Regulation of SRF/CArG‐dependent gene transcription during chronic partial obstruction of murine small intestine

Abstract

Intestinal obstructions lead to a variety of motility disorders. Small intestine smooth muscles undergo dramatic phenotypic changes in response to obstruction, but the underlying molecular mechanisms are unknown. Using RT-PCR, ChIP, Re-ChIP, and Western blots, we examined the effect of small bowel mechanical obstruction on smooth muscle gene expression. Obstruction caused a transient hyperplasia, followed by a prolonged hypertrophic response of small intestine smooth muscle cells. Smooth muscle myosin heavy chain (MHC), alpha-actin, and gamma-actin expression decreased initially, and then increased as hypertrophy developed. Myocardin expression decreased initially and then increased, while kruppel-like factors (KLF)4 and KLF5 expression increased initially, and then decreased. Serum response factor (SRF) expression decreased initially, and then recovered to sham-operated levels as hypertrophy developed. SRF binding to smooth muscle MHC and alpha-actin promoters decreased initially, but then increased above sham-operated levels as hypertrophy developed. Elk-1 binding to smooth muscle myosin heavy chain and alpha-actin promoters increased initially, and then decreased to sham-operated levels as hypertrophy developed. c-fos expression increased initially, which was associated with increased SRF/Elk-1 binding to the c-fos promoter. The Elk-1 phosphorylation inhibitor U-0126 inhibited the increase in c-fos expression. These findings indicate a dynamic response of small intestine smooth muscles to bowel obstruction involving switching between differentiated, proliferative, and hypertrophic phenotypes. These results suggest that changes in the expression and interactions between SRF, myocardin, Elk-1, and c-fos play key roles in the phenotypic switching of small intestine smooth muscles in response to mechanical obstruction.

Serum Response Factor Is Essential for Prenatal Gastrointestinal Smooth Muscle Development and Maintenance of Differentiated Phenotype

“…Whether expression of SRF and contractile proteins (myosin heavy chains) increases or decreases with intestinal smooth muscle hypertrophy has been the subject of controversy. 13,28 The discrepancy between contradictory findings by different investigators may be explained by the dynamic stages of the hypertrophic response at which the tissues were examined. For instance, Chen et al 13 found that SRF and contractile protein in whole muscle tissue were down-regulated at the beginning of hypertrophy but later restored to the control levels.…”

“…13,28 The discrepancy between contradictory findings by different investigators may be explained by the dynamic stages of the hypertrophic response at which the tissues were examined. For instance, Chen et al 13 found that SRF and contractile protein in whole muscle tissue were down-regulated at the beginning of hypertrophy but later restored to the control levels. Our Western blot data indicated that SRF levels increased after 2 weeks of partial obstruction but that MYH11 levels remained unchanged.…”

“…The phenomena of SMC hypertrophy and hyperplasia, which occurs during partial bowel obstruction, has been previously well described. 13,22,23 Inflammation may also cause SMC hypertrophy within the GI tract. 24,25 However, the molecular mechanism remains less understood.…”

“…Since SMCs dedifferentiate into a proliferating phenotype and hypertrophy under pathological obstruction, 13 we hypothesized that SRF expression in GI SMCs decreases during the re- sulting compensatory process. To test our hypothesis, we looked for changes in SRF expression in hypertrophic GI tissue using a surgically induced partial obstruction mouse model.…”

Molecular and Cellular Characteristics of the Colonic Pseudo-obstruction in Patients With Intractable Constipation

“…6 and 7). Extensive luminal dilation with smooth muscle hypertrophy developed proximally to the TZ region closely resembles the results of partial occlusion mechanically induced in laboratory animals, 29,30 which lead us to speculate that the TZ is the site of high resistance due to functional obstruction. Accumulation of the fecal contents proximally may result in passive stretch and luminal dilation, leading to smooth muscle hypertrophy as an adaptation to increased functional demand.…”