The CDK inhibitor, p27(Kip1), is required for IL-4 regulation of astrocyte proliferation

IL-4 is a pleiotrophic cytokine that has been shown to affect cells of the central nervous system, We have demonstrated that IL-4 inhibits DNA synthesis and proliferation in human astroglia expressing IL-4 receptors, In this study, we sought to identify mechanisms that could account for the antimitogenic effects of IL-4. Epidermal growth factor (EGF)-stimulated human astroglia were arrested in G(1) phase by IL-4, even though IL-4 stimulated levels of the G(1) cyclins, D1 and E. Histone H1 kinase activity of cdk2 immunoprecipitates, however, was sharply reduced by IL-4; impairment of kinase activity was also evident in cyclin E immunoprecipitates, which contained evidence of hypophosphorylated (inactive) cdk2 product, Reduced cyclin E-associated cdk2 activity was not due to impaired cyclin-dependent kinase-activating kinase (CAK) activity, which was unaffected by IL-4. Inactive cyclin E/cdk2 complexes from IL-4 + EGF-treated cells contained, however, strikingly elevated p27(Kip1) cdk inhibitor, Elevated p27 was also detectable in whole cell lysates after 24 and 48 h of IL-4 treatment; by 72 h, p27 was no longer elevated. Pretreatment with antisense but not mismatch p27 oligonucleotides attenuated the inhibitory effects of IL-4 on DNA synthesis and histone kinase activity of cyclin E/cdk2 complexes. Antisense p27 also abrogated IL-4-mediated elevation of p27 in whole cell lysates and cyclin E/cdk2 complexes. These findings demonstrate that IL-4 regulates the cell cycle machinery of astroglial cells via a p27(Kip1) braking mechanism.