Harnessing senescence to control PI3K/AKT/mTORC pathway-driven cancer through the metabolic intervention

Abstract

Oncogene-induced senescence (OIS) is a critical anti-tumour mechanism by arresting cell proliferation following oncogenic activation in normal cells. Cell cycle withdrawal along with senescence-associated secretory phenotype, deregulated metabolism and macromolecular changes are the hallmarks of cellular senescence. The PI3K/AKT/mTORC1 pathway is a key signalling pathway for homeostatic control of cell growth, proliferation, and survival. Molecular aberrations in the PI3K/AKT/mTORC1 signalling pathway have been identified in one third of solid tumours and more than 40 cancer types. Paradoxically, PI3K/AKT/mTORC1 hyperactivation causes a senescence-like phenotype in non-transformed cells. While most studies have focused on RAS/MAPK-induced senescence (RIS), little is known about the mechanisms regulating AKT-induced senescence (AIS) and its potential tumour-suppressive function. We hypothesise that understanding the molecular requirements for AIS and determining the essential regulators involved in AIS will identify therapeutic vulnerabilities for treatment of PI3K/AKT/mTORC1-driven cancer.

To characterise the molecular landscape of AIS, we performed the RNA-seq and metabolomics analysis in BJ-TERT human skin fibroblasts overexpressing AKT1 or HRASG12V in Chapter 3. Compared to normal proliferating cells, AIS and RIS cells exhibited distinct metabolomes and transcriptomes, supporting metabolic and transcriptional reprogramming in OIS. The common molecular signatures in both AIS and RIS cells well represent the hallmarks of senescence and the molecular signatures that are unique for AIS or RIS are associated with the distinct molecular mechanisms underpinning these two types of OIS. The similar metabolic profile presented by the AIS and RIS cells with enhanced glycolysis, TCA cycle and oxidative phosphorylation to maximise energy production indicates a highly active metabolic status in OIS.

To gain more insight of metabolic dependency of AIS, we utilised Cystathionine beta-Synthase (CBS), one of the top candidates in an RNAi screen we recently performed to identify key regulators of AIS and characterised its role in AIS maintenance in chapter 4. CBS acts as a hub for coordinating the transsulfuration and transmethylation pathways, regulating the biosynthesis of hydrogen sulfide (H2S) and glutathione (GSH) and modulating mitochondrial functions and cellular bioenergetics. We demonstrated that 1) the upregulation of transsulfuration pathway activity is an AIS-specific antioxidant response. 2) the metabolic reprogramming potentially couples to epigenetic alterations to maintain AIS. 3) CBS depletion releases oxidative stress through suppression of mitochondrial oxidative phosphorylation and causes AIS escape. The finding of loss of CBS resulting in the release of the senescence brake engaged by AKT hyperactivation promoted us to further evaluate the potential tumour suppressor role of CBS in gastric cancer. We provided in vitro and in vivo evidence to support that CBS depletion synergises with PI3K pathway activation to promote gastric cancer initiation and restoration of CBS inhibited gastric tumour growth via H2S. These findings provided an example of harnessing metabolic vulnerabilities therapeutically for treating cancer.

Overall, our study provides novel mechanistic insights into AIS maintenance and more importantly, reveals the essential role of the metabolic reprogramming in AIS maintenance and escape. Further investigation of the molecular mechanisms underpinning PI3K/AKT/mTORC1-driven senescence and tumorigenesis will facilitate developing appropriate strategies to improve human health in aging, aging-related diseases, and cancer.