Alejo Efeyan

mTOR: from growth signal integration to cancer, diabetes and ageing

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing.

Tumour biology: Senescence in premalignant tumours

Oncogene-induced senescence is a cellular response that may be crucial for protection against cancer development, but its investigation has so far been restricted to cultured cells that have been manipulated to overexpress an oncogene. Here we analyse tumours initiated by an endogenous oncogene, ras, and show that senescent cells exist in premalignant tumours but not in malignant ones. Senescence is therefore a defining feature of premalignant tumours that could prove valuable in the diagnosis and prognosis of cancer.

Nutrient-sensing mechanisms and pathways

The ability to sense and respond to fluctuations in environmental nutrient levels is a requisite for life. Nutrient scarcity is a selective pressure that has shaped the evolution of most cellular processes. Different pathways that detect intracellular and extracellular levels of sugars, amino acids, lipids and surrogate metabolites are integrated and coordinated at the organismal level through hormonal signals. During food abundance, nutrient-sensing pathways engage anabolism and storage, whereas scarcity triggers homeostatic mechanisms, such as the mobilization of internal stores through autophagy. Nutrient-sensing pathways are commonly deregulated in human metabolic diseases.

mTOR and cancer: many loops in one pathway

The mammalian target of rapamycin (mTOR) is a master regulator of cell growth and division that responds to a variety of stimuli, including nutrient, energy, and growth factors. In the last years, a significant number of pieces have been added to the puzzle of how mTOR coordinates and executes its functions. Extensive research on mTOR has also uncovered a complex network of regulatory loops that impact the therapeutic approaches aimed at targeting mTOR.

p53: guardian of the genome and policeman of the oncogenes.

The process of malignant transformation universally entails genetic damage and oncogenic signaling, two stresses that are signaled to p53 through different genetic pathways. Based on this, it is possible to distinguish two jobs for p53: “guardian of the genome” that consists in sensing and reacting to DNA damage through the ATM/ATR and Chk1/Chk2 kinases, and “policeman of the oncogenes” that, correspondingly, consists in responding to oncogenic signaling through the p53-stabilizing protein ARF. Contrary to expectation, recent genetic evidence in mice indicates that the response of p53 to DNA damage has little or no impact on cancer protection. In contrast, ARF-dependent activation of p53 is critical for p53-mediated tumor suppression. Here, we discuss the mechanistic implications of these observations and their relevance for cancer therapy.

Amino acids and mTORC1: from lysosomes to disease

The mechanistic target of rapamycin (mTOR) kinase controls growth and metabolism, and its deregulation underlies the pathogenesis of many diseases, including cancer, neurodegeneration, and diabetes. mTOR complex 1 (mTORC1) integrates signals arising from nutrients, energy, and growth factors, but how exactly these signals are propagated await to be fully understood. Recent findings have placed the lysosome, a key mediator of cellular catabolism, at the core of mTORC1 regulation by amino acids. A multiprotein complex that includes the Rag GTPases, Ragulator, and the v-ATPase forms an amino acid-sensing machinery on the lysosomal surface that affects the decision between cell growth and catabolism at multiple levels. The involvement of a catabolic organelle in growth signaling may have important implications for our understanding of mTORC1-related pathologies.

Regulation of mTORC1 by the Rag GTPases is necessary for neonatal autophagy and survival

