Pulmonary arterial hypertension (PAH) is a common complication of scleroderma with a poor prognosis. TGFβ is one of the primary drivers of vascular remodeling in PAH, which is the leading cause of increased vascular resistance. The role of canonical TGFβ signaling (SMADdependent) has been characterized in PAH, but the role of non-canonical TGFβ signaling (SMADindependent) is still poorly understood.
In this study we sought to characterize the role of non-canonical TGFβ-mediated transcriptional mechanisms in PAH pathogenesis. In addition to SMAD signaling, TGFβ also activates Rho GTPases to regulate the activity of myocardin-related transcription factor (MRTF). MRTF is a transcriptional co-activator that binds to serum response factor (SRF) and regulates the expression of contractile genes (αSMA and CNN1). We demonstrated that upregulation of these contractile genes by TGFβ is dependent on both SMAD and Rho/MRTF/SRF transcriptional mechanisms in human pulmonary arterial smooth muscle cells (HPASMC). The MRTF/SRF pathway inhibitor CCG-222740 reduced TGFβ1-induced expression of contractile genes.
Based on the kinetics of TGFβ1-induced MRTF activation, it appeared TGFβ1 stimulated Rho/MRTF/SRF through an indirect mechanism.We found TGFβ1 increased the mRNA level of sphingosine kinase (SphK1), which is the enzyme that produces sphingosine-1-phosphate (S1P). TGFβ1 also increased levels of mRNA for CTGF and EDN1. These factors are all upstream activators of Rho. This suggests that an autocrine/paracrine signaling mechanism downstream of TGFβ1 may be required for Rho/MRTF/SRF activation. Since MRTF/SRF is a critical downstream signaling node for several stimulators (TGFβ1 and S1P), it could be a promising therapeutic target for PAH. Inhibition of the MRTF/SRF pathway by CCG-222740 may limit the pathological progression in PAH.
The beneficial effects of MRTF/SRF inhibitors have also been demonstrated in skin and lung fibrosis models. This proposal will provide valuable therapeutic data about the MRTF/SRF inhibition in PAH patients especially scleroderma patients with PAH.