Role of the JNK/c-Jun/AP-1 signaling pathway in galectin-1-induced T-cell death

Galectin-1 (gal-1), an endogenous beta-galactoside-binding protein, triggers T-cell death through several mechanisms including the death receptor and the mitochondrial apoptotic pathway. In this study we first show that gal-1 initiates the activation of c-Jun N-terminal kinase (JNK), mitogen-activated protein kinase kinase 4 (MKK4), and MKK7 as upstream JNK activators in Jurkat T cells. Inhibition of JNK activation with sphingomyelinase inhibitors (20 mu M desipramine, 20 mu M imipramine), with the protein kinase C-delta (PKC delta) inhibitor rottlerin (10 mu M), and with the specific PKC theta pseudosubstrate inhibitor (30 mu M) indicates that ceramide and phosphorylation by PKCd and PKC delta mediate gal-1-induced JNK activation. Downstream of JNK, we observed increased phosphorylation of c-Jun, enhanced activating protein-1 (AP-1) luciferase reporter, and AP-1/DNA-binding in response to gal-1. The pivotal role of the JNK/c-Jun/AP-1 pathway for gal-1-induced apoptosis was documented by reduction of DNA fragmentation after inhibition JNK by SP600125 (20 mu M) or inhibition of AP-1 activation by curcumin (2 mu M). Gal-1 failed to induce AP-1 activation and DNA fragmentation in CD3-deficient Jurkat 31-13 cells. In Jurkat E6.1 cells gal-1 induced a proapoptotic signal pattern as indicated by decreased antiapoptotic Bcl-2 expression, induction of proapoptotic Bad, and increased Bcl-2 phosphorylation. The results provide evidence that the JNK/c-Jun/AP-1 pathway plays a key role for T-cell death regulation in response to gal-1 stimulation. Cell Death and Disease (2010) 1, e23; doi:10.1038/cddis.2010.1; published online 4 February 2010