Iron overload in aging Bmp6‑/‑ mice induces exocrine pancreatic injury and fibrosis due to acinar cell loss

Abstract

The relationship between hemochromatosis and diabetes has been well established, as excessive iron deposition has been reported to result in impaired function of the endocrine and exocrine pancreas. Therefore, the objective of the present study was to analyze the effects of iron accumulation on the pancreata and glucose homeostasis in a bone morphogenetic protein 6‑knockout (Bmp6^‑/‑) mouse model of hemochromatosis. The sera and pancreatic tissues of wild‑type (WT) and Bmp6^‑/‑ mice (age, 3 and 10 months) were subjected to biochemical and histological analyses. In addition, ¹⁸F‑fluorodeoxyglucose biodistribution was evaluated in the liver, muscle, heart, kidney and adipose tissue of both animal groups. The results demonstrated that 3‑month‑old Bmp6^‑/‑ mice exhibited iron accumulation preferentially in the exocrine pancreas, with no signs of pancreatic injury or fibrosis. No changes were observed in the glucose metabolism, as pancreatic islet diameter, insulin and glucagon secretion, blood glucose levels and glucose uptake in the liver, muscle and adipose tissue remained comparable with those in the WT mice. Aging Bmp6^‑/‑ mice presented with progressive iron deposits in the exocrine pancreas, leading to pancreatic degeneration and injury that was characterized by acinar atrophy, fibrosis and the infiltration of inflammatory cells. However, the aging mice exhibited unaltered blood glucose levels and islet structure, normal insulin secretion and moderately increased α‑cell mass compared with those in the age‑matched WT mice. Additionally, iron overload and pancreatic damage were not observed in the aging WT mice. These results supported a pathogenic role of iron overload in aging Bmp6^‑/‑ mice leading to iron‑induced exocrine pancreatic deficiency, whereas the endocrine pancreas retained normal function.

Keywords: bone morphogenetic protein 6; glucose homeostasis; pancreas; iron metabolism; diabetes.

Figure 1

Young Bmp6^−/− mice exhibit iron overload in the exocrine pancreas with no histological changes. (A-D) Pancreata from 3-month-old WT and Bmp6^−/− mice (n=6 mice/group) were analyzed for morphological changes by (A) hematoxylin and eosin staining, (B) iron measurement by Perl's Prussian blue staining, (C) degree of pancreatic inflammation by neutrophil marker CD15 and (D) macrophage marker KiM2R immunostaining. Original magnification, ×20. Scale bar, 50 µm. Bmp6^−/−, bone morphogenetic protein 6-knockout; WT, wild-type.

Figure 2

Young Bmp6^−/− mice exhibit no signs of pancreatic fibrosis. (A) Pancreata from 3-month-old WT and Bmp6^−/− mice (n=6 mice/group) were analyzed for collagen deposition by Sirius red staining. Original magnification, ×10. Scale bar, 100 µm. (B) Serum amylase and lipase levels were assessed as pancreatic injury markers. Data are presented as the mean ± SD. Bmp6^−/−, bone morphogenetic protein 6-knockout; WT, wild-type.

Figure 3

Young Bmp6^−/− mice exhibit normal glucose metabolism. (A) Blood glucose levels were measured following overnight fasting in 3-month-old WT and Bmp6^−/− mice (n=6 mice/group). (B-D) Pancreatic sections were analyzed for (B) mean diameter of pancreatic islets, (C) insulin and (D) glucagon secretion by immunohistochemistry. Original magnification, ×20. Scale bar, 50 µm. (E) Biodistribution of 18-FDG in target tissues was assessed 60 min post-i.p. injection of 10-18 MBq 18-FDG (n=4 mice/group). 18-FDG uptake was expressed as SUV. Data are presented as the mean ± SD. ^*P<0.05 vs. WT. Bmp6^−/−, bone morphogenetic protein 6-knockout; WT, wild-type; 18-FDG, ¹⁸F-fluorodeoxyglucose; SUV, standardized uptake value.

Figure 4

Aging Bmp6^−/− mice exhibit severe iron overload in the exocrine pancreas, leading to acinar cell loss and macrophage infiltration. (A-D) Pancreata from 10-month-old WT and Bmp6^−/− mice (n=6 mice/group) were analyzed for (A) morphological changes by hematoxylin and eosin staining, (B) iron measurement by Perl's Prussian blue staining, (C) degree of pancreatic inflammation by neutrophil marker CD15 (white arrows) and (D) macrophage marker KiM2R (black arrows) immunostaining. Original magnification, ×20. Scale bar, 50 µm. Bmp6^−/−, bone morphogenetic protein 6-knockout; WT, wild-type.

Figure 5

Aging Bmp6^−/− mice develop pancreatic fibrosis. (A and B) Pancreata from 10-month-old WT and Bmp6^−/− mice (n=6 mice/group) were analyzed for (A) collagen deposition by Sirius red staining and (B) relative amount of pancreatic collagen quantified by morphometric analysis. Original magnification, ×10. Scale bar, 100 µm. (C) Serum amylase and lipase levels were assessed as pancreatic injury markers. Data are presented as the mean ± SD. ^*P<0.05 vs. WT. Bmp6^−/−, bone morphogenetic protein 6-knockout; WT, wild-type.

Figure 6

Damaged exocrine pancreas does not affect glucose metabolism in aging Bmp6^−/− mice. (A) Blood glucose levels were measured following overnight fasting in 10-month-old WT and Bmp6^−/− mice (n=6 mice/group). (B-D) Pancreatic sections were analyzed for (B) mean diameter of pancreatic islets, (C) insulin and (D) glucagon secretion by immunohistochemistry. Original magnification, ×20. Scale bar, 50 µm. Data are presented as the mean ± SD. Bmp6^−/−, bone morphogenetic protein 6-knockout; WT, wild-type.