Mikko Lammi

A variety of stress stimuli induce heat shock genes to express heat shock proteins (Hsps). The Hsps prevent the accumulation of cytotoxic protein aggregates and assist refolding of misfolded proteins in response to stress. They are also associated with the inhibition of stress-induced apoptotic cell death. Expression of heat shock genes is regulated both at the transcriptional and posttranscriptional levels, and the rapid transcriptional induction of heat shcok genes involves activation of a specific transcription factor, heat shock factor 1 (HSF1). However, the transcriptional induction varies both in intensity and kinetics in a signal- and cell type-dependent manner. The posttranscriptional regulation mechanisms, including mRNA stabilization and translation, are not well understood.

     In this thesis, I have found that mechanical loading in the form of hydrostatic pressure (HP) increases hsp70 gene expression in human T/C28a4 chondrocyte-like cells, HeLa cervical carcinoma cells, HaCat keratinocytes, and MG-63 osteosarcoma cells. The response to continuous HP was characterized by elevations in the levels of Hsp70 mRNA and protein, without any accompanying activation of HSF1 and transcriptional induction of hsp70 mRNA molecules. Novel protein synthesis was found to be involved in the mRNA stabilization. In contrast to static pressurization, cyclic hydrostatic loading did not result in an induction of the heat shcok genes.

     Primary bovine chondrocytes, which are cells that are adapted to live under gradients of high compressive HP, showed a negligible Hsp70 response upon HP loading, while primary bovine synovial cells and human skin fibroblasts accumulated hsp70 mRNA and protein by a similar mechanism as the established cell lines during HP stress. Interestingly, preconditioning of the barosensitive fibroblasts with HP or heat shock reduced the Hsp70 response, revealing acquired baroresistance.

     In the heat-shocked Neuro-2a neuroblastoma cells, the Hsp70 response takes place by translational regulation rather than by transcriptional induction of the hsp70 gene or mRNA stabilization, unlike in human IMR-32 neuroblastoma cells, which showed transcriptionally induced stress reponse.

     Taken together, the posttranscriptional gene regulation in the form of mRNA stabilization and translational regulation levels seem to be an important way to control the cell-type specific Hsp70 response.