Cannabinoids Inhibit Insulin Receptor Signaling in Pancreatic Beta-Cells
Objective—Optimal glucose homeostasis requires exquisitely precise adaptation of the number of insulin-secreting β-cells in the islets of Langerhans. Insulin itself positively regulates β-cell proliferation in an autocrine manner through the insulin receptor (IR) signaling pathway. It is now coming to light that cannabinoid 1 receptor (CB1R) agonism/antagonism influences insulin action in insulin-sensitive tissues. However, the cells on which the CB1Rs are expressed and their function in islets have not been firmly established. We undertook the current study to investigate if intraislet endogenous cannabinoids (ECs) regulate β-cell proliferation and if they influence insulin action.
Research Design and Methods—We measured EC production in isolated human and mouse islets and β-cell line in response to glucose and KCl. We evaluated human and mouse islets, several β-cell lines, and CB1R-null (CB1R ) mice for the presence of a fully functioning EC system. We investigated if ECs influence β-cell physiology through regulating insulin action and demonstrated the therapeutic potential of manipulation of the EC system in diabetic (db/db) mice.
Results—ECs are generated within β-cells, which also express CB1Rs that are fully functioning when activated by ligands. Genetic and pharmacologic blockade of CB1R results in enhanced IR signaling through the insulin receptor substrate 2-AKT pathway in β-cells and leads to increased β-cell proliferation and mass. CB1R antagonism in db/db mice results in reduced blood glucose and increased β-cell proliferation and mass, coupled with enhanced IR signaling in β-cells. Furthermore, CB1R activation impedes insulin-stimulated IR autophosphorylation on β-cells in a Gαi-dependent manner.
Conclusions—These findings provide direct evidence for a functional interaction between CB1R and IR signaling involved in the regulation of β-cell proliferation and will serve as a basis for developing new therapeutic interventions to enhance β-cell function and proliferation in diabetes.
Insulin is the prime mediator of glucose homeostasis. A paucity (as occurs in type 1 diabetes) or surplus (due to excessive exogenous insulin administration or insulin-secreting tumors) of insulin causes somatic damage by energy deprivation and neuroglucopenic brain damage. Therefore, the number of insulin-secreting β-cells is tightly regulated to maintain a very narrow blood glucose range. Intriguingly, insulin also has major effects on its own secretory cells. Exogenously infused insulin increases β-cell mass, and mice lacking β-cell insulin receptors (IRs) develop insulin-dependent diabetes because of insufficient β-cell proliferation and defective insulin secretion. IR activation on β-cells, in addition to being necessary for optimal function of the glucose sensing machinery, causes phosphorylation of insulin receptor substrate 2 (IRS2), which then transduces the signal to the AKT-forkhead box protein O1 (FoxO1) cascade and increases β-cell proliferation.
The endogenous cannabinoids (ECs), 2-arachidonoylglycerol (2-AG), and anandamide (AEA), are lipid transmitters synthesized only on demand by Ca-dependent enzymes in the brain and the periphery. The biologic effects of ECs are mediated by two G protein-coupled receptors (CB1R and CB2R) that use the Gαi class of heterotrimeric proteins to regulate intracellular signaling pathways. ECs are key players of feeding behavior through the activation of the CB1Rs in the brain. Initial studies found that CB1Rs are expressed mainly in the brain and modulate food intake and energy balance.
However, new evidence has accumulated that suggests that ECs also influence insulin action through peripheral CB1Rs in insulin-sensitive tissues, such as adipose tissue, liver, and muscle, and that these effects are independent of food intake or central CB1R activation. Indeed, AEA impairs insulin-stimulated AKT phosphorylation and decreases glucose uptake in skeletal muscle cells, and CB1R antagonism enhances insulin responsiveness of skeletal muscle. However, the mechanism by which CB1R regulates insulin action remains unknown.
Recent studies have extended this notion to the endocrine pancreas, where CB1Rs and EC metabolic enzymes were found in rodent and human islets. The cells on which CB1Rs are expressed have not been firmly established, however. Initial studies suggested that CB1Rs are densely located in α-cells and to a lesser degree in β-cells, another reported the absence of CB1R in β-cells, whereas still other reports point to the presence of CB1R in β-cells. The presence of CB2R in β-cells is also controversial. Studies reported the presence of CB2R in β-cells, whereas other studies pointed to the absence of CB2R in β-cells. Here, we tried to settle the controversy over the existence of the EC receptors in β-cells and provide a novel, fundamental, and potentially exploitable function for CB1Rs in insulin-mediated β-cell proliferation. We found that an intraislet EC system (ECS) indeed exists and serves as a negative feedback on insulin-mediated β-cell proliferation. We also demonstrate the therapeutic potential of manipulation of the ECS in a mouse model of type 2 diabetes.
