Cell Volume in the Regulation of Cell Proliferation and Apoptotic Cell Death
Cell proliferation must – at some time point – lead to increase of cell volume and one of the hallmarks of apoptosis is cell shrinkage. At constant extracellular osmolarity those alterations of cell volume must reflect respective changes of cellular osmolarity which are hardly possible without the participation of cell volume regulatory mechanisms. Indeed, as shown for ras oncogene expressing 3T3 fibroblasts, cell proliferation is paralleled by activation of Na+/H+ exchange and Na+,K+,2Cl- cotransport, the major transport systems accomplishing regulatory cell volume increase. Conversely, as evident from CD95-induced apoptotic cell death, apoptosis is paralleled by inhibition of Na+/H+ exchanger and by activation of Cl- channels and release of the organic osmolyte taurine, major components of regulatory cell volume decrease. However, ras oncogene activation leads to activation and CD95 receptor triggering to inhibition of K+ channels. The effects counteract the respective cell volume changes. Presumably, they serve to regulate cell membrane potential, which is decisive for Ca++ entry through ICRAC and the generation of cytosolic Ca++ oscillations in proliferating cells. As a matter of fact ICRAC is activated in ras oncogene expressing cells and inhibited in CD95-triggered cells. Activation of K+ channels and Na+/H+ exchanger as well as Ca++ oscillations have been observed in a wide variety of cells upon exposure to diverse mitogenic factors. Conversely, diverse apoptotic factors have been shown to activate Cl- channels and organic osmolyte release. Inhibition of K+ channels is apparently, however, not a constant phenomenon paralleling apoptosis which in some cells may even require the operation of K+ channels. Moreover, cell proliferation may at some point require activation of Cl- channels. In any case, the alterations of cell volume are obviously important for the outcome, as cell shrinkage impedes cell proliferation and apoptosis can be elicited by increase of extracellular osmolarity. At this stage little is known about the interplay of cell volume regulatory mechanisms and the cellular machinery leading to mitosis or death of the cell. Thus, considerable further experimental effort is required in this exciting area of cell physiology.