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A tyrosine phosphatase is associated with the somatostatin receptor
N Delesque1, L Buscail, J P Estève
1INSERM U151, Institut Louis Bugnard, CHU Rangueil, Toulouse, France.
Abstract:
Regulation of tyrosine phosphorylation is thought to be an essential step in signal transduction mechanisms that mediate cellular responses. In pancreatic tumour cells we demonstrated that somatostatin analogues inhibited cell proliferation and stimulated a membrane protein tyrosine phosphatase (PTP) activity at concentrations at which they bind to the somatostatin receptor. To elucidate the role of PTP in the signal transduction pathway activated by somatostatin receptors we first studied the interaction of PTP with the somatostatin receptor at the membrane. We purified somatostatin receptors by immunoaffinity from pancreatic membranes that strongly expressed the type 2 somatostatin receptor sstr2. We identified the receptor as an 87 kDa protein. We demonstrated that a PTP activity co-purified with somatostatin receptors. The PTP was identified as a 66 kDa protein immunoreactive to antibodies against SHPTP1. These antibodies immunoprecipitated somatostatin receptors either occupied or unoccupied by ligand indicating that SHPTP1 is associated with somatostatin receptors. We then expressed sstr2A in monkey kidney COS-7 cells and mouse NIH/3T3 fibroblasts and demonstrated that somatostatin analogues (RC 160, octreotide and BIM 23014) which exhibited high affinity for sstr2 stimulated a PTP activity and inhibited cell proliferation in proportion to their affinities for sstr2. Under the same conditions these analogues have no effect on the growth of cells expressing sstr1. All these results suggest that a PTP related to SHPTP1 is associated with somatostatin receptors and may be involved in the negative growth signal promoted by sstr2.
Insights
Somatostatin receptor activation inhibits pancreatic tumor cell growth by stimulating a protein tyrosine phosphatase (PTP) related to SHPTP1. This PTP associates with the somatostatin receptor, suggesting a role in signal transduction for cell growth inhibition.
Area of Science:
- Cellular signaling
- Molecular biology
- Oncology
Background:
- Tyrosine phosphorylation is crucial for cellular responses.
- Somatostatin analogues inhibit pancreatic tumor cell proliferation.
- Somatostatin receptor activation stimulates membrane protein tyrosine phosphatase (PTP) activity.
Purpose of the Study:
- To elucidate the role of PTP in the somatostatin receptor signal transduction pathway.
- To investigate the interaction between PTP and somatostatin receptors.
- To identify the specific PTP involved in somatostatin receptor signaling.
Main Methods:
- Immunoaffinity purification of somatostatin receptors (sstr2) from pancreatic membranes.
- Co-purification and identification of associated PTP activity.
- Expression of sstr2A in COS-7 cells and NIH/3T3 fibroblasts.
- Assessment of PTP activity and cell proliferation in response to somatostatin analogues.
Main Results:
- An 87 kDa somatostatin receptor (sstr2) was purified.
- A 66 kDa PTP, immunoreactive to SHPTP1 antibodies, co-purified with sstr2.
- SHPTP1 antibodies immunoprecipitated somatostatin receptors, indicating association.
- Somatostatin analogues stimulated PTP activity and inhibited cell proliferation in sstr2-expressing cells, proportional to receptor affinity.
- No effect on cells expressing sstr1.
Conclusions:
- A PTP related to SHPTP1 is associated with somatostatin receptors (sstr2).
- This association suggests SHPTP1 involvement in the negative growth signaling pathway mediated by sstr2.
- Somatostatin receptor signaling in pancreatic cancer involves PTP activation for growth inhibition.