Role of DHS in translation control of islet β-cell replication during high fat induced obesity and glucose intolerance

Abstract

Insulin resistance in liver, muscle, and adipose tissue almost invariably occurs during obesity. To compensate, the insulin-producing β-cell increases insulin production by expanding cellular mass. The inability of the β-cell to fully compensate leads to hyperglycemia and ultimately type 2 diabetes. The enzyme deoxyhypusine synthase (DHS) catalyzes the spermidine-dependent posttranslational modification of Lys50 of eukaryotic translation initiation factor 5A (eIF5A) to form hypusine (Hyp). Studies have demonstrated this modification of eIF5A to contribute to cellular proliferation in cancerous cells, but its role in the physiologic proliferation of islet β-cells is unknown. I hypothesized eIF5A-Hyp to be required for the proliferation of islet β cells during the early phase of insulin resistance, allowing the β-cell to respond to the increased demand for insulin to maintain glucose homeostasis. To test this hypothesis, deletion of DHS was induced post-developmentally in β-cells by crossing Dhs-fl/fl mice with MIP1-CreERT mice, and animals were fed for 1 or 4 weeks with a 60% kcal from fat diet (HFD) or normal chow diet (NCD, 16% kcal from fat diet). NCD-fed and HFD-fed animals had normal glucose homeostasis after one week feeding, regardless of genotype. However, after 4 weeks of HFD, KO mice had significantly worse glucose intolerance compared to control mice. eIF5A-Hyp levels increased in β-cells of control animals and as expected remained low in the KO mice. β-cell proliferation was significantly increased after 1 week of HFD as measured by PCNA staining, however KO mice showed no increase. Cyclin D2 protein, but not mRNA, was increased in control animals fed a HFD; this protein increase was not observed in KO animals. Furthermore, polyribosomal profile of isolated islets of 1 week HFD-fed mice showed the Ccnd2 mRNA bound to the monoribosome fractions in the KO animals compared to the controls, resulting in changes of global translation. Interestingly, Ccnd1 polyribosome to monoribosome ratio showed no changes in translation compared to Ccnd2. Taken together, these results suggest that DHS (and, consequently, eIF5A-Hyp) is necessary for the adaptive proliferative and functional response of β-cells during high fat diet induced obesity and glucose intolerance.