Preformed STAT3 transducer complexes in human HepG2 cells and rat hepatocytes.

Interleukin 6 (IL-6) is a pleiotropic cytokine that mediates a variety of functions, including induction of the acute-phase response in hepatocytes. IL-6 initiates its action by binding to its cell surface receptor, followed by activation of Janus kinases and tyrosine phosphorylation of the signal transducer and transcription factor (STAT) 3. Although it has been suggested that cholesterol- and sphingolipid-enriched membrane domains, called lipid rafts, and caveolin are involved in this process, their roles in the earliest stages of IL-6-mediated signaling are far from being understood. Here we show that pretreatment of HepG2 hepatoma cells with methyl-beta-cyclodextrin (MbetaCD), which removes cholesterol and destroys lipid rafts, inhibited tyrosine phosphorylation of STAT3 in IL-6-activated, but not PV-activated cells. Furthermore, when the cells were lysed under conditions preserving lipid rafts, no IL-6- or PV-induced phosphorylation of STAT3 was observed. Although most of the STAT3 was found in large MbetaCD-resistant assemblies in both non-activated and IL-6-activated cells, its association with lipid rafts was weak or undetectable. The extent of IL-6-induced tyrosine phosphorylation of STAT3 was comparable in cells expressing low or high levels of caveolin. Similar STAT3 transducer complexes were observed in freshly isolated rat hepatocytes. The combined data suggest that STAT3 tyrosine phosphorylation occurs in preformed transducer complexes that can be activated in the absence of intact lipid rafts or caveolin.