Knockdown of Orphan Transporter SLC22A18 Impairs Lipid Metabolism and Increases Invasiveness of HepG2 Cells
The aim of this work is to investigate the roles of solute carrier family 22 member 18 (SLC22A18) in lipid metabolism and in establishing the tumor phenotype of HepG2 cells.
SLC22A18-knockdown HepG2 cells were established by stable transfection with shRNA. Protein expression levels were measured by quantitative proteomics and Western blot analysis. Cell growth was examined by cell counting kit. Accumulation of triglyceride-rich lipid droplets was measured by Oil-Red O staining. Cell migration and invasion were examined by Transwell assays.
SLC22A18-knockdown HepG2 cells accumulated triglyceride-rich lipid droplets and showed decreased expression levels of lysosomal/autophagic proteins, suggesting that lipid degradation is suppressed. Growth of HepG2 cells was decreased by SLC22A18 knockdown, but was restored by free fatty acid supplementation. In addition, SLC22A18 knockdown decreased the expression of insulin-like growth factor-binding protein 1 (IGFBP-1) and increased the invasion ability of HepG2 cells. Exogenous IGFBP-1 blocked the increase of invasion activity induced by SLC22A18 knockdown.
Our results suggest that suppression of SLC22A18 decreased the supply of intracellular free fatty acids from triglyceride-rich lipid droplets by impairing the lysosomal/autophagy degradation pathway and reduced the invasive activity of HepG2 cells by decreasing IGFBP-1 expression.
KEY WORDSfatty acid IGFBP-1 orphan transporter quantitative proteomics SLC22A18
Free fatty acids
Insulin-like growth factor-binding protein-1
KQT-like subfamily Q, member1
Non-alcoholic fatty liver disease
Short hairpin RNA
Solute carrier family 22 member 18
Type 2 diabetes mellitus
ACKNOWLEDGMENTS AND DISCLOSURES
We are grateful for partial financial support through three Grants-in-Aid for Scientific Research from the Japanese Society for the Promotion of Science (JSPS) for Research Activity Start-up (No. 24890170), Young Scientific Research (B) (No. 26860109) and Scientific Research (C) (No. 16 K08373), as well as a grant from the Takeda Science Foundation. S. Ohtsuki is a full professor at Kumamoto University and is also a director of Proteomedix Frontiers. This study was not supported by the company, and its position at the company did not influence the design of the study, the collection of the data, the analysis or interpretation of the data, the decision to submit the manuscript for publication, or the writing of the manuscript and did not present any financial conflicts. The other authors declare no competing interests.
S. Ito, G. Honda. and S. Ohtsuki contributed to the study design. S. Ito, G.Honda, Y. Fujino, S. Ogata, M. Hirayama-kurogi and S.Ohtsuki conducted experiments and performed data analysis. S. Ito and S. Ohtsuki wrote the manuscript. All authors gave final approval for the manuscript to be published.
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