The impact of mitochondria on cancer treatment resistance.

BACKGROUND: The ability of cancer cells to develop treatment resistance is one of the primary factors that prevent successful treatment. Although initially thought to be dysfunctional in cancer, mitochondria are significant players that mediate treatment resistance. Literature indicates that cancer cells reutilize their mitochondria to facilitate cancer progression and treatment resistance. However, the mechanisms by which the mitochondria promote treatment resistance have not yet been fully elucidated. CONCLUSIONS AND PERSPECTIVES: Here, we describe various means by which mitochondria can promote treatment resistance. For example, mutations in tricarboxylic acid (TCA) cycle enzymes, i.e., fumarate hydratase and isocitrate dehydrogenase, result in the accumulation of the oncometabolites fumarate and 2-hydroxyglutarate, respectively. These oncometabolites may promote treatment resistance by upregulating the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway, inhibiting the anti-tumor immune response, or promoting angiogenesis. Furthermore, stromal cells can donate intact mitochondria to cancer cells after therapy to restore mitochondrial functionality and facilitate treatment resistance. Targeting mitochondria is, therefore, a feasible strategy that may dampen treatment resistance. Analysis of tumoral DNA may also be used to guide treatment choices. It will indicate whether enzymatic mutations are present in the TCA cycle and, if so, whether the mutations or their downstream signaling pathways can be targeted. This may improve treatment outcomes by inhibiting treatment resistance or promoting the effectiveness of anti-angiogenic agents or immunotherapy.

Diabetes and susceptibility to infections: Implication for COVID-19.

A number of mechanisms have been proposed to explain the well-established link between diabetic status and an increased susceptibility to infection. Notably, diabetes has been shown to be one of the strongest factors influencing healthcare outcome in COVID-19 infections. Though it has long been noted that lymphocytes upregulate insulin receptors following immune activation, until recently, this observation has received little attention. Here, we point out key findings implicating dysregulated insulin signalling in immune cells as a possible contributing factor in the immune pathology associated with diabetes. Mechanistically, insulin, by activating the PI3K/Akt/mTOR pathway, regulates various aspects of both myeloid cells and lymphocytes, such as cell survival, metabolic reprogramming and the polarization and differentiation of immune cells. PI3K signalling is also supressed by immune checkpoint proteins, suggesting that insulin signalling may antagonize peripheral tolerance. Remarkably, it has also recently been shown that, following insulin binding, the insulin receptor translocates to the nucleus where it plays a key role in regulating the transcription of various immune-related genes, including pathways involved in viral infections. Taken together, these observations suggest that dysregulated insulin signalling may directly contribute to a defective immune response during COVID-19 infections.

Insulin-mediated immune dysfunction in the development of preeclampsia.

Epidemiological observations implicate insulin resistance as a predisposing factor in the development of preeclampsia (PE). It is also well established that PE manifests in the context of a dysregulated immune response at the maternal-foetal interface, though all the underlying drivers of such immune dysregulation remains to be accounted for. Although it has long been known that various immune cells express insulin receptors following immune activation, it is only recently that insulin signalling has been shown to play a key role in immune cell differentiation, survival and effector function through its canonical activation of the PI3K/Akt/mTOR pathway. Here we argue that hyperinsulinemia, manifesting either from insulin resistance or from intensive insulin therapy, likely plays a direct role in driving immune cell dysfunction which plays a central role in the development of PE. This line of reasoning also explains the superior results of insulin-sparing interventions compared to intensive insulin therapy as monotherapy.

The onco-immunological implications of Fusobacterium nucleatum in breast cancer.

Breast cancer is a leading cause of death worldwide and a better understanding of this disease is needed to improve treatment outcomes. Recent evidence indicates that bacterial dysbiosis is associated with breast cancer, but the bacteria involved remain poorly characterised. Furthermore, an association between periodontal disease, characterised by oral dysbiosis, and breast cancer have also been discovered, but the mechanisms responsible for this association remains to be elucidated. The oral bacterium involved in periodontal disease, Fusobacterium nucleatum, have recently been detected in human breast tumour tissue and it promoted tumour growth and metastatic progression in a mouse model. The mechanisms of how F. nucleatum might colonise breast tissue and how it might promote tumour progression has not been fully elucidated yet. Here we discuss the breast tumour microbiota, its colonisation by F. nucleatum, possible mechanisms by which F. nucleatum might promote breast cancer progression and how this might impact breast cancer treatment. Literature indicates that F. nucleatum might promote breast cancer progression through activating the Toll-like receptor 4 pathway and by supressing the immune system. This results in cell growth and treatment resistance through autophagy as well as immune evasion. Targeted treatment directed at F. nucleatum combined with immunotherapy and autophagy inhibitors might therefore be a feasible treatment strategy for breast cancer patients.

