Late-breaking abstract: Protein tyrosine phosphatase (PTP)α promotes profibrotic responses in fibroblasts via Smad signaling

Idiopathic pulmonary fibrosis (IPF) is a fatal fibrosing lung disease for which treatment remains suboptimal. Transforming Growth Factor (TGF)-β, a pleiotropic cytokine, is critical in the pathogenesis of pulmonary fibrosis. We have reported that genetic deficiency of PTPα prevents the development of pulmonary fibrosis in murine models and that PTPα is required for TGF-β-induced myofibroblast differentiation. To determine the mechanism by which PTPα controls profibrotic responses, we compared phosphorylation of Smad 2/3 in wild type (WT) and PTPα-/- mice treated with intratracheal instillation of adenovirus expressing active TGF-β. These studies revealed that p-Smad2/3 levels were markedly attenuated in the lungs of PTPα-/- compared to WT mice as determined by immunostaining. Phosphorylation of Smad3 was markedly attenuated in PTPα-/- compared to WT fibroblasts after TGF-β stimulation. Further analysis revealed that while early (15 min) Smad3 phosphorylation was comparable between PTPα-/- and WT cells, the levels of pSmad3 diminished rapidly (by 30 min) in PTPα-/- cells whereas they persisted for 90 min in WT cells. This was also true for levels of nuclear pSmad3. Total Smad2/3 levels were unchanged, suggesting that PTPα prolongs the duration of Smad phosphorylation but not degradation. PTPα interacted with TGF-β receptor II as determined by co-immunoprecipitation. We conclude that PTP promotes profibrotic responses in fibroblasts via regulation of Smad-dependent signaling pathways, possibly by control of TGF-β receptor activation. These observations identify a key check point in profibrotic signaling controlled by PTPα that may represent a novel therapeutic target.