Abstract

Cancer cells exhibit a significantly altered metabolic phenotype, characterized by a distinctive reliance on glucose to support their growth, survival, and proliferation through a unique energy extraction mechanism. This metabolic reprogramming is epitomized by an increased glucose demand and a preference for lactic fermentation, even in the presence of oxygen, rather than the conventional citric acid cycle utilized by non-cancerous cells. This phenomenon, known as the Warburg Effect, serves as a hallmark of cancer metabolism. The implications of this metabolic shift extend beyond cellular energy dynamics, as it initiates the development of a novel tumor microenvironment characterized by adverse conditions such as hypoxia and glucose scarcity, particularly pronounced in pancreatic cancer.\nPancreatic ductal adenocarcinoma (PDA), a notably therapy-resistant malignancy, exemplifies this metabolic adaptation. Under conditions of glucose limitation, PDA cells demonstrate a remarkable ability to utilize uridine as a primary energy substrate, a process regulated by uridine phosphorylase 1 (UPP1). UPP1 plays a central role in metabolizing uridine-derived ribose, thereby supporting central carbon metabolism, maintaining redox homeostasis, and promoting cell survival and proliferation under nutrient-restricted conditions. This metabolic pathway is under the regulatory control of KRAS–MAPK signaling, which is further amplified in response to nutrient deprivation. Consistent with these observations, tumor specimens show elevated UPP1 expression compared to normal tissues, correlating with poor clinical outcomes. The availability of uridine in the tumor microenvironment and its active catabolism within tumor cells underscore the clinical importance of these metabolic processes. Notably, genetic ablation of UPP1 disrupts uridine utilization and significantly suppresses PDA growth in murine models.\nIn summary, this review highlights uridine metabolism as a pivotal compensatory mechanism enabling PDA cells to adapt to nutrient-deprived environments. By influencing the Warburg Effect, uridine utilization integrates nucleotide metabolism with energy production, revealing critical molecular pathways that regulate glucose metabolism in tumor cells. These findings provide novel insights into the metabolic flexibility of PDA and identify potential therapeutic targets for its treatment.\n

Content

I.     Introduction
Cancer encompasses more than uncontrolled proliferation, revealing profound alterations in cellular behavior. Central to this complex landscape is the Warburg effect—a metabolic reprogramming first described by Otto Warburg in 1924—where cancer cells preferentially utilize aerobic glycolysis over oxidative phosphorylation ^1,2. Beyond its role in energy production, the Warburg effect initiates a cascade of alterations that shape the tumor microenvironment, which is frequently characterized by low vascularity and hypoxia, leading to glucose scarcity ^3,4. At first glance, glucose limitation appears paradoxical to the mechanisms driven by the Warburg effect, seemingly incompatible with tumor survival, growth, and proliferation. Nevertheless, tumors exhibiting these characteristics often demonstrate extreme invasiveness and rapid progression ⁵.
Pancreatic ductal adenocarcinoma (PDA), renowned for its aggressive nature and resistance to therapies, exemplifies this adaptive complexity through its unconventional reliance on alternative energy substrates. Recent studies highlight uridine utilization as a primary energy source in PDA, underscoring the remarkable metabolic flexibility of cancer cells ⁶. The oncogene KRAS, which is frequently mutated in PDA, drives the synthesis of its protein KRAS ⁷. When mutated, KRAS becomes hyperactivated, serving as a central regulator of signal transduction pathways that promote cell proliferation and growth. This overactivation leads to the upregulation of transcription factors such as uridine phosphorylase 1 UPP1 and uridine phosphorylase 2 UPP2 ⁸. These enzymes catalyze the conversion of uridine, derived from RNA, into ribose, enabling an alternative mechanism for energy extraction.
In this review, I aim to explore the intricate interplay between the Warburg effect and uridine utilization in pancreatic cancer. The molecular mechanisms governing energy production and biosynthesis in the pancreatic tumor microenvironment will be analyzed, offering a new perspective on the role of uridine as a pivotal nutrient for PDA under glucose-deprived conditions. UPP1 emerges as a key regulator of these processes, orchestrated through the KRAS–MAPK signaling pathway.

II. MICROENVIRONMENT OF PDA
2.1 Oncogene Mutations and the Warburg Effect
The Warburg effect is defined by a metabolic shift favoring aerobic glycolysis, even in the presence of oxygen and functional mitochondria ^1,9. At the molecular level, cancer cells exhibit elevated glucose uptake, facilitated by the upregulation of glucose transporters such as SLC2A1 ¹. Once internalized, glucose preferentially undergoes glycolysis rather than oxidative phosphorylation, a process typically restricted to hypoxic conditions in non-cancerous cells. The increased glycolytic flux also activates the pentose phosphate pathway, enhancing biosynthesis and supporting cell division and local pressure.
In most cancers, this metabolic shift is driven by oncogenic mutations, including KRAS activation, MYC overexpression, PI 3-kinase activation, and loss-of-function mutations in PTEN and TP53, rather than direct alterations in mitochondrial respiration complexes ¹⁰. Notably, KRAS mutations occur in up to 95% of pancreatic tumors and activate the KRAS–MAPK cascade, a pivotal driver of metabolic reprogramming [Fig. 1].

