Increased endocytosis and formation of multivesicular bodies in phorbol ester-stimulated human monoblastic U-937...

The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is known to arrest mitotic activity and induce macrophage differentiation in the U-937 monoblastic cell line. The acute effect of TPA on ultrastructural morphology and endocytic activity of U-937 cells was studied. TPA induced within 15 min a marked enlargement of multivesicular bodies (MVBs), comprising both volume and number of inclusion vesicles (other organelles appeared unchanged). At this stage the MBVs frequently showed tubular cytoplasmic extensions. Inclusion vesicles accumulated in MBVs with prolonged incubation (60 min). Horseradish peroxidase (HRP) and cationized ferritin (CF) added to the medium were routed preferentially to MVBs in TPA-stimulated cells. In contrast to MVBs of unstimulated cells many of the TPA-induced MVBs showed a positive cytochemical reaction to acid phosphatase. The MVBs in cells incubated with ionomycin, a calcium ionophore, did not differ from those of unstimulated cells. Cellular uptake of 125I-HRP was increased five times the control values already after 5 min of TPA stimulation. The uptake increased further with prolonged incubation (60 min), but at a slower rate. Together these indicate a TPA-induced transfer by endocytosis of portions of the plasma membrane to the lysosomal system via MVBs. Consideration of MVBs as part of the receptor-mediated endocytic pathway suggests that this effect of TPA might involve down-regulation of cell-surface receptors. The possibility of MVBs as a proton-sequestrating compartment, responsible for the cytoplasmic alkalinization previously reported for TPA-stimulated U-937 monoblastic cells, is discussed.