Abstract
Pituitary tumors (PTs) occur in almost 17 % of the population and they represent about 10 % of all intracranial tumors. Growth hormone-producing pituitary tumors account for about 12% of pituitary tumors. At present, many patients with this type of pituitary tumor suffer from symptoms and complications caused by abnormal hormone production and/or mass effects due to limited treatment options. L-type amino acid transporter 1 (LAT1/SLC7A5) delivers essential amino acids into cells, and higher expression of LAT1 has been detected in various types of cancer including gliomas and pancreatic cancers. Furthermore, several lines of evidence suggest that blockade of LAT1 in tumor cells suppresses mammalian target of rapamycin complex (mTORC) 1 activity, resulting in either cell growth arrest or apoptotic cell death. However, LAT1 expression has not been elucidated in normal pituitary glands or PTs. In addition, the effects of LAT1 inhibition on cell growth and hormone synthesis in PTs also remain unknown. Here we show that LAT1, with expression comparable to LAT2, LAT3 and LAT4 in the normal mouse pituitary gland, is predominantly expressed in rat pituitary tumor GH4 cells, which secrete GH and PRL. An LAT1-specific inhibitor, JPH203, provided by J-Pharma Co., Ltd., Yokohama Japan, suppressed growth of these cells as well as GH and PRL synthesis. Water-Soluble Tetrazolium salt (WST)-8 assay revealed that effective doses of JPH203 on cell proliferation were ranging from 5 to 40 μM. Analyzing Annexin V-Fluorescein isothiocyanate (FITC) binding by flow cytometry, we found that JPH203 (40μM) induced apoptosis within three days and growth arrest for at least three weeks in GH4 cells. JPH203 also reduced the amount of GH and PRL mRNAs 40 % and 50 %, respectively. These results indicate that LAT1 is a potential target of therapy for pituitary tumors. Next, the mechanisms of JPH203 action in GH4 cells were investigated. Unexpectedly, JPH203 did not have any apparent effects on the phosphorylation state of mTOR1 and its downstream effectors. In contrast, rapamycin decreased phosphorylation levels of these proteins, resulting in a reduction in cell proliferation. Our findings suggest that in pituitary tumor cells there could be differential amino acid sensing pathways, which regulate both cell growth and hormone synthesis. Further studies will be required to determine the mechanism of action of the LAT1 inhibitor in GH4 cells.