Induction of differentiation of leukaemic (HL-60) or prostate cancer (LNCaP, JCA-1) cells potentiates apoptosis triggered by onconase

Onconase (Onc) is a ribonuclease from amphibian oocytes that is cytostatic and cytotoxic to many tumour lines. It shows in vivo antitumour activity in mouse tumour models and is currently in Phase III clinical trials. The pre sent study was designed to test whether cytotoxic effects of ONC can be mod ulated by differentiating agents. Human leukaemic HL-60 and prostate cancer LNCaP and JCA-1 cells were treated with Onc in the absence and presence of several inducers of differentiation and frequency of apoptosis was assesse d using three different cytometric methods and confirmed by analysis of cel l morphology. A moderate degree of apoptosis observed after 48-72 h incubat ion of HL-60 cells in the presence of 0.42 muM Onc alone was markedly poten tiated by administration of retinoic acid (all trans), sodium butyrate or d imethylsulfoxide at concentrations known to induce differentiation but be m inimally cytotoxic. Likewise, the frequency of apoptosis of LNCaP and JCA-1 cells treated with Onc was increased in the cultures to which phenylbutyra te was added. Although cell treatment with Onc alone, with each of the diff erentiating agents alone or with Onc in combination with the differentiatin g agents led to an increase in the proportion of G(1) cells, no specific ce ll cycle phase preference in induction of apoptosis was observed. The data suggest that cells undergoing differentiation are particularly vulnerable t o Onc; a combination of Onc and differentiating agents should be considered for further in vivo tests to assess its possible usefulness in the clinic.