Incubation of intact frog erythrocytes with 12-O-tetradecanoyl phorbol-13-acetate (TPA), a
tumor-promoting
phorbol diester which activates
protein kinase C, results in an approximate two- to threefold increase in subsequently tested
beta-adrenergic agonist-stimulated
adenylate cyclase activity. This increase is due to an elevation in the Vmax of the
enzyme rather than to a change in affinity for the agonist. TPA treatment of frog erythrocytes does not alter the affinity (KD) or the binding capacity (Bmax) for the
beta-adrenergic antagonist [125I]
cyanopindolol. In addition, agonist/[125I]
cyanopindolol competition curves are not affected by TPA pretreatment nor is their sensitivity to
guanine nucleotides. Incubation of frog erythrocyte membranes alone with TPA does not promote sensitization or activation of
adenylate cyclase activity. Pretreatment of intact frog erythrocytes with TPA also produces approximately two- to threefold increases in basal,
guanine nucleotide-,
prostaglandin E1-,
forskolin-, NaF-, and MnCl2-stimulated
adenylate cyclase activities in frog erythrocyte membranes. This enhancement of
adenylate cyclase activity by TPA is induced rapidly (t1/2 approximately equal to 5 min) and with an EC50 of about 10(-7) to 10(-6) M. Other
tumor-promoting
phorbol diesters or
phorbol diester-like compounds including 4
beta-phorbol 12,13-dibutyrate, 4
beta-phorbol 12,13-didecanoate, and
mezerein are effective in promoting enhanced
adenylate cyclase activity. In contrast,
phorbols such as 4
beta-phorbol, 4 alpha-
phorbol 12,13-didecanoate, and
4-O-methylphorbol 12-myristate 13-acetate, which are inactive in
tumor promotion and which do not activate
protein kinase C, do not affect frog erythrocyte
adenylate cyclase activity. These data are suggestive of a
protein kinase C-mediated phosphorylation of one of the
adenylate cyclase components that is distal to the receptor, i.e., the
nucleotide regulatory and/or catalytic components.