Targeting PPP1R15 in malignancy

The goal of cancer therapy is to kill malignant cells with no or minimal harm to normal tissue. This can be achieved by identifying and targeting features specific to the cancer cell. A recurring theme in cancer biology is the inactivation of processes that would normally cause cancer cell death. Strategies that reactive these processes are therefore promising targets for the development of new treatments. Because cancer cells commonly make high levels of protein they are prone to accumulating incorrectly synthesised (so called 'misfolded') protein. The accumulation of misfolded protein in an a part of the cell called the endoplasmic reticulum (ER) leads to a toxic state called 'ER stress'. This reduces protein synthesis in the cell and activation of protective processes following the triggering of an ER stress sensor called PERK. PERK appears to be important in cancer progression because cancer cells lacking PERK form smaller tumours, while drugs that block PERK are showing great promise in research studies as anti-cancer agents. A protein called PPP1R15A normally opposes the action of PERK and we previously showed that PPP1R15A is responsible for some of the cell death caused by ER stress. Consequently, cells that have lost PPP1R15A experience more protective by PERK and we recently reported that highly aggressive forms of a cancer called malignant mesothelioma tend to lose PPP1R15A, possibly increasing the cancers resistance to ER stress.

In this project we will explore the potential for targeting PPP1R15A in the treatment of cancer. We have recently reported that PPP1R15A is regulated by the abundance of a protein called G-actin, the levels of which are sensitive to cellular growth signals and to coll movement. We will determine how the interaction between PPP1R15A and G-actin in regulated in health and whether manipulation of this interaction can increase PPP1R15A activity in the cell, since this could potentially increase the effectiveness of anti-cancer PERK inhibitors. PPP1R15A is an unstable protein that is efficiently degraded within the cells. We will identify the cellular machinery that causes PPP1R15A degradation, since inhibition of this machinery could restore PPP1R15A levels and so increase ER stress-induced cancer cell killing. Remarkably, the precise mechanism by which PPP1R15A leads to cell toxicity remains unclear. PPP1R15A can bind to cellular components including the ER, lipid droplets and mitochondria, all of which are crucial for cellular survival. By determining the effect of PPP1R15A binding on the function of these cellular structures we will better understand the toxic effects of this protein.

Together, these studies will enable us to understand the mechanism and functional consequences of PPP1R15A loss from cancer cells. This will be important in the development of anticancer therapies that target ER stress.

Deregulated protein synthesis is a feature of cancer and can lead to endoplasmic reticulum (ER) stress. A rectifying response to ER stress involves the phosphorylation of eIF2a by PERK, which inhibits new protein synthesis while triggering a gene expression programme through enhanced translation of ATF4. A target of ATF4 is PPP1R15A, which dephosphorylates eIF2a to restore normal translation. We showed that PPP1R15A contributes to cell death during unremitting ER stress by promoting accumulation of misfolded protein. Phosphorylation of eIF2a has a permissive effect on tumour growth and so PERK inhibition has emerged as a novel anticancer strategy. We observed the loss of PPP1R15A by some cancers leading us to hypothesise that restoration of PPP1R15A activity could serve to oppose PERK and potentiate the anticancer effects of PERK inhibitors.

Aim 1. Elucidate the physiological regulation of PPP1R15A: We demonstrated that PPP1R15A is regulated by the binding of G-actin. The abundance of G-actin is determined by cytoskeletal dynamics that are themselves regulated by growth and mobility cues. We will will determine how these cues areguate eIF2a phosphatase activity in health, so that we might target these pathways to augment PPP1R15A activity in disease.
Aim 2. Elucidate the regulation of PPP1R15A stability: PPP1R15A has a half-life of <4 hours. We will identify the degradation machinery responsible for this, since inhibiting PPP1R15A degradation could also promote eIF2a phosphatase activity.
Aim 3. Elucidate the role of PPP1R15 membrane binding: Binding to the ER, lipid droplets and mitochondria is a prominent but poorly understood feature of the PPP1R15 proteins. We will determine the effect such binding on the function of the organelles and on the activity and stability of PPP1R15.

By understanding the regulation, stability and membrane association of PPP1R15A we will be able to develop strategies to control its activity in diseases including cancer.

The main outcomes of this research will be:
(a) a new understanding of the role of PPP1R15A and the integrated stress response, the beneficiaries of which include researchers in the broad areas of cell and cancer biology as well as those researching integrated stress and related diseases (such as diabetes and neurodegenerative disorders) but especially those studying stress signalling in malignancy;
(b) extensive research training for the post-doctoral researcher, which will include molecular biology techniques, cell biology, murine models of disease and biochemistry. The practical skills as well as key transferable skills such as work planning, teamwork and project management that the post-doc will acquire will enable them to develop towards being an independent research scientist. The University of Cambridge offers additional courses on bioinformatics, information technology skills, and generic transferable skills.