The processes regulating glucose-stimulated insulin secretion (GSIS) and its modulation by incretins in pancreatic β-cells are only partly understood. Here we investigate the involvement of β-catenin in these processes. Reducing β-catenin levels using siRNA knockdown attenuated GSIS in a range of β-cell models and blocked the ability of GLP-1 agonists and the depolarizing agent KCl to potentiate this. This could be mimicked in both β-cell models and isolated islets by short-term exposure to the β-catenin inhibitory drug pyrvinium. In addition, short-term treatment with a drug that increases β-catenin levels results in an increase in insulin secretion. The timing of these effects suggests that β-catenin is required for the processes regulating trafficking and/or release of pre-existing insulin granules rather than for those regulated by gene expression. This was supported by the finding that the overexpression of the transcriptional co-activator of β-catenin, transcription factor 7-like 2 (TCF7L2), attenuated insulin secretion, consistent with the extra TCF7L2 translocating β-catenin from the plasma membrane pool to the nucleus. We show that β-catenin depletion disrupts the intracellular actin cytoskeleton, and by using total internal reflectance fluorescence (TIRF) microscopy, we found that β-catenin is required for the glucose- and incretin-induced depletion of insulin vesicles from near the plasma membrane. In conclusion, we find that β-catenin levels modulate Ca-dependent insulin exocytosis under conditions of glucose, GLP-1, or KCl stimulation through a role in modulating insulin secretory vesicle localization and/or fusion via actin remodeling. These findings also provide insights as to how the overexpression of TCF7L2 may attenuate insulin secretion.

In healthy individuals, any rise in blood glucose levels is rapidly countered by the release of insulin from the β-cells of the pancreas which in turn promotes the uptake and storage of the glucose in peripheral tissues. The β-cells possess exquisite mechanisms regulating the secretion of insulin to ensure that the correct amount of insulin is released. These mechanisms involve tight control of the movement of insulin containing secretory vesicles within the β-cells, initially preventing most vesicles being able to move to the plasma membrane. Elevated glucose levels trigger an influx of Ca2+ that allows fusion of the small number of insulin containing vesicles that are pre-docked at the plasma membrane but glucose also stimulates processes that allow other insulin containing vesicles located further in the cell to move to and fuse with the plasma membrane. The mechanisms controlling these processes are complex and not fully understood but it is clear that the interaction of the β-cells with other β-cells in the islets is very important for their ability to develop the appropriate machinery for proper regulation of insulin secretion. Emerging evidence indicates one factor that is key for this is the formation of homotypic cadherin mediated adherens junctions between β-cells. Here, we review the evidence for this and discuss the mechanisms by which these adherens junctions might regulate insulin vesicle trafficking as well as the implications this has for understanding the dysregulation of insulin secretion seen in pathogenic states.

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The recent finding that β-catenin levels play an important rate-limiting role in processes regulating insulin secretion lead us to investigate whether its binding partner α-catenin also plays a role in this process. We find that levels of both α-E-catenin and α-N-catenin are rapidly up-regulated as levels of glucose are increased in rat clonal β-cell models INS-1E and INS-832/3. Lowering in levels of either α-catenin isoform using siRNA resulted in significant increases in glucose stimulated insulin secretion (GSIS) and this effect was attenuated when β-catenin levels were lowered indicating these proteins have opposing effects on insulin release. This effect of α-catenin knockdown on GSIS was not due to increases in insulin expression but was associated with increases in calcium influx into cells. Moreover, simultaneous depletion of α-E catenin and α-N catenin decreased the actin polymerisation to a similar degree as latrunculin treatment and inhibition of ARP 2/3 mediated actin branching with CK666 attenuated the α-catenin depletion effect on GSIS. This suggests α-catenin mediated actin remodelling may be involved in the regulation of insulin secretion. Together this indicates that α-catenin and β-catenin can play opposing roles in regulating insulin secretion, with some degree of functional redundancy in roles of α-E-catenin and α-N-catenin. The finding that, at least in β-cell models, the levels of each can be regulated in the longer term by glucose also provides a potential mechanism by which sustained changes in glucose levels might impact on the magnitude of GSIS.

