Lamb Lab

A. Nutrient regulation of cell growth and metabolism

One of the most exciting current questions in biomedical research is how the environment- in the form of nutrients- interacts with genes to affect cell growth and metabolism. Understanding the interplay between these factors is likely to influence how we treat diseases in which nutrients play a major role, including cancer and diabetes.

1) Nutrient signaling to mTOR and cell growth-1: MAP4K3

mTOR/raptor (termed mTORC1) is a key recipient of information that relates nutrients such as glucose and amino acids to the cell for anabolic processes such as protein synthesis. We have identified a protein kinase-MAP4K3- that serves as a key amino acid-regulated upstream regulator of mTORC1 (Findlay GM et al,. (2007). Biochem J. 403, 13-20; Yan et al (2010) Molecular Cell, in press). Projects are based on identifying how amino acids signal to regulate MAP4K3 and mTORC1, involving proteomic and genetic analyses. Another interest is the formation of undefined tubulovesicular structures (see cryo-EM image with immunogold labeling, below) induced upon expression of inactive MAP4K3. The vesicular trafficking events in controlling nutrient signaling is currently under investigation.

Lamb Lab Project

Lamb lab Project1
Fig A-1

2) Nutrient signaling to cell growth-2: Sensing of amino acid nutrients

A major question is how nutrient amino acids are “sensed” and how that information is related to both mTORC1 and other signaling pathways. Branched-chain aliphatic side-chain amino acids (BCAA, leucine-isoleucine-valine), arginine, and latterly glutamine have all been linked to regulation of mTORC1 signaling. Glutamine likely acts as an intracellular efflux substrate allowing transport of branched-chain amino acids. However, although it appears that the intracellular concentration of BCAA or arginine is “sensed”, the sensing mechanism(s) are not known. We plan to identify metabolic signatures following depletion of amino acids to determine how particular amino acids are sensed and transmit this signal to mTORC1.

Fig A-2

B. Nutrient signaling in health and disease

1) Growth and the mTOR pathway

How is growth controlled at the level or the organism, organ and individual cell? Even within Homo sapiens, overall body size can vary widely (see an extreme example; image below, left panel). The mTOR pathway is known to play an important role in cell growth control. An example of such regulation can be seen in overexpression of Rheb in a subset of epidermal cells in Xenopus laevis development (see image below, middle panel). However how overall organ size is determined and the role of mTOR in this process is poorly understood. In postnatal life organ size is generally set within narrow limits and remains proportionate to overall body size. In the case of skeletal or heart muscle, training and exercise can increase overall organ size. In hypertrophic cardiomyopathy, which affects around 1/500 of the population, a pathologic type of ventricular wall growth can occur within the heart (see image below, right panel).

When proliferation and growth are largely coupled during development, the mTOR pathway contributes to setting overall organ size. However what is not known is whether, once size is set, an organ requires continual signaling from mTOR to maintain this size in proportion to body size, or whether this can be manipulated independently of body size. We believe that the use of genetic models of induced loss of mTORC1 components such a Raptor or mTORC1 activators such as the small GTPase Rheb in mice will shed light on this important question.

2) An amino acid requirement for tumours: Achilles heel for cancer?

The biosynthetic requirement for cell growth and proliferation indicates that cancer cells should have an increased avidity for nutrients. The “Warburg effect” indicates cancer cells preferential metabolism of glucose to generate ATP using the relatively inefficient method of glycolysis rather than oxidative phosphorylation, thereby generating lactic acid. To circumvent the inefficient generation of ATP cancer cells are thought to have invoked mechanisms during tumourigenesis that ensure high levels of glucose uptake and flux, thereby generating large amounts of carbon-skeleton intermediates for biosynthesis. This property is used clinically to image cancer in the body using the glucose analogue FDG and PET scanners. Cancer cells are anticipated to be particularly vulnerable to therapeutics that might restrict glycolysis or prevent disposal of lactic acid.

Nonetheless, some cancers cannot be imaged with FDG-PET, suggesting that they use an alternative energy source that similarly fuels biosynthesis. A likely candidate for such a fuel is the abundant amino acid glutamine. Similar to glucose, glutamine metabolism by glutaminolysis generates carbon-skeleton TCA intermediates for biosynthesis and generates lactic acid, but additionally provides nitrogen groups for nucleotide biosynthesis. In cell culture various cancer cells can be shown to require glutamine for survival and proliferation despite the ample presence of glucose (see images of an ovarian cancer cell line, below).

Glutamine is known to be transported into the cell via the SLC1A5 transporter. Intracellular glutamine stimulates uptake of essential amino acids such as leucine-thereby activating mTORC1- via SLCA7, a co-transporter which effluxes intracellular glutamine and imports leucine. Losing glutamine from the cell to maintain mTORC1 signaling seems counterintuitive as glutamine is also rapidly metabolized to glutamate during glutaminolysis by the mitochondrial phosphate-dependant glutaminase. We are interested in how cancer cells utilize glutamine and how is this related to growth control and mTORC1.

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