Differentiate or Die: 3-Bromopyruvate and Pluripotency in Mouse Embryonic Stem Cells

Metabolic control of induced pluripotency

Pluripotent stem cells of the mammalian epiblast and their cultured counterparts-embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs)-have the capacity to differentiate in all cell types of adult organisms. An artificial process of reactivation of the pluripotency program in terminally differentiated cells was established in 2006, which allowed for the generation of induced pluripotent stem cells (iPSCs). This iPSC technology has become an invaluable tool in investigating the molecular mechanisms of human diseases and therapeutic drug development, and it also holds tremendous promise for iPSC applications in regenerative medicine. Since the process of induced reprogramming of differentiated cells to a pluripotent state was discovered, many questions about the molecular mechanisms involved in this process have been clarified. Studies conducted over the past 2 decades have established that metabolic pathways and retrograde mitochondrial signals are involved in the regulation of various aspects of stem cell biology, including differentiation, pluripotency acquisition, and maintenance. During the reprogramming process, cells undergo major transformations, progressing through three distinct stages that are regulated by different signaling pathways, transcription factor networks, and inputs from metabolic pathways. Among the main metabolic features of this process, representing a switch from the dominance of oxidative phosphorylation to aerobic glycolysis and anabolic processes, are many critical stage-specific metabolic signals that control the path of differentiated cells toward a pluripotent state. In this review, we discuss the achievements in the current understanding of the molecular mechanisms of processes controlled by metabolic pathways, and vice versa, during the reprogramming process.

Leucine and Arginine Availability Modulate Mouse Embryonic Stem Cell Proliferation and Metabolism

Amino acids are crucial nutrients involved in several cellular and physiological processes, including fertilization and early embryo development. In particular, Leucine and Arginine have been shown to stimulate implantation, as lack of both in a blastocyst culture system is able to induce a dormant state in embryos. The aim of this work was to evaluate the effects of Leucine and Arginine withdrawal on pluripotent mouse embryonic stem cell status, notably, their growth, self-renewal, as well as glycolytic and oxidative metabolism. Our results show that the absence of both Leucine and Arginine does not affect mouse embryonic stem cell pluripotency, while reducing cell proliferation through cell-cycle arrest. Importantly, these effects are not related to Leukemia Inhibitory Factor (LIF) and are reversible when both amino acids are reconstituted in the culture media. Moreover, a lack of these amino acids is related to a reduction in glycolytic and oxidative metabolism and decreased protein translation in mouse embryonic stem cells (mESCs), while maintaining their pluripotent status.

The role of metabolism in directed differentiation versus trans-differentiation of cardiomyocytes

The advent of induced pluripotent stem cells (iPSCs) and identification of transcription factors for cardiac reprogramming have raised hope to cure heart disease, the leading cause of death in the world. Our knowledge in heart development and molecular barriers of cardiac reprogramming is advancing, but many hurdles are yet to be overcome for clinical translation. Importantly, we lack a full understanding of molecular mechanisms governing cell fate conversion toward cardiomyocytes. In this review, we will discuss the role of metabolism in directed differentiation versus trans-differentiation of cardiomyocytes. Cardiomyocytes exhibit a unique metabolic feature distinct from PSCs and cardiac fibroblasts, and there are multiple overlapping molecular mechanisms underlying metabolic reprogramming during cardiomyogenesis. We will discuss key metabolic changes occurring during cardiomyocytes differentiation from PSCs and cardiac fibroblasts, and the potential role of metabolic reprogramming in the enhancement strategies for cardiomyogenesis. Only when such details are discovered will more effective strategies to enhance the de novo production of cardiomyocytes be possible.

