Epidermal growth factor rapidly activates the hexose monophosphate shunt in kidney cells

Abstract

Epidermal growth factor (EGF) is a potent mitogen that rapidly activates plasma membrane Na+-H+ exchangers, thereby causing intracellular alkalinization. The rise in intracellular pH (pHi) may be an important signal for cell growth. However, recent studies have dissociated Na+-H+ exchange activity and/or alkalinization from cellular proliferation. We have studied the role of EGF in the growth of rat renal proximal tubule (PT) cells in primary culture and monitored the early effects of EGF on pHi in these cells using microfluorimetry and the pHi probe, 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). EGF increased DNA synthesis in growing PT cells and produced transient alkalinization (2-3 min) due to activation of Na+-H+ exchange. In contrast, in the absence of extracellular Na+, EGF administration caused pHi to decrease. This acidification was prevented by 2-deoxy-D-glucose and 6-aminonicotinamide, inhibitors of glucose utilization and the hexose monophosphate shunt (HMP), respectively. EGF was also found to stimulate HMP shunt activity in PT cells using an isotopic method for distinguishing between glucose utilization through the HMP shunt vs. glycolysis. Because EGF caused both cytoplasmic acidifying (HMP activation) and alkalinizing (Na+-H+ exchange activation) processes, we propose that the primary role for the activation of Na+-H+ exchange during growth may be to extrude acid from the cell in order to maintain pHi at levels permissive for cell growth.