Oleanonic acid inhibits glioma growth by inactivating STAT3 <i>via</i> upregulating SIRT6 in nude mice

Patients with glioma have a very high mortality rate, thus improving the poor prognosis of glioma has been the goal in the therapeutic field. Searching for more effective drugs for gliomas from natural compounds is a promising strategy. In this study, both oleanonic acid and oleanolic acid inhibited proliferation of glioma cells and reduced expression of cyclin D1 and E1, but the former has a lower IC₅₀ than the latter. Oleanonic acid reduced the expression of p-STAT3 but not p-STAT1 and 5, and also reducing the expression of STAT3 in the nucleus and its transcriptional activity in glioma cells. Furthermore, knockdown of STAT3 expression inhibited proliferation and migration of glioma cells. Next, the expressions of the upstream regulators such as SIRT6 and p-JAK2 but not SIRT1, p-ERK1/2, p300 were increased by oleanonic acid. The overexpression of SIRT6 not only reduced the expression of p-STAT3 and its transcriptional activity but also inhibited the proliferation and migration of glioma cells. In addition, the effects that oleanonic acid reduced the expression of p-STAT3 and its transcriptional activity and inhibited the proliferation and migration were attenuated by the knockdown of SIRT6. Furthermore, oleanonic acid effectively suppressed glioma growth and extended survival in nude mice bearing intracerebral U87 xenografts, but not in nude mice bearing intracerebral SIRT6-knockdown U87 xenografts. In conclusion, oleanonic acid upregulates the expression of SIRT6 to inactivates STAT3 and then inhibits glioma growth.