(A) Correlation of TKT and CTPS1 expression levels versus the IC50 of gemcitabine in 17 pancreatic cancer cell lines. ‘r’ depicts Pearson correlation value, and p values denote significance of correlation. (B) IHC staining of TKT and CTPS in formalin-fixed paraffin-embedded sections obtained from normal human pancreas, PDAC and metastatic lesions. Scale bars: 250 μm (C) Kaplan-Meier progression-free survival analysis of PDAC patients on gemcitabine/5-FU chemotherapy with high (above a composite score of 9; n=8 for TKT, n=10 for CTPS) or low (below a composite score of 9; n=17 for TKT, n= 15 for CTPS) TKT or CTPS expression levels based on IHC in pancreatic tumors. Comparisons were made by log-rank (Mantel-Cox) test and p values denote significance of alterations in survival in TKT or CTPS high- vs. low-expressing population. (D) Graphical summary for the metabolic basis of gemcitabine resistance in pancreatic cancer. Gemcitabine resistant cells demonstrate increased HIF–1α-mediated glucose uptake and, resultantly, increased flux of glucose through the PPP and pyrimidine biosynthesis to generate dCTP. Inhibition of HIF-1α or pyrimidine biosynthesis increases gemcitabine sensitivity in pancreatic cancer cells. R5P: Ribose-5-Phosphate; UMP: Uridine 5′-monophosphate; dCTP: Deoxycytidine 5′-triphosphate; dFdCTP: 2′,2′-Difluorodeoxycytidine 5′-triphosphate or Gemcitabine 5′-triphosphate. See also Table S1.