Intracellular acidification mediates the proliferative response of PC12 cells induced by potassium ferricyanide and involves MAP kinase activation

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Abstract
Potassium ferricyanide is known to elicit cell growth and mitogenesis in various cells by stimulating a transplasma membrane electron‐transport system. When serum‐starved PC12 cells were treated with potassium ferricyanide, stimulation of mitogenesis was evidenced by enhanced DNA synthesis, as well as by increased cell numbers. Intracellular pH (pHi) of PC12 cells was measured at 37°C by microfluorimetric analysis of 2′,7′‐bis‐(2‐carboxyethyl)‐5(and‐6)‐carboxyfluorescein (BCECF). The resting pHi of unstimulated cells was 7.52 (external pH 7.40). Addition of potassium ferricyanide (100 μM) decreased pHi by about 0.25 pH units. Lowering pHi to a similar extent, either by decreasing external pH (pHo) or by adding a weak acid, also elicited a mitogenic response, indicating that intracellular acidification by itself has growth factor‐mimicking, mitogenic effects. Nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) triggered proliferation without changes in pHi. The mitogenic treatments eliciting intracellular acidification did not activate protein kinase C (PKC) but stimulated the p42/p44 mitogen‐activated protein (MAP) kinase. Our results indicate that 2 distinct mitogenic pathways are active in PC12 cells: the first is independent of pHi and involves activation of the PKC pathway and the second requires a permissive pHi value around 7.25 and involves activation of the p42/p44 MAP kinase pathway. © 1996 Wiley‐Liss, Inc.