Abstract
Glycogen synthase kinase 3α/β (GSK3α/β) is a constitutively active serine/threonine kinase involved in multiple physiological processes, such as protein synthesis, stem cell maintenance and apoptosis, and acts as a key suppressor of the Wnt-β-catenin pathway. In the present study, we examined the therapeutic potential of a novel GSK3 inhibitor, CG0009, in the breast cancer cell lines, BT549, HS578T, MDA-MB-231, NCI/ADR-RES, T47D, MCF7 and MDA-MB-435, from the NCI-60 cancer cell line panel. Assessment of cytotoxicity, apoptosis and changes in estrogen-signaling proteins was performed using cell viability assays, Western blotting and quantitative real-time PCR. CG0009 enhanced the inactivating phosphorylation of GSK3α at Ser21 and GSK3β at Ser9 and simultaneously decreased activating phosphorylation of GSK3β at Tyr216, and induced caspase-dependent apoptosis independently of estrogen receptor α (ERα) expression status, which was not observed with the other GSK3 inhibitors examined, including SB216763, kenpaullone and LiCl. CG0009 treatment (1 µmol/L) completely ablated cyclin D1 expression in a time-dependent manner in all the cell lines examined, except T47D. CG0009 alone significantly activated p53, leading to relocation of p53 and Bax to the mitochondria. GSK3 inhibition by CG0009 led to slight upregulation of the β-catenin target genes, c-Jun and c-Myc, but not cyclin D1, indicating that CG0009-mediated cyclin D1 depletion overwhelms the pro-survival signal of β-catenin, resulting in cell death. Our findings suggest that the novel GSK3 inhibitor, CG0009, inhibits breast cancer cell growth through cyclin D1 depletion and p53 activation, and may thus offer an innovative therapeutic approach for breast cancers resistant to hormone-based therapy.
Highlights
Glycogen synthase kinase 3 (GSK3) is a serine/threonine kinase expressed as two similar isoforms, a and b [1,2]
CG0009 administered between concentrations of 0.001 and 100 mmol/L suppressed the growth of different breast cancer cell lines and the colon cancer cell line, HCT116, with a wide range of IC50 values except T47D cells (Table 1)
GSK3 inhibitors were initially identified as anti-diabetic targets, but more recent studies have highlighted these compounds as an emerging class of molecular therapeutic agents for human cancers
Summary
Glycogen synthase kinase 3 (GSK3) is a serine/threonine kinase expressed as two similar isoforms, a and b [1,2]. GSK3 was initially identified as a metabolic regulator that phosphorylates and inhibits glycogen synthase [3]. GSK3 is a constitutively active enzyme in normal cells and undergoes rapid inhibition by stimuli [2,4]. GSK3 is a key suppressor of the canonical Wnt signaling pathway of adenomatous polyposis coli (APC), axin and b-catenin, which is involved in embryonic cell fate determination and cell renewal [6,7,8]. GSK3b phosphorylates b-catenin, which leads to its destruction, suppressing signals that otherwise promote cell proliferation
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