Glycolytic Profiling of Mouse Embryonic Stem Cells (mESCs)

Mouse embryonic stem cells (mESCs) can be captured in vitro in different pluripotency states through media modulation, mimicking their natural environment during early embryo development. As highly proliferative cells, mESCs prefer to use glycolysis to support the energetic and biosynthetic demands, even in the presence of oxygen. Indeed, glycolysis can not only supply ATP at a much faster rate, when compared to other catabolic pathways, but also provides biosynthetic substrates to meet anabolic requirements. Considering that ESCs cultured in different media conditions display distinct metabolic requirements, it is of utmost importance to have a robust metabolic characterization methodology to understand how subtle metabolic variations may be coupled to ESC identity. Here we describe how to profile the glycolytic activity of naive mouse ESC, using the established Seahorse XFe24 Live-cell Metabolic Assay. This may be a useful protocol for understanding how the glycolytic function of mESCs changes in certain circumstances and how is it coupled to diverse pluripotency/differentiation phenotypes.

MYCN Function in Neuroblastoma Development

Dysregulated expression of the transcription factor MYCN is frequently detected in nervous system tumors such as childhood neuroblastoma. Here, gene amplification of MYCN is a single oncogenic driver inducing neoplastic transformation in neural crest-derived cells. This abnormal MYCN expression is one of the strongest predictors of poor prognosis. It is present at diagnosis and is never acquired during later tumorigenesis of MYCN non-amplified neuroblastoma. This suggests that increased MYCN expression is an early event in these cancers leading to a peculiar dysregulation of cells that results in embryonal or cancer stem-like qualities, such as increased self-renewal, apoptotic resistance, and metabolic flexibility.

Monitoring Mitochondrial Function in Mouse Embryonic Stem Cells (mESCs)

Mouse embryonic stem cells (mESCs) can be grown in culture, recapitulating the different states of pluripotency of their in vivo counterparts, with notably different metabolic profiles. mESCs in a naïve pluripotent state present an ambivalent metabolism, using both glycolysis and oxidative phosphorylation as energy sources. Here, we describe a method to evaluate the oxidative function of naïve mESCs using the Seahorse Extracellular Flux Analyzer coupled to flow cytometry analysis of mitochondrial transmembrane potential using the TMRM fluorescence probe, thus assessing both oxygen consumption and mitochondrial membrane potential. This may be a useful protocol for understanding how mitochondrial oxidative function and potential of mESCs change in certain circumstances, and how is it related with various pluripotency/differentiation phenotypes.

Effects of DMSO on the Pluripotency of Cultured Mouse Embryonic Stem Cells (mESCs)

DMSO is a commonly used solvent in biological studies, as it is an amphipathic molecule soluble in both aqueous and organic media. For that reason, it is the vehicle of choice for several water-insoluble substances used in research. At the molecular and cellular level, DMSO is a hydrogen-bound disrupter, an intercellular electrical uncoupler, and a cryoprotectant, among other properties. Importantly, DMSO often has overlooked side effects. In stem cell research, the literature is scarce, but there are reports on the effect of DMSO in human embryoid body differentiation and on human pluripotent stem cell priming towards differentiation, via modulation of cell cycle. However, in mouse embryonic stem cell (mESC) culture, there is almost no available information. Taking into consideration the almost ubiquitous use of DMSO in experiments involving mESCs, we aimed to understand the effect of very low doses of DMSO (0.0001%-0.2%), usually used to introduce pharmacological inhibitors/modulators, in mESCs cultured in two different media (2i and FBS-based media). Our results show that in the E14Tg2a mESC line used in this study, even the smallest concentration of DMSO had minor effects on the total number of cells in serum-cultured mESCs. However, these effects could not be explained by alterations in cell cycle or apoptosis. Furthermore, DMSO did not affect pluripotency or differentiation potential. All things considered, and although control experiments should be carried out in each cell line that is used, it is reasonable to conclude that DMSO at the concentrations used here has a minimal effect on this particular mESC line.

