Regulation of the proline regulatory axis and autophagy modulates stemness in TP73/p73 deficient cancer stem-like cells.

Cancer stem-like cells (CSLCs) reside as a small population within tumors, which mostly contain a larger population of differentiated cells. With their unique self-renewing abilities, CSLCs remain refractory to various therapeutic interventions, which otherwise kill differentiated cancer cells, and thus are a major culprit behind cancer treatment failures and cancer relapse. Recently, the process of macroautophagy/autophagy has emerged as a potential therapeutic target for eliminating CSLCs, as autophagic homeostasis has been discovered to play an important role in the growth of cancer and normal stem cells, and is required for the maintenance of the non-differentiated state of CSLCs. Our current work now shows that the so-called 'tumor suppressor' TP73/p73 plays an unconventional role in CSLC biology, and positively regulates the growth and stemness of CSLCs through the modulation of autophagy. Our data show that TP73/p73 deficiency, promotes autophagy in CSLCs by activating the autophagy machinery involving AMPK-TSC-MTOR signaling. Mechanistically, TP73/p73 deficiency-induced autophagy occurs as a result of reduced ATP levels resulting from the metabolic perturbations within the proline regulatory axis. Collectively, these findings unveil novel therapeutically-relevant implications for autophagy in the TP73/p73-dependent regulation of stemness within CSLCs.

Regulation of Cancer and Cancer-Related Genes via NAD.

SIGNIFICANCE: NAD+ is an essential redox cofactor in cellular metabolism and has emerged as an important regulator of a wide spectrum of disease conditions, most notably, cancers. As such, various strategies targeting NAD+ synthesis in cancers are in clinical trials. Recent Advances: Being a substrate required for the activity of various enzyme families, especially sirtuins and poly(adenosine diphosphate [ADP]-ribose) polymerases, NAD+-mediated signaling plays an important role in gene expression, calcium release, cell cycle progression, DNA repair, and cell proliferation. Many strategies exploring the potential of interfering with NAD+ metabolism to sensitize cancer cells to achieve anticancer benefits are highly promising, and are being pursued. CRITICAL ISSUES: With the multifaceted roles of NAD+ in cancer, it is important to understand how cellular processes are reliant on NAD+. This review summarizes how NAD+ metabolism regulates various pathophysiological processes in cancer, and how this knowledge can be exploited to devise effective anticancer therapies in clinical settings. FUTURE DIRECTIONS: In line with the redundant pathways that facilitate NAD+ metabolism, further studies should comprehensively understand the roles of the various NAD+-synthesizing as well as NAD+-utilizing biomolecules to understand its true potential in cancer treatment.

HDAC6 differentially regulates autophagy in stem-like versus differentiated cancer cells.

Cancer stem-like cells (CSCs), a small population of pluripotent cells residing within heterogeneous tumor mass, remain highly resistant to various chemotherapies as compared to the differentiated cancer cells. It is being postulated that CSCs possess unique molecular mechanisms, such as autophagic homeostasis, that allow CSCs to withstand the therapeutic assaults. Here we demonstrate that HDAC6 inhibition differentially modulates macroautophagy/autophagy in CSCs as compared to that of differentiated cancer cells. Using human and murine CSC models and differentiated cells, we show that the inhibition or knockdown (KD) of HDAC6 decreases CSC pluripotency by downregulating major pluripotency factors POU5F1, NANOG and SOX2. This decreased HDAC6 expression increases ACTB, TUBB3 and CSN2 expression and promotes differentiation in CSCs in an apoptosis-independent manner. Mechanistically, HDAC6 KD in CSCs decreases pluripotency by promoting autophagy, whereas the inhibition of pluripotency via retinoic acid treatment, POU5F1 or autophagy-related gene (ATG7 and ATG12) KD in CSCs decreases HDAC6 expression and promotes differentiation. Interestingly, HDAC6 KD-mediated CSC growth inhibition is further enhanced in the presence of autophagy inducers Tat-Beclin 1 peptide and rapamycin. In contrast to the results observed in CSCs, HDAC6 KD in differentiated breast cancer cells downregulates autophagy and increases apoptosis. Furthermore, the autophagy regulator p-MTOR, upstream negative regulators of p-MTOR (TSC1 and TSC2) and downstream effectors of p-MTOR (p-RPS6KB and p-EIF4EBP1) are differentially regulated in CSCs versus differentiated cancer cells following HDAC6 KD. Overall these data identify the differential regulation of autophagy as a molecular link behind the differing chemo-susceptibility of CSCs and differentiated cancer cells.

