Evidence of a retinoid signaling alteration involving the activator protein 1 complex in tumorigenic human bronchial epithelial cells and non- small cell lung cancer cells

Retinoids, including retinol and retinoic acid derivatives, inhibit the growth of normal human bronchial epithelial (HBE) cells. Using a lung carcinogenesis model consisting of normal, immortalized, and tumorigenic HBE cells, we showed previously that, compared to normal HBE cells, the tumorigenic HBE cell line 1170I is resistant to the growth-inhibitory effects of all-trans-retinoic acid (t-RA). Retinoid receptor function is preserved in tumorigenic 1170I cells, suggesting that other retinoid signaling components are altered. The activator protein 1 (AP-1) complex is a component of the retinoid signaling pathway and has demonstrated importance in cellular growth and differentiation. Therefore, we investigated whether AP-1 is involved in a retinoid signaling defect in tumorigenic 1170I cells and in retinoid- resistant non-small cell lung cancer (NSCLC) cell lines. We found that t-RA treatment inhibited AP-1 transcriptional activity in normal HBE cells but not in tumorigenic 1170I cells nor in the NSCLC cell lines Calu-1, Calu-6, SKMES- 1, and ChaGo K1. We sought mechanisms for this retinoid signaling alteration involving AP-1 in tumorigenic 1170I cells. Basal AP-1 transcriptional activity; AP-1 DNA-binding activity; and the mRNA levels of c-fos, the AP-1 coactivator Jun activation domain-binding protein 1, and the retinoid receptor corepressor, the silencing mediator for retinoid and thyroid hormone receptors (SMRT), were lower in tumorigenic 1170I cells than in normal HBE cells. Transient transfection of tumorigenic 1170I cells with c-fos or CREB binding protein, which is a coactivator of AP-1 and retinoid receptors, enhanced basal AP-1 transcriptional activity but did not alter the effects of t-RA on AP-1 transcriptional activity. These findings provide evidence of a retinoid signaling alteration involving AP-1 in tumorigenic 1170I and NSCLC cells. Furthermore, the inhibitory effect of t-RA on AP-1 transcriptional activity was not restored in tumorigenic 1170I cells by transfection of c- fos, silencing mediator for retinoid and thyroid hormone receptors, Jun activation domain-binding protein 1, or CREB-binding protein, suggesting the involvement of other transcriptional coregulators in this retinoid signaling defect.