A cullin 4B-RING E3 ligase complex fine-tunes pancreatic δ cell paracrine interactions
Qing Li, … , Yaoqin Gong, Xiao Yu
Qing Li, … , Yaoqin Gong, Xiao Yu
Published June 12, 2017
Citation Information: J Clin Invest. 2017;127(7):2631-2646. https://doi.org/10.1172/JCI91348.
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A cullin 4B-RING E3 ligase complex fine-tunes pancreatic δ cell paracrine interactions

Abstract

Somatostatin secreted by pancreatic δ cells mediates important paracrine interactions in Langerhans islets, including maintenance of glucose metabolism through the control of reciprocal insulin and glucagon secretion. Disruption of this circuit contributes to the development of diabetes. However, the precise mechanisms that control somatostatin secretion from islets remain elusive. Here, we found that a super-complex comprising the cullin 4B-RING E3 ligase (CRL4B) and polycomb repressive complex 2 (PRC2) epigenetically regulates somatostatin secretion in islets. Constitutive ablation of CUL4B, the core component of the CRL4B-PRC2 complex, in δ cells impaired glucose tolerance and decreased insulin secretion through enhanced somatostatin release. Moreover, mechanistic studies showed that the CRL4B-PRC2 complex, under the control of the δ cell–specific transcription factor hematopoietically expressed homeobox (HHEX), determines the levels of intracellular calcium and cAMP through histone posttranslational modifications, thereby altering expression of the Cav1.2 calcium channel and adenylyl cyclase 6 (AC6) and modulating somatostatin secretion. In response to high glucose levels or urocortin 3 (UCN3) stimulation, increased expression of cullin 4B (CUL4B) and the PRC2 subunit histone-lysine N-methyltransferase EZH2 and reciprocal decreases in Cav1.2 and AC6 expression were found to regulate somatostatin secretion. Our results reveal an epigenetic regulatory mechanism of δ cell paracrine interactions in which CRL4B-PRC2 complexes, Cav1.2, and AC6 expression fine-tune somatostatin secretion and facilitate glucose homeostasis in pancreatic islets.

Authors

Qing Li, Min Cui, Fan Yang, Na Li, Baichun Jiang, Zhen Yu, Daolai Zhang, Yijing Wang, Xibin Zhu, Huili Hu, Pei-Shan Li, Shang-Lei Ning, Si Wang, Haibo Qi, Hechen Song, Dongfang He, Amy Lin, Jingjing Zhang, Feng Liu, Jiajun Zhao, Ling Gao, Fan Yi, Tian Xue, Jin-Peng Sun, Yaoqin Gong, Xiao Yu

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Figure 5

Transcriptional upregulation of Cav1.2 and AC6 contributes to the enhanced somatostatin secretion in Cul4b-deficient pancreatic δ cells.

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Transcriptional upregulation of Cav1.2 and AC6 contributes to the enhanc...
(A) The mRNA levels of L-type calcium channels, adenylyl cyclases, G proteins, PKA and transcription factors in CUL4B knockdown and control TGP52 cells, as determined by quantitative reverse-transcriptase PCR (qRT-PCR) (n = 3). (B–E) Cav1.1, Cav1.2, Cav1.3, AC6, AC7 and Gs protein levels in CUL4B knockdown and control TGP52 cells. B and D, representative Western blots from at least 3 experiments are shown. C and E, bar graph representation and statistical analyses of B and D, respectively. (F) qRT-PCR was used to analyze the mRNA levels of L-type calcium channel α subunits and adenylyl cyclases in primary δ cells from Sst-Cre+/– GFPfl/+ Cul4bfl/Y and WT littermates (n = 7). (G) Effects of Cav1.2 or Cav1.3 knockdown on somatostatin secretion from CUL4B knockdown and control TGP52 cells (n = 4). (H) Effects of AC6 or AC7 knockdown on somatostatin secretion from CUL4B knockdown and control TGP52 cells. (I) Glucose-induced cAMP levels in AC6 or AC7 knockdown and control TGP52 cells, with or without CUL4B knockdown. Cells were incubated with 20 mM glucose for 15 minutes.*P < 0.05; **P < 0.01; ***P < 0.001. Sst-Cre+/– GFPfl/+ Cul4bfl/Y mice were compared with their WT littermates, and CUL4B knockdown cells were compared with control shRNA-treated cells. The bars represent mean ± SEM. All data were analyzed using 1-way ANOVA.