Circulating free heme induces cytokine storm and pulmonary hypertension through the MKK3/p38 axis.

Hemolysis is associated with pulmonary hypertension (PH), but the direct contribution of circulating free heme to the PH pathogenesis remains unclear. Here, we show that the elevated levels of circulating free heme are sufficient to induce PH and inflammatory response in mice and confirm the critical role of mitogen-activated protein kinase kinase-3 (MKK3)-mediated pathway in free heme signaling. Following the continuous infusion of heme for 2 wk, wild-type (WT) but not MKK3 knockout (KO) mice develop PH, as evidenced by a significantly elevated right ventricular (RV) systolic pressure, RV hypertrophy, and pulmonary vascular remodeling. The MKK3/p38 axis, markedly activated by heme infusion in WTs, results in upregulated proliferative/cytokine signaling targets Akt, ERK1/2, and STAT3, which were abrogated in MKK3 KO mice. Moreover, the MKK3 KOs were protected against heme-mediated endothelial barrier dysfunction by restoring the tight junction protein zonula occludens-1 expression and diminishing the inflammatory cell infiltration in the lungs. Plasma cytokine multiplex analysis revealed a severe cytokine storm already 24 h after initiation of heme infusion, with a significant increase of 19 cytokines, including IL-1b, IL-2, IL-6, IL-9, and TNF-a, in WT animals and complete attenuation of cytokine production in MKK3 KO mice. Together, these findings reveal a causative role of circulating free heme in PH through activating inflammatory and proliferative responses. The central role of MKK3 in orchestrating the heme-mediated pathogenic response supports MKK3 as an attractive therapeutic target for PH and other lung inflammatory diseases linked to hemolytic anemia.