Bcl-2 is a key factor for cardiac fibroblast resistance to programmed cell death

Cardiac fibroblasts play an essential role in the physiology of the heart. These produce extracellular matrix proteins and synthesize angiogenic and cardioprotective factors. Although fibroblasts of cardiac origin are known to be resistant to apoptosis and to remain metabolically active in situations compromising cell survival, the underlying mechanisms are unknown. Here, we report that cardiac fibroblasts were more resistant than dermal or pulmonary fibroblasts to mitochondria-dependent cell death. Cytochrome c release was blocked in cardiac fibroblasts but not in dermal fibroblasts treated with staurosporine, etoposide, serum deprivation, or simulated ischemia, precluding caspase-3 activation and DNA fragmentation. Resistance to apoptosis of cardiac fibroblasts correlated with the expression of the anti-apoptotic protein Bcl-2, whereas skin and lung fibroblasts did not express detectable levels of this protein. Bcl-x(L,) Bax, and Bak were expressed at similar levels in cardiac, dermal, and lung fibroblasts. In addition, the death of cardiac fibroblasts during hypoxia was not associated with the cleavage of Bid but rather with Bcl-2 disappearance, suggesting the requirement of the mitochondrial apoptotic machinery to execute death receptor-induced programmed cell death. Knockdown of bcl-2 expression by siRNA in cardiac fibroblasts increased their apoptotic response to staurosporine, serum, and glucose deprivation and to simulated ischemia. Moreover, dermal fibroblasts overexpressing Bcl-2 achieved a similar level of resistance to these stimuli as cardiac fibroblasts. Thus, our data demonstrate that Bcl-2 is an important effector of heart fibroblast resistance to apoptosis and highlight a probable mechanism for promoting survival advantage in fibroblasts of cardiac origin.