Substrate-associated macromolecules promote cytodifferentiation of BC3H1 myogenic cells

Establishment and characterization of a conditionally immortalized smooth muscle/myometrial-like cell line

A novel smooth muscle/myometrial-like cell line, SMU1-10, has been generated from the uterus of a H-2Kb-tsA58 transgenic mouse carrying a thermolabile SV40 large T-antigen gene. These cells grow continuously when maintained at the permissive temperature (33 degrees C) for the SV40 large T-antigen but stop dividing when placed at the non-permissive temperature (39 degrees C) and ultimately die within 3 weeks. All of the SMU1-10 cells produce smooth muscle alpha-actin (SMAA) at both 33 degrees C and 39 degrees C. A subset of the cells also contain smooth muscle gamma-actin (SMGA), a hallmark of smooth muscle differentiation, and the fraction of cells staining for this actin increases from about 1% when maintained for three days at 33 degrees C to as much as 30% at 39 degrees C over the same length of time. However, the appearance of SMGA in SMU1-10 cells appears to be regulated mainly at a post-transcriptional level since in situ hybridization indicates that all cells contain SMGA mRNA at both 33 degrees C and 39 degrees C. SMU1-10 cultures also contain smooth muscle myosin heavy chain (SM-MHC) and SM22 alpha, both of which are only found in smooth muscle of the adult mouse. Three additional smooth muscle (myometrium)-related markers, connexin 43, the thromboxane A2 receptor, and the progesterone receptor also are present in these cells. At the nonpermissive temperature for SV40 large T-antigen, the both level of SMGA mRNA and the number of cells staining for this actin are significantly increased in the presence of progesterone, a process that is similar to the upregulation of SMGA in the myometrium late in pregnancy. Overall, SMU1-10 cells provides a potentially useful in vitro model system to study smooth muscle/myometrial differentiation.

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.

Plasticity of vascular smooth muscle alpha-actin gene transcription. Characterization of multiple, single-, and double-strand specific DNA-binding proteins in myoblasts and fibroblasts

Transcriptional activity of the mouse vascular smooth muscle (VSM) alpha-actin promoter was governed by both cell type and developmental stage-specific mechanisms. A purine-rich motif (PrM) located as -181 to -176 in the promoter was absolutely required for activation in mouse AKR-2B embryonic fibroblasts and partially contributed to activation in undifferentiated mouse BC3H1 myoblasts. Transcriptional enhancer factor 1 recognized the PrM and cooperated with other promoter-binding proteins to regulate serum growth factor-dependent transcription in both myoblasts and fibroblasts. Two distinct protein factors (VAC-ssBF1 and VAC-ssBF2) also were identified that bound sequence-specifically to single-stranded oligonucleotide probes that spanned both the PrM and a closely positioned negative regulatory element. VAC-ssBF1 and BF2 binding activity was detected in undifferentiated myoblasts, embryonic fibroblasts, and several smooth muscle tissues in the mouse and human. A myoblast-specific protein (VAC-RF1) also was detected that bound double-stranded probes containing a CArG-like sequence that previously was shown to impart strong, cell type specific repression. The binding activity of transcription enhancer factor 1, VAC-RF1, and VAC-ssBF1 was significantly diminished when confluent BC3H1 myoblasts differentiated into myocytes and expressed VSM alpha-actin mRNA after exposure to serum-free medium. The results indicated that cell type-specific control of the VSM alpha-actin gene promoter required the participation of multiple DNA-binding proteins, including two that were enriched in smooth muscle and had preferential affinity for single-stranded DNA.

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.

The effects of halothane and isoflurane on intracellular Ca2+ regulation in cultured cells with characteristics of vascular smooth muscle

The Ca(2+)-sensitive photoprotein aequorin was used to monitor changes in intracellular [Ca2+] within cultured cells with characteristics of vascular smooth muscle. Two cell lines were investigated: they were A10 cells, which are transformed cells originally derived from rat aorta, and BC3H1 cells obtained from mouse brain neoplasm. Transient increases in intracellular [Ca2+] were induced following exposure to two different volatile anaesthetics (halothane and isoflurane) and various vasoactive substances (acetylcholine, endothelin, histamine, serotonin and vasopressin). The amplitude of the transients induced by isoflurane were more dependent on the presence of extracellular Ca2+ than those induced by halothane, thus the modes and/or locations of action of these two anesthetics are somewhat different. The response of the two cell lines to the vasoactive substances are unique. Receptor activated changes in [Ca2+]i by various agonists were diminished in the presence and absence of either anesthetic. These data suggest that, although the receptor populations within each cell line were slightly different, the prior application of a volatile anesthetic in a clinically-relevant dose induced a transient increase in [Ca2+]i that could subsequently diminish agonist responses.

Ryanodine receptor protein is expressed during differentiation in the muscle cell lines BC3H1 and C2C12

BC3H1 and C2C12, murine cell lines, were assessed as model systems for the expression of ryanodine receptor protein during myogenesis. The ryanodine receptor is a calcium release channel of the sarcoplasmic reticulum and a component of the triad junction, a structure which is essential to excitation-contraction coupling in mature striated muscle. BC3H1 and C2C12 cells do not express the ryanodine receptor at detectable levels in a proliferative, nondifferentiated state. The ryanodine receptor protein is expressed during differentiation in BC3H1 and C2C12 cells, becoming detectable within 24 hr of the onset of differentiation. In both cell lines the ryanodine receptor is assembled in oligomeric form and binds [3H]ryanodine with high affinity. Fusion is not required for expression of the ryanodine receptor in either BC3H1 or nonfusing C2C12 cells. The level of expression of the ryanodine receptor protein is modulated by incubation with the growth factors TGF-beta and bFGF in a manner similar to that of other muscle-specific proteins. These initial observations suggest that the BC3H1 and C2C12 cell lines provide a model system for further investigations of the expression and function of the ryanodine receptor during myogenic differentiation.