Abstract
The heat shock response (HSR) is a well-conserved, cytoprotective stress response that activates the HSF1 transcription factor. During severe stress, cells inhibit mRNA splicing which also serves a cytoprotective function via inhibition of gene expression. Despite their functional interconnectedness, there have not been any previous reports of crosstalk between these two pathways. In a genetic screen, we identified SF3B1, a core component of the U2 snRNP subunit of the spliceosome, as a regulator of the heat shock response in Caenorhabditis elegans. Here, we show that this regulatory connection is conserved in cultured human cells and that there are at least two distinct pathways by which SF3B1 can regulate the HSR. First, inhibition of SF3B1 with moderate levels of Pladienolide B, a previously established small molecule inhibitor of SF3B1, affects the transcriptional activation of HSF1, the transcription factor that mediates the HSR. However, both higher levels of Pladienolide B and SF3B1 siRNA knockdown also change the concentration of HSF1, a form of HSR regulation that has not been previously documented during normal physiology but is observed in some forms of cancer. Intriguingly, mutations in SF3B1 have also been associated with several distinct types of cancer. Finally, we show that regulation of alternative splicing by SF3B1 is sensitive to temperature, providing a new mechanism by which temperature stress can remodel the transcriptome.
Highlights
The heat shock response (HSR) was first identified more than fifty years ago as an increase in the puffing of Drosophila salivary chromosomes in response to elevated temperature [1]
To explore the connections between SF3B1 and the HSR, we first tested the effect of SF3B1 depletion in cultured human cells to determine if SF3B1-mediated regulation of the HSR is conserved from worms to humans
We found that siRNA knockdown of SF3B1 in HeLa cells led to a dramatic decrease in the cells’ ability to mount a heat shock response after exposure to standard heat shock conditions of 42 ̊ for 1 hour (Fig 1A)
Summary
The heat shock response (HSR) was first identified more than fifty years ago as an increase in the puffing of Drosophila salivary chromosomes in response to elevated temperature [1]. The HSR has been established as a cellular stress response that senses protein folding and leads to activation of the HSF1 transcription factor and the upregulation of a set of cytoprotective heat shock genes (reviewed in [2]). Inhibition of mRNA splicing occurs upon exposure to severe, but not mild, heat stress in Drosophila and cultured human cells [7,8]. Severe heat shock has been shown to inhibit protein translation through phosphorylation of eIF-2α[11] and relocalization of mRNAs into cytoplasmic stress granules [12]. The intriguing link between the mRNA splicing machinery and the heat shock response and the important roles of both SF3B1 and HSF1 in cancer prompted us to further analyze the connections between SF3B1 and the HSR. We demonstrate that this regulation is conserved in human cells, delineate two pathways by which SF3B1 regulates HSF1, and discover that regulation of alternative splicing by SF3B1 is sensitive to stress
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