Combined De-Repression of Chemoresistance Associated Mitogen-Activated Protein Kinase 14 and Activating Transcription Factor 2 by Loss of microRNA-622 in Hepatocellular Carcinoma

SIMPLE SUMMARY: Novel approaches to target RAS-signaling have led to the so called “RAS renaissance”. In HCC, it was demonstrated that wild-type RAS-RAF-ERK-signaling strongly contributes to HCC progression and drug resistance. In this context, microRNA-622 has emerged as one of the most promising tumorsuppressive microRNAs, targeting RAS-signaling in HCC. However, the majority of microRNA-622 target genes remained elusive. Moreover, limited chemoresponse of HCC was demonstrated by activation of MAPK14-ATF2-signaling which represents a RAS-collateral-pathway, but the expression and regulation of both MAPK14 and ATF2 remained to be clarified. In this study, we demonstrated co-overexpression of MAPK14 and ATF2 in HCC in vitro and in vivo. Moreover, in contrast to “one-microRNA-one-target” interactions, we identified common binding sites for microRNA-622 in both synergistic drug resistance-associated genes–MAPK14 and ATF2. Our data revealed that microRNA-622 functions as a superior pathway regulator inducing de-repression of the MAPK14-ATF2-axis in HCC. ABSTRACT: Chemoresistance is a major hallmark driving the progression and poor prognosis of hepatocellular carcinoma (HCC). Limited chemoresponse of HCC was demonstrated to be mediated by mitogen-activated protein kinase 14 (MAPK14) and activating transcription factor 2 (ATF2). Recently, we have demonstrated loss of control of RAS-RAF-ERK-signaling as a consequence of miR-622 downregulation in HCC. However, the majority of target genes of this potent tumorsuppressive microRNA had remained elusive. The MAPK14-ATF2-axis represents a collateral pathway ensuring persisting ERK-activation in the presence of sorafenib-mediated RAF-inhibition. In contrast to the function of the MAPK14-ATF2-axis, both the expression and regulation of MAPK14 and ATF2 in human HCC remained to be clarified. We found combined overexpression of MAPK14 and ATF2 in human HCC cells, tissues and in sorafenib resistant cell lines. High expression of MAPK14 and ATF2 was associated with reduced overall survival in HCC patients. Deciphering the molecular mechanism promoting combined upregulation of MAPK14 and ATF2 in HCC, we revealed that miR-622 directly targets both genes, resulting in combined de-repression of the MAPK14-ATF2-axis. Together, miR-622 represents a superior regulator of both RAS-RAF-ERK as well as MAPK14-ATF2-signaling pathways in liver cancer.