Involvement of the role of Chk1 in lithium-induced G2/M phase cell cycle arrest in hepatocellular carcinoma cells.

Lithium, a therapeutic agent for bipolar disorder, can induce G2/M arrest in various cells, but the mechanism is unclear. In this article, we demonstrated that lithium arrested hepatocellular carcinoma cell SMMC-7721 at G2/M checkpoint by inducing the phosphorylation of cdc2 (Tyr-15). This effect was p53 independent and not concerned with the inhibition of glycogen synthase kinase-3 and inositol monophosphatase, two well-documented targets of lithium. Checkpoint kinase 1 (Chk1), a critical enzyme in DNA damage-induced G2/M arrest, was at least partially responsible for the lithium action. The lithium-induced phosphorylation of cdc2 and G2/M arrest was abrogated largely by SB218078, a potent Chk1 inhibitor, as well as by Chk1 siRNA or the over-expression of kinase dead Chk1. Furthermore, lithium-induced cdc25C phosphorylation in 7721 cells and in vitro kinase assay showed that the activity of Chk1 was enhanced after lithium treatment. Interestingly, the increase of Chk1 activity by lithium may be independent of ataxia telangiectasia mutated (ATM)/ATM and Rad3-related (ATR) kinase. This is because no elevated phosphorylation on Chk1 (Ser-317 and Ser-345) was observed after lithium treatment. Moreover, caffeine, a known ATM/ATR kinase inhibitor, relieved the phosphorylation of cdc2 (Tyr-15) by hydroxyurea, but not that by lithium. Our study's results revealed the role of Chk1 in lithium-induced G2/M arrest. Given that Chk1 has been proposed to be a novel tumor suppressor, we suggest that the effect of lithium on Chk1 and cell cycle is useful in tumor prevention and therapy.

Down-regulation of N-acetylglucosaminyltransferase-V induces ER stress by changing glycosylation and function of GLUT1.

N-Acetylglucosaminyltransferase-V (GnT-V) is a key enzyme in the processing of N-glycans during synthesis of glycoproteins. We have reported that down-regulating GnT-V could induce endoplasmic reticulum stress (ER stress) in 7721 cells, a human hepatocarcinoma cell line. In a search for mechanisms of ER stress, we found that there was a prominent decline of glucose uptake in antisense GnT-V transfectant, furthermore, a decrease of tri- or tetra-antannary sugar chain of glucose transporter 1 (GLUT1). However, distribution of GLUT1 in antisense GnT-V transfectant was not affected. Glucose deprivation has been known to activate ER stress in tumor cells. Therefore, the data presented in this study indicate that the glycosylation change and decrease of transport activity of GLUT1 may be one possible mechanism of ER stress induced by down-regulating GnT-V, and GnT-V may contribute to the regulation of glucose uptake by modifying glycosylation of GLUT1 in some tumor cells.