Density‐Dependent modulatioon of vascular smooth muscle α‐actin biosynthetic processing in differntiated BC3H1 myogenic cells

The expression of vasuclar smooth muscle (VSM) α‐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 α‐actin mRNA. Muscle α‐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‐[ 35 S] cysteine to identify muscle actin biosynthetic intermediates revelated that EDTA‐dispersed myocytes expressed nascent forms of both the VSM and skeletal muscle α‐actin polypetide chains. However EDTA‐dispersed myocytes were less effieicent in the post‐translational processing of immautre VSM α‐actin compared to non‐dispersed myocytes. Simple cell‐to‐cell contact may mediate VSM α‐actin processing efficiency since high‐density preparations of EDTA‐dispersed myocytes processed more VSM α‐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 mehcanisms capable of discriminating between different isoform classes of nascent actin polypetide chains. © 1992 Wiley‐Liss, Inc.