Increased expression of the mRNA encoding the somatostatin receptor subtype five in human colorectal adenocarcinoma

Numerous studies have suggested that the anti-proliferative potency of soma tostatin (SS) analogues may be an efficient tool to improve the prognosis o f colorectal cancer. In order to facilitate current efforts to design poten t antitumour SS analogues, we studied the distribution of human SS receptor s (hsst1-5) mRNAs in a large set of tumoural and normal colonic tissues. Lo calisation of hsst1-5 mRNAs in normal and tumoural tissues was performed by in situ hybridisation using radioactive antisense or sense riboprobes. Sem i-quantitative analysis of hsst5 mRNA was performed using a computerised im age analysis system. Hsst binding sites were characterised by studying the relative potency of SS14, SS28 or SS analogues in displacing [I-125]Tyr(o) -D-Trp(8)-SS14 bound to HT29-D4 cells. Hsst5 mRNA was by far the most expre ssed subtype in both normal and transformed epithelial cells as well as in the HT29-D4 cell line. An increased expression of hsst5 mRNA was found in t umours. Hsst1 mRNA was expressed preferentially as clusters in immune cells in lamina propria and in stroma close to the tumour. A low expression of h sst4, hsst3 and hsst2 was seen in normal and tumoural tissue. In HT29-D4, b inding experiments with SS14 demonstrated the existence of one SS binding c lass (K-d=524 nM, B-max=1fmol/10(6) cells). In competition binding studies, SS28 and BIM23268 (an analogue that shows preferential specificity towards hsst5) effectively inhibited binding of [I-125]Tyr(o)-D-Trp(8)-SS14 (IC50= 15 and 157 nM respectively), while BIM23197 (an analogue that shows prefere ntial affinity for hsst2) was ineffective. Our results show a high expressi on of hsst5 mRNA in human tumoural colonic tissue, while hsst5 protein is t he predominant hsst protein subtype in a tumoural colonic cell line.