Second messenger regulation of lipoprotein uptake by an arterial wall model

Second messenger signaling has been shown to regulate a variety of cellular functions in response to external stimuli. The following study was performed to determine the potential role of second messengers on influencing lipoprotein uptake by the arterial wall. An aortic endothelial cell (EC)-smooth muscle cell (SMC) bilayer was preexposed to various mediators of the cyclic AMP and inositol phosphate pathways for 30 min. The permeability, binding, and cellular uptake of 125I-labeled low-density lipoprotein (LDL) (10 micrograms/ml) added to the upper well media of the bilayer were then measured for each cell type after a 3-h incubation period. Forskolin (100 microM), an activator of adenylate cyclase, resulted in an increase in all measured parameters. 8-Br-cAMP (30 microM), a cAMP analogue, showed a similar effect on EC permeability (P < 0.00005) while galanin (0.1 mg/ml), an adenylate cyclase inhibitor, had no effect. GTP-binding protein inhibition with pertussis toxin (10 mg/ml) led to a marked reduction in SMC uptake (P < 10(-7)) without affecting membrane binding. Protein kinase C activation with phorbol myristate acetate (0.1 mg/ml) also increased EC permeability to LDL but, unlike forskolin, had no effect on LDL binding. This effect was further potentiated by calcium ionophore A23187 (5 x 10(-6) M), indicating a contributing role of intracellular calcium. These results would suggest that LDL uptake can be influenced by several second messenger systems, and that EC and SMC may respond differently to these intracellular signals. Second messenger regulation may allow changes in lipoprotein uptake by the arterial wall in response to external stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)