Somatostatin is an inhibitory regulator with important cellular effects whose actions are mediated through the G protein-coupled somatostatin receptors 1-5 (sst1-5). We have previously described the existence of sst2 in human endometrium, but the expression for other isoforms was unknown. The precise control of cellular proliferation and angiogenesis in the endometrium is not yet fully understood, and we hypothesise that somatostatin may have a role.
Archived tissue sections of human endometrium from the proliferative (n=11), early secretory (n=4), late secretory (n=4) and menstrual (n=7) phases of the cycle were stained with anti-sst antibodies. The immunohistochemistry staining was analysed to determine the receptor expression and localisation. The staining intensity was quantified using image analysis software. All samples were histologically normal.
Immunoreactivity was present for all somatostatin receptors in both epithelial and stromal cells. Sst1, sst2B and sst3 were positively expressed in most endometrial samples (21-26/26), but present less frequently for sst4 (10/26) and sst5 (4/26). Quantitative analysis showed the intensity of receptor expression differed significantly between the menstrual phases. Epithelial expression of sst1 was significantly higher (P<0.001) in the early secretory phase than in both the proliferative or late secretory phases. Epithelial expression of sst2B (P<0.0001) and sst3 (P<0.0001) was significantly higher in the proliferative phase compared to the early and late secretory phases. These two receptors also showed a downward trend in intensity from early to late secretory phase. The intensity of expression in stromal cells was lower than that in epithelial cells. Stromal cell expression of sst3 was significantly higher in the proliferative than secretory phases (P<0.001).
We have demonstrated the expression of all sst receptors in the endometrium that has not previously been described. The different expression levels of sst1, sst2B and sst3 in different endometrial phases suggests that they have a role in the normal menstrual cycle.