Effects of somatostatin and its analogues on tyrosine kinase activity in rodent tumors

The effects of native somatostatin-14 (SS-14) and of its two analogues, octreotide and CH-275, on the activity of tyrosine kinases (TK) in two rodent tumors (rat pituitary tumor and murine colonic cancer) were studied in vitro. The activity of TK was measured in tissue homogenates using gamma[(32)P]ATP as the donor of the phosphoryl group and poly(Glu(80), Tyr(20)) as a substrate. It was found that native SS-14 inhibited TK activity in both investigated tumors. Octreotide, which acts preferentially via somatostatin receptor subtype 2 (SSTR2), was very effective in inhibiting TK activity in the rat pituitary tumor, but it is a rather weak inhibitor of TK activity in murine colonic cancer. CH-275, a selective ligand of the SSTR1 subtype of SS receptors, suppressed TK activity in the pituitary tumor but was ineffective in the colonic cancer. It is hypothesized that the effect of neuropeptide somatostatin (SS-14) on murine colonic cancer is exerted via the subtype of receptor which does not interact with CH-275 and has no or low affinity for octreotide (SSTR 4, 3 or 5?).