Submitted on January 16, 2007
Accepted on May 4, 2007
Polyamines up-regulate the mRNA expression of cationic amino acid transporter-1 in human retinal pigment epithelial cells
Shiho Kaneko1, Emiko Okuda-Ashitaka2, Akira Ando1, Kazuhiro Nishimura3, Kazuei Igarashi3, Masahide Maeda4, Kyoji Furuta4, Masaaki Suzuki4, Miyo Matsumura1, and Seiji Ito2*
1 Ophthalmology, Kansai Medical University, Moriguchi, Osaka, Japan
2 Medical Chemistry, Kansai Medical University, Moriguchi, Osaka, Japan
3 Graduate School of Pharmaceutical Sciences, Chiba University, Chiba, Chiba, Japan
4 Regeneration and Advanced Medical Science, Graduate School of Medicine, Gifu University, Gifu, Gifu, Japan
* To whom correspondence should be addressed. E-mail: ito{at}takii.kmu.ac.jp.
We previously showed that ornithine was mainly transported via cationic amino acid transporter (CAT)-1 in human retinal pigment epithelial (RPE) cell line, hTERT-RPE, and that CAT-1 was involved in ornithine cytotoxicity in OAT-deficient cell produced by a OAT specific inhibitor, 5-fluoromethylornithine (5-FMO). We showed here that CAT-1 mRNA expression was increased by ornithne in OAT-deficient RPE cells, which was reversed by an inhibitor of ornithine decarboxylase (ODC), 
-difluoromethylornithine (DFMO). Polyamines, especially spermine, one of the metabolites of ODC, also enhanced the expression of CAT-1 mRNA. ODC mRNA expression was also increased by ornithine and polyamines, and gene silencing of ODC by siRNA decreased ornithine transport activity and its cytotoxicity. In addition, the mRNA of nuclear protein c-myc was also increased in 5-FMO- and ornithine-treated hTERT-RPE cells, and gene silencing of c-myc prevented the induction of CAT-1 and ODC. Increases in expression of CAT-1, ODC, c-myc, and the inhibition of these stimulated expression by DFMO were also observed in primary porcine RPE cells. These results suggest that spermine plays an important role in stimulation of mRNA expression of CAT-1, which is a crucial role in ornithine cytotoxicity in OAT-deficient hTERT-RPE cells.
