CD44 inhibits α-SMA gene expression via a novel G-actin/MRTF-mediated pathway that intersects with TGFβR/p38MAPK signaling in murine skin fibroblasts

Well-regulated differentiation of fibroblasts into myofibroblasts (MF) is critical for skin wound healing. Neoexpression of alpha-smooth muscle actin (alpha-SMA), an established marker for MF differentiation, is driven by TGF beta receptor (TGF beta R)-mediated signaling. Hyaluronan (HA) and its receptor CD44 may also participate in this process. To further understand this process, primary mouse skin fibroblasts were isolated and treated in vitro with recombinant TGF-beta 1 (rTGF-beta 1) to induce alpha-SMA expression. CD44 expression was also increased. Paradoxically, CD44 knockdown by RNA interference (RNAi) led to increased alpha-SMA expression and alpha-SMA-containing stress fibers. Removal of extracellular HA or inhibition of HA synthesis had no effect on alpha-SMA levels, suggesting a dispensable role for HA. Exploration of mechanisms linking CD44 knockdown to alpha-SMA induction, using RNAi and chemical inhibitors, revealed a requirement for noncanonical TGF beta R signaling through p38MAPK. Decreased monomeric G-actin but increased filamentous F-actin following CD44 RNAi suggested a possible role for myocardin-related transcription factor (MRTF), a known regulator of alpha-SMA transcription and itself regulated by G-actin binding. CD44 RNAi promoted nuclear accumulation of MRTF and the binding to its transcriptional cofactor SRF. MRTF knockdown abrogated the increased alpha-SMA expression caused by CD44 RNAi, suggesting that MRTF is required for CD44-mediated regulation of alpha-SMA. Finally, chemical inhibition of p38MAPK reversed nuclear MRTF accumulation after rTGF-beta 1 addition or CD44 RNAi, revealing a central involvement of p38MAPK in both cases. We concluded that CD44 regulates alpha-SMA gene expression through cooperation between two intersecting signaling pathways, one mediated by G-actin/MRTF and the other via TGF beta R/p38MAPK.