterial calcification. The serum concentration of MGP has been proposed to be a marker for arterial calcification. Since MGP has never been identified as a protein in human serum, all published data from human serum samples are questionable. In this study, three different antibodies were used to identify MGP in serum. The fully γ‐carboxylated form of MGP was found associated with α 2 ‐HS glycoprotein. Mass spectrometry (MS/MS) also identified transthyretin, a protein not previously identified to be associated with α 2 ‐HS glycoprotein. We conclude that mature fully γ‐carboxylated, but not incompletely γ‐carboxylated forms of MGP, are present in human plasma bound to α 2 ‐HS glycoprotein. A proteomic approach was used to determine if vitamin K4 acts as a signaling molecule and affects the proteome in proliferating vascular smooth cells (VSMCs). In the first MS/MS analysis of cell proteins from control cells and K4 treated cells separated by 2‐D‐SDS‐PAGE, we identified tropomyosin 4 as an up‐regulated protein in K4 treated cells. Tropomyocin 4 is known to be a marker for VSMCs transforming from a contractile to a synthetic phase. We expect that further work with K4 will identify its regulatory effect on several important proteins in the aortic wall and may indicate a therapeutic use. Funded by NIH grant RO1HL069331