Stress proteins HSP27, HSP70 and HSP90 increase the resistance of cancer cells to cytotoxic drugs and, in experimental models in vivo, increase their tumorigenicity. HSPs can also be found in the extracellular medium where they are believed to have immunogenic properties.
We have shown that HSPs, particularly HSP70 and HSP27, while no expressed at the basal level in non transformed cells, they are abundant in cancer cells, and this over-expression is necessary for cancer cell survival.
We have demonstrated that HSP27 and HSP70 are essential modulators of the apoptotic process through their specific interaction with key apoptotic proteins. As a consequence, HSPs increase the tumorigenic potential of cancer cells. We have designed peptide inhibitors of HSP70 to validate HSP70 as a promising target in cancer therapy. Finally, we have demonstrated that HSPs regulate the function and/or stability of proteins by affecting their sumoylation and /or ubiquitination, two related post-translational modifications.
Three axes of study for the next four years:
Stress or heat shock proteins (HSPs) were first discovered as a set of proteins whose expression was induced by a heat shock. It was subsequently shown that HSPs were ubiquitous and highly conserved proteins whose expression was induced in response to a wide variety of physiological and environmental insults.
HSPs function as chaperones by assisting the folding of newly synthesized polypeptides, the assembly of multi-protein complexes, and the transport of proteins across cellular membranes. Mammalian HSPs have been classified in two groups according to their size: high molecular weights HSPs and small molecular weights HSPs. The first group includes three major families : HSP90, HSP70 and HSP60. Some of them are expressed constitutively whereas expression of the others is induced by stressful conditions. These proteins can be targeted to different sub-cellular compartments. High molecular weight HSPs are ATP-dependent chaperones and require co-chaperones to modulate their conformation and ATP binding. In contrast, small HSPs, such as HSP27, are ATP-independent chaperones. HSP27 chaperone activity is modulated by its phosphorylation/oligomerization state.
Stress proteins allow the cells to survive to otherwise lethal conditions. Several mechanisms account for their cytoprotective effect . 1) as mentioned above, they are powerful chaperones. 2) They participate in the proteasome-mediated degradation of proteins under stress conditions, thereby contributing to the so called “protein triage”. 3) They inhibit key effectors of the apoptotic machinery at the pre- and post-mitochondrial level. Among the different HSPs, HSP27 and HSP70 are the most strongly induced after stresses such as anticancer drugs, oxidative stress, radiation etc. In addition, HSP27 and HSP70, while hardly expressed in non transformed cells, are abundantly expressed in cancer cells and therefore they have been suggested as important prognostic factors in cancer cells’ resistance to chemo-therapeutic treatment.
Apoptosis or programmed cell death is one of the cell death mechanisms triggered by cytotoxic drugs in tumour cells. This active type of cell death involves proteases called caspases whose activation results in chromatin condensation. Two fundamentally different pathways of apoptosis can be distinguished, although crosstalk between the two signal transducing cascades exists (Figure 1). The extrinsic pathway is triggered through plasma membrane proteins of the TNF receptor family known as death receptors (such as Fas and TRAIL) and leads to the direct activation of caspases, in particular the receptor-proximal caspase-8. The intrinsic pathway involves intracellular stress signals that elicit the production or activation of pro-apoptotic molecules, which converge on the mitochondria to trigger their permeabilization.
Outer mitochondrial membrane permeabilization leads to the release of caspase activators, in particular cytochrome c, which interacts with cytosolic apoptosis protease-activating factor-1 (Apaf-1) and pro-caspase-9 to form the apoptosome, the caspase-3 activation complex. The flavoprotein apoptosis inducing factor (AIF) is another mitochondrial intermembrane protein released upon an apoptotic stimulus. AIF translocates to the nucleus and triggers caspase-independent nuclear condensation . Two additional mitochondrial proteins, Smac/Diablo and Htra2/Omi, activate apoptosis by neutralizing the inhibitory activity of the IAPs (inhibitory apoptotic proteins) that associate with and inhibit caspases.
Apoptosis and differentiation are two physiological processes that share different features like chromatin condensation or the need of caspases activity. It has been demonstrated in many differentiation models that the activation of caspases is preceded by a mitochondrial membrane depolarization and release of mitochondria apoptogenic molecules. This suggests that the mitochondrial-caspase dependent apoptotic pathway is a common intermediate for conveying apoptosis and differentiation (Figure 1). Timing, intensity and cellular compartmentalization might determine whether a cell is to die or differentiate. HSPs might be essential to orchestrate this decision.