HDAC inhibitors suppress c-Jun/Fra-1-mediated proliferation through transcriptionally downregulating MKK7 and Raf1 in neuroblastoma cells.

// Weiwen He 1, 2, * , Yanna Wu 1, 2, * , Xiaomei Tang 1, 2 , Yong Xia 2 , Guozhen He 1, 2 , Zhiqun Min 3 , Chun Li 1, 2 , Shiqiu Xiong 4 , Zhi Shi 5 , Yongjian Lu 1, 2 , Zhongmin Yuan 1, 2 1 Department of Neurosurgery, The Second Affiliated Hospital of Guangzhou Medical University, Guangzhou, China 2 Institute of Neuroscience, Key Laboratory of Neurogenetics and Channelopathies of Guangdong Province and Ministry of Education of China, Guangzhou Medical University, Guangzhou, China 3 Clinical Laboratory Center of Molecular Medicine, The Second Affiliated Hospital of Guangzhou Medical University, Guangzhou, China 4 Department of Biochemistry, University of Leicester, Leicester, UK 5 Department of Cell Biology and Institute of Biomedicine, College of Life Science and Technology, Jinan University, Guangzhou, China * These authors contributed equally to this work Correspondence to: Zhongmin Yuan, e-mail: yzm@gzhmu.edu.cn Keywords: neuroblastoma, Fra-1/c-Jun dimer, MKK7, HDAC inhibitor, proliferation Received: August 02, 2015      Accepted: December 23, 2015      Published: December 30, 2015 ABSTRACT Activator protein 1 (AP-1) is a transcriptional factor composed of the dimeric members of bZIP proteins, which are frequently deregulated in human cancer cells. In this study, we aimed to identify an oncogenic AP-1 dimer critical for the proliferation of neuroblastoma cells and to investigate whether histone deacetylase inhibitors (HDACIs), a new generation of anticancer agents, could target the AP-1 dimer. We report here that HDACIs including trichostatin A, suberoylanilidehydroxamic acid, valproic acid and M344 can transcriptionally suppress both c-Jun and Fra-1, preceding their inhibition of cell growth. c-Jun preferentially interacting with Fra-1 as a heterodimer is responsible for AP-1 activity and critical for cell growth. Mechanistically, HDACIs suppress Fra-1 expression through transcriptionally downregulating Raf1 and subsequently decreasing MEK1/2-ERK1/2 activity. Unexpectedly, HDACI treatment caused MKK7 downregulation at both the protein and mRNA levels. Deletion analysis of the 5′-flanking sequence of the MKK7 gene revealed that a major element responsible for the downregulation by HDACI is located at -149 to -3 relative to the transcriptional start site. Knockdown of MKK7 but not MKK4 remarkably decreased JNK/c-Jun activity and proliferation, whereas ectopic MKK7-JNK1 reversed HDACI-induced c-Jun suppression. Furthermore, suppression of both MKK-7/c-Jun and Raf-1/Fra-1 activities was involved in the tumor growth inhibitory effects induced by SAHA in SH-SY5Y xenograft mice. Collectively, these findings demonstrated that c-Jun/Fra-1 dimer is critical for neuroblastoma cell growth and that HDACIs act as effective suppressors of the two oncogenes through transcriptionally downregulating MKK7 and Raf1.