APOBEC3G rescues lymphoma cells from the deleterious effects caused by DNA damage
The human genome encodes for 11 cytidine deaminases of the apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like (APOBEC) family. The catalytically active APOBECs induce mutations on single-stranded (ss) DNA or RNA by deaminating Cytidine to Thymidine. APOBEC3 (A3) proteins function as innate immune factors, and elicit broad retrotransposon and retrovirus-restricting activities. The physiological functions of the cellular deaminases are not limited to innate and acquired immunity and they have roles beyond this setting. The APOBEC3G (A3G) was first described as an anti-HIV-1 restriction factor, acting by directly deaminating reverse transcripts of the viral genome. HIV-1 Vif neutralizes A3G by mediating its degradation to establish effective infection in host target cells and by inhibition of its deaminase activity.
Lymphoma cells, which express high amounts of A3G, can restrict Vif-deficient HIV-1. Moreover, we have shown that several types of cancer cells, such as lymphoma and myeloma cells, e.g. diffuse large B-cell lymphoma (DLBCL) cells, which express high amounts of A3G, show enhanced cell survival after ionizing radiation (IR). Previously, we demonstrated that A3G is responsible for the cell survival following genotoxic treatments and that A3G plays a dual role in promoting survival of cancer cells in vivo, first by enhancing DSB repair following IR treatments, thus preventing cell death, and second by promoting a mutator phenotype that drives tumor progression.
Recently, we have extended our findings, showed that induction of DNA damage by treating the cells with UV, BrdU or by combined BrdU and UV treatments stimulates A3G expression in lymphoma cells, and demonstrated that A3G is responsible for lymphoma cell survival following DNA damage induction. Our results indicate that A3G is involved in rescuing the cells from the detrimental effect of DNA damage mostly from DSBs. Hence, A3G is a potential therapeutic target to increase tumor cells sensitivity to genotoxic treatments and to prevent tumor promotion.
Inhibition of A3G:
Lymphoma cells and glioblastoma cells express high levels of A3G. Both tumors are radio- and chemo-therapies resistant, while A3G suppression converts these cells radiotherapy-sensitive. These findings emphasize the urgent need for A3G inhibitors. We found that HIV-1 Vif and Vif derived peptides of 9-15 amino acids inhibit A3G. These peptides inhibit the deamination activity as well as the binding of ssDNA and RNA molecules. Treatment of lymphoma cells with these peptides convert them X-Ray sensitive. These active peptides penetrate the cells, can persist in the cell cytoplasm, and inhibit the endogenous A3G deaminase activity. Thus, these peptides appear to be initial scaffolds for the development of future potent peptide-mimetic inhibitors that could become efficient anti-A3G small-molecule drugs to convert radio- and chemotherapies resistant cells sensitive.
Activation of AID:
Activation-induced cytidine deaminase (AID) belongs to the AID/APOBEC family. AID is a relatively small enzyme of 198 amino acids with normal expression limited to activated B lymphocytes, though expression in other tissues appears to be inducible in inflammatory situations.
The naïve antibody repertoire is of low affinity. To increase affinity and change effector function of antibodies, activated B cells edit their Immunoglobulin (Ig) loci in the form of somatic hypermutation (SHM) and class switch recombination (CSR). Both SHM and CSR are initiated by the enzyme activation-induced cytidine deaminase (AID). AID deficiency results in Hyper IgM type immunodeficiency. Thus, AID is essential for an effective antibody response.
Unlike most of A3 enzymes AID is a very slow deaminase. In order to get a better understanding of the AID function in B lymphocytes and in tumor cells, we inserted mutations in the AID gene to replace several amino acids in its active site vicinity. The mutated AID enzymes were expressed in mammalian cells and their activity is tested in vitro.