Background: Lysophosphatidic acid (LPA) is a ubiquitously expressed phospholipid that regulates diverse cellular functions. Previously identified LPA receptor subtypes (LPAR1-5) are weakly expressed or absent in the liver. This study sought to determine LPAR expression, including the newly identified LPAR6, in normal human liver (NL), hepatocellular carcinoma (HCC), and non-tumor liver tissue (NTL), and LPAR expression and function in human hepatoma cells in vitro.
Methods: We determined LPAR1-6 expression by quantitative reverse transcriptase polymerase chain reaction, Western blot, or immunohistochemistry in NL, NTL, and HCC, and HuH7, and HepG2 cells. Hepatoma cells were treated with LPA in the absence or presence of LPAR1-3 (Ki16425) or pan-LPAR (α-bromomethylene phosphonate) antagonists and proliferation and motility were measured.
Results: We report HCC-associated changes in LPAR1, 3, and 6 mRNA and protein expression, with significantly increased LPAR6 in HCC versus NL and NTL. Analysis of human hepatoma cells demonstrated significantly higher LPAR1, 3, and 6 mRNA and protein expression in HuH7 versus HepG2 cells. Treatment with LPA (0.05-10 μg/mL) led to dose-dependent HuH7 growth and increased motility. In HepG2 cells, LPA led to moderate, although significant, increases in proliferation but not motility. Pretreatment with α-bromomethylene phosphonate inhibited LPA-dependent proliferation and motility to a greater degree than Ki16425.
Conclusions: Multiple LPAR forms are expressed in human HCC, including the recently described LPAR6. Inhibition of LPA-LPAR signaling inhibits HCC cell proliferation and motility, the extent of which depends on LPAR subtype expression.
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