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Abstract
The screening of two different retroviral cDNA expression libraries to select genes that confer constitutive doxorubicin resistance has in both cases resulted in the isolation of the heat shock factor 1 (HSF1) transcription factor. We show that HSF1 induces a multidrug resistance phenotype that occurs in the absence of heat shock or cellular stress and is mediated at least in part through the constitutive activation of the multidrug resistance gene 1 (MDR-1). This drug resistance phenotype does not correlate with an increased expression of heat shock-responsive genes (heat shock protein genes, or HSPs). In addition, HSF1 mutants lacking HSP gene activation are also capable of conferring multidrug resistance, and only hypophosphorylated HSF1 complexes accumulate in transduced cells. Our results indicate that HSF1 can activate MDR-1 expression in a stress-independent manner that differs from the canonical heat shock-activated mechanism involved in HSP induction. We further provide evidence that the induction of MDR-1 expression occurs at a posttranscriptional level, revealing a novel undocumented role for hypophosphorylated HSF1 in posttranscriptional gene regulation.
Supplemental material for this article may be found at http://mcb.asm.org/.
We thank A. Vervish for flow cytometry analysis; D. Iancu for technical help in performing sensitive run-on assays; J. Wang for the gift of plasmids pcDNA3-dn-hHSF1 (originally named pcDNA3-mHSF1), pcDNA3-c-hHSF1, and pGL3-phsp70B; K. W. Scotto for the gift of pMDR(−1202); C. Baum for the gift of pSF91m3; A. Laurand for the gift of pSF-MDR; R. E. Kingston for the gift of pBabe-puro-hHSF1-flag wt and phenylalanine (3F) mutant derivatives and for pBabe-puro-Flag-Gal4(1-94)HSF-ABC-wt; S. Moore for the gift of HepG2 cells; and L. L. Pritchard for critical reading of the manuscript.
This work was supported by a grant from the Association pour la Recherche sur le Cancer (contract no. 4200XA0031F to T.T.).