RAR beta 2-mediated growth inhibition in HeLa cells

Retinoic acid inhibits the growth of a variety of normal and transformed cells in vitro and in vivo. How retinoic acid inhibits cell growth is poorly understood but involves interactions between the ligand and a series of nuclear and cytoplasmic receptors. The nuclear receptors for retinoic acid are of two types, the RARs and the RXRs. Each can function as a ligand-inducible transcription enhancing factor. In previous studies, we have demonstrated that an isoform of one RAR, RAR beta 2, is transcriptionally up-regulated in senescent human dermal fibroblasts and senescent human mammary epithelial cells. Moreover, we have also shown that RAR beta 2 can inhibit oncogene-induced focus formation, in primary rat embryo fibroblasts, as effectively as the tumor suppressor gene p53. Here, we extend our studies of retinoid-regulated signal transduction pathways that inhibit cell proliferation by demonstrating that HeLa cells expressing an RAR beta 2 construct are growth inhibited by greater than 50% when compared to the parent cell lines. The RAR beta 2-expressing cell lines are inhibited further by the addition of exogenous all-trans-retinoic acid. Finally, soft agar assays show that the RAR beta 2-expressing cell lines also demonstrate an inhibition of growth in soft agar, when compared to the parent growth cell lines, and are inhibited further in the presence of added all-trans-retinoic acid. These data definitively show that RAR beta 2 can inhibit cell proliferation in an established tumor cell line and provide more strength to the notion that this isoform is an effective growth inhibitor in vitro and, most likely, in vivo.