Density-dependent modulation of vascular smooth muscle alpha-actin biosynthetic processing in differentiated BC3H1 myogenic cells.

The expression of vascular smooth muscle (VSM) alpha-actin mRNA during BC3H1 myogenic cell differentiation is specifically stimulated by conditions of high cell density. Non-proteolytic dissociation of cell-cell and cell-matrix contacts in post-confluent cultures of BC3H1 myocytes using EDTA promotes loss of the differentiated morphological phenotype. EDTA-dispersed myocytes exhibit an undifferentiated fibroblastoid appearance and contained reduced levels of both VSM and skeletal alpha-actin mRNA. Muscle alpha-actin mRNA levels in EDTA-dispersed myocytes were not restored to that observed in confluent myocyte preparations by experimental manipulation of cell density conditions. Pulse-labeling techniques using L-[35S]cysteine to identify muscle actin biosynthetic intermediates revealed that EDTA-dispersed myocytes expressed nascent forms of both the VSM and skeletal muscle alpha-actin polypeptide chains. However EDTA-dispersed myocytes were less efficient in the post-translational processing of immature VSM alpha-actin compared to non-dispersed myocytes. Simple cell-to-cell contact may mediate VSM alpha-actin processing efficiency since high-density preparations of EDTA-dispersed myocytes processed more VSM alpha-actin intermediate than myocytes plated at low density. The actin isoform selectivity of the response to modulation of intercellular contacts suggests that actin biosynthesis in BC3H1 myogenic cells involves mechanisms capable of discriminating between different isoform classes of nascent actin polypeptide chains.