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

Tissue and developmental specific expression of murine smooth muscle gamma-actin fusion genes in transgenic mice

Smooth muscle gamma-actin (SMGA) is an excellent marker of smooth muscle differentiation because it is essentially restricted to smooth muscle. As a first step toward unraveling the mechanisms underlying smooth muscle development and differentiation, we have examined the tissue-specific and developmental expression patterns of six constructs carrying portions of the murine SMGA gene linked to chloramphenicol acetyltransferase (CAT) in stable lines of transgenic mice. Based on the transgenic studies most, if not all, of the regulatory elements necessary for proper spatial and temporal expression of SMGA are present within a 13.7 kb segment of the SMGA gene containing 4.9 kb of upstream sequence, exon 1, intron 1, and a portion of exon 2 up to the start codon for translation. A second construct (SMGA11.6CAT) that lacks the distal 2.1 kb of upstream sequence but is otherwise identical to SMGA13.7CAT shows a similar level of smooth muscle-specific CAT activity. However, SMGA9.3CAT fusion gene containing only 571 bp of 5' flanking sequence, but otherwise identical to SMGA13.7CAT, and SMGA6.0CAT containing only the 4.9 kb upstream sequence, exon 1, and a miniintron 1 show a more than a 100-fold reduction of CAT activity in most smooth muscle-rich tissues. Furthermore, removal of most or all of intron 1 from a transgene with 571 bp of upstream sequence (SMGA2.0 CAT and SMGA0.6CAT) results in a near-complete or complete loss of activity, respectively, in all tissues. Overall, the studies suggest that upstream elements between -2.7 kb and -571 bp and elements within intron 1 are required for high levels of SMGA gene expression in an appropriate temporal-spatial fashion.

Proteoglycan biosynthesis is required in BC3H1 myogenic cells for modulation of vascular smooth muscle alpha-actin gene expression in response to microenvironmental signals

Induction of vascular smooth muscle (VSM) alpha-actin mRNA expression during cytodifferentiation of mouse BC3H1 myogenic cells coincides with the accumulation of cell surface- and extracellular matrix-associated sulfated proteoglycans. Inhibition of proteoglycan biosynthesis in myogenic cells using an artificial beta-D-xyloside glycosaminoglycan acceptor was accompanied by a reduction in cell surface/extracellular matrix proteoglycans and VSM alpha-actin mRNA expression while enhancing the secretion of free chondroitin sulfate/dermatan sulfate and heparan sulfate glycosaminoglycans into the culture medium. Maximum inhibition of VSM alpha-actin mRNA expression required that proteoglycan biosynthesis be blocked during the early phase of cytodifferentiation when myoblasts were fully confluent and quiescent. The inhibitory effect of beta-D-xyloside on alpha-actin mRNA expression resulted from attenuation at both the transcriptional and post-transcriptional control points. Sustained proteoglycan biosynthesis was required for induction of VSM alpha-actin mRNA in quiescent myoblasts in response to cytodifferentiation-permissive, substrate-associated macromolecules (SAM) or upon exposure to soluble serum factors capable of transiently stimulating VSM alpha-actin gene transcription. The results suggested that efficient myoblast cytodifferentiation and modulation of VSM alpha-actin mRNA levels depended on intact cell surface proteoglycans to convey signals generated as a consequence of cellular interaction with substrate components and serum factors.

Identification of a 38-kDa heparin-binding glycoprotein (gp38k) in differentiating vascular smooth muscle cells as a member of a group of proteins associated with tissue remodeling

Cultured aortic smooth muscle cells (SMC) exhibit morphological and phenotypic modulation characterized by a change from a substrate attached monolayer culture to a multilayered nodular cell culture in which SMC are imbedded into the extracellular matrix. Associated with nodule formation is a change in the pattern of SMC gene expression including increased expression of a well characterized marker of smooth muscle cell differentiation, SM alpha-actin, and a 38-kDa glycoprotein (gp38k). gp38k has sequence homology with proteins reported to be correlated with tissue remodeling. To characterize the gp38k mRNA we designed degenerate oligonucleotides based on partial polypeptide sequencing to select a cDNA encoding the full-length gp38k. Southern analysis indicates that porcine gp38k is present as a single copy gene. Northern analysis indicates that the increase in gp38k is correlated with an increase in the steady state level of gp38k mRNA; and is present in cultures that have initiated the formation of multilayered foci and nodules. The correlation between SMC differentiation and gp38k expression is further established by using culture conditions that facilitate SMC differentiation. Cultures seeded onto reconstituted extracellular matrix show rapid formation of nodules and increased expression of gp38k mRNA. Comparison of the gp38k and cDNA sequences with nucleotide and protein sequences available through GenBank and SwissProt data banks revealed that molecules homologous to gp38k were present in human, mouse, bovine, and Drosophila tissues, suggesting that the gp38k may be a member of a gene family. Although a function for gp38k has not been identified, this report represents the first report of its correlation with a specific process important in phenotypic and morphological modulation of vascular SMC.