Expression of somatostatin receptors in proliferating and quiescent endothelial cells
RL Adams, IP Adams & SL Atkin
Somatostatin (sst) modulates exocrine and endocrine secretions, cellular proliferation and apoptosis, via a series of 5 G-protein linked transmembrane receptors (sstrs 1-5). Sstrs 2 and 5 have been reported on vasculature but their role remains obscure. Here, we report the expression of sstrs in endothelial cells in vitro, during both proliferative and quiescent states to determine whether these receptors may change dynamically. Endothelial cells were harvested from ten human umbilical veins (HUVECs). Cells were seeded and subsequently harvested in proliferation (70% confluence) or quiescence (100% confluence). Samples were then propidium-iodide stained and cell cycle distribution analysed using flow cytometry, to identify whether the samples were in proliferative or quiescent phases. Sstr mRNA was identified using qRT PCR. HUVECs expressed mRNA for sstrs 1, 2 and 5. Sstrs 1, 2 and 5 were expressed in 5/10, 8/10 and 6/10 proliferative HUVECs respectively. Of these, 1/5, 5/8 and 5/6 corresponding quiescent HUVECs were negative for receptors 1, 2 and 5, respectively. Expression of sstrs 2 and 5 in receptor positive quiescent samples was reduced; it was 1% to 42% of levels found in corresponding proliferating samples. Two quiescent samples were sstr 1 positive when the corresponding proliferating sample was sstr 1 negative. Quiescent HUVECs were not sstr 2 or 5 positive when corresponding proliferating cells were sstr 2 or 5 negative. All samples expressed at least one sstr and all were positive for a receptor when proliferating that they did not express, or expressed at lower levels, when quiescent. Sstr 5 mRNA has not previously been found in HUVECs. These results show that expression of sstr 2 and sstr 5 increases when HUVECs are proliferating, indicating that expression of sstrs 2 and 5 changes dynamically in the cell cycle suggesting their active role in angiogenesis.