TAp73 Modifies Metabolism and Positively Regulates Growth of Cancer Stem-Like Cells in a Redox-Sensitive Manner.

PURPOSE: Stem-like cancer cells, with characteristic self-renewal abilities, remain highly refractory to various clinical interventions. As such, stemness-inhibiting entities, such as tumor suppressor p53, are therapeutically pursued for their anticancer activities. Interestingly, similar implications for tumor suppressor TAp73 in regulating stemness features within stem-like cancer cells remain unknown.Experimental Design: This study utilizes various in vitro molecular biology techniques, including immunoblotting, qRT-PCR, and mass spectrometry-based proteomics, and metabolomics approaches to study the role of TAp73 in human and murine embryonal carcinoma stem-like cells (ECSLC) as well as human breast cancer stem-like cells (BCSLC). These findings were confirmed using patient-derived brain tumor-initiating cells (BTIC) and in vivo xenograft models. RESULTS: TAp73 inhibition decreases the expression of stem cell transcription factors Oct4, Nanog, and Sox-2, as well as tumorsphere formation capacity in ECSLCs. In vivo, TAp73-deficient ECSLCs and BCSLCs demonstrate decreased tumorigenic potential when xenografted in mice. Mechanistically, TAp73 modifies the proline regulatory axis through regulation of enzymes GLS, OAT, and PYCR1 involved in the interconversion of proline-glutamine-ornithine. Further, TAp73 deficiency exacerbates glutamine dependency, enhances accumulation of reactive oxygen species through reduced superoxide dismutase 1 (SOD1) expression, and promotes differentiation by arresting cell cycle and elevating autophagy. Most importantly, the knockdown of TAp73 in CD133HI BTICs, separated from three different glioblastoma patients, strongly decreases the expression of prosurvival factors Sox-2, BMI-1, and SOD1, and profoundly decreases their self-renewal capacity as evidenced through their reduced tumorsphere formation ability. CONCLUSIONS: Collectively, we reveal a clinically relevant aspect of cancer cell growth and stemness regulation through TAp73-mediated redox-sensitive metabolic reprogramming.

Phosphoglycerate dehydrogenase inhibition induces p-mTOR-independent autophagy and promotes multilineage differentiation in embryonal carcinoma stem-like cells.

Cancer cells with a less differentiated stem-like phenotype are more resistant to therapeutic manipulations than their differentiated counterparts, and are considered as one of the main causes of cancer persistence and relapse. As such, induction of differentiation in cancer stem-like cells (CSLCs) has emerged as an alternative strategy to enhance the efficacy of anticancer therapies. CSLCs are metabolically distinct from differentiated cells, and any aberration from the intrinsic metabolic state can induce differentiation of CSLCs. Therefore, metabolism-related molecular targets, with a capacity to promote differentiation within CSLCs, are of therapeutic importance. Here, we demonstrate that phosphoglycerate dehydrogenase (PHGDH), an essential enzyme catalyzing the synthesis of amino acid serine, is important for maintaining the poorly differentiated, stem-like state of CSLCs. Our data shows that PHGDH deficiency impairs the tumorsphere formation capacity in embryonal carcinoma stem-like cells (ECSLCs), breast cancer stem-like cells (BCSLCs) and patient-derived brain tumor-initiating cells (BTICs), which is accompanied by the reduced expression of characteristic stemness-promoting factors, such as Oct4, Nanog, Sox-2, and Bmi-1. Mechanistically, PHGDH deficiency in ECSLCs promotes differentiation to various lineages via degradation of Oct4 and by increasing the stability of differentiation marker ß3-tubulin. Furthermore, PHGDH inhibition promotes p-mTOR independent but Beclin-1-dependent autophagy, independent of apoptosis. When studied in combination, the inhibition of both PHGDH and p-mTOR in ECSLCs causes further augmentation of autophagy, and additionally promotes apoptosis, demonstrating the clinical applicability of PHGDH-based manipulations in cancer therapies. Recapitulating these in vitro findings in CSLC models, the intratumoral PHGDH expression in patient-derived tumors is positively correlated with the mRNA levels of stemness factors, especially Oct4, and cancer patients co-expressing high levels of PHGDH and Oct4 display significantly lower survival than those with low PHGDH/Oct4 co-expression. Altogether, this study identifies a clinically-relevant role for PHGDH in the regulation of stemness-differentiation axis within CSLCs.

