Down-regulation of tumor suppressor A kinase anchor protein 12 in human hepatocarcinogenesis by epigenetic mechanisms.

UNLABELLED: The A kinase anchor protein 12 (AKAP12) is a central mediator of protein kinase A and protein kinase C signaling. Although AKAP12 has been described to act as a tumor suppressor and its expression is frequently down-regulated in several human malignancies, the underlying molecular mechanisms responsible for the AKAP12 reduction are poorly understood. We therefore analyzed the expression of AKAP12 and its genetic and epigenetic regulatory mechanisms in human hepatocarcinogenesis. Based on tissue microarray analyses (n = 388) and western immunoblotting, we observed a significant reduction of AKAP12 in cirrhotic liver (CL), premalignant lesions (DN), and hepatocellular carcinomas (HCCs) compared to histologically normal liver specimens (NL). Analyses of array comparative genomic hybridization data (aCGH) from human HCCs revealed chromosomal losses of AKAP12 in 36% of cases but suggested additional mechanisms underlying the observed reduction of AKAP12 expression in hepatocarcinogenesis. Quantitative methylation analysis by MassARRAY of NL, CL, DN, and HCC tissues, as well as of various tumorigenic and nontumorigenic liver cell lines revealed specific hypermethylation of the AKAP12α promoter but not of the AKAP12β promoter in HCC specimens and in HCC cell lines. Consequently, restoration experiments performed with 5-aza-2'deoxycytidine drastically increased AKAP12α mRNA levels in a HCC cell line (AKN1) paralleled by AKAP12α promoter demethylation. As hypermethylation is not observed in CL and DN, we investigated microRNA-mediated posttranscriptional regulation as an additional mechanism to explain reduced AKAP12 expression. We found that miR-183 and miR-186 are up-regulated in CL and DN and are able to target AKAP12.
CONCLUSION: In addition to genetic alterations, epigenetic mechanisms are responsible for the reduction of the tumor suppressor gene AKAP12 in human hepatocarcinogenesis.