Prevention of ornithine cytotoxicity by proline in human retinal pigment epithelial cells.
Ueda M. Masu Y. Ando A. Maeda H. Del Monte MA. Uyama M. Ito S.
Department of Ophthalmology, Kansai Medical University, Osaka, Japan.
PURPOSE: To investigate the relationship between ornithine-delta-aminotransferase (OAT) deficiency and ornithine accumulation and the specific degeneration of retinal pigment epithelial (RPE) cells in gyrate atrophy. METHODS: Human RPE cells, human hepatoma cells, and human fibroblast cells were treated with 5-fluoromethylornithine (5-FMOrn), a specific irreversible inhibitor of OAT. Ornithine cytotoxicity was determined by using a [3H]thymidine incorporation assay and immunohistochemical staining for cytokeratin. The effects of various metabolites of ornithine and arginine, such as creatine, creatine phosphate, I-delta 1-pyrroline-5-carboxylic acid (L-P5C), and proline, which may be deficient in gyrate atrophy on RPE cell damage by ornithine, were determined by the same procedures. RESULTS: When the human RPE cells, HepG2 hepatoma cells, and WI-38 fibroblast cells were treated with 0.5 mM 5-FMOrn for 30 minutes, which inactivated OAT, ornithine exhibited severe time- and dose-dependent inhibition of DNA synthesis in the human RPE cells but not in the HepG2 hepatoma cells or WI-38 fibroblast cells. The inhibition of DNA synthesis was accompanied by drastic changes in morphologic appearance, disorganization of the cytoskeleton, and cell death. Ornithine or 5-FMOrn alone did not exhibit such cytotoxicity to the RPE cells. Proline prevented the cytotoxicity of ornithine. CONCLUSIONS: These findings suggest that an elevated level of ornithine combined with an increased sensitivity to ornithine as a result of OAT deficiency may be crucial to the specific RPE degeneration in gyrate atrophy. They suggest also that abnormalities of proline metabolism may be involved in the progress of gyrate atrophy.
Role of epidermal growth factor receptor in the metastasis of intraocular melanomas.
Ma D. Niederkorn JY.
Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas 75235, USA.
PURPOSE: To explore the expression and function of epidermal growth factor receptor (EGFR) expression on human uveal melanoma cells. METHODS: Five human uveal melanoma cell lines were examined by flow cytometry for the expression of EGFR. The correlation between EGFR expression and metastasis of uveal melanoma cells was tested in a nude mouse model of intraocular melanoma. The effect of EGFR on liver homing of blood-borne uveal melanoma cells was tested by tracing the fate of radiolabeled cells treated with anti-EGFR monoclonal antibody. The capacity of EGFR to inhibit the cytotoxic effects of tumor necrosis factor-alpha (TNF-alpha) was determined in vitro. The role of EGFR in promoting metastatic disease was studied by infusing intraocular melanoma-bearing mice using a neutralizing antibody against EGFR. RESULTS: EGFR was expressed to varying degrees on all eight human uveal melanoma cell lines. Expression of EGFR correlated with metastatic potential and capacity of blood-borne uveal melanoma cells to localize in the liver. EGFR rendered uveal melanoma cells resistant to the cytolytic effects of TNF-alpha. Blocking EGFR with a neutralizing monoclonal antibody increased the susceptibility of uveal melanoma cells to TNF-mediated cytolysis, inhibited metastases, and prolonged host survival. CONCLUSIONS: The expression of EGFR on five human uveal melanoma cell lines is correlated with an increased capacity to localize in the liver, an increased resistance to TNF-mediated lysis, and decreased survival. Targeting EGFR expression and function may be a fruitful strategy for managing patients with uveal melanoma.