Enhanced SREBP2-driven cholesterol biosynthesis by PKCλ/ι deficiency in intestinal epithelial cells promotes aggressive serrated tumorigenesis

The metabolic and signaling pathways regulating aggressive mesenchymal colorectal cancer (CRC) initiation and progression through the serrated route are largely unknown. Although relatively well characterized as BRAF mutant cancers, their poor response to current targeted therapy, difficult preneoplastic detection, and challenging endoscopic resection make the identification of their metabolic requirements a priority. Here, we demonstrate that the phosphorylation of SCAP by the atypical PKC (aPKC), PKCλ/ι promotes its degradation and inhibits the processing and activation of SREBP2, the master regulator of cholesterol biosynthesis. We show that the upregulation of SREBP2 and cholesterol by reduced aPKC levels is essential for controlling metaplasia and generating the most aggressive cell subpopulation in serrated tumors in mice and humans. Since these alterations are also detected prior to neoplastic transformation, together with the sensitivity of these tumors to cholesterol metabolism inhibitors, our data indicate that targeting cholesterol biosynthesis is a potential mechanism for serrated chemoprevention.

Virulence and Metabolism Crosstalk: Impaired Activity of the Type Three Secretion System (T3SS) in a Pseudomonas aeruginosa Crc-Defective Mutant

Pseudomonas aeruginosa is a ubiquitous nosocomial opportunistic pathogen that harbors many virulence determinants. Part of P. aeruginosa success colonizing a variety of habitats resides in its metabolic robustness and plasticity, which are the basis of its capability of adaptation to different nutrient sources and ecological conditions, including the infected host. Given this situation, it is conceivable that P. aeruginosa virulence might be, at least in part, under metabolic control, in such a way that virulence determinants are produced just when needed. Indeed, it has been shown that the catabolite repression control protein Crc, which together with the RNA chaperon Hfq regulates the P. aeruginosa utilization of carbon sources at the post-transcriptional level, also regulates, directly or indirectly, virulence-related processes in P. aeruginosa. Among them, Crc regulates P. aeruginosa cytotoxicity, likely by modulating the activity of the Type III Secretion System (T3SS), which directly injects toxins into eukaryotic host cells. The present work shows that the lack of Crc produces a Type III Secretion-defective phenotype in P. aeruginosa. The observed impairment is a consequence of a reduced expression of the genes encoding the T3SS, together with an impaired secretion of the proteins involved. Our results support that the impaired T3SS activity of the crc defective mutant is, at least partly, a consequence of a defective protein export, probably due to a reduced proton motive force. This work provides new information about the complex regulation of the expression and the activity of the T3SS in P. aeruginosa. Our results highlight the need of a robust bacterial metabolism, which is defective in the ∆crc mutant, to elicit complex and energetically costly virulence strategies, as that provided by the T3SS.

Hyaluronan driven by epithelial aPKC deficiency remodels the microenvironment and creates a vulnerability in mesenchymal colorectal cancer

Mesenchymal colorectal cancer (mCRC) is microsatellite stable (MSS), highly desmoplastic, with CD8+ T cells excluded to the stromal periphery, resistant to immunotherapy, and driven by low levels of the atypical protein kinase Cs (aPKCs) in the intestinal epithelium. We show here that a salient feature of these tumors is the accumulation of hyaluronan (HA) which, along with reduced aPKC levels, predicts poor survival. HA promotes epithelial heterogeneity and the emergence of a tumor fetal metaplastic cell (TFMC) population endowed with invasive cancer features through a network of interactions with activated fibroblasts. TFMCs are sensitive to HA deposition, and their metaplastic markers have prognostic value. We demonstrate that in vivo HA degradation with a clinical dose of hyaluronidase impairs mCRC tumorigenesis and liver metastasis and enables immune checkpoint blockade therapy by promoting the recruitment of B and CD8+ T cells, including a proportion with resident memory features, and by blocking immunosuppression.

Protein kinase Cλ/ι in cancer: a contextual balance of time and signals

Nononcogenic cancer drivers often impinge on complex signals that create new addictions and vulnerabilities. Protein kinase Cλ/ι (PKCλ/ι) suppresses tumorigenesis by blocking metabolic pathways that regulate fuel oxidation and create building blocks for the epigenetic control of cell differentiation. Reduced levels of PKCλ/ι unleash these pathways to promote tumorigenesis, but the simultaneous activation of the STING-driven interferon cascade prevents tumor initiation by triggering immunosurveillance mechanisms. However, depending on the context of other signaling pathways, such as WNT/β-catenin or PKCζ, and timing, PKCλ/ι deletion can promote or inhibit tumorigenesis. In this review, we discuss in detail the molecular and cellular underpinnings of PKCλ/ι functions in cancer with the perspective of the crosstalk between metabolism and inflammation in the tumor microenvironment.

