Browsing by Author "Hatanaka, Masayuki"

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    Chronic high fat feeding restricts islet mRNA translation initiation independently of ER stress via DNA damage and p53 activation
    (Springer Nature, 2017-06-19) Hatanaka, Masayuki; Anderson-Baucum, Emily; Lakhter, Alexander; Kono, Tatsuyoshi; Maier, Bernhard; Tersey, Sarah A.; Tanizawa, Yukio; Evans-Molina, Carmella; Mirmira, Raghavendra G.; Sims, Emily K.; Pediatrics, School of Medicine
    Under conditions of high fat diet (HFD) consumption, glucose dyshomeostasis develops when β-cells are unable to adapt to peripheral insulin demands. Few studies have interrogated the molecular mechanisms of β-cell dysfunction at the level of mRNA translation under such conditions. We sought to address this issue through polyribosome profile analysis of islets from mice fed 16-weeks of 42% HFD. HFD-islet analysis revealed clear trends toward global reductions in mRNA translation with a significant reduction in the polyribosome/monoribosome ratio for Pdx1 mRNA. Transcriptional and translational analyses revealed endoplasmic reticulum stress was not the etiology of our findings. HFD-islets demonstrated evidence of oxidative stress and DNA damage, as well as activation of p53. Experiments in MIN-6 β-cells revealed that treatment with doxorubicin to directly induce DNA damage mimicked our observed effects in islets. Islets from animals treated with pioglitazone concurrently with HFD demonstrated a reversal of effects observed from HFD alone. Finally, HFD-islets demonstrated reduced expression of multiple ribosome biogenesis genes and the key translation initiation factor eIF4E. We propose a heretofore unappreciated effect of chronic HFD on β-cells, wherein continued DNA damage owing to persistent oxidative stress results in p53 activation and a resultant inhibition of mRNA translation.
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    Divergent compensatory responses to high-fat diet between C57BL6/J and C57BLKS/J inbred mouse strains
    (American Physiological Society (APS), 2013-12-15) Sims, Emily K.; Hatanaka, Masayuki; Morris, David L.; Tersey, Sarah A.; Kono, Tatsuyoshi; Chaudry, Zunaira Z.; Day, Kathleen H.; Moss, Dan R.; Stull, Natalie D.; Mirmira, Raghavendra G.; Evans-Molina, Carmella; Department of Medicine, IU School of Medicine
    Impaired glucose tolerance (IGT) and type 2 diabetes (T2DM) are polygenic disorders with complex pathophysiologies; recapitulating them with mouse models is challenging. Despite 70% genetic homology, C57BL/6J (BL6) and C57BLKS/J (BLKS) inbred mouse strains differ in response to diet- and genetic-induced obesity. We hypothesized these differences would yield insight into IGT and T2DM susceptibility and response to pharmacological therapies. To this end, male 8-wk-old BL6 and BLKS mice were fed normal chow (18% kcal from fat), high-fat diet (HFD; 42% kcal from fat), or HFD supplemented with the PPARγ agonist pioglitazone (PIO; 140 mg PIO/kg diet) for 16 wk. Assessments of body composition, glucose homeostasis, insulin production, and energy metabolism, as well as histological analyses of pancreata were undertaken. BL6 mice gained weight and adiposity in response to HFD, leading to peripheral insulin resistance that was met with increased β-cell proliferation and insulin production. By contrast, BLKS mice responded to HFD by restricting food intake and increasing activity. These behavioral responses limited weight gain and protected against HFD-induced glucose intolerance, which in this strain was primarily due to β-cell dysfunction. PIO treatment did not affect HFD-induced weight gain in BL6 mice, and decreased visceral fat mass, whereas in BLKS mice PIO increased total fat mass without improving visceral fat mass. Differences in these responses to HFD and effects of PIO reflect divergent human responses to a Western lifestyle and underscore the careful consideration needed when choosing mouse models of diet-induced obesity and diabetes treatment.
