Okadaic acid gives concentration-dependent reciprocal effects on the fluid phase endocytosis activated by Ca2+ and phorbol 12-myristate 13-acetate.

Incubation of a human fibrosarcoma cell line HT-1080 in increasing concentration of Ca2+ was found to enhance endocytic internalization of a fluid phase marker, horseradish peroxidase. At 16.8 mM Ca2+, generation of the effect required incubation for more than 45 min. The effect was reversed by removal of the excess ion for 30 min. Monitoring the intracellular concentration showed that the incubation induced a transient large Ca2+ influx followed by a recovery to 230 +/- 50 nM instead of the normal level of 83 +/- 5 nM. The activation was not inhibited by inhibitors of protein kinases nor a cAMP antagonist. In contrast, the effect was prevented by okadaic acid (OKA) at 100 nM without detectable effect on the basal activity. Fluid phase uptake by HT-1080 cells was also enhanced by phorbol 12-myristate 13-acetate (PMA). In contrast to the case with Ca2+, OKA at 100 nM did not prevent the PMA effect but further enhanced the endocytosis. The effect of OKA was concentration-dependent, as the reagent at 1 microM inhibited not only both the activation but also the basal activity. In Ca(2+)- or PMA-stimulated cells, FITC-dextran was delivered to endosomes that had been labeled with TRITC-transferrin. In contrast, following treatment with a combination of PMA and 100 mM OKA, fluid phase was internalized in vesicular compartments devoid of transferrin labeling. These results suggest that, through differential modifications of protein phosphorylation, endocytosis can be enhanced distinctively either by employing conventional receptor-bearing compartments or generating a new endosomal population.