Aging process modulates nonlinear dynamics in liver cell metabolism

The aged liver: Beyond cellular senescence

The aging of the population, the increased prevalence of chronic liver diseases in elderly and the need to broaden the list of potential liver donors enjoin us to better understand what is an aged liver. In this review, we provide a brief introduction to cellular senescence, revisit the main morphological and functional modifications of the liver induced by aging, particularly concerning metabolism, immune response and regeneration, and try to elude some of the signalling pathways responsible for these modifications. Finally, we discuss the clinical consequences of aging on chronic liver diseases and the implications of older age for donors and recipients in liver transplantation.

Rapid Assessment of Mitochondrial Complex I Activity and Metabolic Phenotyping of Breast Cancer Cells by NAD(p)H Cytometry

Cancer cells are known to display a variety of metabolic reprogramming strategies to fulfill their own growth and proliferative agenda. With the advent of high resolution imaging strategies, metabolomics techniques, and so forth, there is an increasing appreciation of critical role that tumor cell metabolism plays in the overall breast cancer (BC) growth. In this report, we demonstrate a sensitive, flow-cytometry-based assay for rapidly assessing the metabolic phenotypes in isolated suspensions of breast cancer cells. By measuring the temporal variation of NAD(p)H signals in unlabeled, living cancer cells, and by measuring mitochondrial membrane potential {Δψ_m } in fluorescently labeled cells, we demonstrate that these signals can reliably distinguish the metabolic phenotype of human breast cancer cells and can track the cellular sensitivity to drug candidates. We further show the utility of this metabolic ratio {Δψ_m /NAD(p)H} in monitoring mitochondrial functional improvement as well as metabolic heterogeneity in primary murine tumor cells isolated from tumor biopsies. Together, these results demonstrate a novel possibility for rapid metabolic functional screening applications as well as a metabolic phenotyping tool for determining drug sensitivity in living cancer cells. © 2018 International Society for Advancement of Cytometry.

An Emergence Framework of Carcinogenesis

Experimental paradigms provide the framework for the understanding of cancer, and drive research and treatment, but are rarely considered by clinicians. The somatic mutation theory (SMT), in which cancer is considered a genetic disease, has been the predominant traditional model of cancer for over 50 years. More recently, alternative theories have been proposed, such as tissue organization field theory (TOFT), evolutionary models, and inflammatory models. Key concepts within the various models have led to them being difficult to reconcile. Progressively, it has been recognized that biological systems cannot be fully explained by the physicochemical properties of their constituent parts. There is an increasing call for a 'systems' approach. Incorporating the concepts of 'emergence', 'systems', 'thermodynamics', and 'chaos', a single integrated framework for carcinogenesis has been developed, enabling existing theories to become compatible as alternative mechanisms, facilitating the integration of bioinformatics and providing a structure in which translational research can flow from both 'benchtop to bedside' and 'bedside to benchtop'. In this review, a basic understanding of the key concepts of 'emergence', 'systems', 'system levels', 'complexity', 'thermodynamics', 'entropy', 'chaos', and 'fractals' is provided. Non-linear mathematical equations are included where possible to demonstrate compatibility with bioinformatics. Twelve principles that define the 'emergence framework of carcinogenesis' are developed, with principles 1-10 encapsulating the key concepts upon which the framework is built and their application to carcinogenesis. Principle 11 relates the framework to cancer progression. Principle 12 relates to the application of the framework to translational research. The 'emergence framework of carcinogenesis' collates current paradigms, concepts, and evidence around carcinogenesis into a single framework that incorporates previously incompatible viewpoints and ideas. Any researcher, scientist, or clinician involved in research, treatment, or prevention of cancer can employ this framework.