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    Loss of mTORC1 signalling impairs β-cell homeostasis and insulin processing
    (Nature Communication Group, 2017-07-12) Blandino-Rosano, Manuel; Barbaresso, Rebecca; Jimenez-Palomares, Margarita; Bozadjieva, Nadejda; Werneck-de-Castro, Joao Pedro; Hatanaka, Masayuki; Mirmira, Raghavendra G.; Sonenberg, Nahum; Liu, Ming; Rüegg, Markus A.; Hall, Michael N.; Bernal-Mizrachi, Ernesto; Pediatrics, School of Medicine
    Deregulation of mTOR complex 1 (mTORC1) signalling increases the risk for metabolic diseases, including type 2 diabetes. Here we show that β-cell-specific loss of mTORC1 causes diabetes and β-cell failure due to defects in proliferation, autophagy, apoptosis and insulin secretion by using mice with conditional (βraKO) and inducible (MIP-βraKOf/f) raptor deletion. Through genetic reconstitution of mTORC1 downstream targets, we identify mTORC1/S6K pathway as the mechanism by which mTORC1 regulates β-cell apoptosis, size and autophagy, whereas mTORC1/4E-BP2-eIF4E pathway regulates β-cell proliferation. Restoration of both pathways partially recovers β-cell mass and hyperglycaemia. This study also demonstrates a central role of mTORC1 in controlling insulin processing by regulating cap-dependent translation of carboxypeptidase E in a 4EBP2/eIF4E-dependent manner. Rapamycin treatment decreases CPE expression and insulin secretion in mice and human islets. We suggest an important role of mTORC1 in β-cells and identify downstream pathways driving β-cell mass, function and insulin processing.
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    Maintenance of Pdx1 mRNA translation in islet β-cells during the unfolded protein response
    (The Endocrine Society, 2014-11) Templin, Andrew T.; Maier, Bernhard; Tersey, Sarah A.; Hatanaka, Masayuki; Mirmira, Raghavendra G.; Department of Pediatrics, IU School of Medicine
    In type 1 diabetes, proinflammatory cytokines secreted by infiltrating immune cells activate the unfolded protein response (UPR) in islet β-cells, which leads to attenuation of global mRNA translation. Under such conditions, privileged mRNAs required for adaptation to the prevailing stress are maintained in an actively translated state. Pdx1 is a β-cell transcription factor that is required for the adaptive UPR, but it is not known how translation of its mRNA is maintained under these conditions. To study translation, we established conditions in vitro with MIN6 cells and mouse islets and a mixture of proinflammatory cytokines (IL-1β, TNF-α, and IFN-γ) that mimicked the UPR conditions seen in type 1 diabetes. Cell extracts were then subjected to polyribosome profiling to monitor changes to mRNA occupancy by ribosomes. Similar to other privileged mRNAs (Atf4 and Chop), Pdx1 mRNA remained partitioned in actively translating polyribosomes under the UPR, whereas the mRNA encoding a proinsulin-processing enzyme (Cpe) and others partitioned into inactively translating monoribosomes. Bicistronic luciferase reporter analyses revealed that the distal portion of the 5'-untranslated region of mouse Pdx1 (between bp -105 to -280) contained elements that promoted translation under both normal and UPR conditions, and this region exhibited conserved sequences and secondary structure similar to those of other known internal ribosome entry sites. Our findings suggest that Pdx1 protein levels are maintained in the setting of the UPR, in part, through elements in the 5'-untranslated region that confer privileged mRNA translation in a 5'-7-methylguanylate cap-independent manner.
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    Palmitate induces mRNA translation and increases ER protein load in islet β-cells via activation of the mammalian target of rapamycin pathway
    (American Diabetes Association, 2014-10) Hatanaka, Masayuki; Maier, Bernhard; Sims, Emily K.; Templin, Andrew T.; Kulkarni, Rohit N.; Evans-Molina, Carmella; Mirmira, Raghavendra G.; Department of Medicine, IU School of Medicine
    Saturated free fatty acids (FFAs) have complex effects on the islet β-cell, acutely promoting adaptive hyperplasia but chronically impairing insulin release. The acute effects of FFAs remain incompletely defined. To elucidate these early molecular events, we incubated mouse β-cells and islets with palmitate and then studied mRNA translation by polyribosomal profiling and analyzed signaling pathways by immunoblot analysis. We found that palmitate acutely increases polyribosome occupancy of total RNA, consistent with an increase in mRNA translation. This effect on translation was attributable to activation of mammalian target of rapamycin (mTOR) pathways via L-type Ca(2+) channels but was independent of insulin signaling. Longer incubations led to depletion of polyribosome-associated RNA, consistent with activation of the unfolded protein response (UPR). Pharmacologic inhibition of mTOR suppressed both the acute effects of palmitate on mRNA translation and the chronic effects on the UPR. Islets from mice fed a high-fat diet for 7 days showed increases in polyribosome-associated RNA and phosphorylation of S6K, both consistent with activation of mTOR. Our results suggest that palmitate acutely activates mRNA translation and that this increase in protein load contributes to the later UPR.