title: C-terminal region of human somatostatin receptor 5 is required for induction of Rb and G1 cell cycle arrest.
abstract: Ligand-activated somatostatin receptors (SSTRs) initiate cytotoxic or cytostatic antiproliferative signals. We have previously shown that cytotoxicity leading to apoptosis was signaled solely via human (h) SSTR subtype 3, whereas the other four hSSTR subtypes initiated a cytostatic response that led to growth inhibition. In the present study we characterized the antiproliferative signaling mediated by hSSTR subtypes 1, 2, 4, and 5 in CHO-K1 cells. We report here that cytostatic signaling via these subtypes results in induction of the retinoblastoma protein Rb and G1 cell cycle arrest. Immunoblot analysis revealed an increase in hypophosphorylated form of Rb in agonist-treated cells. The relative efficacy of these receptors to initiate cytostatic signaling was hSSTR5 > hSSTR2 > hSSTR4 approximately = hSSTR1. Cytostatic signaling via hSSTR5 also induced a marginal increase in cyclin-dependent kinase inhibitor p21. hSSTR5-initiated cytostatic signaling was G protein dependent and protein tyrosine phosphatase (PTP) mediated. Octreotide treatment induced a translocation of cytosolic PTP to the membrane, whereas it did not stimulate PTP activity when added directly to the cell membranes. C-tail truncation mutants of hSSTR5 displayed progressive loss of antiproliferative signaling proportional to the length of deletion, as reflected by the marked decrease in the effects of octreotide on membrane translocation of cytosolic PTP, and induction of Rb and G1 arrest. These data demonstrate that the C-terminal domain of hSSTR5 is required for cytostatic signaling that is PTP dependent and leads to induction of hypophosphorylated Rb and G1 arrest.
authors.affiliation: Department of Medicine, McGill University and Royal Victoria Hospital, Montreal, Quebec, Canada.
authors.forename: K
authors.initials: K
authors.lastname: Sharma
authors.forename: Y C
authors.initials: YC
authors.lastname: Patel
authors.forename: C B
authors.initials: CB
authors.lastname: Srikant
mesh_terms: D000818:Animals; D016466:CHO Cells / Q000187:drug effects / Q000378:metabolism; D006224:Cricetinae; D003600:Cytosol / Q000378:metabolism; D016193:G1 Phase / Q000187:drug effects / Q000502:physiology*; D019204:GTP-Binding Proteins / Q000187:drug effects / Q000378:metabolism; D006728:Hormones / Q000494:pharmacology; D006801:Humans; D008565:Membrane Proteins; D009154:Mutation; D015282:Octreotide / Q000494:pharmacology; D010766:Phosphorylation; D054562:Protein Tyrosine Phosphatase, Non-Receptor Type 1; D017027:Protein Tyrosine Phosphatases / Q000187:drug effects / Q000378:metabolism; D017481:Receptors, Somatostatin / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D016160:Retinoblastoma Protein / Q000235:genetics / Q000378:metabolism*; D013004:Somatostatin / Q000378:metabolism / Q000494:pharmacology; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D006728:Hormones; D008565:Membrane Proteins; D017481:Receptors, Somatostatin; D016160:Retinoblastoma Protein; D010566:Virulence Factors, Bordetella; C098681:somatostatin receptor subtype-4; C101130:somatostatin receptor type 1; D013004:Somatostatin; C094573:somatostatin receptor 5; C093733:somatostatin receptor 2; D054562:Protein Tyrosine Phosphatase, Non-Receptor Type 1; D017027:Protein Tyrosine Phosphatases; D019204:GTP-Binding Proteins; D015282:Octreotide