Citrate-Induced p85α–PTEN Complex Formation Causes G₂/M Phase Arrest in Human Pharyngeal Squamous Carcinoma Cell Lines

Citrate is a key intermediate of the tricarboxylic acid cycle and acts as an allosteric signal to regulate the production of cellular ATP. An elevated cytosolic citrate concentration inhibits growth in several types of human cancer cells; however, the underlying mechanism by which citrate induces the growth arrest of cancer cells remains unclear. The results of this study showed that treatment of human pharyngeal squamous carcinoma (PSC) cells with a growth-suppressive concentration of citrate caused cell cycle arrest at the G2/M phase. A coimmunoprecipitation study demonstrated that citrate-induced cell cycle arrest in the G2/M phase was associated with stabilizing the formation of cyclin B1−phospho (p)-cyclin-dependent kinase 1 (CDK1) (Thr 161) complexes. The citrate-induced increased levels of cyclin B1 and G2/M phase arrest were suppressed by the caspase-3 inhibitor Ac-DEVD-CMK and caspase-3 cleavage of mutant p21 (D112N). Ectopic expression of the constitutively active form of protein kinase B (Akt1) could overcome the induction of p21 cleavage, cyclin B1−p-CDK1 (Thr 161) complexes, and G2/M phase arrest by citrate. p85α−phosphatase and tensin homolog deleted from chromosome 10 (PTEN) complex-mediated inactivation of Akt was required for citrate-induced G2/M phase cell cycle arrest because PTEN short hairpin RNA or a PTEN inhibitor (SF1670) blocked the suppression of Akt Ser 473 phosphorylation and the induction of cyclin B1−p-CDK1 (Thr 161) complexes and G2/M phase arrest by citrate. In conclusion, citrate induces G2/M phase arrest in PSC cells by inducing the formation of p85α−PTEN complexes to attenuate Akt-mediated signaling, thereby causing the formation of cyclin B1−p-CDK1 (Thr 161) complexes

Monacolin K affects lipid metabolism through SIRT1/AMPK pathway in HepG2 cells

Monacolin K is the secondary metabolite isolated from Monascus spp. It is the natural form of lovastatin, which is clinically used to reduce the synthesis of cholesterol by inhibiting 3-hydroxy-3-methylglutaryl coenzyme A reductase. In the present study, monacolin K increased protein expression of SIRT1 and phosphorylation level of AMP-activated protein kinase (AMPK) in HepG2 cells. Through activation of SIRT1/AMPK pathway, monacolin K increased phosphorylation of acetyl CoA carboxylase and caused nuclear translocation of forkhead box O1. The western blotting results showed that monacolin K increased expression of adipose triglyceride lipase but decreased abundances of fatty acid synthase (FAS) and sterol regulatory element-binding protein 1 (SREBP1). Monacolin K also decreased the intracellular accumulation of lipids as demonstrated by Oil Red O staining. In addition, the immunostaining showed that monacolin K prevented the nuclear translocation of SREBP1, indicating the association with down-regulation of FAS. All the demonstrated effects of monacolin K were counteracted by nicotinamide or compound C, the inhibitors of SIRT1 or AMPK. In summary, monacolin K reduces the lipid content through SIRT1/AMPK pathway in HepG2 cells, which promotes catabolism and inhibits anabolism of lipid

A Novel Methodology for Detecting Variations in Cell Surface Antigens Using Cell-Tearing by Optical Tweezers

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