Endoplasmic reticulum stress in beta cells explained

Beta cells are heavily engaged in the synthesis and secretion of insulin. They are therefore particularly sensitive to endoplasmic reticulum (ER) stress and the subsequent unfolded protein response (UPR). Severe or prolonged episodes of ER stress can lead to the death of beta cells,[1] which can contribute to the development of both type I and type II diabetes.[2]

ER stress in beta cells links obesity to type 2 diabetes and inflammation to type 1 diabetes.

ER stress in peripheral cells has also been linked to insulin resistance,[3] a precursor to type 2 diabetes.

Activation of ER stress

ER stress can be activated by a variety of factors. In experimental conditions, excessive lipid (which can happen following obesity, a common condition preceding type 2 diabetes) and pro-inflammatory cytokines (which can occur following an inflammation, a common cause for type 1 diabetes) can activate ER stress in beta cells.

Causes such as defective protein processing[4] and trafficking [5] or inappropriate calcium regulation [6] are likely in lipid-mediated ER stress. On the other hand, cytokines are likely to activate ER stress by decreasing the calcium pump Serca2b (also known as Atp2a2), leading to subsequent depletion in ER calcium stores.[7]

ER stress and inflammation

All three of the pathways involved in the resolution of ER stress by the unfolded protein response (UPR) are also related to inflammation.[8] The two pathways are very connected and both have been shown to activate each other[8]

Resolution of ER stress

Activation of ER stress by lipids results in a typical unfolded protein response (UPR) to primarily restore ER function, whereas cytokine-activated ER stress leads to an atypical UPR that preferentially activate apoptosis in beta cells.[9]

The UPR is activated when GRP78, a.k.a. BiP, a protein-folding chaperone, is recruited to assist in protein folding. In β-cells, insulin production is a major source of improperly folded protein. GRP78 is normally bound to the luminal domain of ATF6, IRE1, and PERK, which prevents them from initiating their respective pathways of the UPR. When GRP78 is recruited to assist in protein folding, unbound ATF6, IRE1 and PERK are able to initiate the UPR.[10] The UPR is also activated by cytokines[8]

ER stress activates apoptosis through C/EBP homologous protein (CHOP)[11]

Measurement

ER stress can be measured using quantitative real-time RT-PCR to measure the rate at which XBP1 is cleaved by IRE1 when the UPR is activated.[12] XBP1 mRNA cleavage leads to the translation of a transcription factor for genes that resolve ER stress. Measuring the rate at which XBP1 is cleaved gives a quantitative measure of ER stress in real time.

Notes and References

  1. Eizirik DL, Cardozo AK, Cnop M. 2008. The role for endoplasmic reticulum stress in diabetes mellitus. Endocrine Reviews. 29. 1. 42-61. 10.1210/er.2007-0015. 18048764. free.
  2. Laybutt DR, Preston AM, Akerfeldt MC, Kench JG, Busch AK, Biankin AV, Biden TJ. 2007. Endoplasmic reticulum stress contributes to beta cell apoptosis in type 2 diabetes. Diabetologia. 50. 4. 752-763. 10.1007/s00125-006-0590-z. 17268797. free.
  3. Wellen. Kathryn E.. Hotamisligil. Gökhan S.. 2005-05-02. Inflammation, stress, and diabetes. Journal of Clinical Investigation. en. 115. 5. 1111–1119. 10.1172/JCI25102. 0021-9738. 1087185. 15864338.
  4. Jeffrey KD, Alejandro EU, Luciani DS, Kalynyak TB, Hu X, Li H, Lin Y, Townsend RR, Polonsky KS, Johnson JD. 2008. Carboxypeptidase E mediates palmitate-induced beta-cell ER stress and apoptosis. Proceedings of the National Academy of Sciences of the United States of America. 105. 24. 8452–8457. 10.1073/pnas.0711232105. free. 18550819. 2448857.
  5. Preston AM, Gurisik E, Bartley C, Laybutt DR, Biden TJ. 2009. Reduced endoplasmic reticulum (ER)-to-Golgi protein trafficking contributes to ER stress in lipotoxic mouse beta cells by promoting protein overload. Diabetologia. 52. 11. 2369–2373. 10.1007/s00125-009-1506-5. 19727664. free.
  6. Cunha DA, Hekerman P, Ladrière L, Bazarra-Castro A, Ortis F, Wakeham MC, Moore F, Rasschaert J, Cardozo AK, Bellomo E, Overbergh L, Mathieu C, Lupi R, Hai T, Herchuelz A, Marchetti P, Rutter GA, Eizirik DL, Cnop M. 6. 2008. Initiation and execution of lipotoxic ER stress in pancreatic beta-cells. Journal of Cell Science. 121. 14. 2308–2318. 10.1242/jcs.026062. free. 18559892. 3675788.
  7. Oyadomari S, Takeda K, Takiguchi M, Gotoh T, Matsumoto M, Wada I, Akira S, Araki E, Mori M. 2001. Nitric oxide-induced apoptosis in pancreatic beta cells is mediated by the endoplasmic reticulum stress pathway. Proceedings of the National Academy of Sciences of the United States of America. 98. 19. 10845–10850. 10.1073/pnas.191207498. free. 11526215. 58562.
  8. Hotamisligil. Gökhan S.. March 2010. Endoplasmic Reticulum Stress and the Inflammatory Basis of Metabolic Disease. Cell. 140. 6. 900–917. 10.1016/j.cell.2010.02.034. 0092-8674. 2887297. 20303879.
  9. Pirot P, Eizirik DL, Cardozo AK. 2006. Interferon-gamma potentiates endoplasmic reticulum stress-induced death by reducing pancreatic beta cell defence mechanisms. Diabetologia. 49. 6. 1229–1236. 16604358. 10.1007/s00125-006-0214-7. free.
  10. Wang. Miao. Kaufman. Randal J.. January 2016. Protein misfolding in the endoplasmic reticulum as a conduit to human disease. Nature. en. 529. 7586. 326–335. 10.1038/nature17041. 26791723. 2016Natur.529..326W. 4446276. 0028-0836.
  11. Hu. Hai. Tian. Mingxing. Ding. Chan. Yu. Shengqing. 2019-01-04. The C/EBP Homologous Protein (CHOP) Transcription Factor Functions in Endoplasmic Reticulum Stress-Induced Apoptosis and Microbial Infection. Frontiers in Immunology. 9. 3083. 10.3389/fimmu.2018.03083. 1664-3224. 6328441. 30662442. free.
  12. van Schadewijk. Annemarie. van’t Wout. Emily F. A.. Stolk. Jan. Hiemstra. Pieter S.. March 2012. A quantitative method for detection of spliced X-box binding protein-1 (XBP1) mRNA as a measure of endoplasmic reticulum (ER) stress. Cell Stress and Chaperones. en. 17. 2. 275–279. 10.1007/s12192-011-0306-2. 1355-8145. 3273559. 22038282.

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