Protein tyrosine kinase activities of the epidermal growth factor receptor and ErbB proteins: correlation of oncogenic activation with altered kinetics

Ras-independent oncogenic transformation by an EGF-receptor mutant

Mutations in the ligand-binding domain of the epidermal growth factor receptor have been identified in several types of human cancers, including malignant gliomas. These mutations render signaling by this receptor to be constitutively ligand-independent. In fibroblasts transformed with ligand-independent epidermal growth factor receptor mutants, there is a correlation between the formation of a unique phosphotyrosine protein complex and oncogenic transformation. This phosphoprotein complex includes Grb2, Shc, Sos, tyrosine-phosphorylated form of caldesmon, and two, as yet, unidentified proteins. The presence of Grb2, Shc, and Sos in this complex implicates Ras in ligand-independent signaling by these oncogenic epidermal growth factor receptor mutants. We, therefore, have used retroviral co-infections of cultured primary fibroblasts to determine if Ras activation is required for phosphoprotein complex formation, stress fiber loss, or transformation. As predicted, expression of a dominant-negative Ras mutant (N17Ras) completely abrogates ligand-stimulated soft agar colony growth of primary fibroblasts. In contrast, N17Ras expression has no effect on v-ErbB mediated stress fiber disassembly, soft agar colony growth, or phosphoprotein complex assembly. In addition, our data suggest that ligand-dependent Ras activation may be suppressed by oncogenic v-ErbB expression. Together these observations suggest that oncogenic signaling by v-ErbB does not require Ras activation, and implicate an alternative signal transduction pathway in ligand-independent epidermal growth factor receptor oncogenic signaling.

Inhibition of tyrosine kinase activity decreases expression of surfactant protein A in a human lung adenocarcinoma cell line independent of epidermal growth factor receptor

Epidermal growth factor (EGF) enhances fetal lung development in vivo and in vitro. Ligand binding to the EGF receptor stimulates an intrinsic receptor tyrosine kinase initiating a signal transduction cascade. We hypothesized that blocking EGF receptor function with tyrosine kinase inhibitors would decrease the expression of surfactant protein A in human pulmonary epithelial cells. Human pulmonary adenocarcinoma cells (NCI-H441) were exposed to genistein (a broad range inhibitor of tyrosine kinases) and tyrphostin AG1478 (a specific inhibitor of EGF receptor tyrosine kinase). Genistein significantly decreased surfactant protein A (SP-A) and SP-A mRNA levels in H441 cells without affecting cell viability. The inhibitory effect of genistein on SP-A content was reversible. In contrast, tyrphostin AG1478 had no effect on SP-A levels despite a greater inhibitory effect than genistein on EGF receptor tyrosine autophosphorylation. Furthermore, treatment of H441 cells with exogenous EGF did not increase SP-A content or mRNA levels beyond baseline. We conclude that inhibition of tyrosine kinase activity other than the EGF receptor decreases the expression of surfactant protein A at a pretranslational level in human pulmonary adenocarcinoma cells. These results suggest the importance of tyrosine kinases in modulating human SP-A synthesis.

Insights into erythroid differentiation obtained from studies on avian erythroblastosis virus

Analysis of the oncogenes v-erbB and v-erbA and their normal proto-oncogene counterparts has revealed several novel aspects of erythroid differentiation. A new erythroid progenitor capable of extended self-renewal has been described, tyrosine kinase receptors and steroid hormone receptors have been found to cooperate in controlling self-renewal, and dramatic alterations in the cell cycle have been found to accompany induction of terminal differentiation.

Tyrosine kinase activity may be necessary but is not sufficient for c-erbB1-mediated tissue-specific tumorigenicity

Expression of mutant avian c-erbB1 genes results in tissue-specific transformation in chickens. Site-directed mutagenesis was used to generate kinase-defective mutants of several tissue-specific v-erbB transforming mutants by replacement of the ATP-binding lysine residue in the kinase domain with an arginine residue. These kinase-defective v-erbB mutants were analyzed for their in vitro and in vivo transforming potentials. Specifically, kinase-defective mutants of erythroleukemogenic, hemangioma-inducing, and sarcomagenic v-erbB genes were assessed for their oncogenic potential. In vitro transformation potential was assessed by soft-agar colony formation in primary cultures of chick embryo fibroblasts (CEF). In vivo transformation potential was determined by infection of 1-day-old line 0 chicks with concentrated recombinant retrovirus and then monitoring of birds for tumor formation. These transformation assays demonstrate that kinase activity is absolutely essential for transformation by tissue-specific transforming mutants of the avian c-erbB1 gene. Since all of the tissue-specific v-erbB mutants characterized to date exhibit tyrosine kinase activity in vitro but do not transform all tissues in which they are expressed, we conclude that v-erbB-associated tyrosine kinase activity may be necessary but is not sufficient to induce tumor formation.

Involvement of the epidermal growth factor receptor in the invasion of cultured mammalian cells by Salmonella typhimurium

Salmonella infection continues to be a major world-wide health problem. One essential pathogenic feature common to all Salmonella is their ability to penetrate the cells of the intestinal epithelium which are normally non-phagocytic. The internalization of Salmonella into mammalian cells is thought to be a receptor-mediated phenomenon and the invasion of cultured epithelial cells depends on several Salmonella genes, but nothing is known about the host determinants participating in this interaction. Protein tyrosine phosphorylation follows stimulation of many cell-surface receptors to initiate signal transduction pathways that stimulate cellular responses. We report here that invasion of cultured Henle-407 cells by Salmonella typhimurium induces the tyrosine phosphorylation of the epidermal growth factor (EGF) receptor. In contrast, an isogenic strain of S. typhimurium that is defective in invasion owing to a mutation in the invA gene is unable to induce such phosphorylation. Addition of EGF to cultured Henle-407 cells allowed the internalization of the invasion-defective S. typhimurium invA mutant although it did not cause the internalization of an adherent, but non-invasive, strain of Escherichia coli. This result indicates that stimulation of the EGF receptor is involved in the invasion of cultured Henle-407 cells by S. typhimurium.