Abstract.The relationship between growth and cytodifferentiation was studied in cultured rat aortic smooth muscle cells (SMCs) using expression of the smooth muscle (SM)-specific isoactins (Vanderkerckhove, J., and K. Weber, 1979, Differentiation, 14:123-133) as a marker for differentiation in these cells. Isoactin expression was evaluated by: (a) measurements of fractional isoactin content and synthesis ([35S]methionine incorporation) by densitometric evaluation of two-dimensional isoelectric focusing sodium dodecyl sulfate gels, and (b) immunocytological examination using SM-specific isoactin antibodies. Results showed the following: (a) Loss of a-SM isoactin was not a prerequisite for initiation of cellular proliferation in primary cultures of rat aortic SMCs. (b) a-SM isoactin synthesis and content were low in subconfluent log phase growth cells but increased nearly threefold in density-arrested postconfluent cells. Conversely, 13-nonmuscle actin synthesis and content were higher in rapidly dividing subconfluent cultures than in quiescent postconfluent cultures. These changes were observed in primary and subpassaged cultures. STUDIES by 4) and others (37, 41) demonstrate that vascular smooth muscle cells (SMCs) ~ undergo marked changes in phenotype when grown in cell culture. These changes include diminished contractile filaments and contractile proteins, loss of contractility, alterations in biosynthetic properties, and changes in sensitivity to mitogens. Based on observations that most cells did not proliferate in cell culture until these changes occurred, Chamley-Campbell et al. (2) proposed that cells normally exist in a nonproliferating contractile state and must modulate to a synthetic state before proliferating. Furthermore, they proposed that established SMC cultures remain permanently in a synthetic state. However, recent studies demonstrate that established cultures of SMCs continue to express smooth muscle (SM)-specific contractile proteins (10, 13,20), retain high affinity receptors to various agonists (5, 18), and undergo a Abbreviations used in this paper: FCS, fetal calf serum; HBSS, Hanks' balanced salt solution; IEF, isoelectric focusing; NM, nonmuscle; SFM, serum-free medium; SM, smooth muscle; SMC, smooth muscle cell. agonist-induced changes in membrane conductance (5,24). Whereas these studies demonstrate that cultured SMCs continue to express a variety of differentiated characteristics, they did not specifically address how growth and cytodifferentiation were related. An understanding of this relationship may have profound implications for investigators interested in studying various aspects of SMC contractile function in cultured cells, as well as in studies of growth control. It is not known, for example, whether present methods for inducing quiescence in cultured SMCs (e.g., serum-free medium [SFM,22], plasma-derived serum [33]) are associated with increased expression of cell-specific proteins indicative of entrance into a true Go state.A major limitation in previous studies ofcytod...

Previously, we demonstrated that treatment of postconfluent quiescent rat aortic smooth muscle cells (SMCs) with platelet-derived growth factor (PDGF)-BB dramatically reduced smooth muscle (SM) a-actin synthesis. In the present studies, we focused on the expression of two other SM-specific proteins, SM myosin heavy chain (SM-MHC) and SM a-tropomyosin (SM-aTM), to determine whether the actions of PDGF-BB were specific to SM a-actin or represented a global ability of PDGF-BB to inhibit expression of cell-specific proteins characteristic of differentiated SMCs. SM-MHC and SM-aTM expression were assessed by one-or

