ACTIVATION IN-VITRO OF SOMATOSTATIN RECEPTOR SUBTYPE-2, SUBTYPE-3, ORSUBTYPE-4 STIMULATES PROTEIN-TYROSINE-PHOSPHATASE ACTIVITY IN MEMBRANES FROM TRANSFECTED RAS-TRANSFORMED NIH 3T3 CELLS - COEXPRESSION WITH CATALYTICALLY INACTIVE SHP-2 BLOCKS RESPONSIVENESS

Somatostatin receptors (sstr) subtypes 1-5 were transiently expressed in NIH 3T3 cells stably transformed with Ha-Ras(G12V) to assess the ab ility of each receptor to stimulate protein tyrosine phosphatase (PTPa se) activity in vitro. Treatment of membranes from sstr(2)-, sstr(3)-, or sstr(4)-expressing cells with somatostatin-14 plus guanyl-5'-yl im idodiphosphate (GMPPNP) increased PTPase activity, and this stimulatio n was pertussis toxin-sensitive. Somatostatin alone, GMPPNP alone, or somatostatin plus GDP were ineffective under these conditions. sstr(1) and sstr(5) failed to increase PTPase activity although both receptor s were expressed, as assessed by appearance of high-affinity binding s ites for [I-125-Tyr(11)]somatostatin-14. Somatostatin plus GMPPNP stim ulated PTPase activity in vitro when sstr(2) was coexpressed with wild type PTP1B or a Cys to Ser (C/S), catalytically inactive PTP1B or wit h wild type SH2-domain containing PTPase SHP-2. However, coexpression with catalytically inactive C/S SHP-2 abrogated this response. Thus, t hree of the five cloned sstr's can couple to activate PTPase in this c ellular background. Abrogation of the response by C/S SHP-2 strongly s uggests, but does not prove, a role for SHP-2 in the mechanism.