Chaperones/heat shock proteins (HSPs) of the HSP90 and HSP70 families show elevated levels in proliferating mammalian cells and a cell cycle-dependent expression. They transiently associate with key molecules of the cell cycle control system such as Cdk4, Wee-1, pRb, p53, p27/Kip1 and are involved in the nuclear localization of regulatory proteins. They also associate with viral oncoproteins such as SV40 super T, large T and small t antigen, polyoma large and middle S antigen and EpsteinBarr virus nuclear antigen. This association is based on a J-domain in the viral proteins and may assist their targeting to the pRb/E2F complex. Small HSPs and their state of phosphorylation and oligomerization also seem to be involved in proliferation and differentiation. Chaperones/HSPs thus play important roles within cell cycle processes. Their exact functioning, however, is still a matter of discussion. HSP90 in particular, but also HSP70 and other chaperones associate with proteins of the mitogen-activated signal cascade, particularly with the Src kinase, with tyrosine receptor kinases, with Raf and the MAP-kinase activating kinase (MEK). This apparently serves the folding and translocation of these proteins, but possibly also the formation of large immobilized complexes of signal transducing molecules (scaffolding function).

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The expression and the nuclear translocation of the constitutive heat shock protein 70 (Hsc70) were determined during the cell cycle in synchronized rat astrocytomic C6 glioma cells. Cells were first shifted to the G0 by serum starvation. Twelve hours after a subsequent growth stimulation by transfer to 20% newborn calf serum, about 50% of the cells entered S phase. Western blot analysis with different monoclonal antibodies showed that only the constitutively expressed and moderately stress-activated Hsc70 is induced during serum stimulation. Maximal cellular Hsc70 content (170% of the control) was observed in early to mid S phase followed by a drastic decline while cells pass through G2/M (20% of the control). Hsp70, the major heat-inducible heat shock protein in C6 cells, is not detected in either asynchronously proliferating, serum-starved or in serum-stimulated C6 cells. Analysis of the nuclear and cytoplasmic protein fractions showed a significant increase of Hsc70 translocation into the nucleus during early S phase. These results indicate a role for Hsc70 but not for Hsp70 in the process of S phase entry and/or progression in C6 cells under physiological conditions.

Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) is a potential anticancer drug since it induces apoptosis preferentially in malignant but not in normal cells. Not all cancer cells, however, are TRAIL susceptible. Chemotherapeutic drugs and ionizing radiation have been found to be able to sensitize resistant tumor cells for TRAIL-induced apoptosis. Since ultraviolet B radiation (UVB) is a potent inducer of apoptosis but exhibits much less adverse effects, we studied whether UVB sensitizes TRAIL-resistant melanoma cells. Therefore, we analyzed the TRAIL-sensitive human cell line A-375 in comparison to the resistant cell line IGR-37. Both cell lines showed expression of the long form of the antiapoptotic FLICE inhibitory protein FLIP(L) which, however, was partially cleaved into the 43 kDa form in A-375 cells. In addition, only IGR-37 cells expressed the short splicing variant FLIP(S), which exerts high antiapoptotic activity. Accordingly, transient overexpression of FLIP(S) rendered A-375 cells resistant to TRAIL. Upon exposure to low UVB doses, TRAIL-treated IGR-37 cells underwent pronounced apoptosis and TRAIL sensitivity of A-375 cells was dramatically increased. In both cases, UVB caused an inhibition of flip expression. This study indicates that (i) the expression level and the processing status of FLIP may play a crucial role in determining the sensitivity of melanoma cells towards TRAIL and (ii) expression of FLIP is regulated by UVB.

It is well known that hyperthermia causes a transient tolerance of cells to a second heat challenge (acquired thermotolerance). The present study addresses the question of whether hyperthermic pre-treatment also increases the tolerance against heat- and hydrogen peroxide-induced apoptosis in rat IPC-81 leukaemia cells. This cell line exhibits an aberrant heat shock response which is characterized by a lack of the inducible Hsp70 isoform, even under conditions of heat or hydrogen peroxide stress, while the constitutively expressed Hsc70 and the inducible isoform of hemoxygenase (HO-1) are strongly enhanced by heat stress (43.5 degrees C; 30 min). In spite of this Hsp70 deficiency, hyperthermic pre-treatment protects IPC-81 leukaemia cells against apoptotic cell death induced by heat or hydrogen peroxide, but is less effective against necrosis induced by higher doses of the applied stressors. Addition of hydrogen peroxide (25 microM) enhances the amount of bax mRNA, while the level of bcl-2 mRNA remains unchanged. No increase of bax mRNA, in contrast, could be detected in heat shock-primed IPC-81 cells when treated with hydrogen peroxide after a 12h recovery. These results indicate that hyperthermic pre-treatment may exert its anti-apoptotic function not only by enhanced expression of constitutive as well as inducible HSPs but also by lowering the level of bax transcripts and thereby increasing the Bcl-2/Bax ratio.