Prevention of TGF-β-induced apoptosis by interlukin-4 through Akt activation and p70S6K survival signaling pathways

In this study, we demonstrate that interleukin-4 (IL-4) protects human hepatocellular carcinoma (HCC) cell line Hep3B from apoptosis induced by transforming growth factor-beta (TGF-beta). Further investigation of IL-4-transduced signaling pathways revealed that both insulin response substrate 1 and 2 (IRS-1/-2) and extracellular signal-regulated kinase (ERK) pathways were activated after IL-4 stimulation. The IRS-1/-2 activation was accompanied by the activation of phosphotidylinositol-3-kinase (PI3K), leading to Akt and p70 ribosomal protein S6 kinase (p70S6K). Interestingly, a protein kinase C (PKC) inhibitor, Go6976, inhibited the phosphorylation of Akt, suggesting that the Akt activation was PKC-dependent. Using specific inhibitors for PI3K or ERK, we demonstrated that the PI3K pathway, but not the ERK pathway, was required for protection. The constitutively active form of PI3K almost completely rescued TGF-beta-induced apoptosis, further supporting the importance of the PI3K pathway in the protective effect of IL-4. Furthermore, a dominant negative Akt and/or Go6976 only partially blocked the anti-apoptotic effect of IL-4. Similarly, rapamycin, which interrupted the activation of p70S6K, also only partially blocked the protective effect of IL-4. However, in the presence of both rapamycin and dominant negative Akt with or without Go6976, IL-4 almost completely lost the anti-apoptotic effect, suggesting that both Akt and p70S6K pathways were required for the protective effect of IL-4 against TGF-beta-induced apoptosis.