Antitumor Ribonucleases

Investigating the effects of double mutation C30A/C75A on onconase structure: Studies at atomic resolution

The structure of onconase C30A/C75A double mutant has been determined at 1.12Å resolution. The structure has high structural homology to other onconase structures. The changes being results of mutation are relatively small, distributed asymmetrically around the two mutated positions, and they are observed not only in the mutation region but expanded to entire molecule. Different conformation of Lys31 side chain that influences the hydrogen bonding network around catalytic triad is probably responsible for lower catalytic efficiency of double mutant. The decrease in thermal stability observed for the onconase variant might be explained by a less dense packing as manifested by the increase of the molecular volume and the solvent accessible surface area.

Generation of new cytotoxic human ribonuclease variants directed to the nucleus

Ribonucleases are promising agents for use in anticancer therapy. Engineering a nuclear localization signal into the sequence of the human pancreatic ribonuclease has been revealed as a new strategy to endow this enzyme with cytotoxic activity against tumor cells. We previously described a cytotoxic human pancreatic ribonuclease variant, named PE5, which is able to cleave nuclear RNA, inducing the apoptosis of cancer cells and reducing the amount of P-glycoprotein in different multidrug-resistant cell lines. These results open the opportunity to use this ribonuclease in combination with other chemotherapeutics. In this work, we have investigated how to improve the properties of PE5 as an antitumor drug candidate. When attempting to develop a recombinant protein as a drug, two of the main desirable attributes are minimum immunogenicity and maximum potency. The improvements of PE5 have been designed in both senses. First, in order to reduce the potential immunogenicity of the protein, we have studied which residues mutated on PE5 can be reverted to those of the wild-type human pancreatic ribonuclease sequence without affecting its cytotoxicity. Second, we have investigated the effect of introducing an additional nuclear localization signal at different sites of PE5 in an effort to obtain a more cytotoxic enzyme. We show that the nuclear localization signal location is critical for the cytotoxicity. One of these variants, named NLSPE5, presents about a 10-fold increase in cytotoxicity respective to PE5. This variant induces apoptosis and kills the cells using the same mechanism as PE5.

In vitro and in vivo anticarcinogenic effects of RNase MC2, a ribonuclease isolated from dietary bitter gourd, toward human liver cancer cells

Hepatocellular carcinoma (HCC) constitutes a predominant part of primary liver cancer which ranks as the fifth most common cancer as well as the third most common cause of cancer mortality. In view of the poor prognosis of unresectable liver cancers, it is of pivotal importance to develop novel chemotherapeutical regimens. RNase MC2 is a 14-kDa ribonuclease isolated from dietary bitter gourd (Momordica charantia) that manifested antitumor potential against breast cancers. In this study, we investigated the potential application of RNase MC2 on Hep G2 cells. We showed that RNase MC2 inhibited cell proliferation and induced cell apoptosis in both in vitro and in vivo studies. RNase MC2 treatment caused cell cycle arrest predominantly at the S-phase and apoptosis, which is associated with the activation of both caspase-8 and caspase-9 regulated caspase pathways. Our further investigation disclosed that RNase MC2 down-regulated the anti-apoptotic protein Bcl-2 and increased the expression of pro-apoptotic protein Bak. Moreover, the phosphorylation of ERK and JNK was involved in the apoptosis process. Importantly, RNase MC2 significantly suppressed the growth of Hep G2 xenograft-bearing nude mice by inducing apoptosis. This notion is supported by data indicating an increased number of caspase-3- and PARP-positive cells, and TUNEL-positive cells in RNase MC2-treated tumor tissues. In summary, we have revealed the antitumor potential of RNase MC2 toward Hep G2 cells. Considering that bitter gourd is a common dietary component in many countries, this study may help to prompt the clinical application of RNase MC2.