Hsp90 inhibition has opposing effects on wild-type and mutant p53 and induces p21 expression and cytotoxicity irrespective of p53/ATM status in chronic lymphocytic leukaemia cells

In chronic lymphocytic leukaemia ( CLL), mutation/ deletion of TP53 is strongly associated with early disease progression, resistance to chemotherapy and short patient survival. Consequently, there is a pressing need to develop novel treatment protocols for this high- risk patient group. The present study was performed to evaluate Hsp90 inhibition as a possible therapeutic approach for such patients. Primary CLL cells of defined ataxia telangiectasia mutated ( ATM)/ p53 status were incubated with the Hsp90 inhibitor geldanamycin ( GA) and analysed by western blotting for the expression of p53, p21, MDM2 and Akt. GA downregulated overexpressed mutant p53 protein ( an oncogene) and upregulated wild- type ( wt) p53 ( a tumour suppressor). The upregulation of wt p53 by GA was independent of ATM and was accompanied by downregulation of Akt and the active form of MDM2, indicating a possible mechanism. GA also produced a p53/ ATM- independent increase in the levels of p21 - a potent inducer of cell- cycle arrest. In- vitro cytotoxicity studies showed that GA killed cultured CLL cells in a dose- and time- dependent fashion irrespective of their p53/ ATM status and more effectively than normal blood mononuclear cells. In summary, our findings reveal important consequences of inhibiting Hsp90 in CLL cells and strongly support the therapeutic evaluation of Hsp90 inhibitors in poor- prognosis patients with p53 defects.