A study of the effect of an anti-proliferative agent on rat glial tumor cells: Effects on insulin -like growth factor -1 gene expression and action

Poly (IC), a synthetic double-stranded RNA co-polymer of inosinic and cytidilic acids, decreases proliferation of normal and tumorigenic cells. I tested the hypothesis that Poly (IC) decreased proliferation of C6 rat glioma cells by disrupting an autocrine IGF-1 growth loop. Poly (IC) decreased the numbers of confluent and sub-confluent C6 cultures. Addition of IGF-1 partially blocked the Poly (IC)-mediated decrease in C6 cell number, suggesting that one mechanism of Poly (IC) action is through down-regulation of IGF-1 gene expression and/or action.;Poly (IC) decreased IGF-1, IGF-1 receptor, IGFBP-4 and IGFBP-5 mRNA levels and decreased immunoreactive IGF-1 peptide, and IGF-1 receptor betaIGFBP-3, IGFBP-4, and IGFBP-5 protein levels in C6 cells. Poly (IC) decreased IGF-1 mRNA independent of new protein synthesis in general and type I interferon (IFN) synthesis in particular. Poly (IC) decreased IGF-1 gene transcription, but did not alter IGF-1 mRNA stability. Poly (IC) activated protein kinase R (PKR) at 5 minutes and increased PKR protein levels at 48 and 72 hours. IGF-1 did not prevent Poly (IC) from activating PKR, but inhibited Poly (IC) from increasing PKR protein levels.;Poly (IC) caused a block in G1 to S transition in confluent C6 cultures whereas in sub-confluent cultures, Poly (IC) decreased the percentage of cells in the G2/M phase. Addition of IGF-1 to Poly (IC)-treated cells decreased the percentage of cells in G0/G1 phase and increased the percentage of cells in G2/M phase in confluent and sub-confluent C6 cultures, indicating reversal of cell cycle blocks. Inhibiting PKR activation also partially prevented the Poly (IC)-mediated cytostasis of C6 cells. Both IGF-1 and a blocking antibody against IFN prevented the increase of PKR levels and the decreased cell proliferation caused by Poly (IC). I conclude that Poly (IC) induces IFN, which mediates the cytostatic effect of Poly (IC) on C6 cells, at least in part through PKR. IGF-1 prevents IFN from inducing PKR, thus explaining the ability of IGF-1 to reverse the cell cycle blocks and the decreased C6 proliferation caused by Poly (IC).