Luteolin As An Inhibitor Of Heat Shock Protein 90 Inhibits Tumor Cell Apoptosis By Inhibiting The Sustained Activation Of STAT3

Flavonoids are polyphenolic compounds occurring in a wide range of plants, which are known to have anti-inflammatory, antioxidant and antibacterial activities. It have also been reported that flavonoids efficiently suppress the proliferation of tumor cells and induce apoptosis by blocking cell cycle progression. Luteolin has been found to possess a potent anticancer activity, and even at low dosage it displays a marked effect on killing malignant cells. To date, however, the detailed mechanism by which luteolin carries out its anticancer activity is unclear.It has been reported that luteolin could induce degradation of Tyr705-phosphorylated STAT3. STAT3is a member of STATs (Signal transducer and activators of transcription) family, which is latent in cytoplasm and becomes activated through tyrosine and/or serine phosphorylation. STAT3can be activated by diverse stimulations and constitutive activation of STAT3is a requirement for the oncogenic transforming property. Phosphorylated STAT3enters into nucleus and works coordinately with other transcriptional co-activators or transcription factors to initiate transcription. In fact, many antiapoptotic genes are downstream targets of STAT3. Abnormal activity of STAT3is associated with a number of human malignancies, including hematologic, breast, head, neck, and prostate cancers.Heat shock protein90(Hsp90) may function as a stabilizer of Tyr-phosphorylated STAT3by directly interacting with it. Hsp90, an ATP dependent protein, is one of the most abundant and ubiquitous molecular chaperones, and has been shown to interact with a variety of cytoplasm proteins including transcription factors, hormone receptors and proteins kinases. Hsp90is increasingly recognized as an important target for molecular cancer therapy due to its role in regulating key proteins in cell growth, survival, and differentiation pathways. Together with the cochaperones, Hsp90sustains the stability and function of its client proteins through forming complexes with them. Inhibition of Hsp90activity will lead to degradation of these client proteins in an ubiquitin-proteasome-dependent pathway and consequently disrupt their function. Many client proteins of Hsp90are crucial in oncogenesis, such as Her-2, Akt, STAT3, and p53.Here we revealed a novel mechanism by which luteolin promoted apoptosis of HeLa and MCF-7cells. In luteolin treated cells, constant phosphorylation of STAT3was obviously repressed resulting in the down-regulation of its downstream targets, which are antiapoptotic factors. Meanwhile, some proteins including STAT3, which are essential to proliferation and antiapopsis of cancer cells, were down-regulated as well. These proteins are generally considered as client proteins of Hsp90. The consistant results were observed by using RNA interference to silent Hsp90expression. Molecular simulation assay revealed that luteolin could bind to Hsp90at its N-terminal ATPase domain. Further investigations through SRP and ATP-coimmunoprecitation confirmed that luteolin specifically binded to Hsp90at ATPase domain. We therefore hypothesized that luteolin inhibits the chaperone action of Hsp90by binding to ATPase domain competently with ATP, thereby promoting the client proteins of Hsp90to undergo degradation through ubiquitin-dependent way.Our investigation demonstrated that luteolin could associate with Hsp90to make STAT3dissociate from Hsp90, and consequently, suppressed STAT3activation. Our observations not only provide a noval mechanism by which luteolin exerts therapeutic value, but also shed light on the future development of anticancer agents targeting Hsp90.