Melatonin stimulates inositol‐1,4,5‐trisphosphate and Ca²⁺release from INS1 insulinoma cells

Abstract

The effects of melatonin in mammalian cells are exerted via specific receptors or are related to its free radical scavenging activity. It has previously been reported that melatonin inhibits insulin secretion in the pancreatic islets of the rat and in rat insulinoma INS1 cells via Gi-protein-coupled MT1 receptors and the cyclic adenosine 3',5'-monophosphate pathway. However, the inositol-1,4,5-trisphosphate (IP3) pathway is involved in the insulin secretory response as well, and the melatonin signal may play a part in its regulation. This paper addresses the involvement of the second messengers IP3 and intracellular Ca2+ ([Ca2+]i) in the signalling cascade of melatonin in the rat insulinoma INS1 cell, a model for the pancreatic beta-cell. For this purpose melatonin at concentrations ranging from 1 to 100 nmol/L, carbachol and the nonselective melatonin receptor antagonist luzindole were used to stimulate INS1 cell batches, followed by an IP3-mass assay and Ca2+ imaging. Molecular biological studies relating to the mRNA of IP3 receptor (IP3R) subtypes and their relative abundance in INS1 cells showed expression of IP3R-1, IP3R-2 and IP3R-3 mRNA. In conclusion, we found that in rat insulinoma INS1 cells there is a dose-dependent stimulation of IP3 release by melatonin, which is accompanied by a likewise transient increase in [Ca2+]i concentrations. The melatonin effect observed mimics carbachol action. It can be abolished by 30 micromol/L luzindole and is sustained in Ca2+-free medium, suggesting a mechanism that includes the depletion of Ca2+ from intracellular stores.

The insulin–melatonin antagonism: studies in the LEW.1AR1-iddm rat (an animal model of human type 1 diabetes mellitus)

“…Recently, it became possible to identify the intracellular signal cascades of pancreatic beta cells that are melatonin-dependent. Our own previous investigations have shown that the MT 1 and MT 2 receptors of pancreatic beta cells are linked to adenylate cyclasecAMP [2], the phospholipase C inositol-1,4,5-triphosphate (IP 3 ) [12,13] and the guanylate cyclase-cGMP pathways [14,15]. It was confirmed that melatonin inhibits the cAMP and cGMP pathways and consequently decreases insulin release [2,8,14,15].…”

“…It was confirmed that melatonin inhibits the cAMP and cGMP pathways and consequently decreases insulin release [2,8,14,15]. In addition, the effects of melatonin are mediated by G q -proteins, phospholipase C and the second messenger IP 3 , which mobilises Ca 2+ from intracellular stores, resulting in increase of insulin [12,13]. In this context, recent investigations have analysed the plasma levels of melatonin in slightly hyperinsulinaemic GotoKakizaki (GK) rats, a rat model of type 2 diabetes [16], as well as in a hypoinsulinaemic streptozotocin-induced rat model of type 1 diabetes.…”

Role of melatonin on diabetes-related metabolic disorders

“…In contrast to the uniform cAMP-dimishing effect of melatonin, both IP3-increasing [61,62] as well as IP3-decreasing [63,64] effects of melatonin have been described in different cell types. Previous studies in INS1 insulinoma cells indicated a dose-dependent stimulation of IP3 release by melatonin, while the competitive melatonin receptor antagonist luzindole was able to completely abolish such IP3-liberating effects of melatonin, thus giving strong evidence for the involvement of melatonin receptors [65] . Furthermore, it was also shown that such a melatonininduced IP3 liberation was able to release Ca 2+ from intracellular stores [65] , a mechanism that is commonly accepted as a trigger for insulin secretion.…”

Melatonin receptors in pancreatic islets: good morning to a novel type 2 diabetes gene

“…MTNR1A has also been implicated in signalling controlled by PLC [37], possibly via coupling to G q/11 [39]. It has been reported that stimulation of INS-1 cells with melatonin provokes the release of inositol 1,4, 5-triphosphate [58,62], and when G i coupling is blocked by pertussis toxin, a stimulatory effect of melatonin is uncovered [58].…”