Role of Mitochondria in Cancer Stem Cell Resistance

Cancer stem cells (CSC) are associated with the mechanisms of chemoresistance to different cytotoxic drugs or radiotherapy, as well as with tumor relapse and a poor prognosis. Various studies have shown that mitochondria play a central role in these processes because of the ability of this organelle to modify cell metabolism, allowing survival and avoiding apoptosis clearance of cancer cells. Thus, the whole mitochondrial cycle, from its biogenesis to its death, either by mitophagy or by apoptosis, can be targeted by different drugs to reduce mitochondrial fitness, allowing for a restored or increased sensitivity to chemotherapeutic drugs. Once mitochondrial misbalance is induced by a specific drug in any of the processes of mitochondrial metabolism, two elements are commonly boosted: an increment in reactive nitrogen/oxygen species and, subsequently, activation of the intrinsic apoptotic pathway.

Metabolic remodeling during somatic cell reprogramming to induced pluripotent stem cells: involvement of hypoxia-inducible factor 1

Induced pluripotent stem cells (iPSCs) were first established from differentiated somatic cells by gene introduction of key transcription factors, OCT4, SOX2, KLF4, and c-MYC, over a decade ago. Although iPSCs can be applicable for regenerative medicine, disease modeling and drug screening, several issues associated with the utilization of iPSCs such as low reprogramming efficiency and the risk of tumorigenesis, still need to be resolved. In addition, the molecular mechanisms involved in the somatic cell reprogramming to pluripotency are yet to be elucidated. Compared with their somatic counterparts, pluripotent stem cells, including embryonic stem cells and iPSCs, exhibit a high rate of glycolysis akin to aerobic glycolysis in cancer cells. This is known as the Warburg effect and is essential for maintaining stem cell properties. This unique glycolytic metabolism in iPSCs can provide energy and drive the pentose phosphate pathway, which is the preferred pathway for rapid cell proliferation. During reprogramming, somatic cells undergo a metabolic shift from oxidative phosphorylation (OXPHOS) to glycolysis trigged by a transient OXPHOS burst, resulting in the initiation and progression of reprogramming to iPSCs. Metabolic intermediates and mitochondrial functions are also involved in the epigenetic modification necessary for the process of iPSC reprogramming. Among the key regulatory molecules that have been reported to be involved in metabolic shift so far, hypoxia-inducible factor 1 (HIF1) controls the transcription of many target genes to initiate metabolic changes in the early stage and maintains glycolytic metabolism in the later phase of reprogramming. This review summarizes the current understanding of the unique metabolism of pluripotent stem cells and the metabolic shift during reprogramming, and details the relevance of HIF1 in the metabolic shift.

Metabolic characterization of a paused-like pluripotent state

Embryonic diapause is a conserved reproductive strategy in which development arrests at the blastocyst phase. Recently mammalian target of rapamycin (mTOR) inhibition was shown to induce diapause on mouse blastocysts and a paused-like state on mouse embryonic stem cells (mESCs). In this work, we aimed to further characterize this new paused-pluripotent state, focusing on its glycolytic and oxidative metabolic function. We therefore exposed mESCs, to the mTOR inhibitor INK-128 and evaluated proliferation, pluripotency status and energy-related metabolism, as well as the mTOR inhibition status and translational function. Unexpectedly, in our hands INK-128 did not inhibit the phosphorylation of mTOR or its downstream targets after 48 h. Accordingly, no alterations on protein translational function were observed. Nonetheless, INK-128 could still successfully induce a paused-like state in naïve mESCs regardless of their culturing conditions, by greatly slowing proliferation without affecting pluripotency status. This effect was more prevalent in 2i cultured cells. Interestingly, in this paused-like state, mESCs present a glucose-related hypometabolic profile, which is a hallmark of diapaused blastocysts, with decreased glycolytic and oxidative metabolism and decreased nutrient uptake. Despite the lack of mTOR inhibition and translational suppression, INK-128 still induced a paused-like pluripotent state through cell cycle and metabolic modulation, rather than by translational suppression, suggesting more than one avenue for this type of pluripotent phenotype.