The immuno-oncological implications of insulin.

Emerging evidence has implicated insulin in regulating the phenotypes of various immune cells through canonical downstream signalling effectors of insulin, namely, the PI3K/Akt/mTOR pathway. Notably, these signalling components also exhibit crosstalk with other immune signalling pathways, such as the JAK/STAT pathway (activated by cytokines and growth factors), and, importantly, are also negatively regulated by the immune checkpoint blockers (ICBs), PD-1 and CTLA-4. Here, we point out recent findings, suggesting that insulin may promote a pro-inflammatory phenotype with potential implications on ICB therapy. As an example, the contemporary paradigm holds that, while T cell receptor recognition of distinct MHC-expressed epitopes ensures specificity, co-activation of CD28 along with signal inputs form various cytokines and insulin operates to 'fine-tune' the immune response via PI3K and other downstream signalling molecules. These considerations highlight the urgent need for focused investigations into the role of insulin in regulating immune cell function in the context of ICB therapies.

Nutritional support in sepsis: when less may be more.

Despite sound basis to suspect that aggressive and early administration of nutritional support may hold therapeutic benefits during sepsis, recommendations for nutritional support have been somewhat underwhelming. Current guidelines (ESPEN and ASPEN) recognise a lack of clear evidence demonstrating the beneficial effect of nutritional support during sepsis, raising the question: why, given the perceived low efficacy of nutritionals support, are there no high-quality clinical trials on the efficacy of permissive underfeeding in sepsis? Here, we review clinically relevant beneficial effects of permissive underfeeding, motivating the urgent need to investigate the clinical benefits of delaying nutritional support during sepsis.

Insulin as an immunomodulatory hormone.

Insulin plays an indispensable role in the management of hyperglycaemia that arises in a variety of settings, including Type I and II diabetes, gestational diabetes, as well as is in hyperglycaemia following a severe inflammatory insult. However, insulin receptors are also expressed on a range of cells that are not canonically implicated in glucose homeostasis. This includes immune cells, where the anti-inflammatory effects of insulin have been repeatedly reported. However, recent findings have also implicated a more involved role for insulin in shaping the immune response during an infection. This includes the ability of insulin to modulate immune cell differentiation and polarisation as well as the modulation of effector functions such as biocidal ROS production. Finally, inflammatory mediators can through both direct and indirect mechanisms also regulate serum insulin levels, suggesting that insulin may be co-opted by the immune system during an infection to direct immunological operations. Collectively, these observations implicate insulin as a bona fide immune-modulating hormone and suggest that a better understanding of insulin's immunological function may aid in optimising insulin therapy in a range of clinical settings.

Mechanisms of doxorubicin-induced drug resistance and drug resistant tumour growth in a murine breast tumour model.

BACKGROUND: Doxorubicin is currently the most effective chemotherapeutic drug used to treat breast cancer. It has, however, been shown that doxorubicin can induce drug resistance resulting in poor patient prognosis and survival. Studies reported that the interaction between signalling pathways can promote drug resistance through the induction of proliferation, cell cycle progression and prevention of apoptosis. The aim of this study was therefore to determine the effects of doxorubicin on apoptosis signalling, autophagy, the mitogen-activated protein kinase (MAPK)- and phosphoinositide 3-kinase (PI3K)/Akt signalling pathway, cell cycle control, and regulators of the epithelial-mesenchymal transition (EMT) process in murine breast cancer tumours. METHODS: A tumour-bearing mouse model was established by injecting murine E0771 breast cancer cells, suspended in Hank's Balances Salt Solution and Corning® Matrigel® Basement Membrane Matrix, into female C57BL/6 mice. Fourty-seven mice were randomly divided into three groups, namely tumour control (received Hank's Balances Salt Solution), low dose doxorubicin (received total of 6 mg/ml doxorubicin) and high dose doxorubicin (received total of 15 mg/ml doxorubicin) groups. A higher tumour growth rate was, however, observed in doxorubicin-treated mice compared to the untreated controls. We therefore compared the expression levels of markers involved in cell death and survival signalling pathways, by means of western blotting and fluorescence-based immunohistochemistry. RESULTS: Doxorubicin failed to induce cell death, by means of apoptosis or autophagy, and cell cycle arrest, indicating the occurrence of drug resistance and uncontrolled proliferation. Activation of the MAPK/ extracellular-signal-regulated kinase (ERK) pathway contributed to the resistance observed in treated mice, while no significant changes were found with the PI3K/Akt pathway and other MAPK pathways. Significant changes were also observed in cell cycle p21 and DNA replication minichromosome maintenance 2 proteins. No significant changes in EMT markers were observed after doxorubicin treatment. CONCLUSIONS: Our results suggest that doxorubicin-induced drug resistance and tumour growth can occur through the adaptive role of the MAPK/ERK pathway in an effort to protect tumour cells. Previous studies have shown that the efficacy of doxorubicin can be improved by inhibition of the ERK signalling pathway and thereby treatment failure can be overcome.

