Inactivation of glycogen synthase kinase-3β, a downstream target of the raf-1 pathway, is associated with growth suppression in medullary thyroid cancer cells

Glycogen synthase kinase-3beta (GSK-3beta) is an important regulator of cell proliferation and survival. Conflicting observations have been reported regarding the regulation of GSK-3beta and extracellular signal-regulated kinase (ERK1/2) in cancer cells. In this study, we found that raf-1 activation in human medullary thyroid cancer cells, TT cells, resulted in phosphorylation of GSK-3beta. Inactivation of GSK-3beta in TT cells with well-known GSK-3beta inhibitors such as lithium chloride (LiCl) and SB216763 is associated with both growth suppression and a significant decrease in neuroendocrine markers such as human achaete-scute complex-like 1 and chromogranin A. Growth inhibition by GSK-3beta inactivation was found to be associated with cell cycle arrest due to an increase in the levels of cyclin-dependent kinase inhibitors such as p21, p27, and p15. Additionally, LiCl-treated TT xenograft mice had a significant reduction in tumor volume compared with those treated with control. For the first time, we show that GSK-3beta is a key downstream target of the raf-1 pathway in TT cells. Also, our results show that inactivation of GSK-3beta alone is sufficient to inhibit the growth of TT cells both in vitro and in vivo.