PPM1A expression positively correlates with CYP3A4 expression in the tumor liver tissue of hepatocellular carcinoma patients: A potential novel mechanism for CYP3A4 repression in hepatocellular carcinoma

Abstract

In most patients with hepatocellular carcinoma (HCC), cytochrome p450 3A4 (CYP3A4) expression has been reported to be significantly reduced in the tumor liver tissue. However, the underlying mechanisms for CYP3A4 repression are not fully understood. We have previously shown that Mg2+/Mn2+‐dependent phosphatase 1A (PPM1A) interacts with human pregnane X receptor (hPXR) and positively regulates hPXR‐mediated CYP3A4 expression in PPM1A expression‐dependent manner. We sought to determine whether PPM1A expression is downregulated in the tumor liver tissue of HCC patients and is positively correlated with CYP3A4 repression. Additionally, we sought to determine whether PPM1A could counteract inhibition of hPXR transactivation function by signaling pathways that are upregulated in HCC. Quantitative RT‐PCR and western blot analyses were performed to study mRNA and protein expression, respectively. Cell‐based reporter gene assays were conducted to examine the hPXR transactivation of CYP3A4 promoter activity. mRNA and protein expression of PPM1A as well as CYP3A4 was found to be significantly repressed in the tumor liver tissues compared to the matched non‐tumor liver tissues. Expression of arginase‐1 mRNA in the tumor and non‐tumor liver tissues, together with upregulation of glypican‐3 mRNA in the tumor liver tissues, verifies that the tumor samples used in our study belong to HCC livers but not non‐hepatocellular tumors metastasized to the liver. In the reporter gene assays, the hPXR‐mediated CYP3A4 promoter activity was inhibited by hepatocyte growth factor, insulin, tumor necrosis factor‐alpha, transforming growth factor‐beta, lipopolysaccharides, cyclin‐dependent kinase 2, protein kinase C activator phorbol 12‐myristate 13‐acetate, and protein kinase A activator dibutyryl‐cAMP. Notably, elevated PPM1A levels counteracted the inhibition of hPXR by these growth factors, cytokines, inflammatory agents, and kinases. The results from the reporter gene assays suggest that decreased PPM1A levels in HCC could not fully counteract the hPXR inhibiting signaling pathways. Together, these results are consistent with the conclusion that PPM1A downregulation in the tumor liver tissue of HCC patients positively correlates with CYP3A4 repression. Downregulation of PPM1A levels in the tumor liver tissue may contribute to reduced hPXR‐mediated CYP3A4 expression, and may provide a novel mechanism for CYP3A4 repression in the tumor liver tissue of HCC patients.Support or Funding InformationThe authors would like to thank Drs. Coleman, Mansour, and Tao for sharing their research facilities. This work was supported by the Auburn University Research Initiative in Cancer Grant and Animal Health and Disease Research Grant to Pondugula SR.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.