Date Published: May 24, 2019
Publisher: Public Library of Science
Author(s): Gretchen L. Bentz, Angela J. Lowrey, Dustin C. Horne, Vy Nguyen, Austin R. Satterfield, Tabithia D. Ross, Abigail E. Harrod, Olga N. Uchakina, Robert J. McKallip, Luwen Zhang.
Cellular sumoylation processes are proposed targets for anti-viral and anti-cancer therapies. We reported that Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) dysregulates cellular sumoylation processes, contributing to its oncogenic potential in EBV-associated malignancies. Ginkgolic acid and anacardic acid, known inhibitors of sumoylation, inhibit LMP1-induced protein sumoylation; however, both drugs have adverse effects in hosts. Here we test the effects of glycyrrhizic acid, a medicinal botanical extract with anti-inflammatory, anti-carcinogenic, and anti-viral properties, on cellular sumoylation processes. While glycyrrhizic acid is known to inhibit EBV penetration, its affect on cellular sumoylation processes remains to be documented. We hypothesized that glycyrrhizic acid inhibits cellular sumoylation processes and may be a viable treatment for Epstein-Barr virus-associated malignancies. Results showed that glycyrrhizic acid inhibited sumoylation processes (without affecting ubiquitination processes), limited cell growth, and induced apoptosis in multiple cell lines. Similar to ginkgolic acid; glycyrrhizic acid targeted the first step of the sumoylation process and resulted in low levels of spontaneous EBV reactivation. Glycyrrhizic acid did not affect induced reactivation of the virus, but the presence of the extract did reduce the ability of the produced virus to infect additional cells. Therefore, we propose that glycyrrhizic acid may be a potential therapeutic drug to augment the treatment of EBV-associated lymphoid malignancies.
Protein post-translational modifications, such as ubiquitination and phosphorylation, allow cells to respond to both external and internal stimuli and are vital to numerous cellular events. The modification of proteins by the small ubiquitin-like modifier or SUMO was identified in 1997 . There are four characterized human SUMO isoforms (SUMO-1, -2, -3, and -4), and SUMO-1 and SUMO-2/3 are ubiquitously expressed in the body. Protein sumoylation is similar to ubiquitination in that it is a dynamic, multi-step process. First, the translated SUMO-pro-peptide undergoes maturation [2–5]. Second, matured SUMO is activated in an ATP-dependent manner by the SUMO-activating enzyme [2–5]. Third, the SUMO-conjugating enzyme, Ubc9, recognizes the conserved sumoylation motif (ΨKxD/E motif, where Ψ represents a hydrophobic amino acid) within the target protein and mediates the formation of an isopeptide bond with the activated protein and the lysine residue within the SUMO motif of the target protein [2–6]. De-sumoylation of the target protein is mediated by sentrin-specific proteases or SENPs .
One proposed therapeutic target for cancer is the sumoylation process [2,21]. Earlier, we identified a novel function for LMP1, in the dysregulation of cellular sumoylation processes during EBV latency [25,62,64], and our recent work documented that SUMO levels are increased in LMP1-positive lymphoma tissues . Therefore, identifying mechanisms by which sumoylation processes can be inhibited may aid the treatment of LMP1-associated malignancies. Our current findings imply that glycyrrhizic acid, a triterpene from licorice root [32,33], inhibits cellular sumoylation processes and can be used to inhibit the growth of EBV-transformed lymphoblastoid cell lines and induce apoptosis. This is the first report using glycyrrhizic acid to target sumoylation processes. In addition, these findings provide further support for the function of sumoylation in the maintenance of latency  and verify that glycyrrhizic acid can inhibit EBV infection, which has been proposed due to inhibition of viral penetration [46,47]