The lactate-NAD+ axis activates cancer-associated fibroblasts by downregulating p62

Reduced p62 levels are associated with the induction of the cancer-associated fibroblast (CAF) phenotype, which promotes tumorigenesis in vitro and in vivo through inflammation and metabolic reprogramming. However, how p62 is downregulated in the stroma fibroblasts by tumor cells to drive CAF activation is an unresolved central issue in the field. Here we show that tumor-secreted lactate downregulates p62 transcriptionally through a mechanism involving reduction of the NAD⁺/NADH ratio, which impairs poly(ADP-ribose)-polymerase 1 (PARP-1) activity. PARP-1 inhibition blocks the poly(ADP-ribosyl)ation of the AP-1 transcription factors, c-FOS and c-JUN, which is an obligate step for p62 downregulation. Importantly, restoring p62 levels in CAFs by NAD⁺ renders CAFs less active. PARP inhibitors, such as olaparib, mimick lactate in the reduction of stromal p62 levels, as well as the subsequent stromal activation both in vitro and in vivo, which suggests that therapies using olaparib would benefit from strategies aimed at inhibiting CAF activity.

NBR1 is a critical step in the repression of thermogenesis of p62-deficient adipocytes through PPARγ

Activation of non-shivering thermogenesis is considered a promising approach to lower body weight in obesity. p62 deficiency in adipocytes reduces systemic energy expenditure but its role in sustaining mitochondrial function and thermogenesis remains unresolved. NBR1 shares a remarkable structural similarity with p62 and can interact with p62 through their respective PB1 domains. However, the physiological relevance of NBR1 in metabolism, as compared to that of p62, was not clear. Here we show that whole-body and adipocyte-specific ablation of NBR1 reverts the obesity phenotype induced by p62 deficiency by restoring global energy expenditure and thermogenesis in brown adipose tissue. Impaired adrenergic-induced browning of p62-deficient adipocytes is rescued by NBR1 inactivation, unveiling a negative role of NBR1 in thermogenesis under conditions of p62 loss. We demonstrate that upon p62 inactivation, NBR1 represses the activity of PPARγ, establishing an unexplored p62/NBR1-mediated paradigm in adipocyte thermogenesis that is critical for the control of obesity.

Stromal SOX2 Upregulation Promotes Tumorigenesis through the Generation of a SFRP1/2-Expressing Cancer-Associated Fibroblast Population

Cancer-associated fibroblasts (CAFs) promote tumor malignancy, but the precise transcriptional mechanisms regulating the acquisition of the CAF phenotype are not well understood. We show that the upregulation of SOX2 is central to this process, which is repressed by protein kinase Cζ (PKCζ). PKCζ deficiency activates the reprogramming of colonic fibroblasts to generate a predominant SOX2-dependent CAF population expressing the WNT regulator Sfrp2 as its top biomarker. SOX2 directly binds the Sfrp1/2 promoters, and the inactivation of Sox2 or Sfrp1/2 in CAFs impaired the induction of migration and invasion of colon cancer cells, as well as their tumorigenicity in vivo. Importantly, recurrence-free and overall survival of colorectal cancer (CRC) patients negatively correlates with stromal PKCζ levels. Also, SOX2 expression in the stroma is associated with CRC T invasion and worse prognosis of recurrence-free survival. Therefore, the PKCζ-SOX2 axis emerges as a critical step in the control of CAF pro-tumorigenic potential.

PKCλ/ι Loss Induces Autophagy, Oxidative Phosphorylation, and NRF2 to Promote Liver Cancer Progression

Oxidative stress plays a critical role in liver tissue damage and in hepatocellular carcinoma (HCC) initiation and progression. However, the mechanisms that regulate autophagy and metabolic reprogramming during reactive oxygen species (ROS) generation, and how ROS promote tumorigenesis, still need to be fully understood. We show that protein kinase C (PKC) λ/ι loss in hepatocytes promotes autophagy and oxidative phosphorylation. This results in ROS generation, which through NRF2 drives HCC through cell-autonomous and non-autonomous mechanisms. Although PKCλ/ι promotes tumorigenesis in oncogene-driven cancer models, emerging evidence demonstrate that it is a tumor suppressor in more complex carcinogenic processes. Consistently, PKCλ/ι levels negatively correlate with HCC histological tumor grade, establishing this kinase as a tumor suppressor in liver cancer.

