Epidermal growth factor receptor induced apoptosis: potentiation by inhibition of Ras signaling.

Previous studies have shown that certain tumor cell lines which naturally express high levels of the epidermal growth factor receptor (EGFR) undergo apoptosis when exposed to epidermal growth factor. Whether this phenomenon is a direct result of receptor overexpression or some other genetic alteration renders these cells sensitive to apoptosis is yet to be established. We show that experimentally increasing the level of EGFR expression predictably leads to apoptosis in a variety of cell types which requires an active tyrosine kinase but not EGFR autophosphorylation sites. Expression of a dominant negative Ras mutant in EGFR overexpressing cells results in a significant potentiation of EGFR induced apoptosis suggesting that Ras activation is a key survival signal generated by the EGFR. We propose that potentiation of EGFR induced apoptosis by dominant negative Ras results, at least in part, by a block of Akt activation.

The OMgp gene, a second growth suppressor within the NF1 gene.

The Oligodendrocyte-Myelin glycoprotein gene (OMgp) is placed within an intron of the NF1 gene. Neurofibromin, the product of NF1, acts as a RasGAP and suppresses growth; inactivating mutations in NF1 lead to neurofibromatosis type 1. We report that OMgp also has growth suppressive effects and downregulates mitogenic signaling pathways closely related to those influenced by neurofibromin. Overexpression of OMgp alters mitogenic signaling in NIH3T3 fibroblasts. Cells overexpressing OMgp grow more slowly in serum compared to controls and show a partial G1 block upon cell cycle analysis. PDGF is the primary mitogen for fibroblasts in serum. Overexpression of OMgp alters PDGF signaling in fibroblasts which results in a block of mitogenic signaling. PDGF induced activation of c-Src is blocked, as is the induction of c-Myc and c-Fos, while tyrosine phosphorylation of the PDGFbeta receptor, PLCgamma1 and induction of c-Jun are intact. Although a number of genes embedded within other genes have been described, the biological significance of this arrangement remains unknown. We demonstrate here that structurally unrelated products of two such genes may exercise closely related functions. Our data also raise the possibility of a role for OMgp in disorders of cell proliferation such as NF1.

The epidermal growth factor receptor associates with and recruits phosphatidylinositol 3-kinase to the platelet-derived growth factor beta receptor.

Receptor tyrosine kinases are classified into subfamilies, which are believed to function independently, with heterodimerization occurring only within the same subfamily. In this study, we present evidence suggesting a direct interaction between the epidermal growth factor (EGF) receptor (EGFR) and the platelet-derived growth factor beta (PDGFbeta) receptor (PDGFbetaR), members of different receptor tyrosine kinase subfamilies. We find that the addition of EGF to COS-7 cells and to human foreskin Hs27 fibroblasts results in a rapid tyrosine phosphorylation of the PDGFbetaR and results in the recruitment of phosphatidylinositol 3-kinase to the PDGFbetaR. In R1hER cells, which overexpress the EGFR, we find ligand-independent tyrosine phosphorylation of the PDGFbetaR and the constitutive binding of a substantial amount of PI-3 kinase activity to it, mimicking the effect of ligand in untransfected cells. In support of the possibility that this may be a direct interaction, we show that the two receptors can be coimmunoprecipitated from untransfected Hs27 fibroblasts and from COS-7 cells. This association can be reconstituted by introducing the two receptors into 293 EBNA cells. The EGFR/PDGFbetaR association is ligand-independent in all cell lines tested. We also demonstrate that the fraction of PDGFbetaR bound to the EGFR in R1hER cells undergoes an EGF-induced mobility shift on Western blots indicative of phosphorylation. Our findings indicate that direct interactions between receptor tyrosine kinases classified under different subfamilies may be more widespread than previously believed.

Expression of the oligodendrocyte-myelin glycoprotein by neurons in the mouse central nervous system.

The oligodendrocyte-myelin glycoprotein (OMgp) is a 110-kDa glycosylphosphatidylinositol-linked protein that was initially identified as a myelin-specific protein but whose precise function remains unknown. In this study, immunohistochemistry, western blots, in situ hybridization, and northern blots were used to determine the distribution of OMgp in the mouse brain. OMgp is present in a concentration detectable on western blots in the brains of newborn mice, and its concentration gradually increases until day 24 of life. OMgp mRNA is also present in amounts detectable on northern blots in the brains of newborn mice, and its concentration gradually increases until day 21 of life, after which the concentration diminishes a little. Most of the OMgp in the mouse brain appears to be expressed in diverse groups of neurons, but it is particularly prominent in large projection neurons such as the pyramidal cells of the hippocampus, the Purkinje cells of the cerebellum, motoneurons in the brainstem, and anterior horn cells of the spinal cord. However, OMgp is not confined to these cells and is expressed in cells in the white matter as well. The OMgp gene is placed within an intron of the neurofibromatosis type I gene and on the opposite strand. This organization raises the possibility that there may be a relationship between the functions of the products of the two genes. In support of this possibility, we show that within the mouse CNS OMgp and neurofibromin are expressed in the same cell types.