Pulmonary hypertension (PH) is a devastating disease characterized by contraction and significant remodeling of the small pulmonary vessels, resulting in increased pulmonary artery pressure, right heart failure, and death. Hypoxia-induced mitogenic factor (HIMF, also known as found in inflammatory zone 1, FIZZ1) is a member of the resistin-like molecule (RELM) family of pleiotropic cytokines we have found critical in development of hypoxia related PH. We have further identified two human proteins correlating to this family, resistin and RELM , to be upregulated in blood and lung (specifically in remodeled vessels and inflammatory cells) from PH patients, confirming the importance of RELM proteins in human disease. Inflammation has been increasingly found to affect initiation and progression of PH morphology, with cellular proliferation and vascular remodeling dependent on immune cells of monocyte or macrophage (MF) origin. Specifically, activated (M2) MF have been implicated in development of hypoxia-induced and inflammation related PH; both Th2-mediated inflammation and hypoxia independently increase expression of HIMF. Recent studies have shown that hypoxia can induce MF to an M2 phenotype in the lung and that this transition is critical to chronic hypoxia induced PH. In addition to HIMF upregulation by chronic hypoxia, we have shown that HIMF can recruit MF to the lung, and that HIMF-mediated cell migration is dependent on direct activation of Bruton's tyrosine kinase (BTK). We have also shown that downstream vascular remodeling by HIMF is dependent on HIMF activation of IL4 and STAT6, both of which are fundamental to M2 MF. In this proposal, we hypothesize that hypoxic upregulation of HIMF is critical in hypoxia-induced transition of MF to an M2 phenotype dependent on HIMF activation of BTK, and resulting in recruitment of MF to the lung and development of PH. We address this hypothesis directly with in vitro studies in Aim 1 and in vivo studies in Aim 2 of this proposal. Under Aim 1, we will first utilize promoter-reporter studies and cellular experiments to determine the specific molecular mechanisms of HIMF/FIZZ1 gene induction in response to hypoxia. We will then investigate whether HIMF is critical in alternative activation of MF in vitro in response to hypoxia. Final investigations under Aim 1 will determine whether HIMF binding to BTK is critical in MF activation and explore the mechanism of this interaction. Aim 2 focuses on in vivo M2 MF activation in response to hypoxia. The first set of experiments will examine HIMF dependent lung inflammation while the second protocol will address the specific interaction of HIMF-BTK activation in PH development. By investigating the role of HIMF in the early stages of the inflammatory response to hypoxia within the lung, we stand to gain valuable knowledge of both PH progression and the origin of critical mediators of the disease. As such, we hope to more fully understand the initiating events in PH with the goal of translating these results to treatment and diagnosis of human pulmonary hypertension.