How Does Inflammation-Induced Hyperglycemia Cause Mitochondrial Dysfunction in Immune Cells?

Inflammatory mediators have an established role in inducing insulin resistance and promoting hyperglycemia. In turn, hyperglycemia has been argued to drive immune cell dysfunction as a result of mitochondrial dysfunction. Here, the authors review the evidence challenging this view. First, it is pointed out that inflammatory mediators are known to induce altered mitochondrial function. In this regard, critical care patients suffer both an elevated inflammatory tone as well as hyperglycemia, rendering it difficult to distinguish between the effects of inflammation and hyperglycemia. Second, emerging evidence indicates that a decrease in mitochondrial respiration and an increase in reactive oxygen species (ROS) production are not necessarily manifestations of pathology, but adaptations taking shape as the mitochondria is abdicating its adenosine triphosphate (ATP)-producing function (which is taken over by glycolysis) and instead becomes "retooled" for an immunological role. Collectively, these observations challenge the commonly held belief that acute hyperglycemia induces mitochondrial damage leading to immune cell dysfunction.

The role of bile acids in nutritional support.

Nutritional support continues to receive much attention as a possible intervention to prevent loss of lean tissue mass, promote recovery and re-establish proper immune function in critical care patients. Yet there remains much controversy regarding the clinical efficacy of such interventions. In addition to the direct effect of nutrition in terms of micro- and macronutrient content, nutritional formulations may exert an effect via the physiological response to feeding. Here, we highlight the key role of postprandial reabsorbed bile acids in attenuating both the inflammatory response and autophagy. These observations suggest that not all patients would benefit from aggressive nutritional support.

Inflammation-induced metabolic derangements or adaptation: An immunometabolic perspective.

Inflammatory mediators have a well-established role in mediating metabolic disturbances. Chronic low-grade inflammation is implicated in the pathogenesis of obesity and the development of metabolic syndrome. This phenomenon is even more pronounced in severe inflammatory states such as in critically ill patients where hyperglycaemia invariably manifests. Similarly, though inflammatory mediators have a well-established role in promoting bone resorption, the adaptive function of this process remains unknown. Here we review emerging evidence from the field of immunometabolism suggesting that these two processes serve a common goal, namely, to sustain the rapid proliferation of immune cells during an infection. Activated immune cells exhibit an increased demand for glucose which not only provides energy, but also glycolytic intermediates which are fluxed into biosynthetic processes. Similarly, phosphate liberated from bone is consumed during the phosphorylation of glycolytic intermediates, which plays a critical role in the synthesis of nucleotides and phospholipids. Taken together, these considerations suggest that metabolic alterations induced by inflammatory mediators do not manifest as an inability to maintain homeostatic levels of metabolites but represent an adaptive shift in the homeostatic set point during an infection.

Role of PKM2 in directing the metabolic fate of glucose in cancer: a potential therapeutic target.

BACKGROUND: Many of the hallmarks of cancer are not inherently unique to cancer, but rather represent a re-enactment of normal host responses and activities. A vivid example is aerobic glycolysis ('Warburg effect'), which is used not only by cancer cells but also by normal cells that undergo rapid proliferation. A common feature of this metabolic adaptation is a shift in the expression of pyruvate kinase (PK) isoform M1 to isoform M2. Here, we highlight the key role of PKM2 in shifting cancer metabolism between ATP production and biosynthetic processes. Since anabolic processes are highly energy dependent, the fate of glucose in energy production versus the contribution of carbon in biosynthetic processes needs to be finely synchronised. PKM2 acts to integrate cellular signalling and allosteric regulation of metabolites in order to align metabolic activities with the changing needs of the cell. CONCLUSIONS: The central role of PKM2 in directing the flow of carbon between catabolic (ATP-producing) and anabolic processes provides unique opportunities for extending the therapeutic window of currently available and/or novel anti-neoplastic agents.

