Clusterin expression in differentiating smooth muscle cells

Clusterin is a heterodimeric glycoprotein, expressed by various cell types and shown to have activity in cell-cell adhesion. Cultured porcine smooth muscle cells (SMC) undergo morphological and phenotypic modulation associated with a change from a substrate-attached monolayer culture to a nodular culture in which most of the cells are present in multicellular aggregations (nodules). During that transition from monolayer to nodular cell culture (> 8 days) the expression of an mRNA and protein with significant homology to rat and human clusterin is increased. Clusterin expression continues in the nodular cell cultures and it is secreted at 0.3 micrograms/ml/24 h as a protein with an apparent molecular mass = 80 kDa. In the presence of beta-mercaptoethanol the molecular mass is approximately 40 kDa. SMC clusterin expression is regulated by culture conditions that also affect culture morphology. SMC cultures seeded on a preformed extracellular matrix composed of Matrigel form nodules within 24 h and cultures seeded on a collagen gel form nodules in 48-72 h. We establish here that Matrigel contains clusterin and propose that endogenous clusterin supports the rapid formation of nodules. The collagen gel does not contain clusterin but facilitates clusterin expression by smooth muscle cells. Nodule formation in SMC cultures growing on collagen gel is inhibited by the addition of anticlusterin antibody to SMC growing on collagen gels and the antibody effect is eliminated by preincubation with purified plasma clusterin. These results demonstrate differential expression of SMC clusterin and suggest that clusterin has a functional role in SMC modulation.