Growth Regulation and Cancer: mTORC1, Exosomes and Cellular Amino Acid Sensing
Tumour cells employ altered metabolic strategies to promote growth and metastasis, which can be exquisitely tuned to the low nutrient and oxygen conditions in which they may develop. We are interested in the adaptive cellular mechanisms within tumour cells that facilitate their growth and whether they could be targeted to selectively impact on cancer cell growth.
Mechanistic Target of Rapamycin Complex 1 (mTORC1) is a key intracellular protein super-complex that senses and integrates the cell's response to a range of environmental cues to regulate organismal growth and homeostasis by control of protein synthesis and other metabolic events. Cancer growth and an increasing number of pathological conditions such as obesity, neurodegeneration and other age-related conditions, including cardiovascular disease, are typically associated with altered, often increased, activity from mTORC1. The central role of amino acids in regulating mTORC1 has been appreciated for some time, but work from our group and others has shed light on the sensing mechanisms involved, and highlighted an important function for late endosomes and lysosomes. We are currently investigating how endolysosomal mTORC1 is regulated by amino acid transporters, and whether other transporters may control mTORC1 from different intracellular compartments.
Out interest in endosomal compartments has led us to focus on understanding the regulation and function of a poorly understood form of communication between cells that involves small vesicles known as exosomes. Classically, exosomes are believed to originate from late endosomal multivesicular bodies and to be secreted by fusion of these compartments to the plasma membrane. Exosomes contain a complex mixture of macromolecules, including many proteins and RNAs. They have been implicated in altering the behaviour of target cells with important physiological and pathological consequences. For example, we have demonstrated that exosomes secreted from the male fly accessory glands alter the behaviour of female flies following mating, while others have shown that cancer exosomes can prime a pre-metastatic niche during tumour progression. Until recently, it has proved challenging to understand how exosome secretion is controlled and how it might be coupled to other events that might occur in late endosomes, such as changes in mTORC1 signalling in response to microenvironmental stress. We are now using our novel in vivo exosome secretion model and human cell-based analysis to investigate this problem in more detail.
Current Research Programme
The Goberdhan group uses a combination of human cell-based assays and in vivo analysis in the fruit fly, Drosophila melanogaster, to understand how amino acids and mTORC1 regulate growth and signalling between cells. The genetics of the fly enabled us to identify the Proton-assisted Amino acid Transporters (PAT) or SLC36 family, as potent amino acid sensors that can activate mTORC1 signalling and cell growth in flies and humans. These studies revealed that the growth-promoting properties of these mTORC1 regulators are increased by PI3K signalling and that this is also associated with a shift in the subcellular localisation of PATs to intracellular compartments. Our work has highlighted an amino acid sensing mechanism that acts at the surface of late endosomes and lysosomes to activate mTORC1. It involves members of the PAT family and appears to become increasingly important for the growth of cancer cells as they are subjected to challenging growth conditions. We are currently investigating whether this mechanism could be targeted to selectively impact on cancer cell growth or other mTORC1-associated human diseases. We have also found that some members of the PAT family can function at very low amino acid levels, which may be particularly relevant to the ability of tumour cells to grow under conditions of microenvironmental stress. These studies are supported by several collaborations focused, for example, on the structural analysis of the PATs (Simon Newstead, Biochemistry Dept, Oxford), the roles of PATs in colorectal and breast cancer growth using in vivo models (Adrian Harris, WIMM, Oxford) and in prostate cancer (Dan Stevens, Clinical Academic Fellow; starting DPhil in October 2015; Freddie Hamdy, NDSS, Oxford; Richard Bryant, NDSS, Oxford; Clare Verrill, Dept of Cell Pathology, John Radcliffe Hospital, Oxford).
We are also investigating the ways in which microenvironmental stress impacts on mTORC1 to regulate exosome secretion. Studies using a mixture of genetic and pharmacological approaches in cancer cell lines are revealing changes in the content and function of secreted exosomes in reponse to such stresses. We have also worked with the Wilson lab (DPAG) to develop a model to analyse exosome signalling in vivo (Corrigan et al., 2014). This uses structures in male flies known as the accessory glands, which shares related functions with the human prostate glands. We have shown that the exosomes produced by male flies following mating can fuse to sperm (as has been reported for human prostate exosomes) and exert physiological effects on the behaviour of the female flies, preventing them from re-mating, and therefore promoting the first male's reproductive interests.
This uses the fly accessory gland, one cell type in this gland, the secondary cell, shares related functions with the human male prostate gland. Remarkably, exosomes from these cells are able to reprogramme female behaviour, so mated females will reject other males, therefore promoting the first male's reproductive interests. This novel system in which membrane trafficking events involved in mTORC1 control and exosome biogenesis can be investigated at remarkably high subcellular resolution, is providing new insights in to several cancer mechanisms.
The fly exosome secretion model is enabling us to analyse the membrane trafficking events involved in mTORC1 control and exosome biogenesis in more detail than previously possible in vivo. This is facilitated by analysis of exosome-secreting cells with particularly large multivesicular bodies, which can be imaged in living tissue. This is providing new insights into several cancer mechanisms. By using a model hopping approach involving both flies and cell-based assays, coupled with clinical collaborations within the CRUK Oxford Centre, we are starting to better understand the regulation and roles of exosomes in cancer progression. This has led to the recent award of a CRUK Programme grant to study the control of exosome secretion by mTORC1.
Prof Katharine E Carr