Antagonism of PI 3-kinase signalling by PTEN and SHIP2

The human body is made up of cells which form the tissues and organs. The different tissues of the body have developed to perform specialised functions which must be coordinated for the organism as a whole to function efficiently and survive. Cell signalling is the process by which cells in our bodies communicate with one another and signal transduction is the means by which a specific signal (perhaps a hormone, such as insulin), arriving at its target tissue, is interpreted to elicit a particular response. Defects in cell signalling are common causes of important human diseases such as cancer and diabetes. We are studying the details of a signal transduction process which malfunctions in more than 50% of human tumours and which accounts for many of the effects of insulin produced after a meal. The central character of this response is a fatty substance or lipid called PIP3 which is made by enzymes called PI 3-kinases. When this substance is produced at the correct time, in the right part of the cell and in small, but sufficient amounts it triggers normal cell responses, but too much, in the wrong place or at an inappropriate time can lead to or promote the development of a tumour. On the other hand, producing too little in response to insulin can be a cause of diabetes. Maintaining this delicate balance of PIP3 involves the PI 3-kinase enzymes which make PIP3 and enzymes called phosphatases which remove it. We are studying the factors which regulate two classes of PIP3 phosphatase called PTEN and SHIP2. PTEN is a tumour suppressor that is mutated or absent in many different kinds of human tumour. We use tissue culture cells to study PTEN and SHIP2 regulation and epithelial cells (the source of most solid tumours) as simple models of disease to examine the consequences of defects in PIP3 phosphatase activity. Lastly, we are developing unique mouse models harbouring specific defects in the PTEN gene to validate the physiological significance of our work using cultured cells. Because of its importance in human disease many pharmaceutical companies are developing drugs which block the PI 3-kinase signalling pathway including inhibitors, currently in clinical trials as anti-cancer or anti-inflammatory agents, of PI 3-kinases themselves. My group has a longstanding, active collaboration with a consortium of 5 international companies to accelerate their drug discovery endeavours in this field.

Class I phosphoinositide 3-kinases (PI 3-kinases) synthesise phosphatidylinositol 3,4,5-trisphosphate (PtdInsP3), a lipid second messenger which controls cell growth, motility, survival and metabolism in response to a wide variety of hormonal and environmental cues. Defects in this regulatory system contribute to the pathology of important human diseases including many forms of cancer, type II diabetes and inflammatory disorders. The integrity of PI 3-kinase signalling depends upon the correct temporal and spatial distribution of inositol lipid second messengers determined by the regulation and targeting of Class I PI 3-kinases and PtdInsP3 phosphatases. Moreover, the metabolism of PtdInsP3 serves at least two distinct purposes. Firstly, the termination of PtdInsP3 signalling via removal of the 3-phosphate by the tumour suppressor phosphatase, PTEN. Secondly, the co-ordinated synthesis of additional lipid signals, including phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2) and phosphatidylinositol 3-phosphate via removal of the 5-phosphate by one or more members of the phosphoinositide 5-phosphatase family such as SHIP and SHIP2. Whilst the functions and molecular targets of PtdInsP3 are well understood, the relative contributions of PtdInsP3 and/or PtdIns(3,4)P2 to PI 3-kinase signalling and which, if any of these functions are driven primarily by PtdIns(3,4)P2 remain ill-defined. We aim to develop our understanding of the distinct biological functions of PtdInsP3 metabolism under four thematic headings. (i) How do cells exert independent temporal and spatial control of PtdInsP3 versus PtdIns(3,4)P2 when the latter is a metabolic product of the former? (ii) The identification, characterisation and biological functions of PtdIns(3,4)P2 effector proteins. (iii) The roles of PTEN and SHIP2 in the development and maintenance of cell polarity in epithelial cells and other relevant models. How does loss of PTEN or oncogenic mutation of p110alpha affect PtdInsP3 distribution and cell polarity? (iv) What, if any, is the role of PTEN s specificity for both PtdInsP3 and phosphopeptide substrates in mediating its tumour suppressor function? Our approach is multidisciplinary, involving molecular characterisation of expressed and purified components; studies of endogenous and exogenously expressed components in cell lines, 3D epithelial cell cultures and other model systems; and the generation of unique mouse models arising from original discoveries in our laboratory concerning substrate recognition by PTEN. We will continue to collaborate with major pharmaceutical companies via the Division of Signal Transduction Therapy of which CPD is a co-director and which aims to accelerate drug discovery in the area of protein and lipid kinases and phosphatases.