The potential role of the Pentose Phosphate Pathway in Chronic Lymphocytic Leukemia disease and chemotherapy resistance.

Chronic lymphocytic leukemia (CLL) is a hematological malignancy displaying a highly heterogeneous clinical course. Several studies highlight the capability of CLL B cells to control the persistent redox stress1,2 by triggering antioxidant pathways that are particularly efficient in aggressive CLL cells3. Antioxidant pathways and anabolic reactions strictly depend on NADPH, whose intracellular pool is provided by the pentose phosphate pathway (PPP). Recent evidence showed that the level of PPP rate-limiting enzyme, glucose-6-phosphate-dehydrogenase (G6PD), is significantly increased in CLL B cells in comparison to normal B cells4,5 and is directly correlated with poor prognosis6, underlying a possible key role of PPP in CLL disease aggressiveness. To this purpose, we thus evaluated i) the catalytic function of G6PD, ii) the mitochondrial respiratory function and iii) the glycolytic flux, in CLL cells derived from patient with ‘indolent’ and ‘aggressive’ disease, classified based on the molecular prognostic marker. The results showed that in vitro stimulation selectively increased G6PD activity only in samples with features of higher aggressiveness (Fig.A). Interestingly, in activated CLL cells, the mitochondrial dependence on fatty acid was significantly higher in patients with more aggressive disease, whereas mitochondrial dependence on pyruvate appears to be reduced (Fig.B). Given that levels of glucose consumed from the culture medium and the lactate produced are similar in ‘indolent’ and ‘aggressive’ activated samples (Fig.C), these data suggested that part of the cell-entering glucose might be processed through PPP, thus configuring the limitation of this enzyme as a possible therapeutic target. In line with these data, we observed a possible involvement of G6PD in the resistance to the Bcl-2 inhibitor Venetoclax. Indeed, recent studies showed that the anti-apoptotic protein Mcl-1 increment might be involved in resistance to Venetoclax7,8. In line with these observations, we observed a possible direct correlation between G6PD and Mcl-1 expression (Fig.D). These data highlight a novel role of G6PD function in the aggressiveness of CLL patients and anti-Bcl-2 therapy resistance, suggesting the PPP-limiting enzyme may represent a new therapeutic target in CLL.