Abstract 1159: BRAF- mediated PHGDH induction establishes a metabolic vulnerability in melanoma
Authors: Jasani Neel , Posorske Benjamin , Karreth Florian 
Published: April 2023

Abstract
Phosphoglycerate Dehydrogenase (PHGDH) gene, encoding the rate limiting enzyme of the serine synthesis pathway, is located at chromosome locus 1p12 and frequently amplified in melanoma. PHGDH is thought to be required for the proliferation of melanoma cells having chromosome 1p12 amplification. However, the role of PHGDH in chromosome 1p12 non-amplified melanoma is poorly understood. We observed that PHGDH is overexpressed in melanoma cell lines that do not harbor chromosome 1p12 amplification compared to melanocytes. Additionally, PHGDH is upregulated in melanomas compared to nevi. Hyperactivation of the MAPK pathway through activating mutations in BRAF or NRAS occurs in nearly all melanomas. Since PHGDH is universally upregulated in melanoma cells, we hypothesized that oncogenic BRAF mutations upregulate PHGDH expression, thereby generating a metabolic vulnerability in melanoma. Pharmacologic inactivation of the MAPK pathway using inhibitors of mutant BRAF/MEK/ERK resulted in the downregulation of PHGDH expression both in vitro and in xenograft tumors in vivo. On the other hand, acute expression of oncogenic BRAF (BRAFV600E) in melanocytes resulted in upregulation of PHGDH expression. Downregulation of PHGDH in 1p12 non-amplified melanoma cell lines decreased their cell proliferation and colony formation ability in vitro. We also observed that genetic knockdown of PHGDH in a BrafV600E/WT; Pten−/- (BPP) melanoma mouse model significantly increased survival. In addition to de-novo synthesis, serine can be taken up from extracellular compartments. Depletion of extracellular serine from the media resulted in upregulation of PHGDH expression and reduced proliferation of melanoma cells in vitro. By contrast, serine-glycine starvation in BPP mice did not affect tumor development or survival. These results implicate the dependency of melanoma tumors on de-novo serine synthesis through PHGDH. To investigate the therapeutic applicability of the PHGDH dependency, we inhibited the MAPK pathway with BRAF inhibitor along with extracellular serine-glycine depletion. We hypothesized that inhibiting the BRAF-mediated PHGDH expression along with extracellular serine-glycine starvation effectively depletes serine specifically in melanoma cells. Notably, cell viability was significantly reduced in vitro when BRAF inhibition was combined with serine-glycine depletion. Furthermore, our preliminary studies suggests that serine-glycine depletion potentiates the effect of mutant BRAF inhibition on melanoma growth in BPP mouse models. Overall, these results suggest the importance of PHGDH for melanoma cell proliferation and tumor growth irrespective of its amplification status. Combined MAPK inhibition and serine starvation may therefore exploit a metabolic vulnerability of melanoma cells and could be explored for melanoma therapy.
Citation Format: Neel Jasani, Benjamin Posorske, Florian Karreth. BRAF- mediated PHGDH induction establishes a metabolic vulnerability in melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1159.