Figure 4. The PPP is active in quiescent fibroblasts.

(A) Isotope labeling dynamics in the PPP for proliferating (P), CI7, and CI14 fibroblasts. The fraction of fully labeled hexose phosphate, pentose phosphate, or sedoheptulose-7-phosphate after addition of [U-¹³C]-glucose is plotted for cells in each condition at each time point. Similarly, the fraction of ATP and UTP with five ¹³C atoms is plotted. The 5×¹³C-ATP and 5×¹³C-UTP are uniformly labeled in their ribose portion and unlabeled in the base portion, as confirmed by tandem mass spectrometry analysis. Data are pooled from five experiments, and error bars indicate standard deviation. -P, phosphate. (B) Schematic diagram of lactate labeling from [1, 2-¹³C]-glucose. [1, 2-¹³C]-glucose is converted into 2×¹³C-lactate through the canonical glycolysis pathway and 1×¹³C-lactate through the PPP. (C) Fibroblasts in different proliferative conditions were incubated with [1, 2-¹³C]-glucose for 4 h. Levels of 1×¹³C-lactate and 2×¹³C-lactate were monitored with mass spectrometry. The ratio of 1×¹³C-lactate to 2×¹³C-lactate is plotted for fibroblasts in each condition. Means ± one standard error (n = 4) are shown. Asterisks indicate p-value<0.01 (proliferating versus CI7, p = 0.006, and proliferating versus CI14 fibroblasts p = 0.002 by Student's t test).