Sequestration of the abrin A chain to the nucleus by BASP1 increases the resistance of cells to abrin toxicity

Abstract

Abrin, a type II ribosome-inactivating protein, comprises A and B subunits wherein the A subunit harbours toxin activity and the B subunit has a galactose-specific lectin activity. The entry of the protein inside the cell is through the binding of the B chain to cell surface glycoproteins followed by receptor-mediated endocytosis and retrograde transport. A previous study from our laboratory showed that different cell lines exhibited differences of as great as ~200-fold in abrin toxicity, prompting the present study to compare the trafficking of the toxin within cells. Observations made in this regard revealed that the abrin A chain, after being released into the cytosol, is sequestered into the nucleus through interaction with a cellular protein of ~25 kDa, BASP1 (brain acid-soluble protein 1). The nuclear localization of the A chain is seen predominantly in cells that are less sensitive to abrin toxicity and dependent on the levels of BASP1 in cells. The sequestration by BASP1 renders cells increasingly resistant to the inhibition of protein synthesis by abrin and the nucleus act as a sink to overcome cellular stress induced by the toxin.

Correlation of abrin‐mediated inhibition of protein synthesis and apoptosis

“…An earlier study had shown that abrin A chain gets sequestered into the nucleus in KB cells , therefore, we wanted to check whether abrin induces direct DNA damage in these cells. We examined phosphorylation of the Histone 2A family protein member gamma‐H2AX (γH2AX), a marker for double strand breaks in DNA .…”

“…Till now, there are no reports on direct DNA damage caused by abrin. However, earlier study showing the nuclear sequestration of the A chain in KB cells by BASP‐1 protein led us to examine the effect of ABA in the nucleus, to determine any possible DNA damage caused by the toxin. Our study revealed that abrin caused DNA damage partially independent of caspases in KB, while in Ovcar‐3 cells, abrin‐induced DNA damage was caspase‐dependent, leading to the conclusion that the DNA damage observed in case of KB cells may be also because of direct DNA damage.…”

“…Observations made in different cell lines show that abrinmediated toxicity can be through more than one pathway. The present study was initiated to gain insights into the mechanisms by which abrin exerts its toxicity in two epithelial cell lines, KB and Ovcar-3, which differ significantly in their sensitivity to abrin mediated PSI due to differences in the trafficking of abrin inside the cell (12). RIPs have been reported to depurinate adenine from substrates other than rRNA, for example, genomic DNA and poly(A) polynucleotides (10,20).…”

“…Different epithelial cells have been shown to exhibit varying level of sensitivity to abrin, despite comparable binding of the toxin to the cell surface . Two cell lines, KB (a nasopharyngeal carcinoma) and Ovcar‐3 (an ovarian carcinoma) have diametrically opposite sensitivities to the toxin (IC 50 ~160 pM and ~2.5 pM, respectively) owing to the nuclear sequestration of abrin A chain . In this study, we analyze the pathways of abrin‐mediated cell death in the two cells to delineate the dependency of apoptosis on abrin‐induced translation inhibition.…”