Bone resorption: supporting immunometabolism.

Activation of the immune system is associated with an increase in the breakdown of various peripheral tissues, including bone. Despite the widely appreciated role of inflammatory mediators in promoting bone resorption, the functional value behind this process is not completely understood. Recent advances in the field of immunometabolism have highlighted the metabolic reprogramming that takes place in activated immune cells. It is now believed that the breakdown of peripheral tissue provides metabolic substrates to fuel metabolic anabolism in activated immune cells. We argue that phosphate, liberated by bone resorption, plays an indispensable role in sustaining immune cell metabolism. The liberated phosphate is then incorporated into macromolecules such as nucleotides and phospholipids, and is also used for the phosphorylation of metabolites (e.g. glycolytic intermediates). In addition, magnesium, also liberated during the breakdown of bone, is an essential cofactor required by various metabolic enzymes which are upregulated in activated immune cells. Finally, calcium activates various additional molecules involved in immune cell migration. Taken together, these factors suggest a key role for bone resorption during infection.

Nutrient excess and autophagic deficiency: explaining metabolic diseases in obesity.

Over-nutrition and a sedentary lifestyle are the driving forces behind the development of metabolic diseases. Conversely, caloric restriction and exercise have proven to be the most effective strategies in combating metabolic diseases. Interestingly, exercise and caloric restriction share a common feature: both represent a potent mechanism for upregulating autophagy. Autophagy is rapidly induced by nutrient deprivation, and conversely, inactivated by amino acids as well as growth factors (e.g. insulin). Here, we review evidence demonstrating that autophagy may indeed be attenuated in metabolic tissue such as liver, muscle, and adipose, in the context of obesity. We also highlight the mechanistic basis by which defective autophagy may contribute to the manifestation of metabolic diseases. This includes a compromised ability of the cell to perform quality control on the mitochondrial matrix, since autophagy plays a pivotal role in the degradation of defective mitochondria. Similarly, autophagy also plays an indispensable role in the clearance of protein aggregates and redundant large protein platforms such as inflammasomes. Autophagy might also play a key role in the metabolism of endotoxins, implicating the importance of autophagy in the pathogenesis of metabolic endotoxemia. These observations underpin an unprecedented role of autophagy in the manifestation of obesity-induced metabolic derangement.

Doxorubicin resistance in breast cancer: A novel role for the human protein AHNAK.

Understanding the response of cancer cells to anti-cancer therapies is crucial to unraveling and preventing the development of therapeutic resistance. The human AHNAK protein is a giant scaffold protein implicated in several diverse cellular functions. The role of AHNAK in cancer is however unclear as the protein has previously been described as a tumor suppressor, as well as being essential for tumor metastasis and invasion, while also being implicated in selected chemotherapeutic responses. To clarify the role of AHNAK in cancer, we investigated the effect of doxorubicin treatment on AHNAK in doxorubicin-sensitive MCF-7 and doxorubicin-resistant MDA-MB-231 breast cancer cell lines, as well as in a tumor-bearing mouse model. The role of AHNAK in the cellular response of breast cancer cells to doxorubicin was also investigated. We report here, for the first time, an association between AHNAK and resistance to doxorubicin. While treatment with doxorubicin modulated AHNAK protein expression both in vitro and in vivo in a dose-dependent manner, no changes in its cellular localization were observed. AHNAK knockdown prevented doxorubicin-induced modulation of cleaved caspase 7 protein expression and cell cycle arrest, while its overexpression decreased cleaved caspase 7 and cleaved PARP levels and induced S-phase arrest, changes that were comparable to the effects of doxorubicin. This novel association was restricted to doxorubicin-resistant cells, implicating the protein in therapeutic resistance. These findings confirm that AHNAK does indeed function in the chemotherapeutic response of breast cancer cells while also emphasizing the need for further investigation into potential implications for AHNAK in terms of predicting and modulating treatment response.

Domesticating Cancer: An Evolutionary Strategy in the War on Cancer.

