Activation of Rac1 and the p38 mitogen-activated protein kinase pathway inresponse to all-trans-retinoic acid

Several signaling pathways are activated by all-transretinoic acid (RA) to mediate induction of differentiation and apoptosis of malignant cells. In t he present study we provide evidence that the p38 MAP kinase pathway is act ivated in a RA-dependent manner in the NB-4, acute promyelocytic leukemia, and the MCF-7, breast carcinoma, cell lines. RA treatment of cells induces a time- and dose-dependent phosphorylation of p38, and such phosphorylation results in activation of its catalytic domain. p38 activation is not induc ible by RA in a variant NB-4 cell line, NB-4.007/6, which is resistant to t he effects of RA, suggesting a role for this pathway in the induction of RA responses. Our data also demonstrate that the small G-protein Rad is activ ated by RA and functions as an upstream regulator of p38 activation, wherea s the MAPKAPH-2 serine kinase is a downstream effector for the RA-activated p38. To obtain information on the functional role of the Rac1/p38/MAPKAPK- 2 pathway in RA signaling, the effects of pharmacological inhibition of p38 on RA-induced gene transcription and cell differentiation were determined Our results indicate that treatment of cells with the SE203580 inhibitor do es not inhibit RA-dependent gene transcription via retinoic acid response e lements or induction of Stat1 protein expression. However, treatment with S B203580 or SB202190 strongly enhances RA-dependent induction of cell differ entiation and RA-regulated growth inhibitory responses. Altogether, our fin dings demonstrate that the Rac1/p38 MAP kinase pathway is activated in a RA -dependent manner and exhibits negative regulatory effects on the induction of differentiation.