Investigating three somatostatin receptor (SSTR)-positive (+) human breast cancer cell lines, Xu et al. found a time- and dose-dependent up- or down-regulation of SSTR2 mRNA expression by 17beta-oestradiol (E(2)) or the anti-oestrogen tamoxifen, respectively, in the two oestrogen receptor-positive (ER+) cell lines but not in the oestrogen receptor-negative (ER-) cell line. This study aimed to confirm the findings of Xu et al. at the protein level by means of western blotting and saturation binding studies using (99m)Tc-depreotide (NeoSpect). The ER+/SSTR+ ZR75-1 and T47D and SSTR+/ER- MDA MB231 breast cancer cell lines were exposed to 1 n M E(2) or a combination of 1 n M E(2) plus 100 n M tamoxifen or ICI 182 780 (Faslodex) for 48 h. Exposed and non-exposed controls were incubated with increasing concentrations of (99m)Tc-depreotide (0.5 n M-15 n M) in the absence and the presence of 20 micro M of octreotide. Scatchard-Rosenthal plots were derived using commercially available software. SSTR subtypes responsible for E(2)-induced changes in (99m)Tc-depreotide binding were identified by means of western blotting. Mean K(d) values for (99m)Tc-depreotide were 13 n M, 7 n M and 4 n M for T47D, ZR75-1 and MDA MB231 cells, respectively. After stimulation with E(2), the ER+ cell line T47D demonstrated a mean increase of 81% ( P<0.05) in (99m)Tc-depreotide binding. Adding the partial agonist tamoxifen and full antagonist ICI 182 780 to E(2) blocked the induced increase in T47D cells, either reducing SSTR expression or restoring it to control levels. ZR75-1 cells stimulated with E(2) showed a mean decrease in (99m)Tc-depreotide binding of 36% as compared to control cells; this difference, however, proved to be not statistically significant. Similarly, B(max) values did not change in ZR75-1 cells exposed to E(2) in combination with an ER antagonist as compared to control cells. Finally, no influence of E(2) on (99m)Tc-depreotide binding was observed in the ER- cell line MDA MB231. Both SSTR2 and SSTR5 were expressed at high levels in T47D cells and ZR75-1 cells. SSTR5 drastically increased in the absence of E(2) and was restored to the original detection level after E(2) treatment. The presented findings support an oestrogen-dependent regulation of SSTR expression in breast cancer cell lines.