Hormonal, nutrient and stress-mediated regulation of glucose and amino acid transport

Research

The work in my lab focuses on the control of intracellular signalling processes that regulate uptake, storage and metabolism of fuels (e.g. glucose) in response to hormonal, stress and nutritional cues.  We are particularly interested in defining the molecular mechanisms by which over supply of certain nutrients, such as saturated fatty acids, induce a marked reduction in insulin responsiveness in tissues such as skeletal muscle. Our research has shown that sustained exposure of muscle cells to palmitate, a saturated fatty acid, results in impaired proximal insulin signalling that leads to an associated loss in the hormonal activation of glucose transport.  A key feature underpinning the inhibition in insulin signalling involves elevated synthesis of ceramide; a palmitate-derived lipid that specifically antagonises the activation of protein kinase B (PKB, also known as Akt), which functions as a critical node in the insulin signalling network.  Over the past few years we have defined the molecular basis of this ceramide-induced inhibition of PKB and recently demonstrated that suppressing the activity or expression of the enzyme (serine palmitoyl transferase) that commits palmitate to synthesis of ceramide partially antagonises the insulin-desensitising effect of palmitate.

Saturated fatty acids also induce an inflammatory response in skeletal muscle, which, in addition to ceramide, may also be a contributing factor in the pathogenesis of insulin resistance.  How this inflammatory response is initiated is poorly understood, but our very recent work suggests that saturated fatty acids induce activation of MEK/Erk signalling and that this appears crucial in supporting the downstream activation of the NFkB pathway and the concomitant increase in the expression of proinflammatory genes.  Intriguingly, we have discovered that this inflammatory response is antagonised by unsaturated fatty acids.  Precisely how this protective anti-inflammatory effect is conferred is unknown, but our on-going work is currently directed at delineating the mechanisms involved.      

Changes in cellular amino acid availability also exert powerful effects on signalling pathways regulating cell growth and differentiation. We are currently exploring the role of membrane amino acid transporters - not only in terms of their capacity to relay nutrients to the intracellular compartment, but as molecular sensors of amino acid availability that can regulate the activity of key intracellular molecules (e.g. mTOR) involved in nutrient signalling. In particular we are interested in defining the sensing/signalling functions associated with the SNAT2 (System A) transporter. Our recent work indicates that SNAT2 substrates can induce activation of the mTOR/S6K1 signalling axis and that System A activity may be important for supporting growth and proliferation of cells.  However, this transporter is also important during periods of amino acid insufficiency. Cellular amino acid deprivation induces a marked increase in SNAT2 expression and activity, but the mechanisms that underlie this effect remain poorly understood. The mTOR pathway is known to be responsive to changes in amino acid availability, but we have no evidence to support the idea that this pathway is responsible for up-regulating SNAT2 function during amino acid limitation. In fact, during periods of amino acid lack the activity of the mTOR pathway is very low, and the mTOR inhibitor, rapamycin, does not antagonise SNAT2 induction under these conditions. This excludes a role for mTOR and implicates another amino acid-responsive pathway. Intriguingly, repression of SNAT2 can be achieved by re-supply of any single System A substrate, including synthetic substrates. The substrate amino acid concentration required to repress SNAT2 expression reflects that required for extracellular binding to the transporter, raising the strong possibility that SNAT2 itself may function as a “transceptor” capable of not only transporting amino acids, but sensing their availability.  Our current work aims to specifically assess (i) how, under amino acid sufficient conditions, SNAT2 signals to the mTOR pathway, (ii) how, during amino acid insufficiency, the transporter “senses” reduced extracellular amino acid availability and how this is transduced to promote an increase in SNAT2 gene expression.  We are particularly keen to identify whether specific SNAT2 domains are important for sensing/signalling functions, which we may gain insight into through domain swapping experiments with structurally related transporters (e.g. System N) that do not exhibit the capacity to adapt to an altered nutrient environment or signal in response to substrate binding.

Teaching

Publications