Mechanisms of Resistance to Temozolomide in Glioblastomas
Glioblastoma multiforme afflicts 12,500 new patients in the U.S. annually (Friedman et al., 2000; Stupp et al., 2009; Silber et al., 2012). Glioblastoma (GBM) is highly lethal, and the average survival expectancy is 14.6 months, and the overall 5-year survival rate for GBM is only 9.8% (Friedman et al., 2000; Stupp et al., 2009). High levels of resistance to current therapeutic modalities and cancer relapse are frequently seen in patients (Haar et al., 2012; Happold et al., 2012). The current standard therapy for GBM mainly includes maximum debulking surgery, radiation, and treatment with the monofunctional alkylating agent temozolomide (TMZ) (Friedman et al., 2000; Nishikawa, 2010). Multiple mechanisms are involved in the TMZ resistance, which may include cancer stem cells, microRNAs, drug efflux, DNA damage repair, tumor cells under hypoxia, histone deacetylation, epithelial-mesenchymal transition, STAT3 kinase, and many others (Haar et al., 2012; Happold et al., 2012; Johannessen and Bjerkvig, 2012; Kitange et al., 2012; Kohsaka et al., 2012; Zhang et al., 2012b).
Temozolomide induces generation of DNA lesions, including O6-methylguanine, N3-methyladenine, and N7-methylguanine (Goellner et al., 2011; Zhang et al., 2012b). The O6-methylguanine lesion is known to trigger autophagy, rather than apoptosis, to cause cell death (Kanzawa et al., 2003). Also, inhibition of antiapoptotic Bcl-2 by a pan-Bcl-2 inhibitor (−)-gossypol leads to autophagic death in gliomas and enhances the action of TMZ (Voss et al., 2010). However, a recent study demonstrated that TMZ-induced autophagy is pro-survival, and may block the eventual apoptosis in GBM cells (Knizhnik et al., 2013). Also, MAPO2 (C1orf201) gene participates in the O6-methylguanine lesion-induced apoptosis (Fujikane et al., 2012). MAPO2 gene encodes a novel 37-kDa protein. It is not determined whether this gene is involved in autophagy.
The O6-methylguanine lesion is a substrate for direct repair by O6-methylguanine-DNA methyltransferase (MGMT) (Pollack et al., 2006; Hegi et al., 2008; Fukushima et al., 2009; Zhang et al., 2012b). Without MGMT repair, O6-methylguanine initiates activation of mismatch repair-deficient (MMR) proteins or Rad3-related protein kinase that ultimately leads to apoptotic cell death (Caporali et al., 2004; Wang and Edelmann, 2006; Roos et al., 2007). High expression of MGMT or loss of MMR contributes significantly to TMZ resistance in many clinical cases (Pollack et al., 2006; Hegi et al., 2008; Sarkaria et al., 2008). The initiation of apoptotic signaling fails in the absence of the MMR system.
Sensitivity to TMZ is significantly associated with the methylation status of MGMT gene promoter in cells committed to differentiation (Villalva et al., 2012). An increase in MGMT gene promoter methylation, which blocks MGMT protein expression, prolongs cancer patient survival. Intriguingly, overexpressed microRNA-21 reduces Bax/Bcl-2 ratio and caspase-3 activity, thereby blocking TMZ-induced apoptosis (Shi et al., 2010). MicroRNA-21 is considered as a pro-survival factor for cancer cells (Li et al., 2012). Integrins play a role in the resistance of advanced cancers to radiotherapy and chemotherapy. α5β1 integrin negatively regulates p53 signaling, and the event induces glioma cell resistance to TMZ (Janouskova et al., 2012). α5β1 integrin is considered as a therapeutic target for high-grade brain tumors. The base excision repair enzyme alkylpurine-DNA-N-glycosylase (APNG), which repairs the cytotoxic lesions N3-methyladenine and N7-methylguanine, also participates in the TMZ resistance (Agnihotri et al., 2012). Upregulation of mitochondrial respiratory chain coupling to suppress the production of reactive oxygen species (ROS) regulated by cytochrome c oxidase contributes in part to TMZ resistance in gliomas (Oliva et al., 2011).
Reversal of TMZ resistance may be achieved by MGMT pseudosubstrates, O6-benzylguanine and lomeguatrib to sensitize tumors to TMZ (Zhang et al., 2012b). Methoxyamine-blocker of base excision repair contributes significantly to TMZ cytotoxicity particularly when O6-methylguanine adducts are repaired or tolerated (Goellner et al., 2011; Zhang et al., 2012b). Dual targeting of base excision repair and NAD(+) biosynthesis may reverse TMZ resistance in patients with resistant and recurrent GBM (Goellner et al., 2011). Interferon-β (IFN-β), levetiracetam (LEV), resveratrol, and valproic acid (VAP) increase the sensitivity of TMZ through MGMT-dependent or -independent mechanisms (Nakada et al., 2012). Resveratrol, a natural polyphenol, reverses TMZ resistance via an NF-κB-dependent mechanism (Huang et al., 2012). STAT3 inhibitor or STAT3 knockdown potentiates TMZ efficacy in resistant GBM cell lines (Kohsaka et al., 2012). Intratumoral hypoxia is common in GBMs and may be associated with the development of TMZ resistance. Induced hyperoxia can be utilized to reverse TMZ resistance in GBMs (Sun et al., 2012). Cancer stem cells are probably the key to failure in TMZ treatment. The concept of cancer stem cell survival from treatment with TMZ and other chemotherapeutic drugs has been more complicated than previously thought (Beier et al., 2011; Chen et al., 2012). CD133-positive cancer stem cells are expressed in both normal stem cells and cancer stem cells (Donovan and Pilkington, 2012). However, the role of CD133 as a marker for glioma cancer stem cells relative to its biological function has yet to be established.