FDG uptake tracks the oxidative damage in diabetic skeletal muscle: An experimental study

Abstract

Objectives: The present study aims to verify the relationship between glucose consumption and uptake of ¹⁸F-2-deoxy-glucose (FDG) in the skeletal muscle (SM) of experimental models of streptozotocin-induced diabetes mellitus (STZ-DM).

Methods: The study included 36 Balb/c mice. Two weeks after intraperitoneal administration of saline (control group, n = 18) or 150 mg streptozotocin (STZ-DM group, n = 18), the two cohorts were submitted to an oral glucose tolerance test and were further subdivided into three groups (n = 6 each): untreated and treated with metformin (MTF) at low or high doses (10 or 750 mg/kg daily, respectively). Two weeks thereafter, all mice were submitted to dynamic micro-positron emission tomography (PET) imaging after prolonged fasting. After sacrifice, enzymatic pathways and response to oxidative stress were evaluated in harvested SM.

Results: On PET imaging, the FDG uptake rate in hindlimb SM was significantly lower in nondiabetic mice as compared with STZ-DM-untreated mice. MTF had no significant effect on SM FDG uptake in untreated mice; however, its high dose induced a significant decrease in STZ-DM animals. Upon conventional analysis, the SM standard uptake value was higher in STZ-DM mice, while MTF was virtually ineffective in either control or STZ-DM models. This metabolic reprogramming was not explained by any change in cytosolic glucose metabolism. By contrast, it closely agreed with the catalytic function of hexose-6P-dehydrogenase (H6PD; i.e., the trigger of a specific pentose phosphate pathway selectively located within the endoplasmic reticulum). In agreement with this role, the H6PD enzymatic response to both STZ-DM and MTF matched the activation of the NADPH-dependent antioxidant responses to the increased generation of reactive oxygen species caused by chronic hyperglycemia. Ex vivo analysis of tracer kinetics confirmed that the enhanced SM avidity for FDG occurred despite a significant reduction in glucose consumption, while it was associated with increased radioactivity transfer to the endoplasmic reticulum.

Conclusions: These data challenge the current dogma linking FDG uptake to the glycolytic rate. They instead introduce a new model considering a strict link between the uptake of this glucose analog, H6PD reticular activity, and oxidative damage in diabetes, at least under fasting condition.

Keywords: Diabetes; Fluorodeoxyglucose; Hexose-6P-dehydrogenase; Oxidative stress; Skeletal muscle.

Figure 1

Effect of STZ and MTF on mice serum glucose level, body weight, and clearance. Serum glucose levels after OGTT in the 18 control (solid line) and in the 18 STZ-DM mice (dashed line) (A) (n = 18). Superimposed on this graph, with the y-axis on the right, is the area under the curve, with solid and dashed columns reporting average values for control and STZ-DM mice, respectively (A). (B) Body weight in control (solid columns) and STZ-DM groups (dashed columns) of mice untreated (green) or under low-dose (red) or high-dose (blue) MTF (B). Average fasting glycemia (C), measured 2 weeks thereafter and soon before PET imaging, in control (solid columns) and STZ-DM groups (dashed columns) untreated (green) and treated with either low-dose (red) or high-dose (blue) MTF (each group includes 6 mice). Blood FDG clearance (D) in control (solid columns) and STZ-DM mice (dashed columns) untreated (green) or under low- (red) or high-dose (blue) MTF (n = 6). *p < 0.05; **p < 0.01 vs the corresponding group of nondiabetic mice.

Figure 2

In vivo effects of STZ-DM and metformin on SM MRGlu, slope of Patlak and SUV. Parametric maps of representative control mice (solid line) or STZ-DM mice (dashed line), untreated (green) or under low (red, MTF10) or high (blue, MTF750) doses of MTF (A). Average glucose consumptions in control (solid columns) and STZ-DM groups (dashed columns) of mice untreated (green) or under low-dose (red) or high-dose (blue) MTF treatment (right) (B). Slope of the Patlak regression line $\frac{k_1\times k_3}{k_2+k_3}$ is expressed in panel C. SM SUV of representative mice (D) and average SUV (E). n = 6; *p < 0.05 vs the corresponding group of nondiabetic mice.

Figure 3

Effect of diabetes and metformin on the enzymatic pathway. Catalytic activities of HK (A), PFK (B), G6PD (C), and H6PD (D) in control (solid columns) and STZ-DM groups (dashed columns) of untreated (green) or treated with low-dose (red) or high-dose (blue) MTF (right) SM homogenate. The NADPH/NADP ratio is represented in panel E. The correlation between H6PD activity and MRGlu (F). GR and G6Pase catalytic activities are expressed in panels G and H, respectively. *p < 0.05 vs the corresponding group of nondiabetic mice.

Figure 4

Ex vivo glucose intake and FDG uptake kinetics. Glucose consumption (orange) and FDG fractional uptake (green) in control (solid columns) and STZ-DM of harvested SM (dashed columns) (n = 3, A). Panel B displays the in vitro lumped constant (the ratio between the extraction fractions of FDG and glucose). Panel C displays the proposed 4-C model to describe the ER role in accumulation and loss of FDG. Panels D–H show values in rate constants.

Figure 5

Muscle sections in the redox state (ROS production). Representative images of muscle sections from control (CTRL) and STZ-treated mice (STZ-DM) stained with H₂DCFDA (A) to assess the presence of ROS. Mean fluorescence index (MFI) of H₂DCFDA (B) of CTRL and STZ-DM mice untreated (green) or under low-dose (red) or high-dose (blue) MTF. Correlation between H₂DCFDA (x-axis) and MRGlu (y-axis) (C). n = 3; *p < 0.05 vs the corresponding group of nondiabetic mice.

Figure 6

Muscle sections in the redox state (antioxidant response). Representative images of muscle sections from control (CTRL) and STZ-treated mice (STZ-DM) stained with Mercury Orange (A) to assess the presence of free thiols. The mean fluorescence index (MFI) of Mercury Orange (B) of the CTRL and STZ-DM mice untreated (green) or under low-dose (red) or high-dose (blue) of MTF. n = 3; **p < 0.01 vs the corresponding group of nondiabetic mice.

Figure 7

Proposed model of SM-FDG uptake and its relationship with NADP reduction to NADPH. The cartoon proposes the ER contribution to FDG kinetics. Pathways of glucose and FDG are depicted in black and red, respectively. G, gluconate; 6PG, 6-P-gluconate; F-2D-G, fluoro-deoxy-gluconate; F-2D-PG, fluoro-deoxy-6P-gluconate. Gluts are depicted as squares, G6P-transporter as a circle.