The PP2A signaling pathway is a druggable target in murine and human hepatocellular carcinoma

Background: Hepatocellular carcinoma (HCC) is one of the most common solid tumor entities worldwide and the third leading cause of cancer related death. For advanced stage disease, systemic pharmacotherapy is the only established therapy, targeting the oncogene protein kinases pathways. However, chemotherapy still yields poor response rates and survival in those patients. Recently, it could be demonstrated, that activating the protein phosphatase pathways may have a therapeutic impact on various types of cancer.

Methods: The murine and human HCC cell lines Hepa 1–6, Hepa 129, HuH7, HepG2 and Hep3B were used for in vitro studies. Cells were treated with either direct (N-hexanoyl-D-sphingosine) or indirect (N,N-dimethylsphingosine, COG449) protein phosphatase 2A (PP2A) activators. Cell proliferation and apoptosis were assessed by MTT-assay, flow cytometry and immunofluorescence. Regulation of the protein phosphatase 2A inhibitory proteins I1PP2A, I2PP2A and CIP2A, as well as signal transduction pathways, were analysed by Western Blot and qRT-PCR. Human HCC tissue samples, obtained from surgical resection and cryo-conserved, were tested for the above mentioned proteins (n=22) in relation to tumor-free surrounding liver tissue and correlated with the Ki67 proliferation index.

Results: The activity of the PP2A inhibitory proteins was markedly up-regulated in mouse and human HCC cell lines. Furthermore, pharmacologic inactivation of I2PP2A, as well as direct or indirect activation of PP2A, led to an AKT mediated growth suppression of HCC cells in vitro. In human HCC, increased levels of I1PP2A, I2PP2A and CIP2A correlated significantly with cell proliferation, as measured by Ki67 expression.

Conclusions: Our data demonstrates that the protein phosphatase pathway is a potential novel treatment target in HCC and PP2A activators represent possible agents in the future therapy of this malignancy.