Citrate-Induced p85-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 G(2)/M phase. A coimmunoprecipitation study demonstrated that citrate-induced cell cycle arrest in the G(2)/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 G(2)/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 G(2)/M phase arrest by citrate. p85-phosphatase and tensin homolog deleted from chromosome 10 (PTEN) complex-mediated inactivation of Akt was required for citrate-induced G(2)/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 G(2)/M phase arrest by citrate. In conclusion, citrate induces G(2)/M phase arrest in PSC cells by inducing the formation of p85-PTEN complexes to attenuate Akt-mediated signaling, thereby causing the formation of cyclin B1-p-CDK1 (Thr 161) complexes.