Syndecans modulate ghrelin receptor signaling
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Ghrelin is a gut hormone that enhances food intake and growth hormone secretion through its G-protein coupled receptor the growth hormone secretagogue receptor (GHSR). Recently, we showed that ghrelin interacts with syndecans (SDCs), a family of membrane proteins known to modulate hypothalamic appetite signaling. Here, we investigated whether SDCs impact ghrelin signaling at GHSR by assessing ghrelin-induced intracellular Ca2+ mobilization (iCa2+) and inositol phosphate1 (IP1) production in HEK293 cells. Compared with controls, the overexpression of SDCs dose-dependently increased the maximum iCa2+ response 2-4-fold, without affecting EC50. The IP1 response was similarly amplified by SDCs, but also indicated that they reduce constitutive (ghrelin-independent) activity of GHSR. These enhanced responses occurred despite a SDC dose-dependent reduction in plasma membrane GHSR levels. Although ghrelin-stimulated Gαq activation was unaltered by SDC1 expression, it failed to restore iCa2+ responsiveness in GNAQ/11 knockout cells, indicating dependence on Gαq/11, not another Gα subunit. This suggests that SDCs modulate either signaling downstream of Gαq/11 or quenching of β-arrestin2 recruitment to GHSR. Indeed, expression of SDCs at levels that only modestly suppress cell surface receptor, reduced ghrelin-induced β-arrestin2 recruitment by ~80%. SDC co-expression also delayed the peak β-arrestin2 response. However, peak β-arrestin2 recruitment follows the peak iCa2+ response, making it unclear if reduced β-arrestin2 recruitment potentiated Ca2+ signaling. Altogether, SDCs enhanced iCa2+/IP1 and reduced β-arrestin2 recruitment by GHSR in response to ghrelin, likely by modulating signaling downstream of Gαq. This could be a novel mechanism through which SDCs affect metabolism and obesity.
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