Autophagy inhibition enhances ursolic acid-induced apoptosis in PC3 cells

The phosphoinositol 3-kinase/Akt pathway plays a critical role in oncogenesis and the dysregulation of this pathway through loss of PTEN is a particularly common phenomenon in aggressive prostate cancers. Several recent studies have indicated that ursolic acid (UA), a pentacyclic triterpenoid, and its derivatives inhibit the growth of cancer cells by cell cycle arrest and the stimulation of apoptosis. In the present study, we report a novel autophagic response of UA in PTEN-deficient PC3 prostate cancer cells. As one of the major types of programmed cell death, autophagy has been observed in response to several anticancer drugs and demonstrated to be responsible for cell death. UA-induced autophagy in PC3 cells is associated with the reduced cell viability and the enhanced expression of LC3-II, an autophagosome marker in mammals, and monodansylcadaverine incorporation into autolysosomes. Furthermore, we found that UA exhibited anti-proliferative effects characterized by G1 phase arrest and autophagy at an early stage that precedes apoptosis. We also show that UA-induced autophagy in PC3 cells are mediated through the Beclin-1 and Akt/mTOR pathways. Inhibition of autophagy by either 3-methyladenine or Beclin-1/Atg5 small interfering RNA enhanced UA-induced apoptosis. Taken together, our data suggest that autophagy functions as a survival mechanism in PC3 cells against UA-induced apoptosis and a rational for the use of autophagy inhibitors in combination with UA as a novel modality of cancer therapy.