LOT1 is a growth suppressor gene down-regulated by the epidermal growth factor receptor ligands and encodes a nuclear zinc-finger protein

We previously reported cloning the rLot1 gene, and its human homolog (hLOT1 ), through analysis of differential gene expression in normal and malignant rat ovarian surface epithelial cells. Both human and rat ovarian carcinoma cell lines exhibited lost or decreased expression of this gene. Interestin gly, the LOT1 gene localized at band q25 of human chromosome 6 which is a f requent site for LOH in many solid tumors including ovarian cancer. In this report we have further characterized the potential role of LOT1 in maligna nt transformation and developed evidence that the gene is a novel target of growth factor signaling pathway. Assays using transient transfections show ed that LOT1 is a nuclear protein and may act as a transcription factor. In vitro and lit vivo studies involving ovarian cancer cell lines revealed th at expression of LOT1 is directly associated with inhibition of cellular pr oliferation and induction of morphological transformations. Additionally, w e show that in normal rat ovarian surface epithelial cells Lot1 gene expres sion is responsive to growth factor stimulation. Its mRNA is strongly down- regulated by epidermal growth factor receptor (EGFR) ligands, namely EGF an d TGF-alpha. Blocking the ligand-activated EGFR signal transduction pathway by the specific EGF receptor inhibitor, tyrphostin AG1478, and the MEK inh ibitor, PD098059, restores the normal level of Lot1 gene expression. It app ears that the regulation of Lot1 gene is unique to these ligands, as well a s the growth promoting agent TPA, since other factors either did not affect Lot1 expression, or the effect was modest and transient. Altogether, the r esults suggest that Lot1 expression is primarily mediated via EGF receptor or a related pathway and it may regulate the growth promoting signals as a zinc-finger motif containing nuclear transcription factor.