The unique hypusine modification of eIF5A promotes islet β cell inflammation and dysfunction in mice

Abstract

In both type 1 and type 2 diabetes, pancreatic islet dysfunction results in part from cytokine-mediated inflammation. The ubiquitous eukaryotic translation initiation factor 5A (eIF5A), which is the only protein to contain the amino acid hypusine, contributes to the production of proinflammatory cytokines. We therefore investigated whether eIF5A participates in the inflammatory cascade leading to islet dysfunction during the development of diabetes. As described herein, we found that eIF5A regulates iNOS levels and that eIF5A depletion as well as the inhibition of hypusination protects against glucose intolerance in inflammatory mouse models of diabetes. We observed that following knockdown of eIF5A expression, mice were resistant to β cell loss and the development of hyperglycemia in the low-dose streptozotocin model of diabetes. The depletion of eIF5A led to impaired translation of iNOS-encoding mRNA within the islet. A role for the hypusine residue of eIF5A in islet inflammatory responses was suggested by the observation that inhibition of hypusine synthesis reduced translation of iNOS-encoding mRNA in rodent β cells and human islets and protected mice against the development of glucose intolerance the low-dose streptozotocin model of diabetes. Further analysis revealed that hypusine is required in part for nuclear export of iNOS-encoding mRNA, a process that involved the export protein exportin1. These observations identify the hypusine modification of eIF5A as a potential therapeutic target for preserving islet function under inflammatory conditions.

Deoxyhypusine synthase haploinsufficiency attenuates acute cytokine signaling

“…As shown in Figure 3A, Dhps +/-MEFs demonstrated response of haploinsufficient cells to the type of stress observed in T2DM, we subjected MEFs to incubation with a cocktail of pro-inflammatory cytokines (IL-1β, TNFα and IFNγ) for 4 h. In prior studies, we showed that pro-inflammatory cytokines cause acute induction of the mRNA and protein for iNOS, and that inhibition of DHS with GC7 blocks activation of iNOS protein, but not its mRNA. 22 As shown in Figure 2C and D, activation of the mRNA encoding iNOS (Nos2) is unaffected in Dhps +/-MEFs, but the production of iNOS protein is reduced by 50%. Although these data are not direct evidence of translational control of Nos2 mRNA, the dissociation between …”

“…25 Inhibition of DHS using GC7 and similar polyamines was shown to have a repressive effect on proliferation in both yeast and mammalian cell cultures as soon as 24 h, with a particularly striking inhibition at the G 1 /S transition of the cell cycle. 22,[26][27][28] The specificity of these inhibitors with respect to the hypusination pathway is at least partially verified in studies showing similar G 1 /S block in temperature-sensitive mutants of eIF5A in yeast, 18,29,30 in DOHH mutants in Drosophila, 31 and by using inhibitors of DOHH in mammalian cells. 32 Whereas all of these studies have been performed in cultured cells or in lower organisms, to date no formal accounts of eIF5A, DHS or DOHH knockouts in intact mice have been reported (of note, one review states in passing that the homozygous knockout of alleles encoding DHS is embryonic lethal).…”

“…20,21 Nonetheless, our own studies in the context of inflammation in β cells suggest that a nuclear species of eIF5A binds and shuttles the mRNA encoding inducible nitric oxide synthase (iNOS) across the nuclear membrane in a manner dependent upon exportin1/CRM1. 22 We interpret this controversy as reflecting more the disparity of cell types and conditions that have been used to study eIF5A rather than actual functionality of the protein. In this respect, we feel that islet β cells represent a particularly notable cell type for which proteins that govern cellular survival have a profound effect on whole-body metabolic homeostasis.…”

“…We feel these findings align with our prior studies, but more importantly, they verify at the level of Dhps haploinsufficiency what we observed in vitro and in vivo with DHS inhibitors. 22 …”

“…With regard to the islet β cell, hypusination may be an especially attractive target for several reasons: (1) hypusinated eIF5A has a very short half-life in islets (~6 h), 22 compared to other cell types (>20 h), 22,[34][35][36] a finding supporting an acute regulatory role for the protein in islets; (2) eIF5A and DHS exhibit rapid and reciprocal nucleo-cytoplasmic shuttling in β cells in response to cytokines or ER stress, 14,22 whereas in other cell types specific compartmentation is less clear or controversial; 20,37 and (3) the islet β cell has a very slow replicative rate and therefore is less susceptible acutely to agents (such as DHS inhibitors) that affect cellular proliferation. Because our data suggest that Dhps +/-mice can maintain normal growth and glucose homeostasis, with evidence that their cells maintain more robust responses to inflammation, we feel that pharmacologic approaches to inhibiting DHS could be successful in mitigating diabetes progression in an intact animal.…”