Akt is a
serine/threonine kinase involved in a variety of cellular responses, including cell proliferation and cell survival. Recent studies from our laboratory suggest that Akt signaling may play an important role in skin
tumor promotion. To explore this premise, we examined epidermal Akt activation and signaling in response to chemically diverse skin
tumor promoters. Mice received single or multiple applications of 12-O-tetradecanoylphorbol-13-acetate (TPA),
okadaic acid, or
chrysarobin. All three
tumor promoters were able to activate epidermal Akt as early as 1 h
after treatment. Activation of Akt following
tumor promoter treatment led to enhanced downstream signaling, including hyperphosphorylation of
glycogen synthase kinase-3beta and Bad. Structure activity studies with
phorbol ester analogues revealed that the magnitude of activation paralleled
tumor-promoting activity. In cultured primary keratinocytes, TPA treatment also led to activation of Akt. Activation of the
epidermal growth factor receptor (EGFR) seemed to underlie the ability of TPA to activate Akt as both
PD153035, an inhibitor of EGFR, and
GW2974, a dual-specific inhibitor of both EGFR and erbB2, were able to effectively reduce TPA-induced Akt phosphorylation as well as TPA-stimulated EGFR and erbB2
tyrosine phosphorylation in a dose-dependent manner. Furthermore, inhibition of
protein kinase C (PKC) activity blocked TPA-stimulated
heparin-binding
EGF production and EGFR transactivation. Inhibition of PKC also led to a decreased association of Akt with the PP2A catalytic subunit, leading to increased Akt phosphorylation. However, combination of EGFR inhibitor and PKC inhibitor completely abrogated TPA-induced activation of Akt. Collectively, the current results support the hypothesis that elevated Akt activity and subsequent activation of downstream signaling pathways contribute significantly to skin
tumor promotion. In addition, signaling through the EGFR via EGFR homodimers or EGFR/erbB2 heterodimers may be the primary event leading to Akt activation during
tumor promotion in mouse skin.