Betulinic Acid Selectively Increases Protein Degradation and Enhances Prostate Cancer-Specific Apoptosis: Possible Role for Inhibition of Deubiquitinase Activity

Abstract

Inhibition of the ubiquitin-proteasome system (UPS) of protein degradation is a valid anti-cancer strategy and has led to the approval of bortezomib for the treatment of multiple myeloma. However, the alternative approach of enhancing the degradation of oncoproteins that are frequently overexpressed in cancers is less developed. Betulinic acid (BA) is a plant-derived small molecule that can increase apoptosis specifically in cancer but not in normal cells, making it an attractive anti-cancer agent. Our results in prostate cancer suggested that BA inhibited multiple deubiquitinases (DUBs), which resulted in the accumulation of poly-ubiquitinated proteins, decreased levels of oncoproteins, and increased apoptotic cell death. In normal fibroblasts, however, BA did not inhibit DUB activity nor increased total poly-ubiquitinated proteins, which was associated with a lack of effect on cell death. In the TRAMP transgenic mouse model of prostate cancer, treatment with BA (10 mg/kg) inhibited primary tumors, increased apoptosis, decreased angiogenesis and proliferation, and lowered androgen receptor and cyclin D1 protein. BA treatment also inhibited DUB activity and increased ubiquitinated proteins in TRAMP prostate cancer but had no effect on apoptosis or ubiquitination in normal mouse tissues. Overall, our data suggests that BA-mediated inhibition of DUBs and induction of apoptotic cell death specifically in prostate cancer but not in normal cells and tissues may provide an effective non-toxic and clinically selective agent for chemotherapy.

Induction of cell cycle arrest and apoptosis by betulinic acid-rich fraction from Dillenia suffruticosa root in MCF-7 cells involved p53/p21 and mitochondrial signalling pathway

“…4). BA is a pentacyclic triterpenoid that was first isolated from the stem bark of Zizyphus mauritiana in 1995 and has been demonstrated to induce apoptosis in various cancer cells including breast, colon, lung, melanoma, neuroblastoma, prostate and leukaemia in vitro and in vivo (Damle et al, 2013;Reiner et al, 2013;Sun et al, 2013;Mertens-Talcott et al, 2012;Kessler et al, 2007;Tan et al, 2003;Pisha et al, 1995). A study reported that BA up-regulated p53 and p21 in MCF-7 cells (Wang et al, 2009).…”

Sensitive detection of proteasomal activation using the Deg-On mammalian synthetic gene circuit

“…Interestingly, overexpression of PA28g was shown to recover proteasome function in an in vitro model of Huntington disease and improve cell viability 15 . Pharmacological agents that enhance UPS activity, however, are extremely rare: oleuropein was shown to enhance all three proteasome peptidase activities in fibroblasts 11 , while betulinic acid activates preferentially the chymotrypsin-like proteasome activity 9 and displays potentially anti-cancer properties 10,45 . Inhibition of the deubiquitinating enzyme USP14 was shown to enhance degradation of oxidized proteins, thereby protecting from oxidative stress 12 .…”

“…On the other hand, pharmacological agents that increase UPS activity remain largely unexplored [9][10][11][12] , although this mode of regulation has great potential to ameliorate the phenotypes associated with the development of protein deposition diseases, such as Parkinson's 13,14 and Huntington's 15,16 . There is evidence that proteasome activity could be upregulated by overexpressing proteasomal subunits 15,17,18 , increasing E3 ligase expression [19][20][21] and modulating the rates of proteasomal conformational changes that are rate limiting for peptidase activity 22,23 .…”

Curcusone D, a novel ubiquitin–proteasome pathway inhibitor via ROS-induced DUB inhibition, is synergistic with bortezomib against multiple myeloma cell growth

“…Curcusone D shares no structural or chemical resemblance to any known DUB inhibitors [19][20][21][22][23][24] despite its ability to inhibit the activities of many cellular DUBs (Fig. 4C), establishing it as a new, partially selective DUB inhibitor.…”