Metabolic imaging in multiple time scales

We report here a novel combination of time-resolved imaging methods for probing mitochondrial metabolism in multiple time scales at the level of single cells. By exploiting a mitochondrial membrane potential reporter fluorescence we demonstrate the single cell metabolic dynamics in time scales ranging from microseconds to seconds to minutes in response to glucose metabolism and mitochondrial perturbations in real time. Our results show that in comparison with normal human mammary epithelial cells, the breast cancer cells display significant alterations in metabolic responses at all measured time scales by single cell kinetics, fluorescence recovery after photobleaching and by scaling analysis of time-series data obtained from mitochondrial fluorescence fluctuations. Furthermore scaling analysis of time-series data in living cells with distinct mitochondrial dysfunction also revealed significant metabolic differences thereby suggesting the broader applicability (e.g. in mitochondrial myopathies and other metabolic disorders) of the proposed strategies beyond the scope of cancer metabolism. We discuss the scope of these findings in the context of developing portable, real-time metabolic measurement systems that can find applications in preclinical and clinical diagnostics.

Loss of caspase-2 accelerates age-dependent alterations in mitochondrial production of reactive oxygen species

Mitochondria are known to be a major source and target of oxidative stress. Oxidative stress increases during aging and is suggested to underlie in part the aging process. We have previously documented an increase in endogenous caspase-2 (casp2) activity in hepatocytes obtained from old (28 months) vs. young mice (5 months). More recently, we have shown that casp2 is activated by oxidative stress and is critical for mitochondrial oxidative stress-induced apoptosis. Since casp2 appears integral to mitochondrial oxidative stress-induced apoptosis, in this study we determined whether loss of casp2 altered the production of mitochondrial reactive oxygen radicals (mROS) as a function of age in intact living hepatocytes. To stimulate mitochondrial metabolic activity, we added a mixture of pyruvate and glutamate to hepatocytes while continuously monitoring endogenous mROS production in the presence or absence of rotenone and/or antimycin A. Our data demonstrate that mROS production and neutralization are compromised in hepatocytes of old mice. Interestingly, casp2 deficient hepatocytes from middle age mice (12 months) had similar mROS neutralization kinetics to those of hepatocytes from old WT mice. Rotenone had no effect on mROS metabolism, whereas antimycin A significantly altered mROS production and metabolism in an age-dependent fashion. Our results indicate that: (1) hepatocytes from young and old mice respond differently to dysfunction of the mitochondrial electron transport chain; (2) age-dependent alterations in mROS metabolism are likely regulated by complex III; and (3) absence of casp2 accelerates age-dependent changes in terms of pyruvate/glutamate-induced mROS metabolism.

Mitochondrial NDUFS3 regulates the ROS-mediated onset of metabolic switch in transformed cells

Aerobic glycolysis in transformed cells is an unique metabolic phenotype characterized by a hyperactivated glycolytic pathway even in the presence of oxygen. It is not clear if the onset of aerobic glycolysis is regulated by mitochondrial dysfunction and, if so, what the metabolic windows of opportunity available to control this metabolic switch (mitochondrial to glycolytic) landscape are in transformed cells. Here we report a genetically-defined model system based on the gene-silencing of a mitochondrial complex I subunit, NDUFS3, where we demonstrate the onset of metabolic switch in isogenic human embryonic kidney cells by differential expression of NDUFS3. By means of extensive metabolic characterization, we demonstrate that NDUFS3 gene silencing systematically introduces mitochondrial dysfunction thereby leading to the onset of aerobic glycolysis in a manner dependent on NDUFS3 protein levels. Furthermore, we show that the sustained imbalance in free radical dynamics is a necessary condition to sustain the observed metabolic switch in cell lines with the most severe NDUFS3 suppression. Together, our data reveal a novel role for mitochondrial complex I subunit NDUFS3 in regulating the degree of mitochondrial dysfunction in living cells, thereby setting a “metabolic threshold” for the observation of aerobic glycolysis phenotype within the confines of mitochondrial dysfunction.