Figure 1: K-Ras signaling pathway: From KRAS transcription to cell growth and proliferation via the KRAS–MAPK and PI3K–AKT–mTOR pathways.
KRAS transcription leads to the synthesis of K-Ras, a protein critical for signal transduction. The pathway initiates with ligand binding to the Receptor Tyrosine Kinase (RTK), triggering phosphorylation of tyrosine residues on the receptor. The SH2 domain of docking proteins like GRB2 binds to these phosphotyrosine residues, enabling GRB2 to interact with the guanine nucleotide exchange factor XYLT2 through its SH3 domains. Activated XYLT2, upon association with phosphorylated EGFR, catalyzes the exchange of GDP for GTP on K-Ras, activating the protein. Once activated, K-Ras propagates the signal through two major pathways: the MAPK cascade and the PIK3CA–AKT1–mTOR pathway. These cascades converge on the activation of transcription factors, ultimately promoting cell growth and proliferation.

Signaling pathways such as PI3K–AKT–mTOR play a central role in promoting glycolytic metabolism, enhancing glucose uptake and utilization ². Furthermore, stabilization of hypoxia-inducible factor 1 subunit alpha HIF1A under hypoxic conditions upregulates glycolytic enzymes, reinforcing the Warburg effect. Beyond energy production, glycolysis supports biosynthetic pathways, providing intermediates essential for nucleotide, lipid, and amino acid synthesis, which fuel the rapid proliferation of cancer cells ^11,12. This understanding of the Warburg effect has significant therapeutic implications, with enzymes like hexokinase 2 and lactate dehydrogenase A emerging as promising targets for intervention. Similarly, disruption of signaling pathways such as PI3K–AKT–mTOR offers potential therapeutic avenues to exploit cancer’s metabolic vulnerabilities ^13,14.

2.2 Creation of the Microenvironment
Metabolic reprogramming in tumors contributes to a series of external changes that shape a distinctive microenvironment. Pancreatic cancer, in particular, exhibits unique and challenging features within its tumor microenvironment, contributing to its aggressiveness and therapeutic resistance ³. This includes the formation of a dense desmoplastic stroma composed of fibrotic tissue and extracellular matrix, which creates physical barriers that impede drug penetration and limit the efficacy of chemotherapy. Additionally, the stroma exacerbates tissue tension and local pressure, influencing the fractal-like architecture of pancreatic tumors ¹⁵.
Poor vascularization and consequent hypoxia further define the microenvironment, activating adaptive responses such as the upregulation of HIF1A ⁵, which promotes angiogenesis, alters glucose metabolism, and supports cancer cell survival. The combination of reduced blood supply and hypoxia poses significant challenges for drug delivery and creates selective pressures that favor more aggressive and treatment-resistant cancer cell phenotypes ^3,16. Moreover, the accumulation of lactic acid from glycolysis contributes to extracellular acidosis, while intracellular alkalosis enhances mitogenic signaling, bypassing growth-inhibitory signals. Carbonic anhydrase isoforms mediate this acid-base imbalance, playing a critical role in maintaining tumor cell survival ¹⁷ [Fig. 2].

Figure 2: KRAS regulation of UPP1 via MAPK signaling in PDA cells.

1.    KRAS oncogene and K-Ras protein: KRAS, located on the short arm of chromosome 12 (12p11.1), encodes the K-Ras protein, a key component of the KRAS–MAPK signaling cascade. Mutations in KRAS result in hyperactivation of the K-Ras protein.
2.    Role in signaling and proliferation: K-Ras transduces signals via the MAPK cascade, promoting cell proliferation and growth as described in Figure 1.
3.    MAPK cascade and transcription factors: The MAPK pathway, involving ZHX2, MAP2K7, and EPHB2, activates transcription of key genes, including UPP1 and UPP2, which are critical for uridine metabolism.
4.    UPP1 upregulation: UPP1, located on chromosome 12 (12p12.3), is transcriptionally upregulated under these conditions, leading to increased production of uridine phosphorylase.
5.    Uridine metabolism: Elevated UPP1 activity facilitates the conversion of uridine into uracil and ribose-1-phosphate (ribose-1-P), as depicted in the accompanying reaction schematic.
6.    Ribose-1-P and glycolysis: Ribose-1-P is converted into ribose-5-phosphate by phosphopentomutase and subsequently metabolized through the phosphoketolase pathway into glyceraldehyde-3-phosphate. This intermediate converges with glycolysis, where it is either:
o    Metabolized into pyruvate for gluconeogenesis and glucose production, or
o    Fermented into lactate, a process preferentially utilized by cancer cells as part of the Warburg Effect.

2.3 Microenvironment in Tumor Progression
The dynamic tumor microenvironment profoundly influences cancer progression. Interactions between cancer cells, stromal cells, extracellular matrix components, and immune cells shape tumor behavior and drive metastasis. Stromal cells, such as cancer-associated fibroblasts and endothelial cells, engage in reciprocal signaling with cancer cells, facilitating processes such as migration, invasion, and the formation of pre-metastatic niches ^3,18. This intricate interplay underscores the complexity of the tumor microenvironment, which remains a key focus for therapeutic strategies.

