The impact of physiological metabolite levels on serine uptake, synthesis and utilization in cancer cells

Abstract

Serine is a non-essential amino acid that is critical for tumour proliferation and depletion of circulating serine results in reduced tumour growth and increased survival in various cancer models. While many cancer cells cultured in a standard tissue culture medium depend on exogenous serine for optimal growth, here we report that these cells are less sensitive to serine/glycine depletion in medium containing physiological levels of metabolites. The lower requirement for exogenous serine under these culture conditions reflects both increased de novo serine synthesis and the use of hypoxanthine (not present in the standard medium) to support purine synthesis. Limiting serine availability leads to increased uptake of extracellular hypoxanthine, sparing available serine for other pathways such as glutathione synthesis. Taken together these results improve our understanding of serine metabolism in physiologically relevant nutrient conditions and allow us to predict interventions that may enhance the therapeutic response to dietary serine/glycine limitation.

Lineage-specific silencing of PSAT1 induces serine auxotrophy and sensitivity to dietary serine starvation in luminal breast tumors

“…Because the non-physiological nutrient levels found in traditional tissue culture medium can affect cellular dependence on certain amino acids ( Muir et al, 2017 ; Tardito et al, 2015 ), we first determined whether physiological medium (human plasma-like medium [HPLM]) ( Cantor et al, 2017 ) affects the response to serine and glycine (S/G) starvation. While breast cancer cells proliferate slightly faster in RPMI than HPLM in the presence of S/G, upon S/G starvation, cells proliferate much more slowly in RPMI than HPLM ( Figures S2A - S2D ), indicating that the use of physiological medium allows for enhanced growth without S/G, as has recently been noted ( Hennequart et al, 2021 ). While at present we do not fully understand the cause of these differences, we used the more physiologically relevant HPLM for the remainder of our experiments, with the exception of the organoid work described in Figure 4 .…”

“…This and other work motivated the development of PHGDH inhibitors as potential cancer treatments (Mullarky et al, 2016;Pacold et al, 2016;Rohde et al, 2018;Wang et al, 2017). While the inhibition of serine synthesis has shown efficacy in some models and is still being evaluated, it is becoming clear that in some circumstances it is not effective because of extracellular serine that can be taken up to offset the inhibition of de novo biosynthesis (Chen et al, 2013;Montrose et al, 2021;Me ´ndez-Lucas et al, 2020;Ngo et al, 2020;Nilsson et al, 2012;Sullivan et al, 2019b;Tajan et al, 2021). Because our goal was to identify cases in which lineage-dependent gene expression reduces pathway redundancy, we examined whether the very low expression of PSAT1 found in luminal tumors limits their ability to synthesize serine and creates a dependence on exogenous serine for growth.…”

Cultivation of Cells in a Physiological Plasmax Medium Increases Mitochondrial Respiratory Capacity and Reduces Replication Levels of RNA Viruses

“…Reduced levels of pyruvate in such a physiological medium prevents artificial stabilization of HIF1 that occurs in convenient media. Biosynthesis of purine nucleotides in Plasmax occurs from hypoxanthine, present in the medium and absent in DMEM [ 35 ]. Plasmax also activates the de novo serine biosynthesis pathway [ 18 , 35 ] but with lower levels of its conversion into glutathione (GSH), glycine, purine nucleotides or thymidylate [ 35 ].…”

“…Biosynthesis of purine nucleotides in Plasmax occurs from hypoxanthine, present in the medium and absent in DMEM [ 35 ]. Plasmax also activates the de novo serine biosynthesis pathway [ 18 , 35 ] but with lower levels of its conversion into glutathione (GSH), glycine, purine nucleotides or thymidylate [ 35 ]. Furthermore, many other pathways have not yet been studied.…”

“…The above discussed activation of the serine biosynthesis pathway, described by Vousden’s group, is mediated via ATF4 transcription factor of the integrated stress response [ 35 ]. The authors identified that ATF4 activation was due to lower levels of arginine in the medium (64 µM vs. 400 µM), as its supplement to the DMEM level blocked this activation.…”