Previous studies have shown that multiple serum response factor (SRF)-binding CArG elements were required for smooth muscle cell (SMC)-specific regulation of smooth muscle (SM) alpha-actin expression. However, a critical question remains as to the mechanisms whereby a ubiquitously expressed transcription factor such as SRF might contribute to SMC-specific expression. The goal of the present study was to investigate the hypothesis that SMC-selective expression of SM alpha-actin is due at least in part to (1) unique CArG flanking sequences that distinguish the SM alpha-actin CArGs from other ubiquitously expressed CArG-dependent genes such as c-fos, (2) cooperative interactions between CArG elements, and (3) SRF-dependent binding of SMC-selective proteins to the CArG-containing regions of the promoter. Results demonstrated that specific sequences flanking CArG B were important for promoter activity in SMCs but not in bovine aortic endothelial cells. We also provided evidence indicating that the structural orientation between CArGs A and B was an important determinant of promoter function. Electrophoretic mobility shift assays and methylation interference footprinting demonstrated that a unique SRF-containing complex formed that was selective for SMCs and, furthermore, that this complex was probably stabilized by protein-protein interactions and not by specific interactions with CArG flanking sequences. Taken together, the results of these studies provide evidence that SM alpha-actin expression in SMCs is complex and may involve the formation of a unique multiprotein initiation complex that is coordinated by SRF complexes bound to multiple CArG elements.

Two putative myosin heavy chains designated SM1 and SM2 were detected on a 3.5% polyacrylamide-sodium dodecyl sulfate gel electrophoresis system loaded with homogenates of several mammalian smooth muscles. The two polypeptides were present in nearly equal amounts in all smooth muscle tissues tested and in myosin purified from swine carotid media and stomach. Both proteins were equally stained by smooth muscle-specific myosin antibodies. The smaller of the polypeptides had a mobility nearly identical to that of the single heavy chain observed in purified fast-twitch skeletal myosin. Electrophoresis of pyrophosphate extracts from swine carotid media, swine stomach, rabbit thoracic aorta, and guinea pig taenia coli on nondenaturing pyrophosphate gels revealed a single protein band. When subsequently electrophoresed on a sodium dodecyl sulfate gel, the native bands from swine tissue extracts revealed the two putative heavy chains in nearly equal amounts, as well as a large amount of a higher molecular weight peptide whose properties reflect those of filamen. Sodium dodecyl sulfate gel analysis of the myosin band from pyrophosphate gels of purified swine stomach myosin showed exclusively the two heavy chains in a nearly 1:1 ratio. Smooth muscle myosin migrates homogeneously on pyrophosphate gels, and the virtual equality of the two heavy chains may reflect the presence of large amounts of a myosin isoenzyme, which is a heavy-chain heterodimer.

The objective of the present study was to examine the molecular mechanisms whereby angiotensin II (Ang II) stimulates smooth muscle (SM) alpha-actin expression in rat aortic smooth muscle cells (SMCs). Nuclear run-on analysis and transfection studies indicated that the effects of Ang II on SM alpha-actin were mediated at least in part at the transcriptional level. Transfection of various rat SM alpha-actin promoter/chloramphenicol acetyltransferase (CAT) constructs into SMCs demonstrated that the first 155 bp of the SM alpha-actin promoter was sufficient to confer maximal Ang II responsiveness, conferring an approximately 4-fold increase in reporter activities in these SMCs compared with vehicle-treated SMCs. Mutation of either of two highly conserved CArG elements, designated A (-62) and B (-112), completely abolished Ang II-induced increases in reporter activity, whereas mutation of a homeodomain-like binding sequence at -145 (ATTA) reduced reporter activity by half. Results of EMSAs showed that nuclear extracts from Ang II-treated SMCs exhibited enhanced binding activity of serum response factor (SRF) to the CArG elements and of a homeodomain factor, MHox, to the ATTA element. Northern analyses showed that Ang II also stimulated marked increases in MHox mRNA levels. Western analyses demonstrated that Ang II-induced increases in SRF binding were not due to increased SRF protein expression. Recombinant MHox markedly enhanced binding activity of SRF in EMSAs. Finally, MHox overexpression transactivated a SM alpha-actin promoter/CAT reporter construct by approximately 3.5-fold in transient cotransfection studies. These results provide evidence for involvement of a homeodomain transcription factor, MHox, in Ang II-mediated stimulation of SM alpha-actin via a CArG/SRF-dependent mechanism.