Stimulation of protein kinase C-dependent and -independent signaling pathways by bistratene A in intestinal epithelial cells

The marine toxin bistratene A (BisA) potently induces cytostasis and differ entiation in a variety of systems. Evidence that BisA is a selective activa tor of protein kinase C (PKC) delta implicates PKC delta signaling in the n egative growth-regulatory effects of this agent. The current study further investigates the signaling pathways activated by BisA by comparing its effe cts with those of the PKC agonist phorbol 12-myristate 13-acetate (PMA) in the IEC-18 intestinal crypt cell line. Both BisA and PMA induced cell cycle arrest in these cells, albeit with different kinetics. While BisA produced sustained cell cycle arrest in G(o)/G(1) and G(2)/M, the effects of PMA we re transient and involved mainly a G(o)/G(1), blockade. BisA also produced apoptosis in a proportion of the population, an effect not seen with PMA. B oth agents induced membrane translocation/activation of PKC, with BisA tran slocating only PKC delta and PMA translocating PKC alpha, delta, and epsilo n in these cells. Notably, while depletion of PKC alpha, delta, and epsilon abrogated the cell cycle-specific effects of PMA in IEC-18 cells, the abse nce of these PKC isozymes failed to inhibit BisA-induced G(o)/G(1), and G(2 )/M arrest or apoptosis. The cell cycle inhibitory and apoptotic effects of BisA, therefore, appear to be PKC-independent in IEG-18 cells. On the othe r hand, BisA and PMA both promoted PKC-dependent activation of Erk 1 and 2 in this system. Thus, intestinal epithelial cells respond to BisA through a ctivation of at least two signaling pathways: a PKC delta -dependent pathwa y, which leads to activation of mitogen-activated protein kinase and possib ly cytostasis in the appropriate context, and a PKC-independent pathway, wh ich induces both cell cycle arrest in G(o)/G(1) and G(2)/M and apoptosis th rough as yet unknown mechanisms. (C) 2001 Elsevier Science Inc. All rights reserved.