p62 is a key regulator of nutrient sensing in the mTORC1 pathway

Rags connect mTOR and autophagy

mTOR, the master regulator of protein metabolism, is activated by growth factor signaling, amino acids and other nutrients. Emerging evidence indicates that an unexpected physical association between mTOR and lysosomes plays a critical role in amino acid mediated mTOR activation. Rag GTPases, together with a multi-protein complex called Ragulator, mediate amino acid-mediated mTOR recruitment to the lysosome surface where mTOR becomes activated. Furthermore, mTOR is also recruited to a unique cytoplasmic compartment composed of autolysosomes, which is observed in oncogenic Ras-induced senescent (RIS) cells. Formation of this TOR-autophagy spatial coupling compartment (TASCC) seems to allow activation of mTOR and autophagy in a mutually reinforcing manner. Proper formation of the TASCC also requires active Rag proteins. Interestingly, inhibition of activity of Rag proteins also suppresses acute induction of secretory protein synthesis during RIS. Thus, the TASCC provides evidence for the functional relevance of the Rag-mediated association between lysosomes and mTOR, and provides a mechanism for the simultaneous activation of anabolic and catabolic processes.

Guidelines for the use and interpretation of assays for monitoring autophagy

In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process vs. those that measure flux through the autophagy pathway (i.e., the complete process); thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from stimuli that result in increased autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field.

Excessive Leucine-mTORC1-Signalling of Cow Milk-Based Infant Formula: The Missing Link to Understand Early Childhood Obesity

Increased protein supply by feeding cow-milk-based infant formula in comparison to lower protein content of human milk is a well-recognized major risk factor of childhood obesity. However, there is yet no conclusive biochemical concept explaining the mechanisms of formula-induced childhood obesity. It is the intention of this article to provide the biochemical link between leucine-mediated signalling of mammalian milk proteins and adipogenesis as well as early adipogenic programming. Leucine has been identified as the predominant signal transducer of mammalian milk, which stimulates the nutrient-sensitive kinase mammalian target of rapamycin complex 1 (mTORC1). Leucine thus functions as a maternal-neonatal relay for mTORC1-dependent neonatal β-cell proliferation and insulin secretion. The mTORC1 target S6K1 plays a pivotal role in stimulation of mesenchymal stem cells to differentiate into adipocytes and to induce insulin resistance. It is of most critical concern that infant formulas provide higher amounts of leucine in comparison to human milk. Exaggerated leucine-mediated mTORC1-S6K1 signalling induced by infant formulas may thus explain increased adipogenesis and generation of lifelong elevated adipocyte numbers. Attenuation of mTORC1 signalling of infant formula by leucine restriction to physiologic lower levels of human milk offers a great chance for the prevention of childhood obesity and obesity-related metabolic diseases.

p62: a versatile multitasker takes on cancer

Since its initial discovery as an atypical protein kinase C (PKC)-interacting protein, p62 has emerged as a crucial molecule in a myriad of cellular functions. This multifunctional role of p62 is explained by its ability to interact with several key components of various signaling mechanisms. Not surprisingly, p62 is required for tumor transformation owing to its roles as a key molecule in nutrient sensing, as a regulator and substrate of autophagy, as an inducer of oxidative detoxifying proteins, and as a modulator of mitotic transit and genomic stability; all crucial events in the control of cell growth and cancer.

Leucine signaling in the pathogenesis of type 2 diabetes and obesity

Epidemiological evidence points to increased dairy and meat consumption, staples of the Western diet, as major risk factors for the development of type 2 diabetes (T2D). This paper presents a new concept and comprehensive review of leucine-mediated cell signaling explaining the pathogenesis of T2D and obesity by leucine-induced over-stimulation of mammalian target of rapamycin complex 1 (mTORC1). mTORC1, a pivotal nutrient-sensitive kinase, promotes growth and cell proliferation in response to glucose, energy, growth factors and amino acids. Dairy proteins and meat stimulate insulin/insulin-like growth factor 1 signaling and provide high amounts of leucine, a primary and independent stimulator for mTORC1 activation. The downstream target of mTORC1, the kinase S6K1, induces insulin resistance by phosphorylation of insulin receptor substrate-1, thereby increasing the metabolic burden of β-cells. Moreover, leucine-mediated mTORC1-S6K1-signaling plays an important role in adipogenesis, thus increasing the risk of obesity-mediated insulin resistance. High consumption of leucine-rich proteins explains exaggerated mTORC1-dependent insulin secretion, increased β-cell growth and β-cell proliferation promoting an early onset of replicative β-cell senescence with subsequent β-cell apoptosis. Disturbances of β-cell mass regulation with increased β-cell proliferation and apoptosis as well as insulin resistance are hallmarks of T2D, which are all associated with hyperactivation of mTORC1. In contrast, the anti-diabetic drug metformin antagonizes leucine-mediated mTORC1 signaling. Plant-derived polyphenols and flavonoids are identified as natural inhibitors of mTORC1 and exert anti-diabetic and anti-obesity effects. Furthermore, bariatric surgery in obesity reduces increased plasma levels of leucine and other branched-chain amino acids. Attenuation of leucine-mediated mTORC1 signaling by defining appropriate upper limits of the daily intake of leucine-rich animal and dairy proteins may offer a great chance for the prevention of T2D and obesity, as well as other epidemic diseases of civilization with increased mTORC1 signaling, especially cancer and neurodegenerative diseases, which are frequently associated with T2D.

