Conditional expression of the ErbB2 oncogene elicits reversible hyperplasia in stratified epithelia and up-regulation of TGFα expression in transgenic mice

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Abstract
The ErbB2 receptor tyrosine kinase (RTK) is expressed in basal cells of squamous epithelia and the outer root sheath of hair follicles. We previously showed that constitutive expression of activated ErbB2 directed to these sites in the skin by the keratin 14 (K14) promoter produces prominent hair follicle abnormalities and striking skin hyperplasia in transgenic mice. However, perinatal lethality precluded the establishment of a transgenic line for analysis of ErbB2 function in adult animals. To investigate the significance of ErbB2 signaling in epithelial tissues during and post development, we developed a K14-rtTA/TetRE-ErbB2 `Tet-On' bitransgenic mouse system. These mice were normal until the ErbB2 transgene was induced by exposure to doxycycline (Dox). Prenatal induction resulted in perinatal death. Postnatally, ErbB2 transgene expression was observed at 4 h after the initiation of Dox, and reached a plateau at 24 h. Skin hyperplasia followed after 2 days and these changes reverted to normal upon Dox withdrawal. In adults, as in the neonates, prolonged ErbB2 induction caused prominent skin and hair follicle hyperplasias. Severe hyperplasias in the cornea, eye lids, tongue and esophagus were also observed. ErbB2 transgene induction was accompanied by increased expression of TGFα, a ligand of epidermal growth factor receptor (EGFR), and to a lesser extent, EGFR, further enhancing RTK signal transduction. We conclude that ErbB2 plays important roles in both development and maintenance of hair follicles and diverse squamous epithelia and that this ligand-inducible and tissue-specific `Tet-On' transgenic mouse system provides a means to study transgenes with perinatal toxicity.