Abstract 13402: FHL-1 Contributes to and Colocalizes With Titin in Cardiac Hypertrophy

Abstract

Increased myocardial stiffness is a hallmark of impaired diastole in heart failure (HF). Hypo-phosphorylation of the N2B unique sequence (N2Bus) of titin (TTN), a giant myofilament protein, increases passive tension leading to diastolic dysfunction in HF. Enhancing the altered N2Bus phosphorylation improves cardiac stiffness and function. FHL-1, an interacting protein potentially modulating N2Bus phosphorylation is increased in HF. FHL-1 knockout mice display blunted cardiac hypertrophy and improved diastolic compliance in response to pressure-overload by transverse aortic constriction, TAC. FHL-1 also regulates skeletal muscle hypertrophy. We hypothesize that FHL-1 contributes to cardiac hypertrophy and colocalizes/interacts with TTN in heart failure. In house IHC data showed site-specific N2Bus hypo-phosphorylation at S4099, S4010 and S4185 in human HCM and/or DCM tissue samples. Longitudinal in-vivo studies showed that FHL-1 and cardiac hypertrophy markers genes were increased in left ventricles (LV) of TAC mice using RNA-seq. A persistently enhanced FHL-1 protein expression by immunoblotting and mass spectrometry strongly correlated with LV hypertrophy at 1, 4, and 6 weeks post-TAC. In addition, LV hypertrophy correlated negatively with function (fractional shortening). Increases in FHL-1 and hypertrophy markers mRNA levels were confirmed by RT-qPCR in neonatal rat ventricular myocytes (NRVM) under phenylephrine (PE) and endothelin-1 (ET-1) induced hypertrophy. Under similar conditions, FHL-1 protein levels were increased by immunofluorescence (IF) in cytoplasmic, perinuclear and nuclear regions of NRVM. Colocalization of phospho-TTN and FHL-1 was observed in NRVM and was enhanced under PE- and ET-1 induced hypertrophy. IF studies in human skeletal myotubes showed that FHL-1 expression was increased during myoblast differentiation and IGF1-E3R induced hypertrophy. Preliminary data using Microscale Thermophoresis showed binding affinity between N2Bus and FHL-1 proteins. Our studies show that FHL-1 contributes to hypertrophy in addition to N2B hypo-phosphorylation status contributing to diastolic dysfunction in HF. Targeting FHL-1 and TTN can be a potential strategy to improve diastolic compliance in HF.