Methionine adenosyltransferase 1A (MAT1A) is a tumor suppressor downregulated in hepatocellular carcinoma and cholangiocarcinoma, two of the fastest rising cancers worldwide. We compared MATα1 (protein encoded by MAT1A) interactome in normal versus cancerous livers by mass spectrometry to reveal interactions with 14-3-3ζ. The MATα1/14-3-3ζ complex was critical for the expression of 14-3-3ζ. Similarly, the knockdown and small molecule inhibitor for 14-3-3ζ (BV02), and ChIP analysis demonstrated the role of 14-3-3ζ in suppressing MAT1A expression. Interaction between MATα1 and 14-3-3ζ occurs directly and is enhanced by AKT2 phosphorylation of MATα1. Blocking their interaction enabled nuclear MATα1 translocation and inhibited tumorigenesis. In contrast, overexpressing 14-3-3ζ lowered nuclear MATα1 levels and promoted tumor progression. However, tumor-promoting effects of 14-3-3ζ were eliminated when liver cancer cells expressed mutant MATα1 unable to interact with 14-3-3ζ. Taken together, the reciprocal negative regulation that MATα1 and 14-3-3ζ exert is a key mechanism in liver tumorigenesis.

Background and aims

Forkhead box M1 (FOXM1) and nuclear factor kappa B (NF-ĸB) are oncogenic drivers in liver cancer that positively regulate each other. We showed that methionine adenosyltransferase 1A (MAT1A) is a tumor suppressor in the liver and inhibits NF-ĸB activity. Here, we examined the interplay between FOXM1/NF-κB and MAT1A in liver cancer.

Approach and results

We examined gene and protein expression, effects on promoter activities and binding of proteins to promoter regions, as well as effects of FOXM1 inhibitors T0901317 (T0) and forkhead domain inhibitory-6 (FDI-6) in vitro and in xenograft and syngeneic models of liver cancer. We found, in both hepatocellular carcinoma and cholangiocarcinoma, that an induction in FOXM1 and NF-κB expression is accompanied by a fall in MATα1 (protein encoded by MAT1A). The Cancer Genome Atlas data set confirmed the inverse correlation between FOXM1 and MAT1A. Interestingly, FOXM1 directly interacts with MATα1 and they negatively regulate each other. In contrast, FOXM1 positively regulates p50 and p65 expression through MATα1, given that the effect is lost in its absence. FOXM1, MATα1, and NF-κB all bind to the FOX binding sites in the FOXM1 and MAT1A promoters. However, binding of FOXM1 and NF-κB repressed MAT1A promoter activity, but activated the FOXM1 promoter. In contrast, binding of MATα1 repressed the FOXM1 promoter. MATα1 also binds and represses the NF-κB element in the presence of p65 or p50. Inhibiting FOXM1 with either T0 or FDI-6 inhibited liver cancer cell growth in vitro and in vivo. However, inhibiting FOXM1 had minimal effects in liver cancer cells that do not express MAT1A.

Conclusions

We have found a crosstalk between FOXM1/NF-κB and MAT1A. Up-regulation in FOXM1 lowers MAT1A, but raises NF-κB, expression, and this is a feed-forward loop that enhances tumorigenesis.

Methionine adenosyltransferase 2 A (MAT2A) and MAT2B are essential for hepatic stellate cells (HSCs) activation. Forkhead box M1 (FOXM1) transgenic mice develop liver inflammation and fibrosis. Here we examine if they crosstalk in male mice. We found FOXM1/MAT2A/2B are upregulated after bile duct ligation (BDL) and carbon tetrachloride (CCl4) treatment in hepatocytes, HSCs and Kupffer cells (KCs). FDI-6, a FOXM1 inhibitor, attenuates the development and reverses the progression of CCl4-induced fibrosis while lowering the expression of FOXM1/MAT2A/2B, which exert reciprocal positive regulation on each other transcriptionally. Knocking down any of them lowers HSCs and KCs activation. Deletion of FOXM1 in hepatocytes, HSCs, and KCs protects from BDL-mediated inflammation and fibrosis comparably. Interestingly, HSCs from Foxm1Hep-/-, hepatocytes from Foxm1HSC-/-, and HSCs and hepatocytes from Foxm1KC-/- have lower FOXM1/MAT2A/2B after BDL. This may be partly due to transfer of extracellular vesicles between different cell types. Altogether, FOXM1/MAT2A/MAT2B axis drives liver inflammation and fibrosis.