Increased Serine and One-Carbon Pathway Metabolism by PKCλ/ι Deficiency Promotes Neuroendocrine Prostate Cancer

Increasingly effective therapies targeting the androgen receptor have paradoxically promoted the incidence of neuroendocrine prostate cancer (NEPC), the most lethal subtype of castration-resistant prostate cancer (PCa), for which there is no effective therapy. Here we report that protein kinase C (PKC)λ/ι is downregulated in de novo and during therapy-induced NEPC, which results in the upregulation of serine biosynthesis through an mTORC1/ATF4-driven pathway. This metabolic reprogramming supports cell proliferation and increases intracellular S-adenosyl methionine (SAM) levels to feed epigenetic changes that favor the development of NEPC characteristics. Altogether, we have uncovered a metabolic vulnerability triggered by PKCλ/ι deficiency in NEPC, which offers potentially actionable targets to prevent therapy resistance in PCa.

Mechanistic insight into the regulation of SQSTM1/p62

SQSTM1/p62 facilitates responses to various cellular stresses and has been implicated in human diseases. This protein functions as a major cytoplasmic signaling hub and has multiple binding partners, including arginylated (Nt-R) proteins that are recognized by the ZZ domain of SQSTM1/p62 (SQSTM1/p62_ZZ). We have determined the molecular mechanism of Nt-R recognition using a combination of biochemical and NMR approaches and obtained the crystal structure of SQSTM1/p62_ZZ in complex with Nt-R. We found that binding of SQSTM1/p62_ZZ to Nt-R induces SQSTM1/p62 puncta formation and macroautophagy/autophagy and identified a regulatory linker (RL) region of SQSTM1/p62 that associates with SQSTM1/p62_ZZ in vitro. Our findings suggest a mechanism for SQSTM1/p62 autoregulation that can be essential in mediating autophagy.

ZZ-dependent regulation of p62/SQSTM1 in autophagy

Autophagic receptor p62 is a critical mediator of cell detoxification, stress response, and metabolic programs and is commonly deregulated in human diseases. The diverse functions of p62 arise from its ability to interact with a large set of ligands, such as arginylated (Nt-R) substrates. Here, we describe the structural mechanism for selective recognition of Nt-R by the ZZ domain of p62 (p62_ZZ). We show that binding of p62_ZZ to Nt-R substrates stimulates p62 aggregation and macroautophagy and is required for autophagic targeting of p62. p62 is essential for mTORC1 activation in response to arginine, but it is not a direct sensor of free arginine in the mTORC1 pathway. We identified a regulatory linker (RL) region in p62 that binds p62_ZZ in vitro and may modulate p62 function. Our findings shed new light on the mechanistic and functional significance of the major cytosolic adaptor protein p62 in two fundamental signaling pathways.

The autophagic receptor p62 recognizes arginylated (Nt-R) substrates through its ZZ domain (p62_ZZ). Here the authors identify a p62 auto regulatory mechanism and provide structural insights into the selective recognition of Nt-R by p62_ZZ and further show that Nt-R binding stimulates p62 oligomerization and macroautophagy.

ATF4-Induced Metabolic Reprograming Is a Synthetic Vulnerability of the p62-Deficient Tumor Stroma

Tumors undergo nutrient stress and need to reprogram their metabolism to survive. The stroma may play a critical role in this process by providing nutrients to support the epithelial compartment of the tumor. Here we show that p62 deficiency in stromal fibroblasts promotes resistance to glutamine deprivation by the direct control of ATF4 stability through its p62-mediated polyubiquitination. ATF4 upregulation by p62 deficiency in the stroma activates glucose carbon flux through a pyruvate carboxylase-asparagine synthase cascade that results in asparagine generation as a source of nitrogen for stroma and tumor epithelial proliferation. Thus, p62 directly targets nuclear transcription factors to control metabolic reprogramming in the microenvironment and repress tumorigenesis, and identifies ATF4 as a synthetic vulnerability in p62-deficient tumor stroma.

p62, Upregulated during Preneoplasia, Induces Hepatocellular Carcinogenesis by Maintaining Survival of Stressed HCC-Initiating Cells

p62 is a ubiquitin-binding autophagy receptor and signaling protein that accumulates in premalignant liver diseases and most hepatocellular carcinomas (HCCs). Although p62 was proposed to participate in the formation of benign adenomas in autophagy-deficient livers, its role in HCC initiation was not explored. Here we show that p62 is necessary and sufficient for HCC induction in mice and that its high expression in non-tumor human liver predicts rapid HCC recurrence after curative ablation. High p62 expression is needed for activation of NRF2 and mTORC1, induction of c-Myc, and protection of HCC-initiating cells from oxidative stress-induced death.

