Abstract 18433: A Novel Intracellular Isoform of Matrix Metalloproteinase-2 Activates Innate Immunity and Leads to Cardiac Failure

Abstract

Modulation of cardiac structure and function by matrix metalloproteinases (MMP) is an area of intense interest. Experimental and clinical evidence has pinpointed a critical role for a specific enzyme, MMP-2, in ventricular remodeling and systolic failure. Transgenic expression of active 68 kDa MMP-2 yields multiple defects, including systolic failure and mitochondrial dysfunction. We hypothesized that an additional intracellular isoform of MMP-2 must exist that initiates mitochondrial dysfunction and inflammation. Western blots of mitochondria from ageing mice and a transgenic model of atherogenesis revealed a smaller 65 kDa MMP-2 isoform. Cardiomyoblast cells subjected to transient inhibition of oxidative phosphorylation also generated the mitochondrial 65 kDa MMP-2 isoform. MMP-2 contains three in-frame methionines in the N-terminus, each with a Kozak sequence supporting translational initiation. The 65 kDa MMP-2 isoform is an N-terminal truncation variant generated by alternative translational start site selection at M 77 , thereby deleting the signal sequence and the inhibitory prodomain: thus the 65 kDa MMP-2 is intracellular and enzymatically active. A portion of the 65 kDa MMP-2 localizes in the mitochondrial intramembranous space, triggers mitochondria-to-nuclear stress signaling and actvitates NF-κB and NFAT. Microarray analysis showed that 65 kDa MMP-2 induces a restricted innate immune response transcriptome, including viral stress response genes (OAS1, IFIT, PRKRA), the crucial innate immunity transcription factor IRF7, chemokines and pro-apoptosis genes. Cardiac-specific transgenic expression of the truncated MMP-2 isoform induced progressive cardiomyocyte and ventricular hypertrophy, followed by mononuclear inflammatory cell infiltration and cardiomyocyte apoptosis, as predicted from the microarray results. Functionally, the transgenics showed cardiomyocyte hypertrophy, contractile defects, systolic failure and enhanced injury following ex vivo ischemia/reperfusion. Conclusion: a novel intracellular isoform of MMP-2 is generated by acute or chronic oxidative stress, initiates inflammatory signaling and drives cardiomyocyte apoptosis, features directly relevant to clinical cardiac disease.