Amino acid sensing and regulation of mTORC1

Amino acids play fundamental roles in the cell both as the building blocks of new proteins and as metabolic precursors. To adapt to their limitation during periods of protein starvation, multiple adaptive mechanisms have evolved, including a rapid cessation of new protein synthesis, an increase in amino acid biosynthesis and transport, and autophagy. Here, we discuss what we currently know about how amino acid limitation is sensed, and how this sensing might be transmitted to mTORC1 to regulate protein synthesis and autophagy.

Sequestosome 1/p62: across diseases

Sequestosome 1/p62 is a signal modulator or adaptor protein involved in receptor-mediated signal transduction. Sequestosome 1/p62 is gaining attention as it is involved in several diseases including Parkinson disease, Alzheimer disease, liver and breast cancer, Paget's disease of bone, obesity and insulin resistance. In this review, we will focus on the most recent advances on the physiological function of p62 relevant to human diseases.

Feedback on fat: p62-mTORC1-autophagy connections

Metabolic homeostasis requires integration of multiple signals and cellular activities. Without this integration, conditions of obesity and diabetes often develop. Recent in vivo studies explore the molecular basis for metabolic homestasis, showing that p62 links autophagy and mTORC1 activation to regulate adipogenesis and energy control.

A new link in the chain from amino acids to mTORC1 activation

A recent study reveals that the scaffold protein p62 plays a role in linking nutritional cues (amino acids) to the activation of mammalian target of rapamycin complex 1 (mTORC1), a protein kinase that controls cell size and proliferation.

Development and characterization of gemcitabine-resistant pancreatic tumor cells

BACKGROUND

Pancreatic cancer is an exceptionally lethal disease with an annual mortality nearly equivalent to its annual incidence. This dismal rate of survival is due to several factors including late presentation with locally advanced, unresectable tumors, early metastatic disease, and rapidly arising chemoresistance. To study the mechanisms of chemoresistance in pancreatic cancer we developed two gemcitabine-resistant pancreatic cancer cell lines.

METHODS

Resistant cells were obtained by culturing L3.6pl and AsPC-1 cells in serially increasing concentrations of gemcitabine. Stable cultures were obtained that were 40- to 50-fold increased in resistance relative to parental cells. Immunofluorescent staining was performed to examine changes in beta-catenin and E-cadherin localization. Protein expression was determined by immunoblotting. Migration and invasion were determined by modified Boyden chamber assays. Fluorescence-activated cell sorting (FACS) analyses were performed to examine stem cell markers.

RESULTS

Gemcitabine-resistant cells underwent distinct morphological changes, including spindle-shaped morphology, appearance of pseudopodia, and reduced adhesion characteristic of transformed fibroblasts. Gemcitabine-resistant cells were more invasive and migratory. Gemcitabine-resistant cells were increased in vimentin and decreased in E-cadherin expression. Immunofluorescence and immunoblotting revealed increased nuclear localization of total beta-catenin. These alterations are hallmarks of epithelial-to-mesenchymal transition (EMT). Resistant cells were activated in the receptor protein tyrosine kinase, c-Met and increased in expression of the stem cell markers CD (cluster of differentiation)24, CD44, and epithelial-specific antigen (ESA).

CONCLUSIONS

Gemcitabine-resistant pancreatic tumor cells are associated with EMT, a more-aggressive and invasive phenotype in numerous solid tumors. The increased phosphorylation of c-Met may also be related to chemoresistance and EMT and presents as an attractive adjunctive chemotherapeutic target in pancreatic cancer.