Signaling and growth responses of LLC-PK1/Cl4 cells transfected with the rabbit AT1 ANG II receptor

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Abstract
Angiotensin II (ANG II) receptors of the AT1 subtype are present on the apical and basolateral membranes of renal proximal tubule cells. Cells of the proximal tubulelike cell line, LLC-PK1/Cl4, were transfected with an expression plasmid containing cDNA encoding the rabbit AT1 ANG II receptor. In transfected cells, specific binding of 125I-ANG II was detected on both apical and basolateral membranes; wild-type LLC-PK1/Cl4 cells did not express ANG II receptors. In transfected cells, apical or basolateral ANG II increased both S6 kinase activity and incorporation of [3H]leucine. In cells pretreated with pertussis toxin, the stimulatory effect of apical or basolateral ANG II on [3H]leucine incorporation was abolished. In contrast, ANG II did not affect mitogenesis, determined by [3H]thymidine incorporation. Apical or basolateral ANG II (10(-6) M) stimulated phosphoinositide turnover by 13.4 +/- 4.4% (n = 8) and 16.3 +/- 4.2% (n = 9), respectively. The activity of protein kinase C, determined by phosphorylation of a specific protein kinase C peptide substrate, was also stimulated by ANG II in transfected cells. Apical or basolateral ANG II had no significant effect on cellular adenosine 3',5'-cyclic monophosphate levels. In permeabilized transfected cells, apical ANG II (10(-6) M) inhibited the phosphorylation of a specific peptide substrate of protein kinase A; lower apical concentrations or basolateral ANG II were without significant effect. These results indicate that AT1 ANG II receptors sort to both apical and basolateral membranes in renal epithelial cells and are coupled to activation of phospholipase C. ANG II stimulates protein synthesis by binding to either apical or basolateral receptors; this effect requires coupling to G proteins and may be mediated by activation of S6 kinase. Because high concentrations of ANG II exist in proximal tubule, binding to apical and basolateral receptors may regulate proximal tubule cell growth under physiological conditions.

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