IL21—STAT3 controls the pentose phosphate pathway to support metabolic reprogramming and tumor progression in chronic lymphocytic leukemia

Studying how microenvironmental cues influence metabolic reprogramming can uncover mechanisms driving tumor progression. Using an in vitro model with proliferative stimuli of the in vivo lymph node niche (LN)-including interleukin-21 (IL-21)-we examined metabolic rewiring in chronic lymphocytic leukemia (CLL) cells. We found that the metabolic intermediates of upper glycolysis and its branching pathways are key in fulfilling metabolic demands of proliferating CLL cells. Among branching pathways, the pentose phosphate pathway (PPP) was the most transcriptionally upregulated in proliferating CLL cells. Increased expression of PPP genes was detected ex vivo at the bulk and single-cell level in the LN-resident and -emigrating CLL cells, with more consistency across enzymes of the nonoxidative PPP branch. Expression of the latter correlated with shorter failure-free survival in CLL patients. At the cellular level, metabolomics and 13C-glucose tracing confirmed high activity of the non-oxidative PPP in proliferating CLL cells. IL-21 regulated the expression of PPP enzymes, with STAT3 serving as the primary downstream effector. CRISPR/Cas9-mediated silencing of PPP enzymes revealed that, in vitro, proliferating CLL cells from most patients were not dependent on these enzymes. In contrast, silencing transketolase (TKT)-the rate-limiting enzyme of the non-oxidative PPP-abolished tumor engraftment in vivo, demonstrating that CLL cells rely on this pathway within the tumor microenvironment. These findings uncover a CLL-specific metabolic reprogramming wherein IL-21-STAT3 drives PPP activity and identify the nonoxidative PPP as a critical in vivo vulnerability of leukemic cells in the murine CLL model.