A macrophage NBR1-MEKK3 complex triggers JNK-mediated adipose tissue inflammation in obesity

The c-Jun NH(2)-terminal kinase (JNK) is a critical determinant of obesity-associated inflammation and glucose intolerance. The upstream mechanisms controlling this pathway are still unknown. Here we report that the levels of the PB1 domain-containing adaptor NBR1 correlated with the expression of proinflammatory molecules in adipose tissue from human patients with metabolic syndrome, suggesting that NBR1 plays a key role in adipose-tissue inflammation. We also show that NBR1 inactivation in the myeloid compartment impairs the function, M1 polarization, and chemotactic activity of macrophages; prevents inflammation of adipose tissue; and improves glucose tolerance in obese mice. Furthermore, we demonstrate that an interaction between the PB1 domains of NBR1 and the mitogen-activated kinase kinase 3 (MEKK3) enables the formation of a signaling complex required for the activation of JNK. Together, these discoveries identify an NBR1-MEKK3 complex as a key regulator of JNK signaling and adipose tissue inflammation in obesity.

K63 polyubiquitination and activation of mTOR by the p62-TRAF6 complex in nutrient-activated cells

The ability of cells to respond to changes in nutrient availability is critical for an adequate control of metabolic homeostasis. Mammalian target of rapamycin complex 1 (mTORC1) is a central complex kinase in these processes. The signaling adaptor p62 binds raptor, and integral component of the mTORC1 pathway. p62 interacts with TNF receptor associated factor 6 (TRAF6) and is required for mTORC1 translocation to the lysosome and its subsequent activation. Here we show that TRAF6 is recruited to and activates mTORC1 through p62 in amino acid-stimulated cells. We also show that TRAF6 is necessary for the translocation of mTORC1 to the lysosomes and that the TRAF6-catalyzed K63 ubiquitination of mTOR regulates mTORC1 activation by amino acids. TRAF6, through its interaction with p62 and activation of mTORC1, modulates autophagy and is an important mediator in cancer cell proliferation. Interfering with the p62-TRAF6 interaction serves to modulate autophagy and nutrient sensing.

Overproduction of the multidrug efflux pump MexEF-OprN does not impair Pseudomonas aeruginosa fitness in competition tests, but produces specific changes in bacterial regulatory networks

It is generally assumed that acquisition of antibiotic resistance leads to non-specific fitness costs. We have tested the alternative hypothesis that acquisition of antibiotic resistance may not always produce a general burden to the microorganisms, as measured in competition tests, but rather lead to specific changes in bacterial physiology. To this end we studied the effect of overproducing the multidrug efflux pump MexEF-OprN on Pseudomonas aeruginosa due to a constitutive activation of MexT, the transcriptional activator of the mexEF-oprN genes. We found that overexpression of MexEF-OprN does not cause a significant decrease in P.aeruginosa fitness in classical competition tests, indicating the absence of a large metabolic burden and that any possible negative effects might be observed only under specific conditions. Transcriptomic analyses revealed that overexpression of MexEF-OprN results in reduced expression of several quorum-sensing regulated genes. We traced back this phenotype to a delay in PQS production due to extrusion of kynurenine, a PQS precursor, through the efflux pump. Type VI secretion was also impaired. A Caenorhabditis elegans model demonstrated that overproduction of MexEF-OprN impairs virulence in P.aeruginosa. This effect was mainly due to the activity of the efflux pump, and not to MexT, despite the fact that the latter regulates Type III and Type VI secretion. Altogether, these data indicate that antibiotic resistance can produce modifications in the bacterial regulatory networks with relevant consequences for the bacterial behaviour in specific ecosystems, including the infected host.

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

The signaling adaptor p62 is a critical mediator of important cellular functions, owing to its ability to establish interactions with various signaling intermediaries. Here, we identify raptor as an interacting partner of p62. Thus, p62 is an integral part of the mTORC1 complex and is necessary to mediate amino acid signaling for the activation of S6K1 and 4EBP1. p62 interacts in an amino acid-dependent manner with mTOR and raptor. In addition, p62 binds the Rags proteins and favors formation of the active Rag heterodimer that is further stabilized by raptor. Interestingly, p62 colocalizes with Rags at the lysosomal compartment and is required for the interaction of mTOR with Rag GTPases in vivo and for translocation of the mTORC1 complex to the lysosome, a crucial step for mTOR activation.

