Regulation of Heat Shock Transcription Factor 1:  Aging at the Molecular Level



    Our lab is interested in studying the response of cells to external stress at the molecular level.  Under stressful conditions such as heat shock, oxidative stress, radiation, and hypoxia, cells induce the expression of a group of proteins called heat shock proteins (Hsps).  Hsps are molecular chaperones that protect cellular proteins by preventing them from becoming damaged upon exposure to harmful conditions, and by repairing them after they become damaged.  One of the primary heat shock proteins highly expressed under stress is Hsp70.  The expression of Hsp70 is regulated by the transcription factor, heat shock factor 1 (HSF1).  Under cellular stress HSF1, which primarily resides in the cytosol trimerizes and migrates to the nucleus.  In the trimeric state, HSF1 has a high affinity for cis-acting DNA sequence elements called heat shock elements (HSEs) in the promoter region of heat shock protein genes.  The bound trimer forms a complex with the potential to activate transcription of the gene.
 
    As living organisms age, they become less able to respond to external stresses and maintain homeostasis.  Therefore older cells are more prone to damage and disease.  This is seen in the relationship between aging and an increase in susceptibility to many diseases, and mortality.  Recent studies have shown that there is a significant decline in the transcription levels of the Hsp70 gene in cells from older organisms, and that this may be related to a decreased fidelity of binding of HSF1 to the HSE.      By studying changes in HSF1 in older organisms, we hope to gain insight into an important aspect of aging—the diminished capacity to respond to environmental stress.  The changes in HSF1 and Hsp70 upon aging may serve as a good model for aging at the molecular level for other proteins that show a decline in expression as cells age.    To study these changes, we are using a variety of biochemical and biophysical tools such as circular dichroism, time-resolved fluorescence anisotropy, dynamic light scattering, and protein-DNA interaction assays.


The primary questions we are currently looking at are:

1.  What causes the differences between HSF1 binding to the HSE in young vs. old organisms?

Is the difference seen in these species due to a misfolded or defective form of the transcription factor in older organisms or is it due to changes in upstream regulatory proteins? Studies to answer these questions are limited by the low abundance of HSF1 in mammalian cells, as well as by the problem of heterogeneity of HSF1 molecules in older tissue.  One way to overcome these difficulties is by using single molecule spectroscopy techniques (SMS), which allow measurements to be done at the single molecule level.  Both conventional tools and SMS-based approaches are being used in the lab to identify the specific structural and functional alterations in HSF1 molecules isolated from whole tissue samples.    

2. How do upstream regulatory factors interplay with HSF1?
 
Although HSF1 has been studied for several years, it is still not known how HSF1 is able to sense stress and become a trimer.  Whether it directly senses stress or is regulated by an upstream signaling cascade is not yet fully understood.  We would like to study how different modifications that HSF1 undergoes (phosphorylation, SUMOylation) affect its activity, and look at differences in these modifications in young vs. old organisms.  We are also looking at what kinds of structural changes are involved in HSF1 trimerization, and how these changes are brought about by stress.  How does HSF1 recognize and bind DNA, and what controls deactivation of the HSF1 transcriptional activation activity are also questions of interest.





Figure 1.  Under normal conditions, HSF1 exists primarily as a latent monomer in the cytosol.  Upon exposure to cytotoxic conditions such as heat shock or oxidative stress, HSF1 trimerizes and migrates to the nucleus.  In the trimeric state, HSF1 binds to the HSE, forming a complex that has the potential to activate the transcription of hsp genes. 
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