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C-terminal region of human somatostatin receptor 5 is required for induction of Rb and G1 cell cycle arrest
K Sharma1, Y C Patel, C B Srikant
1Department of Medicine, McGill University and Royal Victoria Hospital, Montreal, Quebec, Canada.
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.
Insights
Somatostatin receptors (SSTRs) mediate antiproliferative signals. Human SSTR subtypes 1, 2, 4, and 5 induce G1 cell cycle arrest by increasing retinoblastoma protein (Rb), with SSTR5 being most effective.
Area of Science:
- Molecular Biology
- Cell Biology
- Endocrinology
Background:
- Ligand-activated somatostatin receptors (SSTRs) regulate cell proliferation through distinct signaling pathways.
- Human SSTR subtype 3 mediates cytotoxic (apoptotic) signals, while other subtypes induce cytostatic (growth-inhibitory) responses.
Purpose of the Study:
- To characterize the antiproliferative signaling mechanisms of human SSTR subtypes 1, 2, 4, and 5 in CHO-K1 cells.
- To elucidate the role of retinoblastoma protein (Rb) and cell cycle arrest in SSTR-mediated cytostasis.
Main Methods:
- Utilized immunoblot analysis to detect changes in Rb protein phosphorylation.
- Investigated G protein and protein tyrosine phosphatase (PTP) involvement in SSTR5 signaling.
- Employed C-terminal truncation mutants of hSSTR5 to assess domain function.
Main Results:
- Cytostatic signaling via hSSTR1, hSSTR2, hSSTR4, and hSSTR5 induced G1 cell cycle arrest through increased hypophosphorylated Rb.
- The relative efficacy of cytostatic signaling was hSSTR5 > hSSTR2 > hSSTR4 ≈ hSSTR1.
- hSSTR5 signaling was G protein and PTP-dependent, involving cytosolic PTP translocation to the membrane and requiring the C-terminal domain of hSSTR5.
Conclusions:
- hSSTR subtypes 1, 2, 4, and 5 mediate cytostatic effects via induction of hypophosphorylated Rb and G1 cell cycle arrest.
- The C-terminal domain of hSSTR5 is crucial for initiating PTP-dependent cytostatic signaling.
- These findings highlight distinct signaling pathways for SSTR subtypes in regulating cell proliferation.