All-trans retinoic acid increases NF-κB activity in PMA-stimulated THP-1 cells upon unmethylated CpG challenge by enhancing cell surface TLR9 expression

An active metabolite of vitamin A, all-trans retinoic acid (ATRA), is known to exert immunomodulatory functions. This study investigates the possible immune potentiating effect of ATRA on NF-kappa B activity in human monocytic THP-1 cells after exposure to unmethylated CpG DNA ODN2006. We observed that challenge with ODN2006 significantly enhanced the NF-kappa B activity of PMA-differentiated THP-1 cells. ATRA synergistically enhanced NF-kappa B activity of cells, in a concentration- and time-dependent manner. The enhanced NF-kappa B activity of PMA-differentiated THP-1 cells after ODN2006 challenge was dependent on the RAR/RXR pathway. To determine the mechanism involved in increasing in the NF-kappa B activity of stimulated THP-1 cells, we examined the effects of PMA and ATRA on the expression of TLR9 (a receptor of ODN2006) in THP-1 cells. PMA treatment significantly enhanced both the intracellular and cell surface expression of TLR9, while ATRA alone showed no effect. However, ATRA synergistically enhanced the cell surface TLR9 expression of PMA-differentiated cells. To determine whether the ATRA-enhanced NF-kappa B activity is due to the enhanced cell surface TLR9 expression, we examined NF-kappa B activity after treatment with anti-TLR9 blocking antibody. Results revealed that the anti-TLR9 antibody treatment almost completely reverses the ATRA-enhanced NF-kappa B activity, suggesting that ATRA enhances NF-kappa B activity through upregulation of the cell surface TLR9 expression in PMA-differentiated and unmethylated CpG challenged THP-1 cells.