Phosphoglycerate Dehydrogenase Is Required for Germinal Center Formation and Is a Therapeutic Target in MYC-driven Lymphoma

The fields of cancer- and immuno-metabolism have re-emerged as areas of significant translational potential. Even though the upregulation of glycolysis by proliferating lymphocytes is the basis for widely used clinical tests such as FDG-PET, little is known about which metabolic pathways are involved in the utilization of glucose to support B-cell proliferation. The synthesis of serine from glucose has been demonstrated to be a key metabolic pathway supporting cellular proliferation in some healthy and malignant cell types. Importantly, this pathway is regulated by MYC, which is known to be essential for germinal centre formation and is commonly dysregulated in lymphoma. Despite this, the role that the serine synthesis pathway (SSP) plays in germinal center biology and pathology has not been previously investigated. We performed a comprehensive characterization of the role of the SSP in germinal center B cells and lymphomas derived from these cells.

We demonstrate that upregulation of a functional SSP is a metabolic hallmark of B-cell activation and the germinal center reaction. We show that both human and murine resting naïve B cells lack expression of the first two enzymes in this pathway, phosphoglycerate dehydrogenase (PHGDH) and phosphoserine aminotransferase (PSAT1) enzymes. However, B-cell activation, predominantly through the B-cell receptor, robustly induces the expression of these enzymes in vitro, resulting in an acquired ability to synthesize serine, glycine and the purine nucleotides adenosine and guanosine from glucose. This is reflected in striking expression of PHGDH and PSAT1 within germinal centers but not in marginal zones confirming that this upregulation is occurring in germinal B cells activated in vivo. We then proceeded to investigate the impact of inhibiting PHGDH on germinal center formation and high-affinity antibody production in vivo. This was done both genetically, using a conditional B-cell knockout mouse model, and pharmacologically using a specific inhibitor of PHGDH, PH-755. Importantly, we show that PHGDH inhibition impairs germinal center formation with a resultant reduction in high-affinity antibody production. Mechanistic experiments demonstrate that PHGDH inhibition effectively blocks cells from synthesising serine and glycine from glucose, making them unable to proliferate in environments that lack these amino acids.

We then investigated role of PHDGH and PSAT1 in Burkitt Lymphoma (BL), Diffuse Large B Cell Lymphoma (DLBCL) and Chronic Lymphocytic Leukemia (CLL). Notably, very high expression of these two proteins was observed in BL, with intermediate-to-high expression in DLBCL and relatively low expression in CLL, where expression was restricted to proliferation centers. Given the heterogeneity of expression in DLBCL patients, we next interrogated a published GSE database (Lenz et al. NEJM 2008) to investigate the impact on outcome. Notably high expression of PSAT1 was significantly associated with a poorer overall survival rate in DLBCL.

We then investigated whether the SSP could be a therapeutic target in lymphoma. We demonstrate that PHGDH inhibition effectively inhibits de novo serine, glycine and purine nucleotide synthesis from glucose, resulting in impaired proliferation and increased apoptosis in a panel of human BL cell lines in vitro. We then analyzed the impact of PHGDH inhibition on lymphoma development in vivo using Eµ-myc mice, which harbour Myc coupled to the IgH enhancer characteristic of BL. Importantly we confirm the role of MYC by demonstrating that Eµ-Myc B-cells show significantly higher expression of PHGDH and PSAT1 expression resulting in increased serine and glycine synthesis when compared to control cells. We demonstrate that pharmacological inhibition of PHGDH using PH-755 impairs lymphoma progression in this model. We confirm the importance of PHGDH by showing that genetic ablation of Phgdh in Eµ-myc cells in a tamoxifen inducible system (using Eµ-myc/+;Rosa26-CreER ^T2/+;Phgdh ^fl/flmice) also results in a significant reduction in lymphoma progression.

Taken together, this work represents the first report of the role of the SSP in the biology of the germinal centre response and lymphomas derived from these cells. These findings establish PHGDH as a critical player in humoral immunity and a clinically relevant target in MYC-driven lymphoma, which is an area of significant unmet need.