Vasoactive intestinal peptide (VIP) is a widely distributed neuropeptide that has been considered a potential regulator of cell growth and differentiation in various tissues, including the gut. To examine this idea, we used a human colon carcinoma cell line (LoVo) as a model system and measured ornithine decarboxylase (ODC), because this is the rate-limiting enzyme for the formation of polyamines, which are thought to be key factors in regulating cell growth. LoVo cells, grown to about 80% confluence in F-12 medium containing 10% fetal bovine serum, were preincubated for 5 h in low serum medium (1% fetal bovine serum in F-12), and ODC activity was determined by measuring 14C02 liberated from 14C-labeled ornithine. VIP caused a dose-related biphasic change in ODC, with activity increased at 10 pM, maximal (5-fold increase) at 10 nM, and decreased toward basal at 100 nM to 1 μM. Incubation of cells for 6 days with VIP in low serum medium showed similar changes in cell numbers, with growth being increased by doses in the 1 pM to 100 nM range and decreased at higher doses (£100 nM). Exposure of cells to 5 mM a;-difluoromethylornithine blocked both the VIP-induced increase in cell number and the VIP-induced increase in ODC activity. Increased ODC mRNA was detected after 2 h of exposure to VIP, a time at which ODC activity peaked after treatment, and the increase in ODC mRNA caused by VIP was dose-dependent. In related experiments LoVo cells were found to have high affinity VIP receptors (Kd = 0.4 nM), as assessed by examination of [125I]VIP binding in the presence of varying concentrations of unlabeled VIP. Studies of intracellular cAMP revealed a dose-related increase in cAMP in response to VIP (ED60 = 11 pM), and the adenylate cyclase activator forskolin increased both ODC activity and ODC mRNA. The findings support the idea that LoVo cells have VIP receptors linked to cAMP which can stimulate cell growth at least in part by increasing ODC synthesis and activity, thereby altering the production of polyamines. The decreased growth and ODC activity observed with high doses of VIP may involve a second messenger other than cAMP.
ASJC Scopus subject areas