Most malignant tumors have a defect in the Rb pathway that allows them to proliferate autonomously, unrestricted by the availability of mitogens and growth factors. This defect preconditions tumor cells to be less sensitive to inhibitors of growth factor receptors and/or signal transduction pathways that arrest normal cells, predominantly in G0/1. Strategies were proposed, therefore, to combine cytotoxic agents with such inhibitors in order to selectively arrest normal host cells in a relatively resistant part of the cycle and thereby protect them during chemotherapy with agents targeting proliferating cells. The present study was designed to explore whether inhibition of phosphatidylinositol-3 kinase (PI3K), one of the pivotal kinases involved in signal transduction essential for cell proliferation can be considered in these strategies. Indeed, proliferation of phytohemagglutinin stimulated human lymphocytes was prevented at 5 μM concentration of LY294002 (LY), a PI3K inhibitor, whereas several-times higher (≥20 μM) concentrations of LY were needed to affect proliferation of leukemic HL-60, Molt-4, or Jurkat cells. LY prevented the exit of lymphocytes from G0 concomitant with the suppression of induction of cyclins D2, D3 and E, and phosphorylation of pRb. G0/1 lymphocytes, that were initially stimulated in the absence of LY, were also inhibited from proliferating following exposure to LY. The arrest of lymphocytes by LY was reversible and, after its removal, the cells asynchronously re-entered the cell cycle. LY, at a concentration of 5-20 μM, protected lymphocytes from apoptosis induced by ara-C but offered no protection at all to Jurkat cells treated under identical conditions. The data suggest that inhibitors of PI3K such as LY may be considered in strategies designed to shield normal cells from the cytotoxicity of chemotherapy by transiently arresting them in the cycle. An additional advantage of LY was the suppression of the protein level and activity of PKB/Akt, a kinase that, through the phosphorylation of the proapoptotic molecule BAD, protects cells from apoptosis. Because this LY-induced suppression was stronger in Jurkat cells (>70%) than in lymphocytes (20%), the proapoptotic effects of PKB/Akt down-regulation appear to be selective towards tumor cells.

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