MATRIX GLA PROTEIN MESSENGER-RNA EXPRESSION IN CULTURED TYPE-II PNEUMOCYTES

Matrix Gla protein (MGP) was first isolated from the matrix fraction o f bone. This highly conserved vitamin K-dependent protein of 14 kDa ha s been identified in numerous tissues and cells, and its mRNA was rece ntly found to be abundant in rat lung. Relatively low MGP protein leve ls in many soft tissues where its mRNA is high suggests an important s ecretory function for this protein. We have found a high specific acti vity of vitamin K-dependent carboxylase in microsomes of rat pulmonary type II cells and the presence of numerous endogenous substrates, inc luding one of 13-15 kDa. To investigate the possibility that MGP and i ts mRNA could be localized in type II cells, rat MGP and actin cDNA pr obes were hybridized to total RNA obtained from freshly isolated type II cells and from cells cultured for up to 6 days. MGP mRNA increased 5- to 6-fold relative to beta-actin mRNA from days 3 to 6 in primary c ulture and MGP secretion increased nearly 60-fold during that interval . MGP mRNA and MGP secretion decreased 25-75% if cultures were supplem ented with vitamin K quinone. Vitamin K deficiency, caused by carbon s tripping the serum or treatment of cell cultures with warfarin, result ed in an induction of carboxylase activity and elevated MGP mRNA. In p arallel experiments, carboxylase specific activity also increased duri ng culture in the presence or absence of vitamin K. Retinoic acid furt her increased steady-state mRNA levels and MGP secretion at later cult ure intervals, an effect which was serum dependent. These results sugg est that MGP is an important and highly regulated vitamin K-dependent protein in type II cells and that changes at the mRNA and protein leve ls could depend on a feedback system involving mature, fully processed MGP. Increased expression of MGP mRNA as type II cells become more ty pe I cell-like suggests that this protein could serve as an important marker or mediator of cellular differentiation and that retinoids may play a role in this transition.