Retinoid target gene activation during induced tumor cell differentiation: human embryonal carcinoma as a model.

Many agents that exhibit chemopreventive activity are able to mediate a differentiation response in premalignant and malignant tissues. One of the most widely studied classes of tumor differentiation agents is the retinoids. There is rapidly evolving evidence for beneficial retinoid actions in the prevention or treatment of clinical tumors. However, the use of retinoids in the clinic is limited by acquired resistance and toxicity, especially when administered chronically in preventive strategies. Although retinoids are known to regulate gene transcription by activating retinoid receptors, the identity of the target genes that mediate the beneficial effects of retinoids are largely unknown. Here we review a useful model of retinoid-induced tumor cell differentiation: human embryonal carcinoma. The pluripotent nature and ease of use make human embryonal carcinoma cells a valuable and practical complement to human embryonic stem cells as an in vitro model of early human development. In addition, retinoid treatment of human embryonal carcinoma is an important model of induced tumor cell differentiation because retinoids cause the reversal of the malignant phenotype coincident with terminal neuronal differentiation. We have used both de novo and candidate approaches with this system in an effort to uncover critical downstream targets of retinoid receptors during differentiation induction.

Developmentally-related candidate retinoic acid target genes regulated early during neuronal differentiation of human embryonal carcinoma.

Embryonal carcinoma is a model of embryonic development as well as tumor cell differentiation. In response to all-trans retinoic acid (RA), the human embryonal carcinoma (EC) cell line, NT2/D1, differentiates toward a neuronal lineage with associated loss of cell growth and tumorigenicity. Through the use of cDNA-based microarrays we sought to identify the early downstream targets of RA during differentiation commitment of NT2/D1 cells. A total of 57 genes were induced and 37 genes repressed by RA. RA regulated genes were restricted at 8 h with 27 genes induced and five repressed. The total number of RA-responsive transcripts increased at 24 and 48 h and their pattern of expression was more symmetrical. For a given time point less than 1% of the 9128 cDNAs on the expression array were regulated by RA. Many of these gene products are associated with developmental pathways including those of TGF-beta (Lefty A, NMA, follistatin), homeo domain (HoxD1, Meis2, Meis1, Gbx2), IGF (IGFBP3, IGFBP6, CTGF), Notch (manic fringe, ADAM11), Hedgehog (patched) and Wnt (Frat2, secreted frizzled-related protein 1) signaling. In addition a large cassette of genes induced by RA at 24-48 h are associated with cell adhesion, cytoskeletal and matrix remodeling, growth suppression and intracellular signaling cascades. The majority of repressed genes are associated with protein/RNA processing, turnover or metabolism. The early induced genes identified may play a regulatory role in RA-mediated growth suppression and terminal differentiation and may have physiologic or pharmacologic importance during normal human development and retinoid-based cancer therapy or prevention.