Selective vulnerability of neurons to acute toxicity after proteasome inhibitor treatment: implications for oxidative stress and insolubility of newly synthesized proteins

Maintaining protein homeostasis is vital to cell viability, with numerous studies demonstrating a role for proteasome inhibition occurring during the aging of a variety of tissues and, presumably, contributing to the disruption of cellular homeostasis during aging. In this study we sought to elucidate the differences between neurons and astrocytes in regard to basal levels of protein synthesis, proteasome-mediated protein degradation, and sensitivity to cytotoxicity after proteasome inhibitor treatment. In these studies we demonstrate that neurons have an increased vulnerability, compared to astrocyte cultures, to proteasome-inhibitor-induced cytotoxicity. No significant difference was observed between these two cell types in regard to the basal rates of protein synthesis, or basal rates of protein degradation, in the pool of short-lived proteins. After proteasome inhibitor treatment neuronal crude lysates were observed to undergo greater increases in the levels of ubiquitinated and oxidized proteins and selectively exhibited increased levels of newly synthesized proteins accumulating within the insoluble protein pool, compared to astrocytes. Together, these data suggest a role for increased oxidized proteins and sequestration of newly synthesized proteins in the insoluble protein pool, as potential mediators of the selective neurotoxicity after proteasome inhibitor treatment. The implications for neurons exhibiting increased sensitivity to acute proteasome inhibitor exposure, and the corresponding changes in protein homeostasis observed after proteasome inhibition, are discussed in the context of both aging and age-related disorders of the nervous system.

Conceptualizing compensatory responses: implications for treatment and research

Many scientists approach the discovery and application of knowledge of physiological processes from a reductionistic paradigm. A reductionistic approach focuses on treating one or a few key disease-related variables but overlooks the interaction of systems and their dependency on one another to produce homeostasis. The purposes of this article are to examine the current paradigm underlying treatment and its effect on patient outcome and to present an alternative perspective for understanding the body's compensatory responses and their implications for treatment and research. Chaos theory and nonlinear methods are presented as possible ways to conceptualize and explore the complex integration of physiological patterns in response to disease, aging, and treatment.

Renin-angiotensin system inhibitors protect against age-related changes in rat liver mitochondrial DNA content and gene expression

Chronic renin-angiotensin system inhibition protects against liver fibrosis, ameliorates age-associated mitochondrial dysfunction and increases rodent lifespan. We hypothesized that life-long angiotensin-II-mediated stimulation of oxidant generation might participate in mitochondrial DNA "common deletion" formation, and the resulting impairment of bioenergetic capacity. Enalapril (10 mg/kg/d) or losartan (30 mg/kg/d) administered during 16.5 months were unable to prevent the age-dependent accumulation of rat liver mitochondrial DNA "common deletion", but attenuated the decrease of mitochondrial DNA content. This evidence - together with the enhancement of NRF-1 and PGC-1 mRNA contents - seems to explain why enalapril and losartan improved mitochondrial functioning and lowered oxidant production, since both the absolute number of mtDNA molecules and increased NRF-1 and PGC-1 transcription are positively related to mitochondrial respiratory capacity, and PGC-1 protects against increases in ROS production and damage. Oxidative stress evoked by abnormal respiratory function contributes to the pathophysiology of mitochondrial disease and human aging. If the present mitochondrial actions of renin-angiotensin system inhibitors are confirmed in humans they may modify the therapeutic significance of that strategy.

Nonlinear scaling analysis of glucose metabolism in normal and cancer cells

Cancer progression is commonly accompanied by an altered glucose metabolism. In general, spatially resolved imaging of glucose metabolism and its subtle alterations might provide valuable diagnostic information in vivo. A classical example is positron emission tomography that exploits this feature in obtaining preferential accumulation of fluorescent analog of glucose in tumors, thereby achieving an imaging contrast. We report a novel scaling analysis of glucose metabolism in mammary epithelial (NMuMG) cells by detrended fluctuation analysis of Cerulean (cyan fluorescent protein variant) fluorescence. Fluorescence fluctuations of Cerulean are reasoned to be indicative of dynamic pH changes associated with glucose metabolism. Normal parental cells and the spontaneously transformed (cancerous) NMuMG cells displayed robust scaling exponent that reflects nonrandom fluctuations in Cerulean fluorescence. Acute dependence of cancer cells on glycolysis as compared with normal cells is exploited to yield a statistically significant difference in scaling exponent, thereby providing discrimination between normal and cancer cells in vitro. By careful design of experiments in vivo, the proposed scaling approach might even have diagnostic potential for early detection of cancer lesions in small animal models.