Regulation of chicken Hsp70 and Hsp90 family gene expression by transforming growth factor‐β1

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Abstract
Transforming growth factor-β1 (TGFβ) is a regulator of protein synthesis in cultured chicken embryo cells (CEC). Preceding a gradual increase in overall protein synthesis, members of the Hsp70 family (Hsp70, Hsc70, and Grp78) and the Hsp90 family (90-2 and 90-3) of molecular chaperones are induced rapidly and represent a new class of TGFβ-inducible proteins (I.M. Takenaka and L.E. Hightower, J. Cell. Physiol., 152:568–577, 1992). Herein, 32P-labeled cDNA probes encoding Hsc70 and Hsp90 were used to show that levels of the corresponding mRNAs increased as a fraction of total RNA and in polysomes within five hours of treatment of CEC with TGFβ. This cytokine did not increase rates of hsc70 and hsp90 gene transcription as measured by run-on transcription assays of isolated nuclei. However, the Hsp RNA inductions were inhibited by dactinomycin, indicating a requirement for newly synthesized RNA. Both Hsc70 and Hsp90 mRNAs had relatively short half-lives, measured by Northern blot analyses of dactinomycin chases, which were not altered substantially in TGFβ-treated cells. In contrast, Hsp mRNA half-lives increased in heat shocked CEC exposed to dactinomycin during recovery, revealing a difference in regulation of these genes in stressed cells compared with TGFβ-treated cells. Our results support the conclusion that hsc70 and hsp90 gene expression is regulated posttranscriptionally in TGFβ-treated CEC, and the mechanism likely involves a nuclear event such as increasing the half-lives of nuclear RNA transcripts, processing, or transport into the cytoplasm.

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