The presence of adherens junctions and the associated protein β-catenin are requirements for the development of glucose stimulated insulin secretion (GSIS) in β-cells. Evidence indicates that modulation of β-catenin function in response to changes in glucose levels can modulate the levels of insulin secretion from β-cells but the role of β-catenin phosphorylation in this process has not been established. We find that a Ser552Ala version of β-catenin attenuates glucose stimulated insulin secretion indicating a functional role for Ser552 phosphorylation of β-catenin in insulin secretion. This is associated with alterations F/G actin ratio  but not  transcriptional activity of β-catenin.   Both glucose and GLP-1 stimulated phosphorylation of the serine 552 residue on β-catenin.  We investigated the possibility that an EPAC-PAK1 pathway might be involved in this phosphorylation event.  We find that reduction in PAK1 levels using siRNA attenuates both glucose and GLP-1 stimulated phosphorylation of β-catenin Ser552 and the effects of these on insulin secretion in β-cell models. Further, both the EPAC inhibitor ESI-09 and the PAK1 inhibitor IPA3 do the same in both β-cell models and mouse islets. Together this identifies phosphorylation of β-catenin at Ser552 as part of a cell signalling mechanism linking nutrient and hormonal regulation of β-catenin to modulation of  insulin secretory capacity of β-cells and indicates this phosphorylation event is regulated downstream of EPAC and PAK1 in β-cells.

High glucose levels are associated with changes in macrophage polarization and evidence indicates that the sustained or even short-term high glucose levels modulate inflammatory responses in macrophages. However, the mechanism by which macrophages can sense the changes in glucose levels are not clearly understood. We find that high glucose levels rapidly increase the α-E catenin protein level in RAW264.7 macrophages. We also find an attenuation of glucose induced increase of α-E catenin when hexosamine biosynthesis pathway is inhibited either with glutamine depletion or with the drugs azaserine and tunicamycin. This indicates the involvement of hexosamine biosynthesis pathway in this process. Then, we investigated the potential role of α-E catenin in glucose induced macrophage polarization. We find that the reduction of α-E catenin level using siRNA attenuates the glucose induced changes of both IL-1β and IL-12 mRNA levels under LPS stimulated condition but does not affect TNF-α expression. Together this indicates that α-E catenin can sense the changes in glucose levels in macrophages via hexosamine biosynthesis pathway and also can modulate the glucose induced gene expression of inflammatory markers such as IL-1β and IL-12.  This identifies a new part of the mechanism by which macrophages are able to respond to changes in glucose levels.

A central characteristic of insulin resistance is the impaired ability for insulin to stimulate glucose uptake into skeletal muscle. While insulin resistance can occur distal to the canonical insulin receptor‐PI3k‐Akt signaling pathway, the signaling intermediates involved in the dysfunction are yet to be fully elucidated. β‐catenin is an emerging distal regulator of skeletal muscle and adipocyte insulin‐stimulated GLUT4 trafficking. Here, we investigate its role in skeletal muscle insulin resistance. Short‐term (5‐week) high‐fat diet (HFD) decreased skeletal muscle β‐catenin protein expression 27% (p = 0.03), and perturbed insulin‐stimulated β‐cateninS552 phosphorylation 21% (p = 0.009) without affecting insulin‐stimulated Akt phosphorylation relative to chow‐fed controls. Under chow conditions, mice with muscle‐specific β‐catenin deletion had impaired insulin responsiveness, whereas under HFD, both mice exhibited similar levels of insulin resistance (interaction effect of genotype × diet p < 0.05). Treatment of L6‐GLUT4‐myc myocytes with palmitate lower β‐catenin protein expression by 75% (p = 0.02), and attenuated insulin‐stimulated β‐catenin phosphorylationS552 and actin remodeling (interaction effect of insulin × palmitate p < 0.05). Finally, β‐cateninS552 phosphorylation was 45% lower in muscle biopsies from men with type 2 diabetes while total β‐catenin expression was unchanged. These findings suggest that β‐catenin dysfunction is associated with the development of insulin resistance.

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