Yang L, Kim H T, Munoz-Medellin D, Reddy P, Brown P H
Department of Medicine, University of Texas Health Science Center at San Antonio, 78284, USA.
Cancer Res. 1997 Oct 15;57(20):4652-61.
To investigate the role of AP-1 transcription factors in mediating retinoid-induced growth suppression of breast cells, we studied the sensitivity of MCF7 breast cancer cells with different levels of AP-1 activity to all-trans retinoic acid (atRA). AP-1 activity was increased in MCF7 cells by stably transfecting c-jun cDNA into these cells. Parental and vector-transfected MCF7 cells, which were sensitive to the growth-inhibitory effects of atRA, exhibited atRA-dependent retinoic acid receptor (RAR) transactivation and transrepression of 12-O-tetradecanoylphorbol-13-acetate-induced AP-1 activity. The c-jun-transfected MCF7 cells had increased basal AP-1 transactivation activity and increased expression of AP-1-regulated genes but were resistant to the antiproliferative effects of atRA. However, MCF7 cells transfected with a deletion mutant of c-jun, TAM-67, which lacks most of the amino-terminal transactivation domain of cJun and is unable to activate AP-1-dependent gene expression, were sensitive to the growth-inhibitory effects of atRA. These results suggest that the transactivation domain of cJun is required for induction of retinoid resistance in these breast cancer cells. atRA did not activate RAR-dependent gene transcription or transrepress 12-O-tetradecanoylphorbol-13-acetate-induced AP-1 activity in these cJun-overexpressing cells. Investigation of the RAR and retinoic acid X receptor expression level demonstrated that RAR alpha and RAR gamma RNA expression was reduced in the c-jun-transfected MCF7 cells, whereas RAR beta expression was up-regulated. However, retinoic acid responsive element DNA binding activity was intact in c-jun-transfected cells. Therefore, the mechanism by which cJun overexpression induces resistance to the growth-inhibitory effect of atRA may be through interference with atRA-dependent RAR transactivation or AP-1 transrepression, possibly through titration of essential coactivators. These results suggest that the antiproliferative effects of retinoids can be overcome by cJun overexpression.
为了研究AP-1转录因子在介导类维生素A诱导的乳腺细胞生长抑制中的作用，我们研究了具有不同AP-1活性水平的MCF7乳腺癌细胞对全反式维甲酸（atRA）的敏感性。通过将c-jun cDNA稳定转染到MCF7细胞中，可增加其AP-1活性。对atRA生长抑制作用敏感的亲本和载体转染的MCF7细胞，表现出atRA依赖的维甲酸受体（RAR）反式激活以及对12-O-十四烷酰佛波醇-13-乙酸酯诱导的AP-1活性的反式抑制。c-jun转染的MCF7细胞具有增强的基础AP-1反式激活活性和AP-1调节基因的表达增加，但对atRA的抗增殖作用具有抗性。然而，用c-jun的缺失突变体TAM-67转染的MCF7细胞，其缺乏cJun大部分氨基末端反式激活结构域且无法激活AP-1依赖的基因表达，对atRA的生长抑制作用敏感。这些结果表明，cJun的反式激活结构域是这些乳腺癌细胞中诱导类维生素A抗性所必需的。在这些cJun过表达的细胞中，atRA未激活RAR依赖的基因转录或反式抑制12-O-十四烷酰佛波醇-13-乙酸酯诱导的AP-1活性。对RAR和维甲酸X受体表达水平的研究表明，在c-jun转染的MCF7细胞中，RARα和RARγ RNA表达降低，而RARβ表达上调。然而，在c-jun转染的细胞中，维甲酸反应元件DNA结合活性是完整的。因此，cJun过表达诱导对atRA生长抑制作用抗性的机制可能是通过干扰atRA依赖的RAR反式激活或AP-1反式抑制，可能是通过滴定必需的共激活因子。这些结果表明，cJun过表达可克服类维生素A的抗增殖作用。
