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Interleukin (IL)-6 signaling leads to phosphorylation of the small heat shock protein (Hsp)27 through activation of the MAP kinase and MAPKAP kinase 2 pathway in monocytes and monocytic leukemia cells.

Interleukin-6 is a multifunctional cytokine which regulates various aspects of the host immune response. Here we show that signaling events transferred by IL-6 in monocytes and the U937 human monocytic leukemia cell line lead to the phosphorylation of the small heat shock protein (Hsp)27. Phosphorylation of Hsp27 is both dose- and time-dependent. In the absence of NaF, a serine/threonine phosphatase inhibitor, IL-6 failed to initiate Hsp27 phosphorylation in vitro. IL-6 also failed to phosphorylate Hsp27 when cells had been deactivated with tyrosine kinase inhibitors such as genistein. The capacity of cellular extracts to phosphorylate Hsp27 could be, however, restored when either immunoprecipitated activated MAP kinase or purified MAPKAP kinase 2 was added to cell lysates. These findings suggest that IL-6-mediated phosphorylation of Hsp27 results from activation of MAPKAP kinase 2, a serine/threonine kinase which is activated by MAP kinase. Taking together, our findings indicate that IL-6-induced activation of MAP kinase by IL-6 entails the activation of MAPKAP kinase 2 and subsequent phosphorylation of the Hsp27.