Carbamate Insecticides Modulate G Protein‐Dependent Signaling in Cells Expressing Melatonin Receptors

Melatonin targets MT 1 and MT 2 melatonin receptors that are expressed in brain as well as peripheral tissues in order to signal “time‐of‐day” messages for the synchronization of circadian rhythms. In mammals, the homeostatic balance of glucose metabolism and insulin release is modulated by G αi protein‐dependent melatonin receptor (MTR) signaling in pancreatic β‐cells (Kemp et al . 2002). In‐silico screening paired with competition binding has demonstrated that carbamate insecticides with similar pharmacophores to melatonin have affinity for the mammalian MT 1 and MT 2 melatonin receptors (Jones et al . 2015 EB Abs .; Popovska‐Gorevski et al . 2017; Glatfelter et al . 2017 EB Abs .). Therefore we hypothesize that exposure to these environmental chemicals can disrupt melatonin receptor signaling. The objective of this study was to examine the effect of carbamate insecticides on G protein dependent‐signaling pathways on human MT 1 and MT 2 melatonin receptors stably expressed in CHO cells and in rat INS‐1 β‐cells expressing MT 1 . The apparent efficacy of carbamates (ie., carbaryl, fenobucarb, bendiocarb, and carbofuran) assessed by affinity ratios derived from competition with 2‐[ 125 I]‐iodomelatonin binding, with or without G protein inactivation by GTP, suggested antagonist efficacy on hMT 1 receptors (no affinity shift with GTP; K iGTP /K iControl :0.59 – 2.3) and agonist efficacy on hMT 2 receptors (rightward affinity shift with GTP; K iGTP /K iControl : 4.8 – 11). The same insecticides tested at 10 μM did not modify forskolin‐stimulated cAMP levels in CHO‐hMT 1 cells but inhibited stimulated cAMP formation in CHO‐hMT 2 cells (47%–72% of melatonin effect on stimulation). Carbaryl (10 pM – 100 μM) acted as a partial agonist at both hMT 1 (log IC 50 = −5.8 ± 0.3 M; E max = 55% melatonin effect) and hMT 2 (log IC 50 = −10.0 ± 0.4 M; E max = 67% melatonin effect) receptors with over 10,000 times higher potency for hMT 2 compared to hMT 1 . Next, carbaryl at 10 μM, but not at 1 μM, inhibited forskolin‐stimulated insulin release from INS‐1 pancreatic β‐cells by 25% (96% melatonin effect) as determined by radioimmunoassay. These data suggest that carbamate insecticides with similar structures to melatonin can alter G protein‐dependent signaling in tissues that express melatonin receptors. Exposure to these chemicals could potentially contribute to the development of diabetes or other metabolic disorders as disruption of β‐cell melatonin receptor signaling and insulin release are linked to increased risk of disease (Tuomi et al . 2016). Support or Funding Information Supported by ES 023684 to MLD and RVR. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .