Expression and regulation of nuclear retinoic acid receptors in human lymphoid cells

Retinoids are known to play an important role in cellular growth and differentiation and more recently in the immune response. Our laboratory has previously shown that all-trans-retinoic acid (atRA) augments immunoglobulin synthesis of cord blood mononuclear cells by enhancing the synthesis of certain cytokines. Transcriptional regulatory elements, the retinoic acid nuclear receptors (RAR), could mediate the RA-induced regulation of genes, e.g., cytokines whose products are involved in the pathways of immunoglobulin synthesis. Although much is known about RAR in various animal species and tissues, little is known about the expression of RAR and its isotypes in human lymphoid cells. In this study, we examined the RAR isotypes (RAR-alpha, RAR-beta, RAR-gamma) and their respective isoforms in T- and B-lymphoid cells using a quantitative RT-PCR assay. RAR-alpha1 and -gamma1 were both constitutively expressed and did not change with the addition of atRA to human T- and B-cell lines or adenoidal T and B lymphocytes. In contrast, RAR-beta2 was not detected. The addition of atRA to cell culture produced a marked increase in the amounts of RAR-beta2 mRNA (2.2- to 41-fold). As with the RAR-beta2 isoform, the addition of atRA increased RAR-alpha2 mRNA levels (3.4- to 17-fold), but only in EBV-transformed B cells and adenoidal B lymphocytes. The RAR-beta1 and -beta3 isoforms were undetectable in lymphoid cells and not inducible with atRA. RAR-gamma2 was expressed at very low levels and was not inducible with atRA. Our results suggest that the expressions of the RAR-alpha2 and -beta2 isoforms in lymphoid cells are highly controlled by atRA. Differences in the regulation of RAR isoforms by atRA in human lymphoid cells may be an important factor in the modulation of cytokine production and the augmentation in Ig synthesis by atRA.