Nerve growth factor-induced accumulation of PC12 cells expressing cyclin D1: evidence for a G1 phase block.

The anti-proliferative effect of nerve growth factor (NGF) on the rat pheochromocytoma cell line PC12 has been previously shown to be accompanied by the accumulation of cells in either the G1 phase with a 2c DNA content, or with a 4c DNA content characteristic for G2/M, as evidenced by flow cytometric analysis of DNA distribution using propidium iodide. Herein, these apparently conflicting results are clarified. The present studies indicate that a simple DNA distribution profile obtained by this technique can confound interpretation of the biological effects of NGF on cell-cycle distribution due to the presence of tetraploid cells. Using cyclin D1 and incorporation of bromodeoxyuridine as markers of respectively, G1 and S phase, we show that PC12 cultures can have a considerable amount of tetraploid cells which, when in the G1 phase, have a 4c DNA content and express cyclin D1. During exposure to NGF, this population increases, reflecting the accumulation of cells in the G1 phase of the cell cycle. The data presented, support the possibility that events affecting the expression or action of G1 regulatory proteins may be involved in the molecular mechanism of the anti-mitogenic effect of NGF.