Ali-Rahmani, F. | bioRxiv Science

bioRxiv2022

The methyltransferases METTL7A and METTL7B confer resistance to thiol-based histone deacetylase inhibitors

Histone deacetylase inhibitors (HDACis) are part of a growing class of epigenetic therapies used for the treatment of cancer. While elevated levels of the efflux pump P-gp are associated with in vitro resistance to romidepsin, this mechanism does not translate to the clinic. We developed a romidepsin-resistant cell line with a resistance mechanism independent of P-gp function that acts upstream of the deacetylation process. We found that expression of the methyltransferase METTL7A is necessary for resistance, and that expression of METTL7A in naive cells can drive resistance to thiol-containing HDACis. We demonstrate that METTL7A can methylate romidesin in vitro and that the ability of METTL7A to drive resistance to thiol-containing HDACis can be blocked by the methyltransferase inhibitor DCMB. Our data supports a model whereby exposure of cells to romidepsin selects for upregulation of the methyltransferase METTL7A, which in turn modifies the zinc-binding thiol, inactivating the drug.

cancer biology↗

bioRxiv2021

Characterization and tissue localization of zebrafish homologs of the human ABCB1 multidrug transporter

Given its similarities with mammalian systems, the zebrafish has emerged as a potential model to study the blood-brain barrier (BBB). Capillary endothelial cells at the human BBB express high levels of P-glycoprotein (P-gp, encoded by the ABCB1 gene) and ABCG2 (encoded by the ABCG2 gene). However, little information has been available about ATP-binding cassette transporters expressed at the zebrafish BBB. In this study, we focus on the characterization and tissue localization of two genes that are similar to human ABCB1, zebrafish abcb4 and abcb5. Cytotoxicity assays with stably-transfected cell lines revealed that zebrafish Abcb5 cannot efficiently transport the substrates doxorubicin and mitoxantrone compared to human P-gp and zebrafish Abcb4. Additionally, zebrafish Abcb5 did not transport the fluorescent probes BODIPY-ethylenediamine or LDS 751, while they were readily transported by Abcb4 and P-gp. A high-throughput screen conducted with 90 human P-gp substrates confirmed that zebrafish Abcb4 has overlapping substrate specificity with P-gp. Basal ATPase activity of zebrafish Abcb4 and Abcb5 was comparable to that of human P-gp. In the brain vasculature, RNAscope probes to detect abcb4 colocalized with staining by the P-gp antibody C219, while abcb5 was not detected. Zebrafish abcb4 also colocalized with claudin-5 expression in brain endothelial cells. Abcb4 and Abcb5 had different tissue localizations in multiple zebrafish tissues, consistent with different functions. The data suggest that zebrafish Abcb4 most closely phenocopies P-gp and that the zebrafish may be a viable model to study the role of the multidrug transporter P-gp at the BBB.

pharmacology and toxicology↗