Keratin 23 is a PPARA‐dependent, MYC‐amplified oncogene that promotes hepatocyte proliferation

Abstract

Chronic activation of the nuclear receptor peroxisome proliferator‐activated receptor alpha (PPARA) promotes MYC‐linked hepatocellular carcinoma in mice. Recent studies have shown that MYC can function as an amplifier of transcription where MYC does not act as an 'on‐off' switch for gene expression, but rather accelerates transcription rates at active promoters by stimulating transcript elongation. Considering the possibility that MYC may amplify the expression of PPARA target genes to potentiate cell proliferation and liver cancer, gene expression was analyzed from livers of wild‐type and hepatocyte‐specific Myc knockout mice (Myc ^ΔHep) treated with the PPARA agonist Wy‐14643. A subset of PPARA target genes was amplified in the presence of MYC, including keratin 23 (Krt23). The induction of Krt23 was significantly attenuated in Myc ^ΔHep mice and completely abolished in Ppara‐null mice. Reporter gene assays and chromatin immunoprecipitation confirmed direct binding of both PPARA and MYC to sites within the Krt23 promoter. Forced expression of KRT23 in primary hepatocytes induced cell cycle‐related genes. These data indicate that PPARA activation elevates MYC expression which in turn potentiates the expression of select PPARA target genes involved in cell proliferation. Finally, KRT23 protein are highly elevated in human hepatocellular carcinomas. These results revealed that MYC‐mediated transcriptional potentiation of select PPARA target genes, such as Krt23, may remove rate‐limiting constraints on hepatocyte growth and proliferation leading to liver cancer.

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