Multiple Akt/PKB-independent survival signaling pathways inhibit apoptosis in LNCaP prostate cancer cells

Deregulation of survival signaling pathways may comprise a significant means by which prostate cancer cells become resistant to chemotherapy or persist at sites of metastasis. In LNCaP prostate cancer cells, apoptotic resistance can be stimulated by multiple agonists, which, in turn, activate multiple survival pathways. We discovered that the Akt/PKB pathway is constitutively active, and treatment of LNCaP cells with PI3K inhibitors results in loss of Akt activity and apoptosis. Akt/PKB-independent survival pathways also exist, however, since various agonists (EGF, androgen, serum, forskolin, VIP, epinephrine, and IL-6) can protect these cells from apoptosis mediated by LY294002 in the absence of Akt/PKB activity. Anti-apoptotic signaling via EGF is likely to involve multiple downstream effectors that may each contribute to distinct survival activities, since inhibitors of PI3K, MEK, PLC-γ, PKC, and Src added alone or in combination, do not block protection, and EGF is still protective when intracellular calcium is disrupted. Nevertheless, one EGF-induced survival pathway could involve the Ras-to-MAPK pathway since activated mutants of Ras, Raf, and MEK are sufficient to protect LNCaP cells from apoptosis. Since EGF-mediated protection persists in cells transfected with dominant negative (K44A) dynamin, it is unlikely that EGFR internalization is a requisite event in the survival signal. While the EGF-triggered anti-apoptotic signal fails to intercept pro-apoptotic signals mediated by TNF-alpha, it prevents redistributions of Smac, cytochrome c, and Bax. We therefore conclude that the Akt-independent survival signal occurs upstream of mitochondria and likely entails the modification and inactivation of pro-apoptotic proteins that target mitochondria.