Effect of EGF on rat parotid gland secretory function

Subcellular localization of epidermal growth factor receptor in human submandibular gland

The subcellular distribution of the epidermal growth factor receptor (EGFr) was demonstrated in the normal human submandibular gland by means of immunogold cytochemistry. EGFr labelling appeared in both acinar and ductal cells, where strong immunoreactivity was associated with a tubulovesicular system near the basolateral surfaces. In addition, groups of reactive vesicles were highlighted among secretory granules of both serous and mucous cells and at the apex of ductal cells. Basolateral vesicles were interpreted as being a result of EGFr internalization after activation by an exogenous ligand, although the functional meaning of those located apically remains unclear.

Effects of EGF and bFGF on Irradiated Parotid Glands

Radiotherapy is common treatment for head-and-neck cancer, during which the salivary glands are often included within the radiation field. The most common side effect of this treatment is the development of oral dryness (xerostomia). This study considers the administration of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF or FGF2) at physiological concentrations before and after irradiation in order to repair radiation-induced damage in salivary gland cells. As a preliminary examination of the efficacy of this approach we have characterized the effects of EGF and bFGF on the apoptotic response of 15-Gy irradiated rat salivary glands in vitro. Also, we have developed a controlled-release delivery system to effectively administer the growth factor to the gland since local delivery is essential to avoid unwanted protection of cancer cells. In vitro administration of bFGF prior to and immediately after irradiation partially protected (44%) the rat parotid gland. EGF did not show any significant radioprotective effect on parotid glands after a single 15-Gy irradiation dose. Encapsulation, storage and release of bFGF from biodegradable 50/50 PLGA microspheres did not affect the functionality of the growth factor in vitro.

EGF inhibits muscarinic receptor-mediated calcium signaling in a human salivary cell line

The effects of epidermal growth factor (EGF) on intracellular calcium ([Ca(2+)](i)) responses to the muscarinic agonist carbachol were studied in a human salivary cell line (HSY). Carbachol (10(-4) M)-stimulated [Ca(2+)](i) mobilization was inhibited by 40% after 48-h treatment with 5 x 10(-10) M EGF. EGF also reduced carbachol-induced [Ca(2+)](i) in Ca(2+)-free medium and Ca(2+) influx following repletion of extracellular Ca(2+). Under Ca(2+)-free conditions, thapsigargin, an inhibitor of Ca(2+) uptake to internal stores, induced similar [Ca(2+)](i) signals in control and EGF-treated cells, indicating that internal Ca(2+) stores were unaffected by EGF; however, in cells exposed to thapsigargin, Ca(2+) influx following Ca(2+) repletion was reduced by EGF. Muscarinic receptor density, assessed by binding of the muscarinic receptor antagonist L-[benzilic-4,4'-(3)HCN]quinuclidinyl benzilate ([(3)H]QNB), was decreased by 20% after EGF treatment. Inhibition of the carbachol response by EGF was not altered by phorbol ester-induced downregulation of protein kinase C (PKC) but was enhanced upon PKC activation by a diacylglycerol analog. Phosphorylation of mitogen-activated protein kinase (MAP kinase) and inhibition of the carbachol response by EGF were both blocked by the MAP kinase pathway inhibitor PD-98059. The results suggest that EGF decreases carbachol-induced Ca(2+) release from internal stores and also exerts a direct inhibitory action on Ca(2+) influx. A decline in muscarinic receptor density may contribute to EGF inhibition of carbachol responsiveness. The inhibitory effect of EGF is mediated by the MAP kinase pathway and is potentiated by a distinct modulatory cascade involving activation of PKC. EGF may play a physiological role in regulating muscarinic receptor-stimulated salivary secretion.

The role of phosphotyrosine signaling pathway in a parotid gland proliferation and function

Tyrosine phosphorylation and the intracellular signaling processes associated with it have been the focus of intense study due to its importance in the regulation of biological processes as diverse as cell proliferation and cell differentiation. While much of what we now understand has been derived from the study of cell lines and tumor cells, the salivary glands provide a model to examine the effects of tyrosine kinases and tyrosine phosphatases in a normal differentiated tissue. This review will focus, therefore, on the role tyrosine kinases and phosphatases play in inducing the transition from stasis to active proliferation and their potential role in mediating secretory function of the salivary glands.