Retinoic acid-induced blr1 expression requires RAR alpha, RXR, and MAPK activation and uses ERK2 but not JNK/SAPK to accelerate cell differentiation

Upstream signaling requirements of retinoic acid (RA)induced blr1 expressio n and downstream signaling consequences of blr1 over-expression in a human myeloid leukemia cell line demonstrate that mitogen-activated protein kinas e (MAPK) signaling complexes are involved in both avenues. RA-induced myelo id differentiation and G(1)/G(0) growth arrest of HL-60 cells is known to r equire the activation of the RAR alpha and RXR retinoid receptors, as well as activation of the MAPK, ERK2. Transcriptional activation of the Burkitt' s lymphoma receptor 1 (blr1) gene occurs early during RA-induced differenti ation of HL-60 cells and requires these same three activating processes. Th e use of retinoid ligands that activate either the RAR alpha or the RXR ret inoid receptors revealed that blr1 mRNA induction was detectable only when both RAR alpha and RXR were activated. Neither the RAR alpha nor RXR select ive ligands alone induced expression of blr1, but the combination of the tw o ligands induced the expression of blr1 to the same extent as RA. The MAPK K (MEK) inhibitor, PD98059, was used to determine whether extracellular sig nal-regulated kinase (ERK2) activation was necessary for induction of blr1 mRNA, PD98059 inhibited induced blr1 mRNA expression, due to RA or activate d RAR alpha plus RXR ligands, indicating that ERK2 activation is necessary for blr1 mRNA expression. Previous studies showed that ectopic expression o f blr1 also caused increased MAPK activation, in particular ERK2, and subse quently accelerated RA-induced differentiation and G(1)/G(0) growth arrest. Inhibition of ERK2 activation inhibited differentiation of blr1 transfecta nts, suggesting that the accelerated differentiation reflected blr1-enhance d ERK2 activation. The present data also demonstrate that ectopic expressio n of blr1 increased JNK/SAPK activity, but JNK/SAPK activation was not need ed for accelerated RA-induced differentiation and growth arrest. The result s show that the signals known to be required for HL-60 differentiation, act ivated RAR alpha, RXR, and ERK2, are necessary for blr1 mRNA expression. Do wnstream consequences of blr1 overexpression include enhanced MAPK signalin g.