A novel function of BCL-2 overexpression in regulatory volume decrease. Enhancing swelling-activated Ca²⁺ entry and Cl^- channel activity

The cellular function of the oncogene bcl-2, a key regulator of apoptosis, is still debated. The goal of this study was to explore the relationship between BCL-2 overexpression and cell volume regulation by using two independent models, Madin-Darby canine kidney (MDCK) cells stably transfected with BCL-2 and MDCK clones with inducible BCL-2 expression by the lac operator/repressor. BCL-2 overexpression enhanced the capability of regulatory volume decrease (RVD), a cellular defensive process against hypotonic stress. In various clones of MDCK cells, hypotonic stress induced an outwardly rectified Cl^- current that was significantly upregulated by BCL-2 overexpression. Other fundamental characteristics of this channel were similar among different MDCK clones, such as sensitivity to Cl^- channel inhibitor, anion permeability, and time-dependent inactivation at more positive potential. Most importantly, BCL-2 overexpression up-regulates the swelling-activated Ca²⁺ transient that is a critical signaling for normal RVD and the activation of swelling-activated Cl^- channel in MDCK cells. BCL-2 overexpression also enhances the capacitative Ca²⁺ entry that can be differentiated from the swelling-activated Ca²⁺ transient by kinetic analysis and sensitivity to Gd³⁺. Moreover, neutralization of endogenous BCL-2 by antibody blocks the normal RVD response and the activation of swelling-activated Cl^- channel in human cervical cancer HT-3 cells. These results provide a new insight into the novel function of BCL-2 overexpression in the regulation of cell volume and ion flux.