Epithelial-mesenchymal transition induction is associated with augmented glucose uptake and lactate production in pancreatic ductal adenocarcinoma

Cancer cells with EMT features are observed under various experimental settings and clinical situations. In recent years, several studies have been conducted investigating the metabolic changes during EMT in breast, lung, and ovarian cancers, following an increased recognition of metabolic reprogramming as a hallmark of tumor development [20–27]. In view of the important role EMT plays in PDAC metastasis and chemoresistance, we sought to understand the metabolic adaptations underlying PDAC EMT. Here, we showed that exposure to TNF? and TGF?, two factors commonly present in the tumor microenvironment that signal through distinct pathways, induced overt EMT in the Panc-1 cell line of human PDAC, reflected by EMT-like morphological, molecular, and functional changes. The induction of EMT by both of these factors was accompanied by augmentations of glucose uptake and lactate secretion, while no major changes to oxidative metabolism were observed. In addition, ¹³C-glucose metabolomics combined with flux analysis revealed that the increase in lactate production by these two treatments might be achieved via different mechanisms and suggested possible contributions of nutrients other than glucose.

While there is relatively limited literature in this area, the nature of the metabolic alterations appears to vary widely across different cancer types and EMT models. One of the earliest studies reported findings similar to our current results, with increased Glut1 expression and glucose uptake in the breast cancer cell line MCF-7 undergoing TGF?-induced EMT [20]. The downregulation of gluconeogenic enzyme FBP1 was described as a predominant EMT-related change by Dong et al. [21], on the basis of comparison of luminal and the more metastatic basal subtype of breast cancer. The enhancement of glycolysis and reduction in gluconeogenesis were also observed in two breast cancer cell lines exhibiting EMT features after mammosphere cultures by Kondaveeti and colleagues, who also noted decreases in enzymes involved in the pentose phosphate and hexosamine synthesis pathways [26]. In contrast, three KRAS- or EGFR-driven non-small cell lung cancer (NSCLC) cell lines treated with TGF? or erlotinib to induce EMT displayed reduced glycolysis to oxidation ratio and lower PDK4 expression [23]. Increased oxygen consumption rate and coordinated suppression of lipogenesis were reported in a separate study on a TGF?-induced EMT model in NSCLC [27]. Surprisingly, increased expression of GLUT3 transporter was also associated with TGF? treatment in the same cancer type [24]. In ovarian cancer, Aspuria et al. induced EMT via succinate dehydrogenase B knockdown and saw decreased maximal OCR, accompanied by increased glucose contribution to pentose phosphate pathway and nucleotide synthesis [22].

Physiologically, the induction of EMT is the result of contextual cues originating from surrounding cells and the circulation acting upon cancer cells that are sensitized by oncogenic mutations. Therefore, the diversity of the tumor microenvironment and genetic composition could partially underscore the heterogeneity of EMT responses. In this context, it is worth noting however that within the same cancer type, there is no current evidence that the status of common mutations appears to have a major impact on EMT-related metabolic reprogramming, as NSCLC cell lines harboring mutant or wild-type KRAS, EGFR, and TP53, as well as breast cancer cell lines with or without HER2 overexpression do not display mutation-specific changes [23, 26]. Given the complexity of genetic events in cancers, further comprehensive studies are needed to uncover any possible interactions between genomic and metabolic landscapes in relation to EMT.

The disparity of EMT metabolic reprogramming in different contexts could additionally be related to variations in the accompanying phenotypic transformations, which are potentially linked to differential metabolic modulations. For instance, stem cells exhibit heightened rates of glycolysis [44, 45] and inhibition of glycolysis was reported to suppress stemness features in glioblastoma stem-like cells [46]. Hence, the increased glycolytic flux observed in some EMT models may be related to the stemness conferred by EMT. Increases in glucose uptake would presumably promote cell survival under hypoxic and nutrient poor conditions, while the lactate secreted could facilitate matrix degradation [47] and evasion of immune surveillance [48, 49], echoing the promotion of apoptotic resistance and invasiveness by EMT. On the other hand, elevated mitochondrial biogenesis and oxidative phosphorylation have, in some cases, been associated with chemoresistance and metastatic behaviors which are also promoted by EMT [50–52].

One prominent feature of PDAC is the presence of dense stromal compartment surrounding the tumors. The dense stroma both constraints the delivery of oxygen/nutrients to the tumor cells and can also act as a physical barrier for extravasation. For PDAC cells that have undergone EMT, the increased lactate excretion may be essential for traversing the stromal layer before reaching the circulation and maintaining viability along the way. Recent reports by Fisher et al. [53] and Zheng et al. [54] questioned the role of EMT in cancer metastasis but provided in vivo evidence that EMT cells are resistant to chemotherapy. It is known that after chemotherapy or experimental induction of gemcitabine resistance, a proportion of PDAC cells exhibit EMT features and display signature stem cell surface markers [12, 55]. Treatment with the anti-glycolytic agent 3-bromopyruvate sensitized primary PDAC cancer stem cells to gemcitabine [56]. The metabolic alterations seen in PDAC cells in the present study are consistent with an increase in stem-like properties with EMT, which could contribute to chemoresistance.

Pinpointing the molecular changes underlying the observed metabolic reprogramming with EMT is important for developing targeted therapies. It could be reasoned that increased lactate output was driven by increased expression of GLUT transporters (1 and 3) and the consequent increase in glycolytic influx, as no major changes in glycolytic and lactate-producing enzymes were observed. However, ¹³C enrichment data suggested that lactate was simultaneously produced from both glucose and non-glucose sources, albeit the former contributing to the majority. While glutamine could act as a significant non-glucose substrate for cancer cells, data from radioactive and stable tracer experiments revealed that neither the TCA cycle activity was enhanced nor its intermediates contributed to lactate production. Further research will therefore be needed to identify the full spectrum of lactate precursors in the EMT models when strategies to “starve” the malignant cells are pursued. In addition, enrichment data from TGF?-treated cells raised the prospect that a greater pyruvate-to-lactate conversion during EMT could also be achieved by downregulating pyruvate consumption pathways, e.g., PDH. The observed changes of PDK isoforms, which control the partitioning of pyruvate to the TCA cycle, were disparate, possibly providing a mechanism of fine tuning glucose metabolism.