Matrix metalloproteinases (MMPs) are well known for their extracellular matrix remodeling function in the heart. Besides their extracellular function, growing number of evidence for intracellular actions of MMPs appear. Concerning the heart, particularly, intracellular localization of MMP2 has been shown, which is related to troponin degradation leading to contractile dysfunction. In this study, we now analysed MMP type expression in isolated cardiomyocytes of rat and tested MMP influence on cardiomyocyte hypertrophy.

First, we analysed in isolated ventricular cardiomyocytes of adult rat, mRNA expression of MMP types that were described to be present in hearts. mRNA expression of MMP 1, 2, 3, 9, 12 and 14 was detected, while expression of MMP 7, 8 and 13 was not observed in real time PCR. Then, cardiomyocytes were stimulated with different hypertrophic stimuli (PE, 10 µM, AngII, 100 nM, and TGFβ1, 1 µg/ml) for 24 h. mRNA-expression of MMP2, 3 and 12 did not change under these stimulations. MMP1-mRNA was increased by PE. Furthermore, MMP9- and 14-mRNA was reduced under PE-stimulation. AngII down regulated expression of MMP14- and MMP1-mRNA, and TGFβ1 reduced MMP9-mRNA. Thus, most MMPs are down regulated under hypertrophic stimulation of cardiomyocytes. Therefore, we hypothesized that MMPs may act as repressors of hypertrophy, and that down regulation of MMPs may induce hypertrophic growth. To test this hypothesis, cardiomyocytes were incubated with different MMP inhibitors (TAPI, 50 µM; MMP-inhibitor III, 50 µM; TIMP-2, 50 ng/ml). Indeed, all three MMP inhibitors increased the rate of proteinsynthesis (128 ± 7 % under TAPI, n=17; 107 ± 3 % under MMP-Inh.III, n=12, and 124 ± 8 % under TIMP-2, n=11, p<0.05 vs. control). Cross sectional area of cells increased to 126 ± 4 % under TAPI (n=10, p<0.05 vs. control) and to 117 ± 3 % under TIMP-2 stimulation (n=7, p<0.05 vs. control). Hypertrophic growth responses to MMP-inhibition could be blocked by inhibition of PI3K with LY294002 (10 µM) or wortmannin (10 nM) and ERK by PD98059 (10 µM).

In conclusion, expression of several MMP types was identified in ventricular cardiomyocytes. Hypertrophic growth stimulation of cardiomyocytes led prevailingly to down regulation of MMP expression. Broad inhibition of MMPs induced hypertrophic growth in cardiomyocytes, thereby, characterizing MMPs as repressor of hypertrophy in the heart.