Evidence for involvement of Wnt signaling pathway in IB-MECA mediated suppression of melanoma cells

Abstract

The A3 adenosine receptor, A3AR, belongs to the family of Gi proteins, which upon induction, suppresses the formation of cAMP and its downstream e ectors. Recent studies have indicated that activation of A3AR by its agonist, IB-MECA, results in growth inhibition of malignant cells. Here we demonstrate the ability of IB-MECA to decrease the levels of protein kinase A, a downstream e ector of cAMP, and protein kinase B/Akt in melanoma cells. Examination of glycogen synthase kinase 3b, GSK-3b, whose phosphorylation is controlled by protein kinase A and B, showed a substantial decrease in the levels of its phosphorylated form and an increase in total GSK-3b levels in IB-MECA treated melanoma cells. This observation suggests that the treatment of cells with IB-MECA augments the activity of GSK-3b in the cells. Evaluation of b-catenin, a key component of Wnt signaling pathway which, upon phosphorylation by GSK-3b rapidly ubiquitinates, showed a substantial decrease in its level after IB-MECA treatment. Accordingly, the level of b-catenin responsive cell growth regulatory genes including c-myc and cyclin D1 was severely declined upon treatment of the cells with IB-MECA. These observations which link cAMP to the Wnt signaling pathway provide mechanistic evidence for the involvement of Wnt pathway via its key elements GSK-3b and b-catenin in the anti-tumor activity of A3AR agonists.

Anti-arthritic effects of clematichinenoside (AR-6) on PI3K/Akt signaling pathway and TNF-α associated with collagen-induced arthritis

“…It has been documented that PI3K/AKT pathway potentiated synovial overgrowth and joint destruction in RA and the translocation of NF-κB p65 into the nucleus was inhibited by a PI3K/AKT inhibitor, indicating that the activation of NF-κB in synovial cells was AKT-dependent (Hashiramoto et al, 2007). Akt, acting as a survival signal in the PI3K cascade, controls apoptosis via the modulation of downstream key signaling protein NF-κB by phosphorylating downstream proteins such as IKK and Iκ-B, which in turn release NF-κB from its complex (Fishman et al, 2002). Indeed, AR-6 treatment diminished the NF-κB protein expression levels .…”

Adenosine receptors as therapeutic targets

“…However, the novel anticancer effect discovered by Fishman and colleagues is caused by a cytostatic effect on tumours related to the WNT pathway 32 , rather than by induction of apoptosis. Recently, it was shown that the A 3 AR is more highly expressed in tumour than in normal cells, which may justify A 3 AR as a potential target for tumour growth inhibition 201 .…”

“…RhoA-phospholipase D1 signalling has been demonstrated to mediate the antiischaemic effect of A 3 ARs 31 . The WNT signalling pathway is involved in A 3 AR agonist-mediated suppression of melanoma cells 32 . In addition, like other ARs, the A 3 AR couples to MAPK, which could give it a role in cell growth, survival, death and differentiation 33,34 .…”

Exploring distal regions of the A3 adenosine receptor binding site: sterically constrained N6-(2-phenylethyl)adenosine derivatives as potent ligands

“…In certain tumor cells, a cytostatic effect of the A 3 AR agonist appears to be related to its downstream activation of the Wnt pathway. 5 We have studied the microscopic interactions of ligands with the A 3 AR and other members of the AR family from the perspectives of both ligand modification and structure-function aspects of the receptors. [6][7][8][9] Extensive mutagenesis studies and molecular modeling based on a high-resolution template of rhodopsin have implicated TM (transmembrane domain) regions 3,6, and 7 in the coordination of adenosine agonists and a putative rotation of TM6 in the activation of the A 2A and A 3 ARs.…”