Research Article HASF (C3orf58) is a novel ligand of the insulin-like growth factor 1 receptor Akshay Bareja 1,2, *, Conrad P. Hodgkinson 1,2, *, Alan J. Payne 1,2 , Richard E. Pratt 1,2 and Victor J. Dzau 1,2 1 Mandel Center for Hypertension and Atherosclerosis Research, Duke University Medical Center, Durham, NC, U.S.A. and 2 Cardiovascular Research Center, Duke University Medical Center, Durham, NC, U.S.A. Correspondence: Victor J. Dzau (victor.dzau@duke.edu) We have recently shown that hypoxia and Akt-induced stem cell factor (HASF) protects the heart from ischemia-induced damage and promotes cardiomyocyte proliferation. While we have identified certain signaling pathways responsible for these protective effects, the receptor mediating these effects was unknown. Here, we undertook studies to identify the HASF receptor. A yeast two-hybrid screen identified a partial fragment of insulin-like growth factor 1 receptor (IGF1R) as a binding partner of HASF. Subsequent co-immunoprecipitation experiments showed that HASF bound to full-length IGF1R. Binding assays revealed a high affinity of HASF for IGF1R. The treatment of neonatal ventricular cardiomyocytes with HASF resulted in the phosphorylation of IGF1R and other proteins known to be involved in IGF1R-mediated signaling pathways. HASF-mediated ERK activation was abrogated by IGF1R pharmacological inhibitors and siRNAs that targeted IGF1R. However, siRNA-mediated knockdown of either IGF2R or the insulin receptor had no effect on HASF-induced cell signaling. Additionally, pharmacologic inhib- ition of IGF1R impeded HASF’s ability to induce cardiomyocyte proliferation. Finally, we documented that in vivo deletion of the IGF1R completely abolished the ability of HASF to promote cardiomyocyte proliferation in an overexpression mouse model providing further evidence in vivo that the IGF1R is the functional receptor for HASF. Introduction The adult mammalian heart displays a very limited capacity to regenerate following injury [1] due to the low proliferative ability of cardiomyocytes. Consequently, there has been much interest in develop- ing therapeutic strategies that either prevent the initial loss of cardiomyocytes or promote their replen- ishment through proliferation of the surviving pool. We, as well as others, have reported that mesenchymal stems cells (MSCs) protect and repair the infarcted heart when injected into the myocardium [2–5]. Our group was amongst the first to show that the beneficial effects of MSCs were found to occur primarily via the release of therapeutic proteins or paracrine factors [6,7]. Identification of biologically active paracrine factors is important for the development of future cardiac therapeutic strategies as these proteins could be used as an alternative to adult stem cells. To this end, we performed a microarray analysis of MSCs and identified many genes encoding secreted proteins. One of these genes encoded for a protein of relatively unknown function, C3orf58, which we named HASF for hypoxia and Akt-induced stem cell factor. Little is known about this protein except for a possible link with human familial autism [8–10]. Recently, our group has shown that HASF has significant effects on cardiomyocyte biology. We found that HASF potently protects cardiomyocytes from cell death and promotes cardiomyocyte proliferation [11,12]. Though we had identified that beneficial effects of HASF required PI3K and PKCε signaling [11,12], a key part of the puzzle remained missing; namely, the identity of the HASF receptor. In this paper, we identify the *These authors contributed equally to this work. Accepted Manuscript online: 17 January 2017 Version of Record published: 20 February 2017 Received: 31 October 2016 Revised: 13 January 2017 Accepted: 17 January 2017 © 2017 The Author(s); published by Portland Press Limited on behalf of the Biochemical Society 771 Biochemical Journal (2017) 474 771–780 DOI: 10.1042/BCJ20160976