Independent activation of CREB3L2 by glucose fills a regulatory gap in mouse β-cells by co-ordinating insulin biosynthesis with secretory granule formation

Objective

Although individual steps have been characterized, there is little understanding of the overall process whereby glucose co-ordinates the biosynthesis of insulin with its export out of the endoplasmic reticulum (ER) and incorporation into insulin secretory granules (ISGs). Here we investigate a role for the transcription factor CREB3L2 in this context.

Methods

MIN6 cells and mouse islets were analysed by immunoblotting after treatment with glucose, fatty acids, thapsigargin and various inhibitors. Knockdown of CREB3L2 was achieved using si or sh constructs by transfection, or viral delivery. In vivo metabolic phenotyping was conducted after deletion of CREB3L2 in β-cells of adult mice using Ins1-CreER⁺. Islets were isolated for RNAseq and assays of glucose-stimulated insulin secretion (GSIS). Trafficking was monitored in islet monolayers using a GFP-tagged proinsulin construct that allows for synchronised release from the ER.

Results

With a K_m ≈3.5 mM, glucose rapidly (T_1/2 0.9 h) increased full length (FL) CREB3L2 followed by a slower rise (T_1/2 2.5 h) in its transcriptionally-active cleavage product, P60 CREB3L2. Glucose stimulation repressed the ER stress marker, CHOP, and this was partially reverted by knockdown of CREB3L2. Activation of CREB3L2 by glucose was not due to ER stress, however, but a combination of O-GlcNAcylation, which impaired proteasomal degradation of FL-CREB3L2, and mTORC1 stimulation, which enhanced its conversion to P60. cAMP generation also activated CREB3L2, but independently of glucose. Deletion of CREB3L2 inhibited GSIS ex vivo and, following a high-fat diet (HFD), impaired glucose tolerance and insulin secretion in vivo. RNAseq revealed that CREB3L2 regulated genes controlling trafficking to-and-from the Golgi, as well as a broader cohort associated with β-cell compensation during a HFD. Although post-Golgi trafficking appeared intact, knockdown of CREB3L2 impaired the generation of both nascent ISGs and proinsulin condensates in the Golgi, implying a defect in ER export of proinsulin and/or its processing in the Golgi.

Conclusion

The stimulation of CREB3L2 by glucose defines a novel, rapid and direct mechanism for co-ordinating the synthesis, packaging and storage of insulin, thereby minimizing ER overload and optimizing β-cell function under conditions of high secretory demand. Upregulation of CREB3L2 also potentially contributes to the benefits of GLP1 agonism and might in itself constitute a novel means of treating β-cell failure.

Graphical abstract

Regulation of β-cell function by glucose activation of CREB3L2. Glucose stimulates flux through the hexosamine biosynthesis pathway (HBP), enhancing O-GlcNAcylation and thus impairing degradation of full-length (FL) CREB3L2 by the proteasome. Independently, glucose also activates mTORC1 to enhance the conversion of FL-CREB3L2 to its transcriptionally active form, P60. This leads to enhanced expression of a select cohort of genes including many controlling steps in the early secretory pathway. Thus, CREB3L2 helps co-ordinate the synthesis of proinsulin with its incorporation into nascent insulin secretory granules (ISG)s. This mitigates against induction of the terminal ER stress marker, CHOP, and contributes to β-cell compensation in response to obesity.