Mechanical stresses induce paracrine beta-2 microglobulin from cardiomyocytes to activate cardiac fibroblasts through epidermal growth factor receptor

By employing a proteomic analysis on supernatant of mechanically stretched cardiomyocytes, we found that stretch induced a significantly high level of beta-2 microglobulin (beta 2M), a non-glycosylated protein, which is related to inflammatory diseases but rarely known in cardiovascular diseases. The present data showed that serum beta 2M level was increased in patients with hypertension and further increased in patients with chronic heart failure (HF) as compared with control group, and the high level of serum beta 2M level correlated to cardiac dysfunction in these patients. In pressure overload mice model by transverse aortic constriction (TAC), beta 2M levels in serum and heart tissue increased progressively in a time-dependent manner. Exogenous beta 2M showed pro-fibrotic effects in cultured cardiac fibroblasts but few effects in cardiomyocytes. Adeno-associated virus 9 (AAV9)-mediated knockdown of beta 2M significantly reduced cardiac beta 2M level and inhibited myocardial fibrosis and cardiac dysfunction but not cardiac hypertrophy at 4 weeks after TAC. In vitro, mechanical stretch induced the rapid secretion of beta 2M mainly from cardiomyocytes by activation of extracellular-regulated protein kinase (ERK). Conditional medium (CM) from mechanically stretched cardiomyocytes activated cultured cardiac fibroblasts, and the effect was partly abolished by CM from beta 2M-knockdown cardiomyocytes. In vivo, knockdown of beta 2M inhibited the increase in phosphorylation of epidermal growth factor receptor (EGFR) induced by TAC. In cultured cardiac fibroblasts, inhibition of EGFR significantly attenuated the beta 2M-induced the activation of EGFR and pro-fibrotic responses. The present study suggests that beta 2M is a paracrine pro-fibrotic mediator and associated with cardiac dysfunction in response to pressure overload.