Glaucocalyxin A-induced oxidative stress inhibits the activation of STAT3 signaling pathway and suppresses osteosarcoma progression in vitro and in vivo

Osteosarcoma (OS) is ranked as the most common primary bone malignancy in children and adolescents worldwide, and the 5-year overall survival rate of OS is not optimistic. Constitutive activation of signal transducer and activator of transcription 3 (STAT3) has been implicated in tumor cell growth, proliferation, and anti-apoptosis in OS. Therefore, the discovery of novel molecular compounds that can effectively block STAT3 activation, is essential for the treatment of OS and improving prognosis. Here, we investigate whether Glaucocalyxin A (GLA), derived from Rabdosia japonica, exhibit the potential anticancer effects in OS. First of all, we identify that GLA potently suppressed cell proliferation, induced G2/M phase arrest and promoted substantial apoptosis in OS. Next, we conclude that GLA could induce Reactive oxygen species (ROS)-mediated oxidative stress via an imbalance of GSH and GSSG. Then, we elucidate for the first time that GLA could significantly inhibit both constitutive and IL-6-inducible activation of STAT3 (Tyr705) and JAK2, the upstream regulator of STAT3. Furthermore, we elucidate that the inhibition of STAT3 is mainly induced by ROS-mediated oxidative stress. Overall, our findings demonstrate that GLA could exhibit potent anticancer effects through effectively blocking the STAT3 signaling pathway, which was induced by ROS-mediated oxidative stress in OS in vitro and in vivo.