Article

Mammalian type I gonadotropin-releasing hormone receptors undergo slow, constitutive, agonist-independent internalization

Medical Research Council Human Reproductive Sciences Unit, The Queen's Medical Research Institute, 47 Little France Crescent, Edinburgh EH16 4TJ, United Kingdom.
Endocrinology (Impact Factor: 4.64). 04/2008; 149(3):1415-22. DOI: 10.1210/en.2007-1159
Source: PubMed

ABSTRACT Regulatory elements present in the cytoplasmic carboxyl-terminal tails of G protein-coupled receptors contribute to agonist-dependent receptor desensitization, internalization, and association with accessory proteins such as beta-arrestin. The mammalian type I GnRH receptors are unique among the rhodopsin-like G protein-coupled receptors because they lack a cytoplasmic carboxyl-terminal tail. In addition, they do not recruit beta-arrestin, nor do they undergo rapid desensitization. By measuring the internalization of labeled GnRH agonists, previous studies have reported that mammalian type I GnRH receptors undergo slow agonist-dependent internalization. In the present study, we have measured the internalization of epitope-tagged GnRH receptors, both in the absence and presence of GnRH stimulation. We demonstrate that mammalian type I GnRH receptors exhibit a low level of constitutive agonist-independent internalization. Stimulation with GnRH agonist did not significantly enhance the level of receptor internalization above the constitutive level. In contrast, the catfish GnRH and rat TRH receptors, which have cytoplasmic carboxyl-terminal tails, displayed similar levels of constitutive agonist-independent internalization but underwent robust agonist-dependent internalization, as did chimeras of the mammalian type I GnRH receptor with the cytoplasmic carboxyl-terminal tails of the catfish GnRH receptor or the rat TRH receptor. When the carboxyl-terminal Tyr325 and Leu328 residues of the mammalian type I GnRH receptor were replaced with alanines, these two mutant receptors underwent significantly impaired internalization, suggesting a function for the Tyr-X-X-Leu sequence in mediating the constitutive agonist-independent internalization of mammalian type I GnRH receptors. These findings provide further support for the underlying notion that the absence of the cytoplasmic carboxyl-terminal tail of the mammalian type I GnRH receptors has been selected for during evolution to prevent rapid receptor desensitization and internalization to allow protracted GnRH signaling in mammals.

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    • "Some species have been shown to be particularly resistant to rapid desensitization by GnRH agonists at the pituitary level because they have evolved cell surface GnRH receptors that are slow to internalize (lacking a phosphorylated tail on the receptor) making desensitization and eventual downregulation take longer or never occur [Pawson et al., 2008]. Perhaps female sea otters have this special adaptation within GnRH cell surface receptors in both the pituitary and perhaps the ovaries as well to prevent rapid desensitized to GnRH stimulation as an adaptation to protracted high reproductive hormone levels needed for effective ovulation and the maintenance of a long lived corpus luteum observed in this species [Aspden et al., 1996; Larson et al., 2003; Pawson et al., 2008]. "
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    • "Whereas these studies were rather measuring ligand internalization, investigations conducted by Millar's group, using an epitope-tagged receptor allowed to trace the receptor itself. These studies established that GnRH-R exhibits a low level of constitutive internalization and does not undergo rapid agonistdependent internalization as compared to non-mammalian GnRH or the TSH-releasing hormone receptors (Pawson et al., 2008). The refractory state of gonadotrope cells under a sustained GnRH challenge is thus believed to occur through desensitization mechanism affecting downstream signaling entities such as Gαq/11, PKC isoforms , or IP3 receptors (Willars et al., 2001; Liu et al., 2003) rather than the receptor itself. "
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    • "desensitization of the GnRH receptor and, hence, medical castration (Maudsley et al., 2004; Pawson et al., 2008). Moreover , according to their " dual pharmacology, " GnRH analogs are also characterized by direct anticancer potential via their interference with growth and survival factors, such as insulin-like growth factor-I, epidermal growth factor, and keratinocyte growth factor (Culig et al., 1994; Montagnani Marelli et al., 2007). "
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