Cytochrome c Oxidase Activity as a Metabolic Regulator in Pancreatic Beta-Cells

Abstract

Pancreatic β-cells couple glucose-stimulated insulin secretion (GSIS) with oxidative phosphorylation via cytochrome c oxidase (COX), a mitochondrial respiratory-chain enzyme. The Cohen diabetic-sensitive (CDs) rats exhibit hyperglycemia when fed a diabetogenic diet but maintain normoglycemia on a regular diet. We have previously reported a decreased COX activity in CDs rats and explored its relevance for type 2 diabetes (T2D). In this study, we investigated the relation between COX activity in islets, peripheral-blood mononuclear cells (PBMCs), and GSIS during diabetes development in CDs rats fed a diabetogenic diet for 4, 11, 20, and 30 days and during reversion to normoglycemia in hyperglycemic CDs rats fed a reversion diet for 7, 11, and 20 days. An oral glucose-tolerance test was performed at different periods of the diets measuring blood glucose and insulin concentrations. COX activity was determined in islets and PBMCs isolated from rats at the different periods of the diets. We demonstrated a progressive reduction in COX activity in CDs-islets that correlated positively with the decreasing GSIS (R2 = 0.9691, p < 0.001) and inversely with the elevation in blood glucose levels (R2 = 0.8396, p < 0.001). Hyperglycemia was initiated when islet COX activity decreased below 46%. The reversion diet restored >46% of the islet COX activity and GSIS while re-establishing normoglycemia. Interestingly, COX activity in PBMCs correlated significantly with islet COX activity (R2 = 0.8944, p < 0.001). Our data support islet COX activity as a major metabolic regulator of β-cells function. The correlation between COX activity in PBMCs and islets may serve as a noninvasive biomarker to monitor β-cell dysfunction in diabetes.

Keywords: blood glucose levels; cytochrome c oxidase; glucose stimulated insulin secretion; pancreatic β-cells.

Figure 1

(A,B) Area under the curve (AUC) of (A) blood glucose and (B) plasma insulin (calculated as insulin/glucose, “insulinogenic-index”) determined before (overnight fast, 0) and after (30, 60, 90, 120 min) glucose administration (350 mg/100 g b.wt) in CDs and Control rats fed RD and a diabetogenic diet for 4, 11, 20 and 30 days and a reversion diet for 4, 7, 11, and 20 days. The light (CDs) and dark (CDr) grey curves represent the two-hour post-OGTT glucose and insulin levels. Data are means ± SEM of 5–8 independent experiments, * p < 0.05 RD, # p < 0.05 vs. rats on diabetogenic diet, $ p < 0.05 vs. control, n = 5–8 rats/feeding period.

Figure 1

(A,B) Area under the curve (AUC) of (A) blood glucose and (B) plasma insulin (calculated as insulin/glucose, “insulinogenic-index”) determined before (overnight fast, 0) and after (30, 60, 90, 120 min) glucose administration (350 mg/100 g b.wt) in CDs and Control rats fed RD and a diabetogenic diet for 4, 11, 20 and 30 days and a reversion diet for 4, 7, 11, and 20 days. The light (CDs) and dark (CDr) grey curves represent the two-hour post-OGTT glucose and insulin levels. Data are means ± SEM of 5–8 independent experiments, * p < 0.05 RD, # p < 0.05 vs. rats on diabetogenic diet, $ p < 0.05 vs. control, n = 5–8 rats/feeding period.

Figure 2

(A,B) COX-activity in (A) islets and (B) PBMCs isolated from CDs and control rats fed RD and a diabetogenic diet for 4, 11, 20, and 30 days and the reversion diet for 4, 7, 11, and 20 days, (n = 5–8 rats/feeding period). Data are means ± SEM of COX/CS activity, * p < 0.05 RD, # p < 0.05 vs. diabetogenic diet; $ p < 0.05 vs. control.

Figure 3

(A,D) Pearson correlation between islets COX/CS activity ratio (open circle), PBMCs COX/CS activity ratio (black circle), and (A) the insulinogenic index_AUC (insulin/glucose) or (B) AUC glucose of CDs rats fed diabetogenic and reversion-diets. (C) Pearson correlation between islets vs. PBMCs COX activity (D). Data are means ± SEM of 5–8 independent experiments.

Figure 4

A proposed scheme describing the relationship between islet COX activity and blood glucose levels in CDs and Control rats according to the different times spent on the diabetogenic and reversion diets. Each circle represents a time period in the diabetogenic/reversion diets. Normoglycemia (blue zone, 2 h post-OGTT blood glucose levels < 140 mg/dL) was observed in rats that had islet COX activity above the minimal COX activity threshold (46% of baseline), while hyperglycemia (red zone 2 h post-OGTT blood glucose levels > 200 mg/dL) was observed when COX activity decreased below the threshold. This “islet COX activity threshold” seems to be mandatory to sustain normoglycemia in this rat model.