Mechanisms of the Metabolic Shift during Somatic Cell Reprogramming

Pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), hold a huge promise for regenerative medicine, drug development, and disease modeling. PSCs have unique metabolic features that are akin to those of cancer cells, in which glycolysis predominates to produce energy as well as building blocks for cellular components. Recent studies indicate that the unique metabolism in PSCs is not a mere consequence of their preference for a low oxygen environment, but is an active process for maintaining self-renewal and pluripotency, possibly in preparation for rapid response to the metabolic demands of differentiation. Understanding the regulatory mechanisms of this unique metabolism in PSCs is essential for proper derivation, generation, and maintenance of PSCs. In this review, we discuss the metabolic features of PSCs and describe the current understanding of the mechanisms of the metabolic shift during reprogramming from somatic cells to iPSCs, in which the metabolism switches from oxidative phosphorylation (OxPhos) to glycolysis.

The Sweet Surrender: How Myeloid Cell Metabolic Plasticity Shapes the Tumor Microenvironment

Immune cells are one of the most versatile cell types, as they can tailor their metabolic activity according to their required function. In response to diverse environmental cues, immune cells undergo metabolic reprogramming to support their differentiation, proliferation and pro-inflammatory effector functions. To meet a dramatic surge in energetic demand, immune cells rewire their metabolism to utilize aerobic glycolysis. This preferential use of glycolysis even under aerobic conditions is well established in tumor cells, and is known as the "Warburg effect." Tumor cells avidly use glucose for aerobic glycolysis, thereby creating a nutrient-starved microenvironment, outcompeting T cells for glucose, and directly inhibiting T-cell anti-tumoral effector function. Given that both immune and tumor cells use similar modes of metabolism in the tumor stroma, it is imperative to identify a therapeutic window in which immune-cell and tumor-cell glycolysis can be specifically targeted. In this review, we focus on the Warburg metabolism as well as other metabolic pathways of myeloid cells, which comprise a notable niche in the tumor environment and promote the growth and metastasis of malignant tumors. We examine how differential immune-cell activation triggers metabolic fate, and detail how this forbidding microenvironment succeeds in shutting down the vigorous anti-tumoral response. Finally, we highlight emerging therapeutic concepts that aim to target immune-cell metabolism. Improving our understanding of immunometabolism and immune-cell commitment to specific metabolic fates will help identify alternative therapeutic approaches to battle this intractable disease.

Proteostatic and Metabolic Control of Stemness

Adult stem cells, particularly those resident in tissues with little turnover, are largely quiescent and only activate in response to regenerative demands, while embryonic stem cells continuously replicate, suggesting profoundly different regulatory mechanisms within distinct stem cell types. In recent years, evidence linking metabolism, mitochondrial dynamics, and protein homeostasis (proteostasis) as fundamental regulators of stem cell function has emerged. Here, we discuss new insights into how these networks control potency, self-renewal, differentiation, and aging of highly proliferative embryonic stem cells and quiescent adult stem cells, with a focus on hematopoietic and muscle stem cells and implications for anti-aging research.

Data on the potential impact of food supplements on the growth of mouse embryonic stem cells

The use of new compounds as dietary supplements is increasing, but little is known in terms of possible consequences of their use. Pluripotent stem cells are a promising research tool for citotoxicological research for evaluation of proliferation, cell death, pluripotency and differentiation. Using the mouse embryonic stem cell (mESC) model, we present data on three different compounds that have been proposed as new potential supplements for co-adjuvant disease treatments: kaempferol, berberine and Tauroursodeoxycholic acid (TUDCA). Cell number and viability were monitored following treatment with increased concentrations of each drug in pluripotent culture conditions.

Pyruvate and Metabolic Flexibility: Illuminating a Path Toward Selective Cancer Therapies

Dysregulated metabolism is an emerging hallmark of cancer, and there is abundant interest in developing therapies to selectively target these aberrant metabolic phenotypes. Sitting at the decision-point between mitochondrial carbohydrate oxidation and aerobic glycolysis (i.e., the 'Warburg effect'), the synthesis and consumption of pyruvate is tightly controlled and is often differentially regulated in cancer cells. This review examines recent efforts toward understanding and targeting mitochondrial pyruvate metabolism, and addresses some of the successes, pitfalls, and significant challenges of metabolic therapy to date.