Abstract
Smooth muscle contraction is a dynamic process driven by acto-myosin interactions that are controlled by multiple regulatory proteins. Our studies have shown that members of the AP-1 transcription factor family control discrete behaviors of smooth muscle cells (SMC) such as growth, migration and fibrosis. However, the role of AP-1 in regulation of smooth muscle contractility is incompletely understood. In this study we show that the AP-1 family member JunB regulates contractility in visceral SMC by altering actin polymerization and myosin light chain phosphorylation. JunB levels are robustly upregulated downstream of transforming growth factor beta-1 (TGFβ1), a known inducer of SMC contractility. RNAi-mediated silencing of JunB in primary human bladder SMC (pBSMC) inhibited cell contractility under both basal and TGFβ1-stimulated conditions, as determined using gel contraction and traction force microscopy assays. JunB knockdown did not alter expression of the contractile proteins α-SMA, calponin or SM22α. However, JunB silencing decreased levels of Rho kinase (ROCK) and myosin light chain (MLC20). Moreover, JunB silencing attenuated phosphorylation of the MLC20 regulatory phosphatase subunit MYPT1 and the actin severing protein cofilin. Consistent with these changes, cells in which JunB was knocked down showed a reduction in the F:G actin ratio in response to TGFβ1. Together these findings demonstrate a novel function for JunB in regulating visceral smooth muscle cell contractility through effects on both myosin and the actin cytoskeleton.
Highlights
The function of hollow organs such as the urinary bladder is dependent on appropriate contractility of smooth muscle (SM)
We determined the impact of transforming growth factor-beta 1 (TGFb1) on the contractile phenotype of primary human bladder smooth muscle cells
We provide evidence to support a role for JunB as a novel regulator of contractility in visceral smooth muscle cells (SMC)
Summary
The function of hollow organs such as the urinary bladder is dependent on appropriate contractility of smooth muscle (SM). In response to pathologic stimuli, such as mechanical stress, or altered innervation, smooth muscle cells (SMC) undergo phenotypic changes that result in loss of differentiation markers, cellular hypertrophy, increased production of extracellular matrix proteins and eventual loss of contractile function [1]. Consistent with a role for TGFb1 in contractile protein expression regulation, elevation of TGFb1 in hollow organs has been linked to alterations in muscle contractility through direct effects on SM marker expression. Exposure of cells in culture to TGFb1 increases stress fiber formation, which in turn can feed forward to regulate SM marker expression in SMC in a RhoA-dependent manner [10], suggesting functional interaction between the actin cytoskeleton and the smooth muscle contractile apparatus
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