The global regulator Crc modulates metabolism, susceptibility to antibiotics and virulence in Pseudomonas aeruginosa

The capacity of a bacterial pathogen to produce a disease in a treated host depends on the former's virulence and resistance to antibiotics. Several scattered pieces of evidence suggest that these two characteristics can be influenced by bacterial metabolism. This potential relationship is particularly important upon infection of a host, a situation that demands bacteria adapt their physiology to their new environment, making use of newly available nutrients. To explore the potential cross-talk between bacterial metabolism, antibiotic resistance and virulence, a Pseudomonas aeruginosa model was used. This species is an important opportunistic pathogen intrinsically resistant to many antibiotics. The role of Crc, a global regulator that controls the metabolism of carbon sources and catabolite repression in Pseudomonas, was analysed to determine its contribution to the intrinsic antibiotic resistance and virulence of P. aeruginosa. Using proteomic analyses, high-throughput metabolic tests and functional assays, the present work shows the virulence and antibiotic resistance of this pathogen to be linked to its physiology, and to be under the control (directly or indirectly) of Crc. A P. aeruginosa strain lacking the Crc regulator showed defects in type III secretion, motility, expression of quorum sensing-regulated virulence factors, and was less virulent in a Dictyostelium discoideum model. In addition, this mutant strain was more susceptible to beta-lactams, aminoglycosides, fosfomycin and rifampin. Crc might therefore be a good target in the search for new antibiotics.

Towards an ecological approach to antibiotics and antibiotic resistance genes

Antibiotics are likely the most important compounds used for human therapy. Conversely, antibiotic resistance is a relevant medical problem. However, besides their relevance for human health, antibiotics and their resistance genes are important elements that can influence the structure of microbial populations. In this article, we discuss antibiotics and antibiotic resistance genes in non-clinical environments.

The signaling adaptor p62 is an important NF-kappaB mediator in tumorigenesis

The balance between cell death and survival, two critical aspects of oncogenic transformation, determines the outcome of tumorigenesis. Nuclear factor-kappaB (NF-kappaB) is a critical regulator of survival; it is induced by the oncogene Ras and, when inhibited, accounts for the cell death response of Ras-transformed cells. Here, we show that the signaling adaptor p62 is induced by Ras, its levels are increased in human tumors, and it is required for Ras-induced survival and transformation. p62-/- mice are resistant to Ras-induced lung adenocarcinomas. p62 is necessary for Ras to trigger IkappaB kinase (IKK) through the polyubiquitination of tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), and its deficiency produces increased reactive oxygen species (ROS) levels, which account for the enhanced cell death and reduced tumorigenicity of Ras in the absence of p62.

Overexpression of the multidrug efflux pumps MexCD-OprJ and MexEF-OprN is associated with a reduction of type III secretion in Pseudomonas aeruginosa

The Pseudomonas aeruginosa genome contains several different multidrug resistance (MDR) efflux pumps. Overproduction of these pumps reduces susceptibility to a variety of antibiotics. Some recently published works have analyzed the effect of the overproduction of MDR efflux pumps on bacterial virulence. Here we have studied the effect of overproduction of the efflux pumps MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY on type III secretion (T3S) in P. aeruginosa. The type III secretion system (T3SS) is used by P. aeruginosa to deliver toxins directly into the cytoplasm of the host cell. Our data indicate that overexpression of either MexCD-OprJ or MexEF-OprN is associated with the impairment of T3S in P. aeruginosa. No effect on overexpression of either MexAB-OprM or MexXY was detected. The observed defect in T3S was due to a lack of expression of genes belonging to the T3SS regulon. Transcription of this regulon is activated by ExsA in response to environmental signals. Overexpression of this transcriptional regulator complemented the defect in T3S observed in the MexCD-OprJ- and MexEF-OprN-overproducing strains. Taken together, these results suggest that overproduction of either MexCD-OprJ or MexEF-OprN is associated with a reduction in the transcription of the T3SS regulon due to the lack of expression of the exsA gene, encoding the master regulator of the system. The relevance of potential metabolic and quorum-sensing imbalances due to overexpression of MDR pumps associated with this phenotype is also discussed.