III. URIDINE AS A PRIMARY ENERGY SOURCE
3.1 Uridine and Glucose Limitation
The pivotal role of altered metabolism in tumor progression raises critical questions regarding the specific metabolites that sustain pancreatic ductal adenocarcinoma (PDA) cells. Recent studies have demonstrated the significant role of uridine as an essential energy substrate under glucose-deprived conditions. A comprehensive analysis of over 175 metabolites across 21 pancreatic cancer cell lines established uridine’s critical function as a metabolic fuel for PDA cells ¹⁹.
Uridine’s utilization as a metabolic adaptation is intricately linked to the expression of UPP1, which facilitates the release of uridine-derived ribose. This process supports central carbon metabolism, maintains redox homeostasis, and ensures the survival and proliferation of glucose-deprived PDA cells ^6,20. Mechanistically, stable expression of UPP1 and UPP2 open reading frames (ORFs) significantly enhances cell proliferation in galactose-based media, provided uridine is available ^19,21. This proliferative effect is contingent on the presence of uridine; neither the expression of UPP1/UPP2 nor the exogenous addition of uridine yields similar effects in glucose-containing media ²².

3.2 UPP1 Expression and RNA-Derived Uridine
The link between uridine consumption and UPP1 expression has been confirmed through mRNA and protein analyses, which underscore its association with nucleoside metabolism. Given the inherent instability of RNA and its susceptibility to RNase-mediated degradation, the potential role of RNA-derived uridine in supporting cell growth in a UPP1-dependent manner was investigated ¹⁹. Experimental data demonstrated that supplementing glucose-free media with purified yeast RNA enabled proliferation of UPP1-expressing cells. This finding highlights the relationship between elevated uridine phosphorylase activity and the capacity of PDA cells to grow in the absence of both glucose and uridine ^23,24.

3.3 Transcriptional Regulation of UPP1
The physiological significance of UPP1 in facilitating uridine metabolism is underscored by its role in enabling PDA cells to adapt to uridine availability. Elevated UPP1 expression, particularly in PDA tumors compared to non-tumoral tissues, is associated with poor patient outcomes. Experimental deletion of UPP1 in immunocompetent murine models resulted in reduced tumor growth, emphasizing its functional relevance ^25,26. Novel regulatory mechanisms, including TP53-mediated pathways, further highlight the importance of UPP1 in PDA metabolism, presenting promising avenues for translational research ²⁷.

IV. KRAS–MAPK SIGNALING
4.1 Transduction Cascade in UPP1 Regulation
The regulation of UPP1 in PDA is closely linked to the KRAS–MAPK signaling pathway, which is further enhanced under nutrient-restricted conditions ²⁸. UPP1 facilitates the conversion of uridine into ribose-1-phosphate, integrating uridine-derived ribose into central carbon metabolism as a compensatory mechanism. This metabolic adaptation is influenced by KRAS mutations, which are present in up to 95% of PDA cases and drive the synthesis of mutated K-Ras protein. This aberrant protein hyperactivates the downstream MAPK signaling cascade, promoting protein synthesis and cell proliferation [Fig. 2] ^27,28. Furthermore, KRAS mutations contribute to the metabolic phenotype of PDA, including upregulation of the SLC2A1 glucose transporter, a hallmark of the Warburg effect and its associated glycolytic reliance.

4.2 MAPK Cascade as a Therapeutic Target
Direct targeting of mutant KRAS has proven challenging, prompting efforts to explore downstream effectors within the pathway, including ZHX2 → MAP2K7 → EPHB2 ²⁹. However, recent findings suggest that inhibiting MAP2K7 in isolation often triggers survival mechanisms, such as autophagy, which diminish therapeutic efficacy. Combining therapies that target multiple nodes within the KRAS–MAPK signaling cascade holds promise for addressing the metabolic and proliferative adaptations of PDA ^7,29.

CONCLUSIONS
A century after the discovery of the Warburg effect, our understanding of tumor dynamics has evolved substantially, revealing intricate mechanisms that sustain cancer survival. Energy acquisition and metabolism in pancreatic ductal adenocarcinoma exemplify this complexity, as PDA demonstrates a unique reliance on alternative substrates like uridine. Key enzymes, UPP1 and UPP2, play critical roles in converting uridine into ribose-1-phosphate, providing a metabolic substitute for glucose. Central to this process is KRAS, which orchestrates transcriptional regulation of genes like UPP1 and UPP2 through the MAPK pathway.
The exploration of the KRAS–MAPK signaling cascade as a therapeutic target has opened new frontiers in PDA treatment. While direct targeting of KRAS remains elusive, inhibitors acting on downstream components such as ZHX2, MAP2K7, or EPHB2 hold potential for disrupting the tumor's metabolic dependencies. Advances in understanding these pathways provide hope for developing innovative therapies against PDA, one of the most aggressive and challenging cancers to treat.