An Intraislet Paracrine Signaling Pathway That Enables Glucagon to Stimulate Pancreatic β-Cells

Because of our shared interest in islet biology, we have read the article by Weir and Bonner-Weir (1) in this issue of Diabetes with great interest. Recent in vitro and in vivo studies strongly suggest the existence of an intraislet paracrine signaling pathway through which glucagon released from α-cells can stimulate the activity of pancreatic β-cells (2,3). Weir and Bonner-Weir (1) express strong concerns about the existence/physiological relevance of this pathway. Their skepticism is primarily based on the contention that blood flow in rodent islets is directed from the islet core, which is densely populated by β-cells, to the islet mantle, where α- and δ-cells are preferentially localized. Because the authors adhere to a strict “core-to-mantle model” of intraislet blood flow, they consider it unlikely that glucagon released from α-cells can modulate the activity of pancreatic β-cells. However, we already know that there is a flow of information from the islet mantle to the core from in vivo studies in mice examining the cross talk between peripherally located δ-cells and β-cells. Deletion of the Sst (4) or Hhex (5) genes, resulting in absent somatostatin secretion or the loss of δ-cells, respectively, caused an increase in glucose-stimulated insulin secretion (GSIS) ex vivo and elevated plasma insulin levels in vivo. Stated differently, the fact that δ-cells are positioned upstream of β-cells establishes precedent for the possibility of a similar flow of information from α- to β-cells, in agreement with recent work carried out by different investigators using different experimental approaches (see below).

Recent in vivo studies using sophisticated imaging techniques (6,7) indicate that intraislet blood flow is more variable than previously thought and does not exclusively involve a core-to-mantle pattern. These studies showed that blood flow between islets and exocrine tissue is bidirectional, that blood flow has different patterns depending on the islet investigated (in-out, out-in), and that blood flow can reverse even within the same capillary. These observations indicate that the pattern of blood flow alone cannot determine the hierarchy of intraislet paracrine interactions. Although Weir and Bonner-Weir cite these studies, they do not discuss the possibility that these new findings could explain intraislet glucagon action on β-cells. This is unfortunate as it clouds the discussion on the more relevant question raised by Weir and Bonner-Weir, namely, the importance of endogenous glucagon in amplifying GSIS in vivo.

Independent of the discussion surrounding intraislet blood flow, it is also possible that glucagon or other bioactive compounds released from pancreatic α-cells can reach β-cells simply by diffusion through the interstitial milieu. In fact, it is not possible with current technologies to distinguish between short-range signaling via the islet microcirculation (by definition endocrine) and paracrine signaling via the interstitial space. The authors also make claims about the hierarchy of paracrine interactions based on anatomical features of rodent islets, but extensive work by Hara and colleagues has revealed a striking plasticity of islet architecture and cellular composition among various species, with each species having its own islet cytoarchitecture (8,9). In the case of adult human islets, different endocrine cell types are mostly intermingled, and α-cells are located in close proximity to β-cells (10,11). Thus, it is likely that the anatomical arrangement of the human islet further facilitates paracrine interactions between different types of islet cells, including reciprocal paracrine interactions between α- and β-cells.

Weir and Bonner-Weir also emphasize that experimental evidence suggests that insulin inhibits the activity of α-cells via intraislet β-cell–to–α-cell signaling. This concept is consistent with a study showing that the lack of α-cell insulin receptors causes reduced glucagon release (12). However, the existence of this β-cell–to–α-cell signaling pathway does not automatically exclude the possibility that a reciprocal α-cell–to–β-cell pathway is also operative in pancreatic islets. A good example for this type of reciprocal regulation is the well-known cross talk between islet β- and δ-cells. Whereas the release of insulin and other biologically active agents (e.g., Ucn3) secreted from β-cells promotes the release of somatostatin from δ-cells, somatostatin released from δ-cells in turn acts on β-cells to reduce insulin secretion (3).

