Transforming Growth Factor-β Receptor–Mediated, p38 Mitogen-Activated Protein Kinase–Dependent Signaling Drives Enhanced Myofibroblast Differentiation during Skin Wound Healing in Mice Lacking Hyaluronan Synthases 1 and 3

ABSTRACT

Normal myofibroblast differentiation is critical for proper skin wound healing. Neoexpression of α-smooth muscle actin (α-SMA), a marker for myofibroblast differentiation, is driven by TGF-β receptor (TGFβR) mediated signaling. Hyaluronan (HA) and its three synthesizing enzymes, hyaluronan synthases (Has1, 2, 3), also participate in this process. In Has1/3 double knockout (Has1/3 null) mice, closure of skin wounds is significantly accelerated. Here we show that TGF-β activity and dermal collagen maturation are increased in Has1/3 null healing skin. Cultures of primary skin fibroblasts isolated from Has1/3 null mice have higher levels of TGF-β activity, α-SMA expression, and phosphorylation of p38MAPK at Thr180/Tyr182, compared to wildtype fibroblasts. We also show that p38α MAPK is a necessary element in a non-canonical TGF-βR signaling pathway driving α-SMA expression in Has1/3 null fibroblasts. Myocardin-related Transcription Factor (MRTF), a cofactor that binds to the transcription factor Serum Response Factor (SRF), is also critical. Nuclear localization of MRTF is increased and MRTF binding to SRF is enhanced in Has1/3 null fibroblasts. Inhibition of MRTF or SRF expression by RNA interference suppresses α-SMA expression at baseline and diminishes its overexpression in Has1/3 null fibroblasts. Interestingly, total matrix metalloproteinase activity is increased in healing skin and fibroblasts from Has1/3 null mice, possibly explaining the increased TGFβ activation.