The heat shock response is transcriptionally regulated by an evolutionarily conserved protein termed heat shock factor (HSF). We report the purification to homogeneity and the partial peptide sequence of HSF from HeLa cells.

As part of our studies on the mechanisms controlling the synthesis of the neutral proteinase collagenase by rabbit synovial cells, we used a cDNA clone to measure total collagenase mRNA levels and to determine mRNA half-life. Phorbol myristate acetate was used to induce collagenase synthesis while all-trans-retinoic acid and dexamethasone were used to inhibit it. Cells stimulated with phorbol myristate acetate contained substantial amounts of collagenase mRNA, but cells treated with all-trans-retinoic acid or dexamethasone contained decreased amounts of collagenase mRNA which correlated well with levels of collagenase protein. Studies on mRNA half-life showed that the t1/2 for total poly(A+) RNA was about 25 h, while that of collagenase varied from as short as 12 h to as long as 36 h. The half-life was not affected by treatment with all-trans-retinoic acid or dexamethasone but was affected by the level of induction of collagenase mRNA: the greater the amount of collagenase mRNA induced, the longer the t1/2. We conclude that our data are consistent with the hypothesis that retinoic acid and dexamethasone act at the level of transcription to decrease collagenase production and the increased level of collagenase mRNA resulting from stimulation with phorbol esters is, in part, due to increased stability of the induced collagenase mRNA.

HSF2, one of the heat shock transcription factors in mammalian cells, is localized to the cytoplasm during normal growth and moves to the nucleus upon activation. Heat shock transcription factors in metazoans contain four hydrophobic heptad repeat sequences, three in the amino terminus and one in the carboxyl terminus, which are predicted to form s-helical coiled-coil structures analogous to the leucine zipper. Here, we show that point mutations in either of two amino-terminal zippers or in the carboxy-terminal zipper disrupt normal localization of HSF2 and cause it to be constitutively nuclear. We demonstrate further that two sequences immediately surrounding the amino-terminal zipper domain are required for nuclear localization. These sequences fit the consensus for a bipartite nuclear localization signal (NLS). We suggest that interactions between the amino-and carboxy-terminal zippers normally mask the NLS sequences of HSF2 and that these interactions are disrupted upon activation to expose the NLS sequences and allow transport of HSF2 to the nucleus. We conclude that zipper domains can regulate subcellular localization.

The regulation of cell cycle progression by steroid hormones and growth factors is important for maintaining normal cellular processes including development and cell proliferation. Deregulated progression through the G1/S and G2/M cell cycle transitions can lead to uncontrolled cell proliferation and cancer. The transcription factor E2F1, a key cell cycle regulator, targets genes encoding proteins that regulate cell cycle progression through the G1/S transition as well as proteins important in DNA repair and apoptosis. E2F1 expression and activity is inhibited by inorganic arsenic (iAs) that has a dual role as a cancer therapeutic and as a toxin that leads to diseases including cancer. An understanding of what underlies this dichotomy will contribute to understanding how to use iAs as a more effective therapeutic and also how to treat cancers that iAs promotes. Here, we show that quiescent breast adenocarcinoma MCF-7 cells treated with 17-β estradiol (E2) progress through the cell cycle, but few cells treated with E2 + iAs progress from G1 into S-phase due to a block in cell cycle progression. Our data support a model in which iAs inhibits the dissociation of E2F1 from the tumor suppressor, retinoblastoma protein (pRB) due to changes in pRB phosphorylation which leads to decreased E2F1 transcriptional activity. These findings present an explanation for how iAs can disrupt cell cycle progression through E2F1-pRB and has implications for how iAs acts as a cancer therapeutic as well as how it may promote tumorigenesis through decreased DNA repair.

To facilitate our studies on the mechanisms controlling collagenase production at a molecular level in rabbit synovial fibroblasts (22) or nonimmune IgG was added to the supernatant and the mixture was incubated at 4°C overnight. The antigen-antibody complexes were isolated with protein A-Sepharose. After 1 hr, the Sepharose beads were washed twice with 1 M NaCl/5 mM EDTA/50 mM Tris HCl, pH 7.5/0.5% Triton X-100, twice with 0.1 M NaCI/1 mM EDTA/10 mM Tris HCl, pH 7.5/1% Triton X-100/0.5% deoxycholate (Sigma)/0.1% NaDodSO4, once with 0