Cardiotoxin III (CTX III), a basic
polypeptide with 60
amino acid residues isolated from Naja naja atra
venom, has been reported to have anticancer activity. Exposure of MDA-MB-231 cells with 0.03, 0.09, and 0.15 microM of CTX III for 18 h, CTX III-induced cell apoptosis, as evidenced by accumulation of sub-G1 population, externalization of
phosphatidylserine, loss of mitochondrial membrane potential (DeltaPsim) with subsequent release of
cytochrome c, and activation of both capases-9 and
caspase-3. This correlated with up-regulation in Bax and Bad, and down-regulation of various
anti-apoptotic proteins, including Bcl-2, Bcl-X(L), and
survivin in CTX III-treated cells. Mechanistic studies showed that CTX III suppressed the phosphorylation of JAK2, STAT3, Akt, and activation of PI3K. Moreover, the PI3K inhibitor
wortmannin blocked activation of STAT3 and Akt without affecting the JAK2 activation, whereas JAK2 inhibitor
AG490 suppressed the levels of phospho-STAT3, phospho-Akt, and PI3K, suggesting that PI3K activation occurs after JAK2 phosphorylation, and both PI3K and JAK2
kinases cooperate to mediate STAT3 and Akt phosphorylation. Both
AG490 and
wortmannin also led to up-regulation in Bax and Bad, and down-regulation of Bcl-2, Bcl-X(L), and
survivin in MDA-MB-231 cells. Taken together, these results indicate that CTX III induces apoptosis in MDA-MB-231 cells via concomitant inactivation of the JAK2, STAT3, PI3K, and Akt signaling pathways.