The role of serine metabolism on the evolution of oral squamous cell carcinoma

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ABSTRACT Oral squamous cell carcinoma (OSCC) is a devastating malignancy associated with high morbidity, poor survival, and few therapeutic options. OSCC is characterized by heterogeneous cell states, including cancer stem cells (CSCs), which drive metastasis and therapy resistance. Although there has been some modest success with targeted therapies, there remains a need for more effective therapeutic options for OSCC. Increasing evidence has shown that changes in serine metabolism can direct cell fate via epigenomic changes. Endogenous serine synthesis generates a by-product, alpha-ketoglutarate (αKG), which is a co-substrate for nuclear αKG- dependent dioxygenases to demethylate histone marker H3K27me3 and de-repress differentiation genes. We have shown that under serine starvation conditions, metastatic OSCC HSC3 cells slowed proliferation concomitant with a change in morphology from mesenchymal to epithelial, compared to cells grown in complete medium. This was associated with a statistically significant increase in the steady-state mRNA and enzyme levels in the serine synthesis pathway under serine starvation conditions, indicating that HSC3 cells rely on exogenous serine for growth. The observed switch from exogenous serine uptake to endogenous serine synthesis in HSC3 cells was accompanied by an increase in αKG concentration. Furthermore, serine starvation and increased αKG were associated with decrease of repressive histone marker H3K27me3, thus de-repressing terminal epithelial differentiation genes. Simultaneously, there is a decrease in H3K4me3, a marker of open chromatin structure, associated with the promotion of stemness genes and aggressive cancer traits in OSCC. Initial studies into the plasticity of the stem-like identity using the tumorsphere formation assay shows that serine starvation leads to a lack of tumorspheres stability. Our findings suggest that a switch from exogenous serine uptake to the endogenous serine biosynthesis promotes OSCC cell differentiation concomitant with the loss of CSC identity. Given my preliminary studies, I hypothesize that serine deprivation promotes epigenetic changes that inhibit CSCs and OSCC progression to advanced disease. I will investigate our hypotheses through two aims. My first aim will be to determine mechanisms underlying epigenetic modifications in a panel of OSCC cell lines in response to serine deprivation in vitro. This will be investigated through use of migration and invasion assays, in depth tumorsphere analysis, and epigenomic profiling. My second aim will be to define changes in tumor size and tumor cell populations in response to dietary serine restriction in vivo, utilizing a syngeneic 4MOSC1 isograft mouse model of OSCC to interrogate the heterogeneity of tumor cell populations through scRNAseq with biochemical validation. These studies will fill the gap in knowledge how changes in serine metabolism impact the epigenomic environment and provide insight into novel therapeutic options for OSCC.