Zfra invokes a novel mitochondrial pathway of cell death bypassing cytochrome c release

Abstract

Zfra is a small size 31-amino-acid C2H2 zinc finger-like protein, and is known to regulate TNF-mediated cell death. Exposure of cells with inducers of mitochondrial permeability transition (MPT), such as staurosporine, atractyloside and betulinic acid, stimulated phosphorylation of Zfra at Ser8 and its translocation to the mitochondria. Overexpressed Zfra induced loss of mitochondrial membrane potential (LMMP), significantly downregulated MPT inhibitors Bcl-2 and Bcl-xL (without causing cytochrome c release), and eventually mediated cell death. In contrast, at low levels Zfra enhanced cell growth and blocked LMMP in dying cells treated with staurosporine. An S8G-Zfra mutant did not to translocate to the mitochondria during MPT and failed to induce cell death (when overexpressed). In comparison, T20G mutant translocated to the mitochondria during MPT but could not inhibit Bcl-2 expression. Together, phosphorylation at Ser8 is essential for overexpressed Zfra to exert its apoptotic function emanating from the mitochondria, where Zfra blocks cytochrome c release, downregulates the expression of Bcl-2 and Bcl-xL and induces LMMP.