The human 5-HT(1A) receptor expressed in HeLa cells stimulates sodium-dependent phosphate uptake via protein kianse C

Regulation of phosphate uptake was studied in HeLa cell lines after transfection with DNA encoding the human 5-HT(1A) receptor. Phosphate uptake was saturable and >90% sodium-depEndent, with V(max) ~ 4.3 nmol/mg protein/3 min and K(m) ~ 0.4 mM phosphate. Activators of protein kinase A decreased V(max) by ~ 30-35% without changing K(m). Treatment with 5-HT or the 5-HT(1A)-specific agonist 8-OH-2-(di-n-propylamino)1,2,3,4-tetrahydronaphthalene increased V(max) ~ 40% without affecting K(m). This effect was blocked by pretreatment with the 5-HT₁ antagonists, methiothepine and spiperone, or pertussis toxin. Surprisingly, the stimulation was not secondary to an inhibition of adenylyl cyclase because 5-HT stimulated phosphate uptake ~ 20% in the presence of 1 mM 8-Br-cAMP. Rather, the primary pathway linked to the stimulation of phosphate uptake involved activation of protein kinase C because (i) 5-HT measurably activated protein kinase C in these cells, (ii) activators of protein kinase C (phorbol esters and diacylglycerol analogues) stimulated phosphate uptake in these cells (iii) the half-maximal doses for 5-HT-induced phosphatidylinositol hydrolysis and stimulation of phosphate uptake were virtually equivalent, and both effects were equally sensitive to pertussis toxin, and (iv) the stimulation was markedly attenuated in cells made deficient in protein kinase C. These results demonstrate that the stimulation of phosphatidylinositol hydrolysis by the 5-HT(1A) receptor can generate physiologically measurable effects on cellular transport and suggest that such accessory pathways may play a prominent role in signal transduction.