mTOR and autophagy: a dynamic relationship governed by nutrients and energy.

Mechanistic target of rapamycin (mTOR) functions as a key homeostatic regulator of cell growth and orchestrates whether anabolic or catabolic reactions are favoured. mTOR complex 1 (mTORC1) manages multiple biosynthetic pathways and promotes cell growth when nutrients are in plentiful supply. Many advances have been made over the last decade on nutrient sensing centred on mTORC1. Recent research reveals that mTORC1 maintains nutrient homeostasis through lysosomal biogenesis and autophagic processes. Cells utilise autophagy to recycle damaged or unwanted organelles and macromolecules and in so doing, generate energy and recover precursor building blocks necessary for normal growth. It is clear that mTOR and autophagy are closely integrated within cells, where defects in signalling through both pathways are known to drive the onset of a range of human diseases, such as cancer and neurodegenerative disease. This review focuses on the dynamic signalling interplay between mTOR and autophagy, which is governed by a core set of proteins that sense nutrients at lysosomal membranes.

Rab35-dependent extracellular nanovesicles are required for induction of tumour supporting stroma.

Communication between diseased cells and the microenvironment is a complex yet crucial element in progression of varied pathological processes. Recent studies in cancer highlight an important role for small extracellular nanovesicles secreted by cancer cells as modulators of cancer-associated stroma, leading to enhanced angiogenesis and metastatic priming. The intrinsic factors regulating extracellular nanovesicle biogenesis and secretion are therefore relevant in studies of nano-communication in the cancer milieu. We generated prostate cancer cells bearing stable knockdown of several candidate vesicle regulating factors and examined the impact on cell health, vesicle secretion and on communication with fibroblastic stromal cells. We highlight that RAB11B and RAB35 regulate phenotypically distinct nanovesicle populations, each accounting for only around 20% of the total. Depleting RAB35, but not RAB11B leaves a remaining population of vesicles whose phenotype is insufficient for driving fibroblast to myofibroblast differentiation, leading to attenuated motile behaviours in 3D in vitro models. Co-implantation of tumour cells with stromal fibroblasts in xenografts similarly showed that RAB11B knockdown had little effect on growth rates in vivo. In contrast, significant attenuation in growth, and attenuation of myofibroblasts at the tumour site was evident when using RAB35-knockdown cells. The study concludes that a RAB35 regulated nanovesicle sub-population is particularly important for communication between cancer and stromal cells, and is required for generating a tumour-supportive microenvironment.

Absence of the Birt-Hogg-Dubé gene product is associated with increased hypoxia-inducible factor transcriptional activity and a loss of metabolic flexibility.

Under conditions of reduced tissue oxygenation, hypoxia-inducible factor (HIF) controls many processes, including angiogenesis and cellular metabolism, and also influences cell proliferation and survival decisions. HIF is centrally involved in tumour growth in inherited diseases that give rise to renal cell carcinoma (RCC), such as Von Hippel-Lindau syndrome and tuberous sclerosis complex. In this study, we examined whether HIF is involved in tumour formation of RCC in Birt-Hogg-Dubé syndrome. For this, we analysed a Birt-Hogg-Dubé patient-derived renal tumour cell line (UOK257) that is devoid of the Birt-Hogg-Dubé protein (BHD) and observed high levels of HIF activity. Knockdown of BHD expression also caused a threefold activation of HIF, which was not as a consequence of more HIF1α or HIF2α protein. Transcription of HIF target genes VEGF, BNIP3 and CCND1 was also increased. We found nuclear localization of HIF1α and increased expression of VEGF, BNIP3 and GLUT1 in a chromophobe carcinoma from a Birt-Hogg-Dubé patient. Our data also reveal that UOK257 cells have high lactate dehydrogenase, pyruvate kinase and 3-hydroxyacyl-CoA dehydrogenase activity. We observed increased expression of pyruvate dehydrogenase kinase 1 (a HIF gene target), which in turn leads to increased phosphorylation and inhibition of pyruvate dehydrogenase. Together with increased protein levels of GLUT1, our data reveal that UOK257 cells favour glycolytic rather than lipid metabolism (a cancer phenomenon termed the 'Warburg effect'). UOK257 cells also possessed a higher expression level of the L-lactate influx monocarboxylate transporter 1 and consequently utilized L-lactate as a metabolic fuel. As a result of their higher dependency on glycolysis, we were able to selectively inhibit the growth of these UOK257 cells by treatment with 2-deoxyglucose. This work suggests that targeting glycolytic metabolism may be used therapeutically to treat Birt-Hogg-Dubé-associated renal lesions.

Mammalian target of rapamycin complex 1: signalling inputs, substrates and feedback mechanisms.

The mammalian target of rapamycin (mTOR) signalling pathway is implicated in the pathogenesis of a number of cancers and inherited hamartoma syndromes which have led to mTOR inhibitors, such as rapamycin, being tested in clinical trials. Knowledge of the mTOR pathway is rapidly expanding. This review provides an update on the most recent additions to the mTOR pathway with particular emphasis on mTORC1 signalling. mTORC1 signalling is classically known for its role in regulating cell growth and proliferation through modulation of protein synthesis. Recent research has identified novel mTORC1 cell signalling mechanisms that modulate mitochondrial biogenesis, hypoxia signalling and cell cycle progression and uncovered novel mTORC1 targets; YY1, HIF and SGK1. It is unsurprising that regulation of mTORC1 is multifaceted with many positive and negative signalling inputs. We discuss the recent advances that have been made to determine the upstream mechanisms that control mTORC1 through hypoxia, energy sensing and nutrient signalling. Also discussed are current findings that have unravelled a series of novel mTORC1-associated proteins that directly control the activity of mTORC1 and include PRAS40, FKBP38, Rag GTPases and RalA.

Induction of signalling in non-erythroid cells by pharmacological levels of erythropoietin.

Erythropoiesis is maintained by the hormone erythropoietin (Epo) binding to its cognate receptor (EpoR) on erythroid progenitor cells. The Epo-EpoR interaction initiates a signal transduction process that regulates the survival, growth and differentiation of these cells. Originally perceived as highly lineage-restricted, Epo is now recognised to have pleiotropic effects extending beyond the maintenance of red cell mass. Functional interactions between Epo and EpoR have been demonstrated in numerous cells and tissues. EpoR expression on neoplastic cells leads to concern that recombinant human erythropoietin, used to treat anaemia in cancer patients, may augment tumour growth. Here we demonstrate that EPO, at pharmacological concentrations, can activate three major signalling cascades, viz. the Jak2/STAT5, Ras/ERK and PI3K/Akt pathways in non-small cell lung carcinoma (NSCLC) cell lines. EpoR synthesis is normally under the control of GATA-1, but NSCLC cells exhibit decreased GATA-1 levels compared to GATA-2, -3 and -6, suggesting that GATA-1 is not essential for EpoR production. The increased Epo-induced signalling was not associated with a growth advantage for the NSCLC cells.