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Contribution of AP-1 Interference Induced by TAC-101 to Tumor Growth Suppression in a Hepatocellular Carcinoma Model

TAC-101, 4-[3,5-bis(trimethylsilyl)benzamido] benzoic acid, is a synthetic ligand for retinoic acid receptor (RAR)-α. Here, we demonstrate the contribution of TAC-101-induced AP-1 interference to stabilization of tumor growth. TAC-101 induced transcriptional activation of RAR, resulting in marked elevation of RARβ, a representative retinoid response marker, and it also significantly repressed the transcriptional activity of AP-1 in JHH-7 cells. In contrast to JHH-7, JHH-6 is another RARα-expressing human hepatocellular carcinoma (HCC) cell line with constitutive activation of AP-1, but it is retinoid insensitive and did not respond to the TAC-101-induced RAR signal. TAC-101 did not inhibit AP-1 activity of the JHH-6 cell line, showing that AP-1 interference by TAC-101 must be in parallel with RAR activation. Interleukin-8 (IL-8), one of the AP-1-regulated factors which correlate with a poor prognosis in HCC patients, was found to be overexpressed in JHH-7 cells. TAC-101 reduced IL-8 production without cytotoxicity and inhibited the progression of HCC in the orthotopic mouse model with decreased tumor IL-8 level. These results suggest that downregulation of the extracellular biomarker for AP-1 interference via the induction of retinoid signals will enhance the pharmacological effect of TAC-101 on HCC and it could be useful as a surrogate biomarker of therapeutic efficacy.