Effects of vasoactive intestinal peptide (VIP) and somatostatin (SST) on lipoprotein receptor expression by A431 tumor cells

A variety of tumor cells have been shown to express lipoprotein receptors. Recent data suggest that lipoprotein receptors may play a regulatory role in the growth of certain tumor cells. We investigated the effects of vasoactive intestinal peptide (VIP) and somatostatin-14 (SST-14) on the binding of 111Indium-labeled lipoproteins [(111)In-low density lipoprotein ((111)In-LDL), (111)In-high density lipoprotein ((111)In-HDL) and (111)In-very low density lipoprotein ((111)In-VLDL)] onto the epidermoid mammary carcinoma cell line A431. Scatchard analyses of the binding data indicated one class of specific high affinity binding sites for LDL, HDL and VLDL expressed by A431 cells, respectively. VIP increased significantly the binding capacity for (111)In-LDL on A431 cells. The VIP-induced increase of (111)In-LDL binding sites was inhibited by SST-14. Furthermore, SST-14 inhibited VIP-induced 3H-thymidine incorporation and adenosine 3'-5' cyclic monophosphate (cAMP) formation in A431 cells with IC50 values in the range of 5-7 nM. However, SST-14 showed no effect on dibutyryl-cAMP-induced increase of (111)In-LDL binding sites expressed on A431 cells. In contrast to (111)In-LDL binding, no effects of VIP or SST-14 on HDL or VLDL binding to A431 tumor cells were found. Our results suggest a direct effect of VIP and SST-14 on LDL-binding onto tumor cells. The complex interactions between VIP and SST-14 on LDL receptor expression of tumor cells may play a role in tumor cell lipid metabolism.