Indiana University-Purdue University Indianapolis (IUPUI) / 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.
| Identifer | oai:union.ndltd.org:IUPUI/oai:scholarworks.iupui.edu:1805/14786 |
| Date | 12 July 2017 |
| Creators | Levasseur, Esther Marie |
| Contributors | Mirmira, Raghu |
| Source Sets | Indiana University-Purdue University Indianapolis |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation |
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