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Proliferation modulates intestinal smooth muscle phenotype in vitro and in colitis in vivo.

Intestinal inflammation causes an increased intestinal wall thickness, in part, due to the proliferation of smooth muscle cells, which impairs the contractile phenotype elsewhere. To study this, cells from the circular muscle layer of the rat colon (CSMC) were isolated and studied, both in primary culture and after extended passage, using quantitative PCR, Western blot analysis, and immunocytochemistry. By 4 days in vitro, both mRNA and protein for the smooth muscle marker proteins α-smooth muscle actin, desmin, and SM22-α were reduced by >50%, and mRNA for cyclin D1 was increased threefold, evidence for modulation to a proliferative phenotype. Continued growth caused significant further decrease in expression, evidence that phenotypic loss in CSMC was proportional to the extent of proliferation. In CSMC isolated at day 2 of trinitrobenzene sulfonic acid-induced colitis, flow cytometry and Western blotting showed that these differentiated markers were reduced in mitotic CSMC, while similar to control in nonmitotic CSMC. By day 35 post-trinitrobenzene sulfonic acid, when inflammation has resolved, CSMC were hypertrophic, but, nonetheless, showed markedly decreased expression of smooth muscle protein markers per cell. In vitro, day 35 CSMC displayed an accelerated loss of phenotype and increased thymidine uptake in response to serum or PDGF-BB. Furthermore, carbachol-induced expression of phospho-AKT (a marker of cholinergic response) was lost from day 35 CSMC in vitro, while retained in control cells. Therefore, proliferation reduces the expression of smooth-muscle-specific markers in CSMC, possibly leading to altered contractility. However, inflammation-induced proliferation in vivo also causes lasting changes that include unexpected priming for an exaggerated response to proliferative stimuli. Identification of the molecular mechanisms of intestinal smooth muscle cell phenotypic modulation will be helpful in reducing the detrimental effects of inflammation.