The cytoskeleton of neuroblastoma cells, clone Neuro 2A, is altered by two stress conditions: heat shock and arsenite treatment. Microtubules are reorganized, intermediate filaments are aggregated around the nucleus, and the number of stress fibers is reduced. Since both stress modalities induce similar cytoskeletal alterations, no thermic denaturation of one or more cytoskeletal components can be involved in this process. Heat shock proteins are induced both by heat and by arsenite. However, cells treated with arsenite synthesize hsp28 which is not detected in heat-treated cells. Synthesis of all hsps is prevented by addition of actinomycin D or cycloheximide. Under these conditions no alterations are observed in the organization of microtubules and intermediate filaments during heat or arsenite treatment. However, these drugs are not able to prevent the rapid loss of stress fibers. A re-formation of the cytoskeleton during the recovery period proceeds within 3 h and is also found to occur in the presence of a protein synthesis inhibitor. These data suggest that reorganization of microtubules and intermediate filamerits during a stress treatment requires the synthesis of a new protein(s), probably hsp(s). 0 1987 Academic Press, Inc.Most living cells respond to exposure to temperatures a few degrees above their normal physiological temperature by activation of a small set of genes and preferential synthesis of proteins encoded by these genes: heat shock proteins (hsps) (for reviews see Refs. [l-lo]). This response was first described to occur in Drosophila [ 111, and since then it has been observed in several other organisms [121. Besides a heat treatment, hsp synthesis can also be induced by a variety of agents like sodium arsenite [13-161, ethanol [15], and amino acid analogs [17, 181 (for a summary of inducers see Ref.[S]). In the last few years the understanding of the mechanism of heat shock gene activation has increased considerably [2,9, lo], but the function of the heat shock response is still not well understood.We have been studying the heat shock response in cultured mammalian cells. We could show both by immunofluorescence microscopy and by immunoelectron microscopy that heat treatments resulting in hsp synthesis concomitantly induce drastic alterations in the cytoskeleton [19]. These cytoskeletal changes were observed 15-20 min after the temperature elevation and were found to be cell specific.The mechanism which leads to the cytoskeletal alteration during the heat treatment is not known. Recent studies have shown that calmodulin activation is ' To whom reprint requests should be addressed.
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