Epidermal growth factor system is a physiological regulator of development of the mouse fetal submandibular gland and regulates expression of the α6-integrin subunit

Abstract

Epidermal growth factor (EGF) and transforming growth factor-α (TGF- α) regulate branching morphogenesis of fetal mouse submandibular gland (SMG) rudiments in vitro. The EGF system (EGF, TGF-α, and their shared receptor, EGFR) also regulates expression of integrins and their ligands in the extracellular matrix. We show here that inhibition of EGFR tyrosine-kinase activity by a tyrphostin retards in vitro development of SMGs. Using total RNA isolated from pooled SMGs taken from intact mouse fetuses, mRNA transcripts for EGF, TGF-α, and EGFR were detected by reverse transcription- polymerase chain reaction (RT-PCR), and age-dependent variations in the levels of these mRNA were quantitatively determined by nuclease protection assays. These findings suggest that the EGF system is operative in the in vivo development of this gland. α6-Integrin subunit was localized by immunofluorescence at the basal surface of epithelial cells. Branching morphogenesis of cultured SMG rudiments was inhibited by anti-α6 antibodies. Synthesis of α6-subunit in cultured SMGs, detected by metabolic labeling and immunoprecipitation, was increased by EGF and drastically reduced by tyrphostin. RT-PCR revealed that mRNAs for α6- and β1- and β4-integrin subunits are expressed at all ages between embryonic day 13 and postnatal day 7. These findings suggest that 1) the EGF system is a physiologic regulator of development of fetal mouse SMG, and 2) one mechanism by which it acts may be by regulating expression of integrins, which in turn control interaction of epithelial cells with the extracellular matrix.