Upregulation of RGS4 and downregulation of CPI-17 mediate inhibition of colonic muscle contraction by interleukin-1β

The pro-inflammatory cytokine IL-1 beta contributes to the reduced contractile responses of gut smooth muscle observed in both animal colitis models and human inflammatory bowel diseases. However, the mechanisms are not well understood. The effects of IL-1 beta on the signaling targets mediating acetylcholine (ACh)-induced initial and sustained contraction were examined using rabbit colonic circular muscle strips and cultured muscle cells. The contraction was assessed through cell length decrease, myosin light chain (MLC20) phosphorylation, and activation of PLC-beta and Rho kinase. Expression levels of the signaling targets were determined by Western blot analysis and real-time RT-PCR. Short interfering RNAs (siRNAs) for regulator of G protein signaling 4 (RGS4) were used to silence endogenous RGS4 in muscle strips or cultured muscle cells. IL-1 beta treatment of muscle strips inhibited both initial and sustained contraction and MLC20 phosphorylation in isolated muscle cells. IL-1 beta treatment increased RGS4 expression but had no effect on muscarinic receptor binding or G alpha(q) expression. In contrast, IL-1 beta decreased the expression and phosphorylation of CPI-17 but had no effect on RhoA expression or ACh-induced Rho kinase activity. Upregulation of RGS4 and downregulation of CPI-17 by IL-1 beta in muscle strips were corroborated in cultured muscle cells. Knockdown of RGS4 by siRNA in both muscle strips and cultured muscle cells blocked the inhibitory effect of IL-1 beta on initial contraction and PLC-beta activation, whereas overexpression of RGS4 inhibited PLC-beta activation. These data suggest that IL-1 beta upregulates RGS4 expression, resulting in the inhibition of initial contraction and downregulation of CPI-17 expression during sustained contraction in colonic smooth muscle.