Original Contribution
β-cell death and proliferation after intermittent hypoxia: Role of oxidative stress

https://doi.org/10.1016/j.freeradbiomed.2008.11.026Get rights and content

Abstract

Intermittent hypoxia (IH), such as occurs in sleep apnea, induces increased oxidative stress and is associated with altered glucose homeostasis. Because pancreatic β cells are very sensitive to oxidative stress we tested whether they could be affected by IH. The effects of IH exposure (24 h/day, 5.7 and 21% O2 alternation) in mice on β-cell proliferation and β-cell death were tested using Ki67 staining and TUNEL staining, respectively. To assess the role of oxidative stress in these processes, transgenic mice with β-cell-specific overexpression of the antioxidant protein MnSOD were exposed to IH. After 4 days of IH exposure, β-cell proliferation was increased almost fourfold. Coinciding with the increase in proliferation, the subcellular localization of the cell cycle regulator cyclin D2 was increased in the nucleus. In addition, β-cell death was increased approximately fourfold. MnSOD transgene did not alter the effects of IH on β-cell proliferation, but completely abrogated the IH effects on cell death. Thus, IH exposure that mimics sleep apnea can lead to increased β-cell proliferation and cell death. Furthermore, the cell death response seems to be due to oxidative stress.

Section snippets

Animals

FVB mice were ordered from the National Cancer Institute. Male transgenic mice overexpressing the antioxidant MnSOD in pancreatic β cells in the FVB background were produced and maintained in our laboratories [23], [24]. All mice for all in vivo and in vitro studies were male and between 70 and 90 days of age. The study was approved by the University of Louisville animal use and care committee and complied with the American Physiological Society Guidelines for Animal Studies.

Chemicals

Rabbit antiserum to

Effects of continuous IH exposure on pancreatic β-cell proliferation, β-cell death, and islet area

The number of proliferating pancreatic islet β cells was normalized to insulin stained area. As shown in Figs. 1A, B, and C, IH exposure resulted in a 3.5-fold higher number of Ki67-positively stained β cells compared to samples from mice exposed to room air (p < 0.001). To confirm this unexpected finding, we examined proliferation in a subgroup of mice using additional independent indicators of proliferation, namely BrdU and PCNA. Immunostaining with these two markers confirmed that

Discussion

This study shows that mimicking sleep apnea through IH exposure increases β-cell proliferation, β-cell death, and β-cell area. Furthermore, the β-cell death response to IH was reversed by the mitochondrial antioxidant transgene MnSOD.

Summary

IH exposure stimulated proliferation and cell death of pancreatic β cells and promoted the movement of cyclin D2 into the β-cell nucleus. IH-induced cell death, but not proliferation, seems to depend upon the level of mitochondrial oxidative stress, because increased MnSOD expression reversed the β-cell death responses. Our results support the possibility that ROS produced during sleep apnea induce β-cell death.

Acknowledgments

This work was funded by NIH Grant HL075080. We thank Sherry Clark for careful care of animals.

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