The mechanistic target of rapamycin complex 1 (mTORC1) pathway regulates organismal growth in response to many environmental cues, including nutrients and growth factors. Cell-based studies showed that mTORC1 senses amino acids through the RagA–D family of GTPases (also known as RRAGA, B, C and D), but their importance in mammalian physiology is unknown. Here we generate knock-in mice that express a constitutively active form of RagA (RagAGTP) from its endogenous promoter. RagAGTP/GTP mice develop normally, but fail to survive postnatal day 1. When delivered by Caesarean section, fasted RagAGTP/GTP neonates die almost twice as rapidly as wild-type littermates. Within an hour of birth, wild-type neonates strongly inhibit mTORC1, which coincides with profound hypoglycaemia and a decrease in plasma amino-acid concentrations. In contrast, mTORC1 inhibition does not occur in RagAGTP/GTP neonates, despite identical reductions in blood nutrient amounts. With prolonged fasting, wild-type neonates recover their plasma glucose concentrations, but RagAGTP/GTP mice remain hypoglycaemic until death, despite using glycogen at a faster rate. The glucose homeostasis defect correlates with the inability of fasted RagAGTP/GTP neonates to trigger autophagy and produce amino acids for de novo glucose production. Because profound hypoglycaemia does not inhibit mTORC1 in RagAGTP/GTP neonates, we considered the possibility that the Rag pathway signals glucose as well as amino-acid sufficiency to mTORC1. Indeed, mTORC1 is resistant to glucose deprivation in RagAGTP/GTP fibroblasts, and glucose, like amino acids, controls its recruitment to the lysosomal surface, the site of mTORC1 activation. Thus, the Rag GTPases signal glucose and amino-acid concentrations to mTORC1, and have an unexpectedly key role in neonates in autophagy induction and thus nutrient homeostasis and viability.

Pten Positively Regulates Brown Adipose Function, Energy Expenditure, and Longevity

Aging in worms and flies is regulated by the PI3K/Akt/Foxo pathway. Here we extend this paradigm to mammals. Pten(tg) mice carrying additional genomic copies of Pten are protected from cancer and present a significant extension of life span that is independent of their lower cancer incidence. Interestingly, Pten(tg) mice have an increased energy expenditure and protection from metabolic pathologies. The brown adipose tissue (BAT) of Pten(tg) mice is hyperactive and presents high levels of the uncoupling protein Ucp1, which we show is a target of Foxo1. Importantly, a synthetic PI3K inhibitor also increases energy expenditure and hyperactivates the BAT in mice. These effects can be recapitulated in isolated brown adipocytes and, moreover, implants of Pten(tg) fibroblasts programmed with Prdm16 and Cebpβ form subcutaneous brown adipose pads more efficiently than wild-type fibroblasts. These observations uncover a role of Pten in promoting energy expenditure, thus decreasing nutrient storage and its associated damage.

Tumour biology: Policing of oncogene activity by p53.

The tumour-suppressor protein p53 provides the most important genetic defence against cancer and is activated in response to DNA damage and to oncogenic signalling, both of which occur almost universally in malignant tumours. But the relative contribution of these two pathways in inducing p53-dependent protection against cancer is unclear. Here we show that p53-dependent protection against cancer is lost in mice that have been genetically manipulated so that their p53 is activated in response to DNA damage but not to oncogenic signalling. We conclude that oncogenic signalling is the critical event that elicits p53-dependent protection and that the DNA-damage stimulus is less important.

Induction of p53-Dependent Senescence by the MDM2 Antagonist Nutlin-3a in Mouse Cells of Fibroblast Origin

Cellular senescence is emerging as an important in vivo anticancer response elicited by multiple stresses, including currently used chemotherapeutic drugs. Nutlin-3a is a recently discovered small-molecule antagonist of the p53-destabilizing protein murine double minute-2 (MDM2) that induces cell cycle arrest and apoptosis in cancer cells with functional p53. Here, we report that nutlin-3a induces cellular senescence in murine primary fibroblasts, oncogenically transformed fibroblasts, and fibrosarcoma cell lines. No evidence of drug-induced apoptosis was observed in any case. Nutlin-induced senescence was strictly dependent on the presence of functional p53 as revealed by the fact that cells lacking p53 were completely insensitive to the drug, whereas cells lacking the tumor suppressor alternative reading frame product of the CDKN2A locus underwent irreversible cell cycle arrest. Interestingly, irreversibility was achieved in neoplastic cells faster than in their corresponding parental primary cells, suggesting that nutlin-3a and oncogenic signaling cooperate in activating p53. Our current results suggest that senescence could be a major cellular outcome of cancer therapy by antagonists of the p53-MDM2 interaction, such as nutlin-3a.