Interactions of STAT3 with caveolin-1 and heat shock protein 90 in plasma membrane raft and cytosolic complexes - Preservation of cytokine signaling during fever

Interleukin-6 (IL-6) initiates STAT3 signaling in plasma membrane rafts with the subsequent transit of Tyr-phosphorylated STAT3 (PY-STAT3) through the cytoplasmic compartment to the nucleus in association with accessory proteins. We initially identified caveolin-1 (cav-1) as a candidate STAT3-associated accessory protein due to its co-localization with STAT3 and PY-STAT3 in flotation raft fractions, and heat shock protein 90 (HSP90) due to its inclusion in cytosolic STAT3-containing 200-400-kDa complexes. Subsequent immuno-magnetic bead pullout assays showed that STAT3, PY-STAT3, cav-1, and HSP90 interacted in plasma membrane and cytoplasmic complexes derived from uninduced and stimulated Hep3B cells. This was a general property of STAT3 in that these interactions were also observed in alveolar epithelial type II-like cells, lung fibroblasts, and pulmonary arterial endothelial cells. Exposure of Hep3B cells to the raft disrupter methyl-beta-cyclodextrin for 1-10 min followed by IL-6 stimulation for 15 min preferentially inhibited the appearance of PY-STAT3 in the cav-1-enriched sedimentable cytoplasmic fraction, suggesting that these complexes may represent a trafficking intermediate immediately downstream from the raft. Because IL-6 is known to function in the body in the context of fever, the possibility that HSP90 may help preserve IL-6-induced STAT3 signaling at elevated temperature was investigated. Geldanamycin, an HSP90 inhibitor, markedly inhibited IL-6-stimulated STAT3 signaling in Hep3B hepatocytes cultured overnight at 39.5 degreesC as evaluated by DNA-shift assays, trafficking of PY-STAT3 to the nucleus, cross-precipitation of HSP90 by anti-STAT3 polyclonal antibody, and reporter/luciferase construct experiments. Taken together, the data show that IL-6/raft/STAT3 signaling is a chaperoned pathway that involves cav-1 and HSP90 as accessory proteins and suggest a mechanism for the preservation of this signaling during fever.