The heat shock response in HeLa cells is accompanied by elevated expression of the c-fos proto-oncogene

Molecular biology of cell activation

This article summarizes common features of activation of different types of cells along different physiological lines such as proliferation, differentiation, and execution of function of terminally differentiated cells. The common basis of many of these phenomena includes (i) first messengers (growth factors, cytokines, neurotransmitters, etc.) acting on membrane receptors, (ii) second messengers (cAMP, IP3, DAG, Ca2+) spreading an activating signal inside the cell, and (iii) elevated expression of some genes (c-fos, c-myc, ornithine decarboxylase). The role of the genetic correlate in cell activation is emphasized, and it is concluded that the aforementioned genes (their protein products) should be called third messengers, whose function is mediation of long-term phenotypic changes.

Adrenergic receptors mediate changes in c-fos mRNA levels in brain

Recent evidence supports a role for transient c-fos expression as one step in signalling pathways by which membrane receptor-ligand interactions are transduced into appropriate intracellular responses. The activity of adrenergic receptors is mediated by second messenger systems which include ion fluxes, changes in cAMP concentration and enhanced phosphoinositide turnover. In order to determine if C-fos induction was also a step in adrenergic signal transduction in the brain, we performed in vivo studies with drugs specific for different adrenergic receptor types. Unexpectedly, we found that the stress associated with a single intraperitoneal (i.p.) injection of drug vehicle produced a transient increase (averaging 4.0-fold) in c-fos mRNA levels in rat brain. Injection of the alpha 2-adrenoreceptor antagonist, yohimbine, produced a transient increase which was larger in magnitude (averaging 9.6-fold) and longer in duration than that produced by injection of the drug vehicle alone. In experiments designed to ask whether either of these inductions was mediated by specific types of adrenergic receptors, we found that the alpha 2- and beta-adrenoreceptors were involved in both responses, while the alpha 1-receptor played a role in mediating the yohimbine induction, but no detectable role in the solvent induction. One hypothesis consistent with our results is that the norepinephrine (NE) released due to the stress associated with an i.p. injection interacts with postsynaptic beta-adrenergic receptors, resulting in the observed c-fos mRNA induction. When the negative feedback effect of NE, mediated by presynaptic alpha 2-receptors, is blocked by yohimbine, the postsynaptic response is enhanced and prolonged.

Elevated expression of proto-oncogenes accompany enhanced induction of heat-shock genes after exposure of rat embryos in utero to ionizing irradiation

We have recently found that the effects of exposing rat embryos in utero to teratogens capable of producing cardiac anomalies were expressed later as enhanced induction of heat-shock proteins (hsp70 family) when embryonic hearts were cultured in vitro. However, it remained to be determined whether heat-shock proteins are induced in vivo after exposure to teratogens. The heat-shock response in some mammalian systems is known to be accompanied by elevated expression of proto-oncogenes. Using gene-specific DNA probes, we examined the levels of the expression (transcription) of heat-shock protein genes and two nuclear proto-oncogenes, c-fos and c-myc, in the embryos removed from irradiated pregnant mother rats 4 or 5 days after the irradiation. We found that the levels of expression in vivo of the hsp70 and c-myc genes in the irradiated embryos increased by approximately twofold as compared with those in the control. The expression in vivo of the c-fos gene was not detected in either the irradiated or non-irradiated embryos. After 0.5-hr incubation in vitro of the embryos, however, the expression of the c-fos gene in the irradiated embryos was highly enhanced whereas the control showed no changes. Although the exact functions of these gene products still remain obscure, the enhanced expression of hsp70 gene(s) and the nuclear proto-oncogenes observed in the present study may reflect repair of intracellular damages and/or regeneration of tissue by compensatory cell proliferation, processes that may disturb the normal program of organogenesis.

Regulation of the heat-shock response by interferon in mouse L cells

Interferon (IFN) is not able to induce heat-shock protein (HSP) synthesis. However IFN pretreatment of mouse L cells has been shown to enhance the decrease of overall protein synthesis which follows a heat shock, and to stimulate the accumulation of HSPs. We show here that the synthesis of a protein (the hepatitis B virus surface antigen) under the control of a Drosophila HSP 70 promoter is also stimulated in IFN-pretreated cells. The regulation by IFN takes place at two levels: first, the rate of HSP gene transcription is increased in nuclei isolated from IFN-treated cells; second, the synthesis of HSPs is prolonged after pretreatment with IFN. Experiments performed in the presence of actinomycin D show that this effect is due to a stabilization by IFN of mRNAs coding for HSPs.

