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Nuclear MMP‐2: presence and activity in cardiac myocytes

MMP‐2 is expressed in almost all heart cells and is found in subcellular compartments including the nucleus. MMP‐2 can be directly activated by oxidative stress (as peroxynitrite). Its activity is further regulated by its phosphorylation. To better study nuclear MMP‐2 in the heart, we extracted the nuclear, cytosolic and membrane fractions from isolated rat hearts (briefly perfused to clear them of blood). Western Blots for lamin A/C (nucleus), GAPDH (cytosol) and VDAC/Porin (membrane) were used as specific markers to confirm fraction purity. Gelatin zymography of these fractions revealed MMP‐2 but not MMP‐9 activities and MMP‐2 activity was about 15 times higher in the cytoplasm than in the nucleus. Interestingly, Western blot analysis showed that MMP‐2 is present in similar amounts both in the nucleus and in the cytoplasm. In order to characterize possible regulators of MMP‐2 activity, we analyzed for the presence of tissue inhibitors of metalloproteases (TIMPs). Western blots revealed that all forms of TIMPs (1–4) are found in the cytoplasm, TIMP‐4 is also present in the membrane fraction, and no TIMP was detected in the nucleus. Our data suggest that myocardial MMP‐2 activity in the nucleus is regulated by a mechanism different from inhibition by TIMPs, possibly by its phosphorylation.