Physiological levels of the PTEN-PI3K-AKT axis activity are required for maintenance of Burkitt lymphoma

In addition to oncogenic MYC translocations, Burkitt lymphoma (BL) depends on the germinal centre (GC) dark zone (DZ) B cell survival and proliferation programme, which is characterized by relatively low PI3K-AKT activity. Paradoxically, PI3K-AKT activation facilitates MYC-driven lymphomagenesis in mice, and it has been proposed that PI3K-AKT activation is essential for BL. Here we show that the PI3K-AKT activity in primary BLs and BL cell lines does not exceed that of human non-neoplastic tonsillar GC DZ B cells. BLs were not sensitive to AKT1 knockdown, which induced massive cell death in pAKT(high) DLBCL cell lines. Likewise, BL cell lines show low sensitivity to pan-AKT inhibitors. Moreover, hyper-activation of the PI3K-AKT pathway by overexpression of a constitutively active version of AKT (myrAKT) or knockdown of PTEN repressed the growth of BL cell lines. This was associated with increased AKT phosphorylation, NF-kappa B activation, and downregulation of DZ genes including the proto-oncogene MYB and the DZ marker CXCR4. In contrast to GCB-DLBCL, PTEN overexpression was tolerated by BL cell lines. We conclude that the molecular mechanisms instrumental to guarantee the survival of normal DZ B cells, including the tight regulation of the PTEN-PI3K-AKT axis, also operate in the survival/proliferation of BL.