Many studies postulating the existence of an intraislet paracrine α-cell–to–β-cell signaling pathway involved the static incubation or perifusion of isolated pancreatic islets. (See the references in the article by Weir and Bonner-Weir [1].) Weir and Bonner-Weir correctly point out that the proper interpretation of the outcome of such studies requires consideration of the disruption of physiological anatomical relationships including circulation and innervation. However, recent work indicates that the key conclusions drawn from investigations with isolated islets where the potentiating actions of glucagon on GSIS were established are fully supported by studies with use of perifused pancreata and by in vivo approaches with genetically modified mice (Table 1). Because these latter studies avoid the pitfalls associated with the use of isolated islets, we will quickly summarize these key studies here.

Recently, Svendsen et al. (13) used the perfused mouse pancreas to investigate the role of intraislet glucagon in regulating insulin release. This model makes it possible to explore paracrine relationships in islets in situ. Specifically, the authors explored glucagon-induced insulin secretion using isolated perfused pancreata from wild-type, global glucagon and glucagon-like peptide 1 (GLP-1) receptor knockout mice, as well as β-cell–specific glucagon receptor knockout mice. They found that intraislet glucagon can promote insulin release by activating both β-cell glucagon and GLP-1 receptors. This concept was supported by the observation that active GLP-1 was not detected in the pancreatic perfusate from wild-type mice (13). These data strongly support the existence of an intraislet paracrine α-cell–to–β-cell pathway mediated by glucagon to ensure appropriate insulin secretion. This conclusion is fully supported by various additional studies using different experimental approaches (see below). For example, Zhu et al. (14) established a novel mouse line that selectively expressed a G_i/o-coupled designer receptor exclusively activated by designer drugs (Gi DREADD: GiD) in pancreatic α-cells. Treatment of these mutant mice with clozapine-N-oxide (CNO), an agent that selectively activates DREADDs (15), almost completely shut off glucagon secretion in vivo. This effect was accompanied by a strong reduction in insulin secretion, hyperglycemia, and glucose intolerance (14). These observations, together with in vitro data obtained with perifused islets derived from the GiD-expressing mutant mice, strongly support the concept that intraislet paracrine α-cell–to–β-cell signaling is essential for maintaining proper glucose homeostasis in vivo. In their article, Weir and Bonner-Weir raise the concern that the outcome of this study may have been affected by off-target effects of CNO. However, such a scenario is unlikely, since CNO treatment of control mice that did not express the GiD receptor had little or no effect on plasma glucagon or insulin levels (14).

There are several additional in vivo examples supporting the importance of α-cell–derived proglucagon products that stimulate insulin secretion from β-cells. Chambers et al. (16) demonstrated that the hyperglycemic actions of the GLP-1R antagonist exendin(9-39) administered during intraperitoneal glucose tolerance tests require proglucagon gene expression in pancreatic α-cells. Capozzi et al. (17) showed that stimulating glucagon secretion with alanine lowers blood glucose levels in a manner that is dependent on the expression of the glucagon and GLP-1 receptors in β-cells. Finally, El et al. (18) reported that α-cell glucose-dependent insulinotropic peptide (GIP) receptors are required for the full incretin effect in response to a mixed nutrient meal, an intervention that robustly enhances both insulin and glucagon secretion. The authors demonstrated that GIP action on α-cells potentiates amino acid–stimulated glucagon secretion, resulting in α-cell–to–β-cell communication via activation of β-cell glucagon and GLP-1 receptors to ensure appropriate insulin secretion and glucose tolerance. The three studies highlighted in this paragraph emphasize that proglucagon products released from α-cells are required for optimal insulin secretion from β-cells.

In conclusion, findings of several recent studies using various experimental approaches strongly support the existence of an intraislet paracrine α-cell–to–β-cell pathway through which glucagon can stimulate insulin secretion, at least in rodents. While additional work is required to confirm that a similar pathway is functional and physiologically relevant in the human organism, paracrine glucagon signaling was shown to adjust human insulin secretion to sustain the human glycemic set point in mice transplanted with human islets (19). Hopefully, research in this area will facilitate the development of novel strategies aimed at targeting this pathway for therapeutic purposes.