Spontaneous activation and signaling by overexpressed epidermal growth factor receptors in glioblastoma cells.
Overexpressed epidermal growth receptor factor receptors (EGFRs) are thought to contribute to the malignant phenotype of human glioblastomas (GBMs), but the mechanism is not well understood. We found that SKMG-3 cells, a rare GBM cell line that maintains EGFR gene amplification in vitro, produced high levels of EGFR protein. The cells also expressed the related receptors HER2/neu and HER4, but not HER3. Immunoblots and tryptic phosphopeptide maps showed that the SKMG-3 EGFRs were intact and functional and that a subset of these receptors were spontaneously autophosphorylated. EGF treatment stimulated phosphorylation of the EGFRs as well as the downstream effectors Erk, AKT1, stat3 and c-Cbl. Under minimal growth conditions, the unstimulated SKMG-3 cells contained constitutively phosphorylated Erk and AKTI but no detectable stat3 DNA-binding complexes. The EGFR kinase inhibitor PD158780 reduced the constitutive phosphorylation of the receptor and Erk but not that of AKT1. In contrast, inhibition of phosphatidylinositol-3-kinase (PI3K) blocked the constitutive phosphorylation of Erk and AKT-1 but not the EGFR. We conclude that the SKMG-3 cells represent the subset of GBMs with amplified EGFR genes that overexpress intact receptors. The results also suggest that in some GBMs, signals from overexpressed EGFRs contribute to the constitutive phosphorylation of Erk, but these signals may not required for the constitutive activation of PI3K or AKT1.
Thomas
Christopher Y
CY
Division of Hematology/Oncology, Mayo Clinic-Jacksonville, Mayo Medical School, Jacksonville, Jacksonville, FL, USA. cyt@virginia.edu
Chouinard
Michael
M
Cox
Michael
M
Parsons
Sarah
S
Stallings-Mann
Melody
M
Garcia
Roy
R
Jove
Richard
R
Wharen
Robert
R
D001943:Breast Neoplasms / Q000473:pathology; D002294:Carcinoma, Squamous Cell / Q000473:pathology; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000378:metabolism; D004789:Enzyme Activation; D066246:ErbB Receptors / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D005784:Gene Amplification; D015972:Gene Expression Regulation, Neoplastic; D018773:Genes, erbB-1; D018734:Genes, erbB-2; D005909:Glioblastoma / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D006801:Humans; D007822:Laryngeal Neoplasms / Q000473:pathology; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009363:Neoplasm Proteins / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D009457:Neuroglia / Q000166:cytology; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D050721:Proto-Oncogene Proteins c-cbl; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000096:biosynthesis / Q000502:physiology*; D066247:Receptor, ErbB-4; D050796:STAT3 Transcription Factor; D015398:Signal Transduction / Q000502:physiology*; D015534:Trans-Activators / Q000378:metabolism; D014407:Tumor Cells, Cultured / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases
D004268:DNA-Binding Proteins; D066246:ErbB Receptors; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D009363:Neoplasm Proteins; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D050721:Proto-Oncogene Proteins c-cbl; D018719:Erb-b2 Receptor Tyrosine Kinases; D066247:Receptor, ErbB-4; D050796:STAT3 Transcription Factor; D015534:Trans-Activators; D044767:Ubiquitin-Protein Ligases; C494086:STAT3 protein, human; C587689:ERBB4 protein, human; C494918:AKT1 protein, human; C067419:CBL protein, human