Expression of c-fos antisense RNA inhibits the differentiation of F9 cells to parietal endoderm

To test the putative role of c-fos in F9 differentiation, we have attempted to inhibit c-fos expression in these cells using an SV40-based expression vector (pSVneo-sof) that programs expression of c-fos antisense (sof) sequences as a 3' extension of a neo mRNA transcript. Of six G418-resistant clones isolated in transfection experiments, five expressed neo-sof transcripts. Two clones synthesized polyadenylated mRNA of the expected size (3.8 kb), two were smaller than expected, and one was larger. Two clones that expressed reduced levels of c-fos protein were inhibited in the induction of laminin, type IV collagen, and proteoglycan-19 RNA transcripts measured after 4 days of differentiation induction with RA and dibutyryl cyclic AMP. Also inhibited was the induction of the differentiation markers, TROMA-1 and TROMA-3. Antisense-expressing cells were not inhibited in the differentiation pathway to visceral endoderm since the alpha-fetoprotein gene was activated normally. We conclude that c-fos antisense expression inhibits some aspects of differentiation in F9 cells.

Destabilization of tubulin mRNA during heat shock in Tetrahymena pyriformis

The regulation of tubulin gene expression was studied in Tetrahymena pyriformis cells during heat shock (shift from 28 degrees C to 34 degrees C). Fluorograms of two-dimensional gels of radiolabelled proteins synthesized during thermal stress revealed that tubulin synthesis is highly repressed when compared with that of exponentially growing cells. The variation in the levels of alpha and beta-tubulin mRNAs was analyzed by Northern-blot hybridization using homologous genomic probes (alpha TT and beta TT1). The results obtained show that heat shock induces a drastic and coordinate reduction in the amount of alpha and beta-tubulin mRNAs isolated from polysomes. This decrease is not due to a shift from the polysomes to the post-polysomal fraction because it was also observed when total cytoplasmic mRNAs were analyzed. Run-on transcription experiments were performed in order to examine whether repression of transcription in heat-shocked cells could explain that reduction. The results obtained show that the apparent rates of tubulin gene transcription are not significantly modified, but on the contrary increase slightly in cells heat-shocked for 15 min and 30 min. The effects of inhibitors of protein synthesis, cycloheximide and pactamycin, on the destabilization of tubulin mRNAs were tested in heat-shocked Tetrahymena cells. Our results revealed that in the presence of these inhibitors, tubulin mRNAs become more stable thus suggesting that an induced factor may be involved in the degradation of alpha and beta-tubulin mRNAs during heat shock.

Effect of heat shock on RNA metabolism in HeLa cells

Incubation of HeLa cells at 42 degrees C results in pronounced inhibition of the accumulation of 18S and 28S ribosomal RNA (rRNA) and non-heat shock polyadenylated messenger RNA (mRNA) in the cytoplasm. Accumulation of transfer RNA and 5S ribosomal RNA is not affected. Transcription of rRNA precursor is reduced to approximately 50% of the 37 degrees C rate after 10 min of hyperthermia and declines to 30% of the control rate after 1 hr. In contrast, the accumulation of mature rRNA in the cytoplasm is inhibited more than 95%. Quantitative hybridization experiments and Northern blot analysis detect little accumulation of rRNA precursor sequences in nuclei, suggesting that the majority of the rRNA that is synthesized is degraded. Heat stress at 42 degrees C was found to have little effect on transcription of most non-heat shock mRNAs. However, accumulation of individual non-heat shock mRNAs in the cytoplasm proceeds at reduced rates. These results indicate that the primary effect of elevated temperature on RNA metabolism in mammalian cells is inhibition of processing and/or transport. Despite this, steady-state levels of abundant and rapidly turning over mRNA species remain unchanged during prolonged heat stress. We find that the half-life of c-myc mRNA increases greater than twofold at 42 degrees C. Thus, 42 degrees C heat stress appears to inhibit both accumulation and turnover of non-heat shock mRNA.