The NAD+ Salvage Pathway Supports PHGDH-Driven Serine Biosynthesis.

NAD+ is a key metabolic redox cofactor that is regenerated from nicotinamide through the NAD+ salvage pathway. Here, we find that inhibiting the NAD+ salvage pathway depletes serine biosynthesis from glucose by impeding the NAD+-dependent protein, 3-phosphoglycerate dehydrogenase (PHGDH). Importantly, we find that PHGDHhigh breast cancer cell lines are exquisitely sensitive to inhibition of the NAD+ salvage pathway. Further, we find that PHGDH protein levels and those of the rate-limiting enzyme of NAD+ salvage, NAMPT, correlate in ER-negative, basal-like breast cancers. Although NAD+ salvage pathway inhibitors are actively being pursued in cancer treatment, their efficacy has been poor, and our findings suggest that they may be effective for PHGDH-dependent cancers.

Autophagic homeostasis is required for the pluripotency of cancer stem cells.

Pluripotency is an important feature of cancer stem cells (CSCs) that contributes to self-renewal and chemoresistance. The maintenance of pluripotency of CSCs under various pathophysiological conditions requires a complex interaction between various cellular pathways including those involved in homeostasis and energy metabolism. However, the exact mechanisms that maintain the CSC pluripotency remain poorly understood. In this report, using both human and murine models of CSCs, we demonstrate that basal levels of autophagy are required to maintain the pluripotency of CSCs, and that this process is differentially regulated by the rate-limiting enzyme in the NAD+ synthesis pathway NAMPT (nicotinamide phosphoribosyltransferase) and the transcription factor POU5F1/OCT4 (POU class 5 homeobox 1). First, our data show that the pharmacological inhibition and knockdown (KD) of NAMPT or the KD of POU5F1 in human CSCs significantly decreased the expression of pluripotency markers POU5F1, NANOG (Nanog homeobox) and SOX2 (SRY-box 2), and upregulated the differentiation markers TUBB3 (tubulin ß 3 class III), CSN2 (casein ß), SPP1 (secreted phosphoprotein 1), GATA6 (GATA binding protein 6), T (T brachyury transcription factor) and CDX2 (caudal type homeobox 2). Interestingly, these pluripotency-regulating effects of NAMPT and POU5F1 were accompanied by contrasting levels of autophagy, wherein NAMPT KD promoted while POU5F1 KD inhibited the autophagy machinery. Most importantly, any deviation from the basal level of autophagy, either increase (via rapamycin, serum starvation or Tat-beclin 1 [Tat-BECN1] peptide) or decrease (via ATG7 or ATG12 KD), strongly decreased the pluripotency and promoted the differentiation and/or senescence of CSCs. Collectively, these results uncover the link between the NAD+ biosynthesis pathway, CSC transcription factor POU5F1 and pluripotency, and further identify autophagy as a novel regulator of pluripotency of CSCs.

NAD+ salvage pathway in cancer metabolism and therapy.

Nicotinamide adenine dinucleotide (NAD+) is an essential coenzyme for various physiological processes including energy metabolism, DNA repair, cell growth, and cell death. Many of these pathways are typically dysregulated in cancer cells, making NAD+ an intriguing target for cancer therapeutics. NAD+ is mainly synthesized by the NAD+ salvage pathway in cancer cells, and not surprisingly, the pharmacological targeting of the NAD+ salvage pathway causes cancer cell cytotoxicity in vitro and in vivo. Several studies have described the precise consequences of NAD+ depletion on cancer biology, and have demonstrated that NAD+ depletion results in depletion of energy levels through lowered rates of glycolysis, reduced citric acid cycle activity, and decreased oxidative phosphorylation. Additionally, depletion of NAD+ causes sensitization of cancer cells to oxidative damage by disruption of the anti-oxidant defense system, decreased cell proliferation, and initiation of cell death through manipulation of cell signaling pathways (e.g., SIRT1 and p53). Recently, studies have explored the effect of well-known cancer therapeutics in combination with pharmacological depletion of NAD+ levels, and found in many cases a synergistic effect on cancer cell cytotoxicity. In this context, we will discuss the effects of NAD+ salvage pathway inhibition on cancer cell biology and provide insight on this pathway as a novel anti-cancer therapeutic target.