Despite progress in treatment approaches for oral cancer, there has been only modest improvement in patient outcomes in the past three decades. The frequent treatment failure is due to the failure to control tumor recurrence and metastasis. These failures suggest that new targets should be identified to reverse oral epithelial dysplastic lesions. Recent developments suggest an active role of glycogen synthase kinase 3 beta (GSK3 β) in various human cancers either as a tumor suppressor or as a tumor promoter. GSK3β is a Ser/Thr protein kinase, and there is emerging evidence that it is a tumor suppressor in oral cancer. The evidence suggests a link between key players in oral cancer that control transcription, accelerated cell cycle progression, activation of invasion/metastasis and anti-apoptosis, and regulation of these factors by GSK3β. Moreover, the major upstream kinases of GSK3β and their oncogenic activation by several etiological agents of oral cancer support this hypothesis. In spite of all this evidence, a detailed analysis of the role of GSK3β in oral cancer and of its therapeutic potential has yet to be conducted by the scientific community. The focus of this review is to discuss the multitude of roles of GSK3β, its possible role in controlling different oncogenic events and how it can be targeted in oral cancer.

Of late, nimbolide, a limonoid from the neem tree (Azadirachta indica) has gained increasing research attention owing to its potent antiproliferative and apoptosis-inducing effects. The present study was designed to investigate the effect of nimbolide on autophagy and the time point at which the phosphorylation status of GSK-3β and PI3K dictate the choice between autophagy and apoptosis in SCC131 and SCC4 oral cancer cells. Additionally, we analysed changes in the expression of proteins involved in autophagy and apoptosis after therapeutic intervention with nimbolide in a hamster model of oral oncogenesis. Furthermore, we also demonstrate changes in the expression of key genes involved in apoptosis and autophagy during the stepwise evolution of hamster and human OSCCs. Nimbolide-induced stereotypical changes in oral cancer cells characteristic of both apoptosis and autophagy. Time-course experiments revealed that nimbolide induces autophagy as an early event and then switches over to apoptosis. Nimbolide negatively regulates PI3K/Akt signalling with consequent increase in p-GSK-3βTyr216, the active form of GSK-3β that inhibits autophagy. Downregulation of HOTAIR, a competing endogenous RNA that sponges miR-126 may be a major contributor to the inactivation of PI3K/Akt/GSK3 signalling by nimbolide. Analysis of key markers of apoptosis and autophagy as well as p-AktSer473 during sequential progression of hamster and human OSCC revealed a gradual evolution to a pro-autophagic and antiapoptotic phenotype that could confer a survival advantage to tumors. In summary, the results of the present study provide insights into the molecular mechanisms by which nimbolide augments apoptosis by overcoming the shielding effects of cytoprotective autophagy through modulation of the phosphorylation status of Akt and GSK-3β as well as the ncRNAs miR-126 and HOTAIR. Development of phytochemicals such as nimbolide that target the complex interaction between proteins and ncRNAs that regulate the autophagy/apoptosis flux is of paramount importance in cancer prevention and therapeutics.

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Mixed lineage kinase 3 (MLK3) is a mitogen-activated protein kinase kinase kinase member that activates the c-Jun N-terminal kinase (JNK) pathway. Aberrant activation of MLK3 has been implicated in neurodegenerative diseases. Similarly, glycogen synthase kinase (GSK)-3␤ has also been shown to activate JNK and contribute to neuronal apoptosis. Here, we show a functional interaction between MLK3 and GSK-3␤ during nerve growth factor (NGF) withdrawal-induced cell death in PC-12 cells. The protein kinase activities of GSK-3␤, MLK3, and JNK were increased upon NGF withdrawal, which paralleled increased cell death in NGFdeprived PC-12 cells. NGF withdrawal-induced cell death and MLK3 activation were blocked by a GSK-3␤-selective inhibitor, kenpaullone. However, the MLK family inhibitor, CEP-11004, although preventing PC-12 cell death, failed to inhibit GSK-3␤ activation, indicating that induction of GSK-3␤ lies upstream of MLK3. In GSK-3␤-deficient murine embryonic fibroblasts, ultraviolet light was unable to activate MLK3 kinase activity, a defect that was restored upon ectopic expression of GSK-3␤. The activation of MLK3 by GSK-3␤ occurred via phosphorylation of MLK3 on two amino acid residues, Ser 789 and Ser 793, that are located within the C-terminal regulatory domain of MLK3. Furthermore, the cell death induced by GSK-3␤ was mediated by MLK3 in a manner dependent on its phosphorylation of the specific residues within the C-terminal domain by GSK-3␤. Taken together, our data provide a direct link between GSK-3␤ and MLK3 activation in a neuronal cell death pathway and identify MLK3 as a direct downstream target of GSK-3␤. Inhibition of GSK-3 is thus a potential therapeutic strategy for neurodegenerative diseases caused by trophic factor deprivation.