Thioredoxin (Trx) is a pro-oncogenic molecule that underlies
tumor initiation, progression and chemo-resistance. PX-12, a Trx inhibitor, has been used to treat certain
tumors. Currently, factors predicting
tumor sensitivity to PX-12 are unclear. Given that
hydrogen sulfide (H2S), a gaseous bio-mediator, promotes Trx activity, we speculated that it might affect
tumor response to PX-12. Here, we tested this possibility. Exposure of several different types of
tumor cells to PX-12 caused cell death, which was reversely correlated with the levels of H2S-synthesizing
enzyme CSE and endogenous H2S. Inhibition of CSE sensitized
tumor cells to PX-12, whereas addition of exogenous H2S elevated PX-12 resistance. Further experiments showed that H2S abolished PX-12-mediated inhibition on Trx. Mechanistic analyses revealed that H2S stimulated Trx activity. It promoted Trx from the oxidized to the reduced state. In addition, H2S directly cleaved the
disulfide bond in PX-12, causing PX-12 deactivation. Additional studies found that, besides Trx, PX-12 also interacted with the
thiol residues of other
proteins. Intriguingly, H2S-mediated cell resistance to PX-12 could also be achieved through promotion of the
thiol activity of these
proteins. Addition of H2S-modified
protein into culture significantly enhanced cell resistance to PX-12, whereas blockade of extracellular sulfhydryl residues sensitized cells to PX-12. Collectively, our study revealed that H2S mediated
tumor cell resistance to PX-12 through multiple mechanisms involving induction of
thiol activity in multiple
proteins and direct inactivation of PX-12. H2S could be used to predict
tumor response to PX-12 and could be targeted to enhance the therapeutic efficacy of PX-12.