Cancer heterogeneity constitutes the major source of disease progression and therapy failure. Tumors comprise functionally diverse subpopulations, with cancer stem cells (CSCs) as the source of this heterogeneity. Since these cells bear in vivo tumorigenicity and metastatic potential, survive chemotherapy and drive relapse, its elimination may be the only way to achieve long-term survival in patients. Thanks to the great advances in the field over the last few years, we know now that cellular metabolism and stemness are highly intertwined in normal development and cancer. Indeed, CSCs show distinct metabolic features as compared with their more differentiated progenies, though their dominant metabolic phenotype varies across tumor entities, patients and even subclones within a tumor. Following initial works focused on glucose metabolism, current studies have unveiled particularities of CSC metabolism in terms of redox state, lipid metabolism and use of alternative fuels, such as amino acids or ketone bodies. In this review, we describe the different metabolic phenotypes attributed to CSCs with special focus on metabolism-based therapeutic strategies tested in preclinical and clinical settings.

Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest tumors, partly due to its intrinsic aggressiveness, metastatic potential, and chemoresistance of the contained cancer stem cells (CSCs). Pancreatic CSCs strongly rely on mitochondrial metabolism to maintain their stemness, therefore representing a putative target for their elimination. Since mitochondrial homeostasis depends on the tightly controlled balance between fusion and fission processes, namely mitochondrial dynamics, we aim to study this mechanism in the context of stemness. In human PDAC tissues, the mitochondrial fission gene DNM1L (DRP1) was overexpressed and positively correlated with the stemness signature. Moreover, we observe that primary human CSCs display smaller mitochondria and a higher DRP1/MFN2 expression ratio, indicating the activation of the mitochondrial fission. Interestingly, treatment with the DRP1 inhibitor mDivi-1 induced dose-dependent apoptosis, especially in CD133⁺ CSCs, due to the accumulation of dysfunctional mitochondria and the subsequent energy crisis in this subpopulation. Mechanistically, mDivi-1 inhibited stemness-related features, such as self-renewal, tumorigenicity, and invasiveness and chemosensitized the cells to the cytotoxic effects of Gemcitabine. In summary, mitochondrial fission is an essential process for pancreatic CSCs and represents an attractive target for designing novel multimodal treatments that will more efficiently eliminate cells with high tumorigenic potential.

Pancreatic ductal adenocarcinoma (PDAC), the most common form of pancreatic cancer, is an extremely lethal disease due to late diagnosis, aggressiveness and lack of effective therapies. Considering its intrinsic heterogeneity, patient stratification models based on transcriptomic and genomic signatures, with partially overlapping subgroups, have been established. Besides molecular alterations, PDAC tumours show a strong desmoplastic response, resulting in profound metabolic reprogramming involving increased glucose and amino acid consumption, as well as lipid scavenging and biosynthesis. Interestingly, recent works have also revealed the existence of metabolic subtypes with differential prognosis within PDAC, which correlated to defined molecular subclasses in patients: lipogenic subtype correlated with a classical/progenitor signature, while glycolytic tumours associated with the highly aggressive basal/squamous profile. Bioinformatic analyses have demonstrated that the representative genes of each metabolic subtype are up-regulated in PDAC samples and predict patient survival. This suggests a relationship between the genetic signature, metabolic profile, and aggressiveness of the tumour. Considering all this, defining metabolic subtypes represents a clear opportunity for patient stratification considering tumour functional behaviour independently of their mutational background.

Lipid droplets as metabolic determinants for stemness and chemoresistance in cancer

The pronounced desmoplastic response in pancreatic ductal adenocarcinoma (PDAC) contributes to the development of a microenvironment depleted of oxygen and nutrients. To survive in this hostile environment, PDAC cells use various adaptive mechanisms that may represent therapeutic targets. In this study, we showed that nutrient starvation and microenvironmental signals commonly present in PDAC tumors activate PPARδ to rewire cellular metabolism and promote invasive and metastatic properties both in vitro and in vivo. Mild mitochondrial inhibition induced by low-dose etomoxir or signals from tumor-associated macrophages altered the lipidome and triggered the downstream transcriptional program of PPARδ. Specifically, PPARδ reduced mitochondrial oxygen consumption and boosted the glycolytic capacity by altering the ratio of MYC and PGC1A expression, two key regulators of pancreatic cancer metabolism. Notably, genetic or pharmacologic inhibition of PPARδ prevented this metabolic rewiring and suppressed both invasiveness in vitro and metastasis in vivo. These findings establish PPARδ as a central driver of metabolic reprogramming in response to starvation and tumor microenvironmental cues that promotes a prometastatic phenotype in PDAC, suggesting that PPARδ inhibition could serve as a therapeutic strategy to combat PDAC progression. Significance: Nutrient starvation and microenvironmental signals activate PPARδ in pancreatic cancer to support survival and metastasis by promoting metabolic plasticity and invasiveness, providing a strong rationale for developing PPARδ-targeted therapies for pancreatic cancer.

The peroxisome proliferator-activated receptors (PPAR-α, PPAR-δ, and PPAR-γ) are transcription factors that belong to the nuclear hormone receptor superfamily. Upon activation by specific lipids, they regulate gene expression by directly binding to PPAR response elements (PPREs) in the DNA. Although the functions of the different PPARs are specific to the isoform, tissue, and context, all three PPARs are generally involved in energy homeostasis through lipid sensing in physiological conditions. Importantly, there is increasing evidence linking PPARs with malignant behavior in cancer, regulating features frequently attributed to the aggressive subpopulation of cancer stem cells (CSCs): self-renewal, tumor initiation, chemoresistance, metastasis, and immune evasion. However, contradictory effects have been described for each isoform in various cancer types, and their implication in these malignant features may not consistently follow a pro- or anti-tumoral pattern. In this review, we revise the current knowledge on the role of the PPAR family members in cancer, with a special focus on cancer stemness, and discuss the potential for PPARs as therapeutic targets in CSC-driven relapse and resistance.