Human parotid
tumors were evaluated for the activation of the
phosphotyrosine signaling pathway by Western blot,
enzyme activity assay, and
reverse transcriptase-polymerase chain reaction. Warthin's
tumor and
mucoepidermoid carcinomas had the greatest level of
tyrosine phosphorylated
proteins identified in plasma membrane fractions. These
tumors, along with pleomorphic
adenocarcinoma, showed high levels of membrane expression of the
tyrosine kinase receptor, c-erbB-2, and phosphatidylinositol-3-kinase. Expression of the
epidermal growth factor receptor was confined to normal tissue. The level of
mRNA for c-erb was elevated only in
mucoepidermoid carcinomas.
Messenger RNA levels for ras were unchanged from control levels in all
tumors, while the level of src
mRNA was higher in the
tumor samples than the normal parotid tissue. The activities of several signal transduction
kinases, including
protein kinase A and C were elevated in
tumor tissue (7.7- to 18.9- and 0.4- to 3.7-fold higher, respectively), relative to surrounding normal tissue. While the level of glandular
amylase was reduced (22%-0% of normal levels) in the
tumor tissue,
epidermal growth factor (
EGF) and
transforming growth factor-alpha (
TGFalpha) content was dramatically higher in the neoplastic tissue (10- to 170-fold and 4.6- to 6.0-fold, respectively). These results suggest that with the presence of elevated levels of
EGF,
TGFalpha, and the
oncoprotein receptor c-erbB-2 in the membrane of parotid
tumors, cell proliferation and activation of the
phosphotyrosine signal transduction pathway may involve autocrine stimulation through the expression of high levels of
growth factor and receptor in the same tissue.