The effects of culture conditions on cell metabolism and function
The *in vitro* microenvironment profoundly affects the capacity of cell culture to model physiological and pathophysiological states. Central to the modern optimisation of cell culture is the development of physiologic media, which highlight the major impact that specific nutrients can have on a variety of cellular processes. One of the most commonly used media today – DMEM, lacks vitamins B7 and B12, yet little is known about the impact of this on cultured cells. Here, we demonstrate that 3T3-L1 adipocytes accumulate large amounts of odd-chain fatty acids *in vitro*. Since vitamins B7 and B12-dependent enzymes regulate the activity of several pathways that converge at the point of lipid metabolism, we further show that the addition of both vitamins are sufficient to eliminate most odd-chain fatty acids, whilst boosting total fatty acid synthesis. Vitamins B7 and B12 supplementation also reduced methylmalonate production and altered nutrient fluxes towards the TCA cycle. Functionally, insulin sensitivity was not affected, however vitamin B7 supplementation increased leptin secretion in these adipocytes, representing a new avenue to study the link between leptin and cellular metabolism or lipid burden. These results highlight the importance of vitamins to adipocyte metabolism as well as how the nutrient microenvironment in standard cell culture can inadvertently compromise our understanding and discoveries of cell metabolism and function. Apart from nutrient composition, oxygen is another critical parameter of cell culture that is frequently overlooked. Cell culture is often considered to be hyperoxic, however pericellular oxygen, which is affected by oxygen diffusivity and consumption, is rarely reported. Here, we provide evidence that several cell types in culture actually experience local hypoxia, with important implications for cell metabolism and, critically, cell function. We focused initially on adipocytes, as adipose tissue hypoxia is frequently observed in obesity and precedes diminished adipocyte function. We show that under standard conditions, cultured adipocytes are highly glycolytic and exhibit a transcriptional profile indicative of physiological hypoxia. Increasing pericellular oxygen diverted glucose flux toward mitochondria, lowered HIF1α activity and resulted in widespread transcriptional rewiring. Functionally, adipocytes increased adipokine secretion and sensitivity to insulin and lipolytic stimuli, recapitulating a healthier adipocyte model. The functional benefits of increasing pericellular oxygen were also observed in macrophages, hPSC-derived hepatocytes and cardiac organoids. Our findings demonstrate that oxygen is limiting in many terminally-differentiated cell types, and that considering pericellular oxygen improves the quality, reproducibility and translatability of cell culture models.
Adipocytes, Cell culture, Metabolism, Oxygen, Vitamins
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