The role of polycomb repressive complex 2 and effects of lobarstin in glioblastoma multiforme

Abstract

Glioblastoma multiforme (GBM) is a grade IV primary malignant brain tumor. The elevated expression of polycomb repressive complex 2 (PRC2) proteins such as EZH2 and SUZ12 have been found in GBM and their expression levels often correlate with poor prognosis. In order to find anti-proliferative factors and to increase the potential of anti-cancer drug for glioblastoma treatment, I studied two diffetent approaches. First, I tested that PRC2 may play as master regulator in GBM. Second, I investigated the influence of co-treatment with lobarstin and TMZ on chemosesitivity of glioblastoma cells. Upon knock-down of PRC2, cell growth was attenuated and cells were accumulated at G1 phase of the cell cycle. Among several G1 regulators, the total and nuclear CDKN1B protein levels were drastically up-regulated by PRC2 depletion. Interestingly, the expression of SKP2, a component of the SCFSKP2 E3 ubiquitin ligase which is known to promote the degradation of CDKN1B protein degradation was significantly reduced under depleted expression of PRC2. Furthermore, PRC2-depletion led to down-regulation of MYC expression at both mRNA and protein levels. In agreement with the in vitro results, glioblastoma patients with elevated EZH2 expression showed high MYC expression but reduced CDKN1B expression. Temozolomide (TMZ) has been used as standard therapy for glioblastoma patients, however, this treatment does not improve the prognosis and survival of patients because of rapid DNA repair system. Upon transient PRC2-depletion, TMZ-induced cytotoxicity and the DNA damage were found to are extended. Interestingly, PRC2-depletion had no effect on MGMT expression, but the expressions of the base excision repair (BER) genes such as PARP1, XRCC1, and LIG3 were significantly decreased. In agreement with the in vitro results, glioblastoma patients with high EZH2 expression showed increased PARP1 expression level. In order to evaluate the possibility of developing combinatorial therapy for GBM, I tested the combination effect of lobarstin and TMZ on T98G cells. Co-treatment with lobarstin and TMZ resulted in enhanced cytotoxicity for GBM cells through repressing DNA-recovery. Expressions of DNA repair genes such as MGMT, PARP1 and LIG 3 were significantly reduced in lobarstin and TMZ co-treated cells. Taken together, these results showed that the PRC2-depletion enhances inhibitory effect on GBM growth and potentiates TMZ antitumor activity. In summary, the effects of PRC2 can be considered as a potential targets for developing a novel therapeutic approaches. Additionally, lobarstin can be used as a potential combination drugs with TMZ for efficient GBM therapy.