Browsing by Author "Hui, Sheng"

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    As Extracellular Glutamine Levels Decline, Asparagine Becomes an Essential Amino Acid
    (Elsevier, 2018-02-06) Pavlova, Natalya N.; Hui, Sheng; Ghergurovich, Jonathan M.; Fan, Jing; Intlekofer, Andrew M.; White, Richard M.; Rabinowitz, Joshua D.; Thompson, Craig B.; Zhang, Ji; Pediatrics, School of Medicine
    When mammalian cells are deprived of glutamine, exogenous asparagine rescues cell survival and growth. Here we report that this rescue results from use of asparagine in protein synthesis. All mammalian cell lines tested lacked cytosolic asparaginase activity and could not utilize asparagine to produce other amino acids or biosynthetic intermediates. Instead, most glutamine-deprived cell lines are capable of sufficient glutamine synthesis to maintain essential amino acid uptake and production of glutamine-dependent biosynthetic precursors, with the exception of asparagine. While experimental introduction of cytosolic asparaginase could enhance the synthesis of glutamine and increase tricarboxylic acid cycle anaplerosis and the synthesis of nucleotide precursors, cytosolic asparaginase suppressed the growth and survival of cells in glutamine-depleted medium in vitro and severely compromised the in vivo growth of tumor xenografts. These results suggest that the lack of asparaginase activity represents an evolutionary adaptation to allow mammalian cells to survive pathophysiologic variations in extracellular glutamine.
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    Coordination of bacterial proteome with metabolism by cyclic AMP signalling
    (Springer Nature, 2013) You, Conghui; Okano, Hiroyuki; Hui, Sheng; Zhang, Zhongge; Kim, Minsu; Gunderson, Carl W.; Wang, Yi-Ping; Lenz, Peter; Yan, Dalai; Hwa, Terence; Microbiology and Immunology, School of Medicine
    The cyclic AMP (cAMP)-dependent catabolite repression effect in Escherichia coli is among the most intensely studied regulatory processes in biology. However, the physiological function(s) of cAMP signalling and its molecular triggers remain elusive. Here we use a quantitative physiological approach to show that cAMP signalling tightly coordinates the expression of catabolic proteins with biosynthetic and ribosomal proteins, in accordance with the cellular metabolic needs during exponential growth. The expression of carbon catabolic genes increased linearly with decreasing growth rates upon limitation of carbon influx, but decreased linearly with decreasing growth rate upon limitation of nitrogen or sulphur influx. In contrast, the expression of biosynthetic genes showed the opposite linear growth-rate dependence as the catabolic genes. A coarse-grained mathematical model provides a quantitative framework for understanding and predicting gene expression responses to catabolic and anabolic limitations. A scheme of integral feedback control featuring the inhibition of cAMP signalling by metabolic precursors is proposed and validated. These results reveal a key physiological role of cAMP-dependent catabolite repression: to ensure that proteomic resources are spent on distinct metabolic sectors as needed in different nutrient environments. Our findings underscore the power of quantitative physiology in unravelling the underlying functions of complex molecular signalling networks.