sstr2 has been reported to desensitize following continuous agonist pretreatment (23). Pretreatment of cells expressing sstr2 with agonist has been reported to reduce high-affinity agonist binding to sstr2 and reduce the ability of SRIF to inhibit cAMP accumulation. The enzyme beta-adrenergic kinase (BARK) has been proposed to be involved in SRIF receptor desensitization (35) and sstr2 becomes phosphorylated when stimulated with agonist (36). However, recent mutagenesis studies suggest that a more complex mechanism may exist for sstr2 desensitization (37).
To test the role of the carboxy terminus of sstr2 in desensitization, this domain was excised from the receptor (37). The truncated receptor was expressed in CHO cells, had similar pharmacological properties as the wild-type receptor and coupled to G proteins and adenylyl cyclase. Furthermore, it desensitized, suggesting that the carboxy terminus is not essential for agonist regulation (37).
The third intracellular loop of sstr2 has been proposed to have a role in G-protein coupling and has several serine residues at positions 244 and 245 which could serve as phosphate acceptors. Mutation of serine 244 to a tryptophan did not affect agonist regulation of the receptor nor did mutation of serine 245 to an alanine. This finding suggests that phosphorylation of these residues is not essential for the receptor to desensitize.
However, most importantly, mutation of serine 245 to a glutamine made sstr2 much more resistent to agonist regulation. This result was unexpected and indicates that the region around this serine has a role in the regulating the receptor. Conceivably, the positive charge of the glutamine may affect the interaction of sstr2 with cellular factors involved in receptor desensitization.
In addition to desensitizing, sstr2 is rapidly internalized when bound to agonist (38). sstr2A transfected in COS-7 cells bound fluorescent labeled SRIF analogs and internalized within an hour of incubation at 37°C. There is no information on whether similar molecular events are involved in internalizing and desensitizing sstr2. Furthermore, it is not clear whether the internalization is physiologically relevant, since the studies were conducted on transfected cells overexpressing the receptor.
Although sstr2 can be regulated, SRIF inhibition of GH release does not desensitize. This suggests that either sstr2 does not desensitize under physiological conditions or somatotrophs expressing sstr2 lack critical factors needed for desensitization. Although sstr2 coupling to adenylyl cyclase desensitizes, its link to Ca2+ channels is maintained following prolonged agonist stimulation (25). If the inhibition of Ca2+ conductance and influx by SRIF is directly linked to the inhibition of GH release by SRIF, the maintenance of SRIF's control of GH secretion is likely to be owing to the resistance of sstr2/Ca2+ coupling to desensitize. Alternatively, if sstr2 desensitizes, the continued ability of SRIF to inhibit GH secretion may be owing to other receptor subtypes that do not desensitize to SRIF, compensating for sstr2 desensitization and maintaining SRIF control of GH secretion. In this regard, sstr1, is relatively resistent to SRIF regulation (39). Agonist pretreatment does not affect high-affinity agonist binding to sstr1, and this receptor is slow to internalize (38). sstr1 mRNA is expressed in pituitary (1,20) and could link up to GH secretion if sstr2 were desensitized and compensate for the loss of sstr2 responsivity.
sstr2 may also be involved in mediating antiproliferative effects of SRIF analogs in the treatment of cancer (40-42). Octreotide, which is used to treatment pituitary ade nomas and other tumors, potently stimulates sstr2. Buscail et al. (40,41) reported that sstr2 stimulation can lead to an inhibition of cell proliferation. sstr2 was found to couple to a tyrosine phosphatase and activation of the phosphatase parallels the reduction in growth of the cells transfected with sstr2. The coupling of sstr2 to the tyrosine phos-phatase is mediated by G proteins and is yet another cellular response coupled to this receptor subtype. Whether SRIF's ability to inhibit cell proliferation desensitizes is not known, although in general SRIF analogs have not proven to be extremely effective as anticancer agents.
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