PMA withdrawal in PMA-treated monocytic THP-1 cells and subsequent retinoic acid stimulation modulate the induction of apoptosis and the appearance of dendritic cells

OBJECTIVES: To analyse proliferation, differentiation and apoptosis in THP-1 cells after stimulation with phorbol 12-myristate 13-acetate (PMA) and retinoic acid (RA). MATERIALS AND METHODS: PMA and RA were used in a three-step-procedure: (i) treatment with 6, 30, 60 nm PMA, that induced initial, intermediate and advanced levels of monocyte-macrophage transition, respectively; (ii) recovery in PMA-free medium; (iii) incubation with 4 μm RA. Cultures were characterized cytokinetically (flow cytometry/bromodeoxyuridine uptake) and immunocytochemically (static cytometry) for expression of CD14, CD11b (monocyte-macrophage) and DC-SIGN (dendritic cell: DCs) markers. RESULTS: Some treatments determined appearance of monocyte/macrophage, dendritic and apoptotic phenotypes, percentages of which were related to PMA dose used in step 1, and dependent on presence/absence of PMA and RA. PMA withdrawal induced dedifferentiation and partial restoration of proliferative activity, specially in 6 and 30 nm PMA-derived cells. Recovery in the presence of serum (fundamental to DC appearance) indicated that depending on differentiation level, cell proliferation and apoptosis were inversely correlated. Treatment with 30 nm PMA induced intermediate levels of monocytic-macrophagic differentiation, with expression of alternative means of differentiation and acquisition of DCs without using cytokines, after PMA withdrawal and RA stimulation. CONCLUSIONS: Our experimental conditions favoured differentiation, dedifferentiation and transdifferentiational pathways, in monocytic THP-1 cells, the balance of which could be related to both cell proliferation and cell death.