The focus of this project is to examine the mechanism(s) by which prostaglandins and linoleic acid metabolites modulate growth factor signaling pathways particularly the EGFR pathway. Syrian hamster embryo (SHE) cells and human colorectal cells are used as models to study the interaction of lipids with signaling pathways. In SHE cells, the linoleate metabolite, 13(S)-HODE stimulates EGF-dependent DNA synthesis while prostaglandins inhibited growth. PGE2 inhibited the activity of Raf-1 which attenuated the interaction between the Ras and Raf-1 to cause a down regulation of the MAP kinase pathway and thus the inhibition of MAP kinase. The addition of 13(S)-HODE up-regulated the EGFR phosphorylation events including a stimulation of MAP kinase activity in the supB+ cells but not the supB- cells. The linoleic acid metabolite attenuated the de-phosphorylation of the EGFR in the supB+ cells thereby up-regulating the entire EGFR pathway. Currently, we are studying the interaction of the EGFR with the adapter proteins that connect the receptor to the MAP kinase pathway. We have evidence that a newly described protein Gab-1, links EGFR to SHP-2 a tyrosine phosphatase associated with neoplastic progression. A ternary complex forms between the EGFR, Gab-1 and SHP-2. We cloned the hamster Gab1 cDNA and expressed it in both 10W+8 (non-tumorigenic) and 10W2T (tumorigenic) cells. Exogenous Gab1 expression in 10W2T cells restored the EGF-dependent association of SHP-2 with EGFR, which indicates that Gab1 functions as a linker between EGFR and SHP-2. Furthermore, exogenous Gab1 expression significantly enhanced EGF-dependent mitogenic activity in 10W+8 cells, while it only showed a marginal effect in 10W2T cells. These results identify Gab1 as an important co-player with SHP-2 in the EGFR signaling pathway via the formation of the EGFR?Gab1?SHP-2 complex. Altered expression of Gab1 is likely to be involved in the altered responsiveness to EGF, therefore, may be a key event in neoplastic progression of SHE cells. In addition, we are exploring the relationship between Cox/Lox and PPARg. Our hypothesis is that high Cox/Lox expression observed in tumors attenuates the anti-tumorigenic activity of PPARg and this is mediated effect through the MAP kinase signaling pathway.
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