Lab Head: Nick Leslie Institute of Biological Chemistry, Biophysics and Bioengineering

PTEN's mechanisms of action

PTEN: both a lipid and a protein phosphatase

For many years, PTEN has been known to have both lipid and protein phosphatase activity in vitro. We know that PTEN's evolutionarily conserved lipid phosphatase activity plays a key role in the regulation of many cell processes through suppressing PI 3-kinase dependent signalling. However, the significance of PTEN's protein phosphatase activity is unclear. To study this, for some years we and other groups have been using a PTEN mutant (PTEN G129E) that has protein, but not lipid phosphatase activity. More recently we have developed a converse mutant of PTEN that has lipid, but not protein, phosphatase activity (PTEN Y138L). Much of our current work is using these mutants to distinguish between the contributions of the lipid and protein phosphatase activities of PTEN to its cellular functions. Our initial data in cultured glioblastoma cells show that whereas the regulation of cell proliferation is dominated by the lipid phosphatase activity of PTEN, in contrast, the inhibition of cell invasion by PTEN requires both activities working together (see Figure and Davidson et al) in a manner that correlates closely with tumour suppression independently of AKT (Tibarewal et al, 2012).

Inhibitors of PI3K, Akt and downstream components, such as mTOR are being developed as drugs to treat cancers, especially those that have lost PTEN function. Our aim therefore is to provide a clearer picture of the mechanisms by which PTEN acts and the signalling pathways that it controls in its roles as a tumour suppressor.

PTEN and epithelial tissue architecture

Multicellular organisms require their cells to communicate and assemble cooperatively into the correct tissue structures. Conversely, the development of almost all solid tumours is accompanied by a loss of tissue architecture, and in some cases this loss of structure appears to be a central process driving tumour growth, rather than its by-product. Experimentally these events can be studied using 3D cell culture techniques in which epithelial cells can be induced to form both hollow spheres and tubules.

For some years our lab has been studying PTEN, which is a lipid phosphatase and important tumour suppressor. Knocking down PTEN expression in 3D culture by interfering RNA has dramatic effects on cell proliferation and tissue morphology that are not evident in standard 2D culture. We are studying these effects, addressing cell signalling events, and cell polarity and architecture in these systems. Within this work, efforts are going into identifying PTEN binding proteins involved in controlling and localizing PTEN within these polarized cells. Importantly, this work shows that the ability of PTEN to control 3D architecture in these mammary epithelial cells also requires both its lipid and protein phosphatase activities and correlates with tumour suppression, but not AKT regulation (Berglund et al 2013).