The stimulation of human
tumor cells overexpressing
epidermal growth factor receptor (EGFR) with
EGF enhances
tumor development and
malignancy. Therefore, compounds that modulate the
EGF-mediated signal inducing apoptosis in EGFR-overexpressing cells would represent a new class of
antitumor drug and might be useful in the treatment of a subset of human
tumors. In the course of screening for compounds that induce apoptosis in EGFR-overexpressing human epidermal
carcinoma A431 cells from secondary metabolites of microorganisms, we found that vacuolar-type
H(+)-ATPase (V-
ATPase) inhibitors, such as
concanamycin B and
destruxin E, induced apoptosis only when the cells were stimulated with
EGF. The
EGF-dependent apoptosis by V-
ATPase inhibitors was not observed in other types of human
tumor cells which do not overexpress EGFR. The apoptosis in A431 cells was inhibited by anti-FasL antibody which neutralized the cytotoxic effect of FasL, indicating that the Fas/FasL system was involved. The expression of cell surface FasL was upregulated by stimulation with
EGF and increased further by V-
ATPase inhibitors. Moreover,
EGF inhibited cytotoxic Fas antibody-induced apoptosis, whereas V-
ATPase inhibitors disrupted the protective effect of
EGF on apoptosis in A431 cells. Taken together, these results suggested that V-
ATPase inhibitors induced
EGF-dependent apoptosis in A431 cells, possibly through both the enhancement of
EGF-induced cell surface expression of FasL and the disruption of an
EGF-induced survival signal.