Abstract and Introduction
Abstract
Objective—Optimal glucose homeostasis requires exquisitely precise adaptation of the number of insulin-secreting β-cells in the islets of Langerhans. Insulin itself positively regulates β-cell proliferation in an autocrine manner through the insulin receptor (IR) signaling pathway. It is now coming to light that cannabinoid 1 receptor (CB1R) agonism/antagonism influences insulin action in insulin-sensitive tissues. However, the cells on which the CB1Rs are expressed and their function in islets have not been firmly established. We undertook the current study to investigate if intraislet endogenous cannabinoids (ECs) regulate β-cell proliferation and if they influence insulin action.
Research Design and Methods—We measured EC production in isolated human and mouse islets and β-cell line in response to glucose and KCl. We evaluated human and mouse islets, several β-cell lines, and CB1R-null (CB1R ) mice for the presence of a fully functioning EC system. We investigated if ECs influence β-cell physiology through regulating insulin action and demonstrated the therapeutic potential of manipulation of the EC system in diabetic (db/db) mice.
Results—ECs are generated within β-cells, which also express CB1Rs that are fully functioning when activated by ligands. Genetic and pharmacologic blockade of CB1R results in enhanced IR signaling through the insulin receptor substrate 2-AKT pathway in β-cells and leads to increased β-cell proliferation and mass. CB1R antagonism in db/db mice results in reduced blood glucose and increased β-cell proliferation and mass, coupled with enhanced IR signaling in β-cells. Furthermore, CB1R activation impedes insulin-stimulated IR autophosphorylation on β-cells in a Gαi-dependent manner.
Conclusions—These findings provide direct evidence for a functional interaction between CB1R and IR signaling involved in the regulation of β-cell proliferation and will serve as a basis for developing new therapeutic interventions to enhance β-cell function and proliferation in diabetes.
Introduction
Insulin is the prime mediator of glucose homeostasis. A paucity (as occurs in type 1 diabetes) or surplus (due to excessive exogenous insulin administration or insulin-secreting tumors) of insulin causes somatic damage by energy deprivation and neuroglucopenic brain damage. Therefore, the number of insulin-secreting β-cells is tightly regulated to maintain a very narrow blood glucose range. Intriguingly, insulin also has major effects on its own secretory cells. Exogenously infused insulin increases β-cell mass, and mice lacking β-cell insulin receptors (IRs) develop insulin-dependent diabetes because of insufficient β-cell proliferation and defective insulin secretion. IR activation on β-cells, in addition to being necessary for optimal function of the glucose sensing machinery, causes phosphorylation of insulin receptor substrate 2 (IRS2), which then transduces the signal to the AKT-forkhead box protein O1 (FoxO1) cascade and increases β-cell proliferation.
The endogenous cannabinoids (ECs), 2-arachidonoylglycerol (2-AG), and anandamide (AEA), are lipid transmitters synthesized only on demand by Ca-dependent enzymes in the brain and the periphery. The biologic effects of ECs are mediated by two G protein-coupled receptors (CB1R and CB2R) that use the Gαi class of heterotrimeric proteins to regulate intracellular signaling pathways. ECs are key players of feeding behavior through the activation of the CB1Rs in the brain. Initial studies found that CB1Rs are expressed mainly in the brain and modulate food intake and energy balance.
However, new evidence has accumulated that suggests that ECs also influence insulin action through peripheral CB1Rs in insulin-sensitive tissues, such as adipose tissue, liver, and muscle, and that these effects are independent of food intake or central CB1R activation. Indeed, AEA impairs insulin-stimulated AKT phosphorylation and decreases glucose uptake in skeletal muscle cells, and CB1R antagonism enhances insulin responsiveness of skeletal muscle. However, the mechanism by which CB1R regulates insulin action remains unknown.
Recent studies have extended this notion to the endocrine pancreas, where CB1Rs and EC metabolic enzymes were found in rodent and human islets. The cells on which CB1Rs are expressed have not been firmly established, however. Initial studies suggested that CB1Rs are densely located in α-cells and to a lesser degree in β-cells, another reported the absence of CB1R in β-cells, whereas still other reports point to the presence of CB1R in β-cells. The presence of CB2R in β-cells is also controversial. Studies reported the presence of CB2R in β-cells, whereas other studies pointed to the absence of CB2R in β-cells. Here, we tried to settle the controversy over the existence of the EC receptors in β-cells and provide a novel, fundamental, and potentially exploitable function for CB1Rs in insulin-mediated β-cell proliferation. We found that an intraislet EC system (ECS) indeed exists and serves as a negative feedback on insulin-mediated β-cell proliferation. We also demonstrate the therapeutic potential of manipulation of the ECS in a mouse model of type 2 diabetes.
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