Abstract 77: Fibronectin Signals Through Pim-1 and β1 Integrin in Cardiac Progenitor Cells to Induce Proliferation and Protection

Abstract

Background: Cardiac Progenitor Cells (CPC) are pivotally involved in cardiac repair. Fibronectin (FN), an extracellular matrix protein, is highly expressed during cardiac development. In adult heart FN is a component of the cardiac stem cell niche and re-appears after myocardial infarction (MI). The role of FN signaling in CPC function and cardiac remodeling following MI has not been elucidated. We demonstrate here proliferative and protective effects of FN signaling in CPC as mediated by β1 integrin receptor and cardioprotective serine/threonine kinase Pim1. Methods: Cell death and proliferation of CPCs was measured using propidium iodide and CyQuant assays. Signaling pathways were analyzed by immunoblotting, qRT-PCR and siRNA depletion of targets. FN was localized in heart sections by immunohistochemistry and cardiac function assessed by echocardiography in control and conditional FN knockout (KO) mouse hearts following MI. Results: FN inhibits starvation and staurosporine induced cell death in CPCs and promotes proliferation in conjunction with induction of Pim1 expression. Protective and pro-proliferative effects of FN are abrogated by inhibition of Pim1 or deletion of β1 integrin receptor. In vivo CPC expansion correlates with FN expression following MI, and CPC localize to regions of up-regulated FN protein in the infarct. Cardiac function in control and KO mice remains equivalent up to 2 weeks post MI, however by 4 weeks KO heart function worsens compared to control as evidenced by ejection fraction values measuring 16.4 +/- 1.5% vs 26.6 +/- 1.8% at 12 weeks post MI in KO and control hearts, respectively. Conclusion: FN provides pro-survival and pro-proliferative effects to CPCs in a Pim1 kinase and β1 integrin dependent manner. CPCs and FN colocalize in vivo in an infarction injury model, while conditional KO of FN in mice leads to further impairment of cardiac function after MI. Taken together these results indicate previously unidentified cardioprotective and regenerative roles for FN in pathologically challenged heart.