β-Catenin Signaling Contributes to Platelet Derived Growth Factor Elicited Bladder Smooth Muscle Cell Contraction Through Up-Regulation of Cx43 Expression

Abstract

Increased gap junctions contribute to bladder overactivity but the factors and mechanisms involved in gap junction regulation in the bladder are not well established. We examined whether and how platelet derived growth factor regulates connexin43 in bladder smooth muscle cells. Cultured rat bladder smooth muscle cells were treated with growth factors with or without agents that interfere with phosphatidylinositol 3-kinase, mitogen activated protein kinase and β-catenin signaling pathways. Connexin43 expression was examined by Western and Northern blot, and immunochemistry. Functional gap junctions were evaluated by scrape-loading dye transfer assay. Bladder smooth muscle cell contraction was measured by collagen gel contraction. 1) Platelet derived growth factor induced phosphatidylinositol 3-kinase and mitogen activated protein kinase dependent accumulation of nuclear β-catenin. This was followed by increased connexin43 expression. 2) Down-regulation of β-catenin by specific siRNA abolished the connexin43 increasing effect of platelet derived growth factor while β-catenin stimulation due to glycogen synthase kinase inhibition mimicked that effect. 3) Basic fibroblast growth factor and epidermal growth factor also induced connexin43 expression. Their effects were potentiated by platelet derived growth factor. 4) Gap junction inhibition attenuated the bladder smooth muscle cell contraction induced by platelet derived growth factor. Consistently fibroblasts from connexin43 knockout (Cx43-/-) mice showed a much weaker contractile response to platelet derived growth factor than cells from connexin43-wild (Cx43+/+) litter mates. Platelet derived growth factor induces connexin43 expression and bladder smooth muscle cell contraction by activating β-catenin signaling. As a convergence point for many signal pathways, β-catenin may be targeted to treat bladder overactivity.