Release of cardiac troponin I from viable cardiomyocytes is mediated by integrin stimulation

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Abstract
Elevated cardiac troponin-I (cTnI) levels have been demonstrated in serum of patients without acute coronary syndromes, potentially via a stretch-related process. We hypothesize that this cTnI release from viable cardiomyocytes is mediated by stimulation of stretch-responsive integrins. Cultured cardiomyocytes were treated with (1) Gly–Arg–Gly–Asp–Ser (GRGDS, n = 22) to stimulate integrins, (2) Ser–Asp–Gly–Arg–Gly (SDGRG, n = 8) that does not stimulate integrins, or (3) phosphate-buffered saline (control, n = 38). Cells and media were analyzed for intact cTnI, cTnI degradation products, and matrix metalloproteinase (MMP)-2. Cell viability was examined by assay of lactate dehydrogenase (LDH) activity and by nuclear staining with propidium iodide. GRGDS-induced integrin stimulation caused increased release of intact cTnI (9.6 ± 3.0%) as compared to SDGRG-treated cardiomyocytes (4.5 ± 0.8%, p < 0.001) and control (3.0 ± 3.4%, p < 0.001). LDH release from GRGDS-treated cardiomyocytes (15.9 ± 3.8%) equalled that from controls (15.2 ± 2.3%, p = n.s.), indicating that the GRGDS-induced release of cTnI is not due to cell necrosis. This result was confirmed by nuclear staining with propidium iodide. Integrin stimulation increased the intracellular and extracellular MMP2 activity as compared to controls (both p < 0.05). However, despite the ability of active MMP2 to degrade cTnI in vitro, integrin stimulation in cardiomyocytes was not associated with cTnI degradation. The present study demonstrates that intact cTnI can be released from viable cardiomyocytes by stimulation of stretch-responsive integrins.