Production of two variant laminin forms by endothelial cells and shift of their relative levels by angiostatic steroids.

Organization of endothelium as the lining of the cardiovascular system is supported by basement membrane. The important role of laminin and other basement membrane proteins is assumed in the angiogenesis. We show here that cultured endothelial cells produce two forms of laminin, and their relative levels are changed by antiangiogenic steroids. The synthesis of laminin subunits by endothelial cells isolated from bovine aorta and from bovine pulmonary artery was studied by metabolic labeling with [35S]methionine. Both endothelial cells produced a novel laminin-related polypeptide (A' subunit) in addition to the A, B1, and B2 subunits. Two-dimensional sodium dodecyl sulfate gel electrophoretic analysis showed that the B1B2 complex was first formed and the A subunit joined it to form the AB1B2 complex or the A' subunit joined it to form A'B1B2 complex. This mechanism implied that replacement of subunits in the complex by a corresponding variant produces variety in the structure and function of laminin. The A'B1B2 complex was the major product in endothelial cells under normal culture conditions. An angiostatic steroid, medroxyprogesterone, suppressed the A' synthesis and stimulated the A synthesis. Consequently, the major product of bovine aorta endothelial cells was converted to AB1B2. Two types of intracellular precursors were identified for each laminin-related polypeptide. Since the precursors in a given complex were synchronized with regard to maturation, the assembly of AB1B2 and A'B1B2 complexes was suggested to occur at an early step of intracellular processing.