Relation of P-glycoprotein expression with spontaneous in vitro apoptosis in B-cell chronic lymphocytic leukemia

Prolonged lifespan of monoclonal lymphocytes in B-cell lymphocytic leukemia (B-CLL) arises from their resistance to programmed cell death. In contrast, when cultured in vitro, B-CLL tumour cells rapidly undergo apoptosis. There is mounting evidence that P-glycoprotein (P-gp), an adenosine triphosphate-binding cassette (ABC) family transporter, plays a significant role in the regulation of apoptosis induced by various stimuli. Since P-gp is commonly expressed in B-CLL cells, we aimed to establish whether its expression level influences resistance to spontaneous apoptosis in B-CLL. For that purpose, P-gp expression by UIC2 antibody staining and P-gp activity by rhodamine 123 (Rh123) efflux in presence or absence of P-gp inhibitor verapamil were studied in peripheral blood lymphocytes obtained from 43 previously untreated B-CLL patients. Simultaneously, the percentage of cells undergoing spontaneous in vitro apoptosis (apoptotic index, AI) by means of activation of caspases and annexin-V-based assays was evaluated. The AI were higher in B-CLL cells than in normal peripheral blood mononuclear cells (medians of AI 27.7% vs 3.9%, p=0.0001 and 34.7% vs 7.4%, p=0.0038, in 24 and 48-hour culture respectively). The AI were also higher among female patients as compared to male patients (medians: 29.7 vs 19.2 p=0.048). Interestingly, we found moderate inverse correlation between P-gp protein expression and AI after 24-hour culture in analysed B-CLL samples (r= -0.36, p=0.019). Moreover, P-gp positive B-CLL samples expressed significantly higher AI than P-gp negative samples with an arbitrary cut-off at Kolmogorov-Smirnov statistics D-value 0.2 (medians of AI 18.4% vs 29.7%, p=0.026). Based on these results we suggest that P-gp expression has some protective effect on B-CLL cell survival in vitro. The difference in the rates of spontaneous apoptosis among male and female patients may contribute to gender-dependent variations in clinical outcome in B-CLL.