Since cancer shares the same molecular machinery as the host, most therapeutic interventions that aim to target cancer would inadvertently also adversely affect the host. In addition, cancer continuously evolves, streamlining its host-derived genome for a new single-celled existence. In particular, short-term clinical success observed with most antineoplastic therapies directly relate to the fact that cancer is constantly evolving. However, the clonal evolution of cancer occasionally also render cancer cells uniquely susceptible to therapeutic interventions, as is exemplified by the clinical relevance of synthetic lethality. Synthetic lethality describes a situation where the simultaneous loss of function in two genes results in lethality, but where a loss of function in either single gene is tolerated. This observation suggests that the evolution of cancer, usually seen as a major clinical challenge, may also afford a key opportunity in lowering on-target toxicities accosted with chemotherapy. As an example, by subjecting cancer to specific selection regimes, cancer can in effect be placed on evolutionary trajectories leading to the development of "targetable" phenotypes such as synthetic lethal interactions. However, such a selection regime would have to overcome a range of obstacles such as on-target toxicity and the selection of an evolvable trait. Since the majority of cancer evolution manifests as a loss of function, we suggest that the induction of auxotrophic phenotypes (i.e., where an organism lose the ability to synthesize specific organic compounds required for growth and thus become dependent on it from dietary sources) may represent an attractive therapeutic option. As an example, animals can obtain vitamin C either by de novo synthesis or from their diet. However, since the maintenance of synthetic pathways is costly, such pathways are often lost if no longer necessary, resulting in the organism being auxotrophic toward the dietary compound. Similarly, increasing the maintenance cost of a redundant pathway in cancer cells is likely to select for clones that have lost such a redundant pathway. Inhibition of a pathway, while supporting the activity of a compensating pathway, may thus induce auxotrophism in cancer cells but not in genomic stable host cells.

Hyperglycaemia in critically ill patients: the immune system's sweet tooth.

There is an ongoing debate regarding the efficacy of glycaemic control in critically ill patients. Here we briefly highlight the key function of elevated glucose in critically ill patients, namely, to enable elevation of aerobic glycolysis in rapidly dividing cells. In particular, aerobic glycolysis provides metabolic intermediates necessary for expansion of biomass in immune cells and promotion of tissue repair. Furthermore, we emphasise that insulin may inhibit autophagy, a cell survival process used in the bulk degradation of cellular debris and damaged organelles. These observations provide a rational basis for tolerating elevated glucose levels in certain critically ill patients.

Cancer stem cells: A product of clonal evolution?

The cancer stem cell (CSC) model has emerged as a prominent paradigm for explaining tumour heterogeneity. CSCs in tumour recurrence and drug resistance have also been implicated in a number of studies. In fact, CSCs are often identified by their expression of drug-efflux proteins which are also highly expressed in normal stem cells. Similarly, pro-survival or proliferation signalling often exhibited by stem cells is regularly reported as being upregulated by CSC. Here we review evidence suggesting that many aspects of CSCs are more readily described by clonal evolution. As an example, cancer cells often exhibit copy number gains of genes involved in drug-efflux proteins and pro-survival signalling. Consequently, clonal selection for stem cell traits may result in cancer cells developing "stemness" traits which impart a fitness advantage, without strictly following a CSC model. Finally, since symmetric cell division would give rise to more cells than asymmetric division, it is expected that more advanced tumours would depart from a CSC. Collectively, these observations suggest clonal evolution may explain many aspects of the CSC.

Enhanced Therapeutic Efficacy in Cancer Patients by Short-term Fasting: The Autophagy Connection.

Preclinical studies suggest that fasting prior to chemotherapy may be an effective strategy to protect patients against the adverse effects of chemo-toxicity. Fasting may also sensitize cancer cells to chemotherapy. It is further suggested that fasting may similarly augment the efficacy of oncolytic viral therapy. The primary mechanism mediating these beneficial effects is thought to relate to the fact that fasting results in a decrease of circulating growth factors. In turn, such fasting cues would prompt normal cells to redirect energy toward cell maintenance and repair processes, rather than growth and proliferation. However, fasting is also known to upregulate autophagy, an evolutionarily conserved catabolic process that is upregulated in response to various cell stressors. Here, we review a number of mechanisms by which fasting-induced autophagy may have an impact on both chemo-tolerance and chemo-sensitization. First, fasting may exert a protective effect by mobilizing autophagic components prior to chemo-induction. In turn, the autophagic apparatus can be repurposed for removing cellular components damaged by chemotherapy. Autophagy also plays a key role in epitope expression as well as in modulating inflammation. Chemo-sensitization resulting from fasting may in fact be an effect of enhanced immune surveillance as a result of better autophagy-dependent epitope processing. Finally, autophagy is involved in host defense against viruses, and aspects of the autophagic process are also often targets for viral subversion. Consequently, altering autophagic flux by fasting may alter viral infectivity. These observations suggest that fasting-induced autophagy may have an impact on therapeutic efficacy in various oncological contexts.