Endoplasmic reticulum-targeting doxorubicin: a new tool effective against doxorubicin-resistant osteosarcoma

Doxorubicin is one of the most effective drugs for the first-line treatment of high-grade osteosarcoma. Several studies have demonstrated that the major cause for doxorubicin resistance in osteosarcoma is the increased expression of the drug efflux transporter ABCB1/P-glycoprotein (Pgp). We recently identified a library of H₂S-releasing doxorubicins (Sdox) that were more effective than doxorubicin against resistant osteosarcoma cells. Here we investigated the molecular mechanisms of the higher efficacy of Sdox in human osteosarcoma cells with increasing resistance to doxorubicin. Differently from doxorubicin, Sdox preferentially accumulated within the endoplasmic reticulum (ER), and its accumulation was only modestly reduced in Pgp-expressing osteosarcoma cells. The increase in doxorubicin resistance was paralleled by the progressive down-regulation of genes of ER-associated protein degradation/ER-quality control (ERAD/ERQC), two processes that remove misfolded proteins and protect cell from ER stress-triggered apoptosis. Sdox, that sulfhydrated ER-associated proteins and promoted their subsequent ubiquitination, up-regulated ERAD/ERQC genes. This up-regulation, however, was insufficient to protect cells, since Sdox activated ER stress-dependent apoptotic pathways, e.g., the C/EBP-β LIP/CHOP/PUMA/caspases 12-7-3 axis. Sdox also promoted the sulfhydration of Pgp that was subsequently ubiquitinated: this process further enhanced Sdox retention and toxicity in resistant cells. Our work suggests that Sdox overcomes doxorubicin resistance in osteosarcoma cells by at least two mechanisms: it induces the degradation of Pgp following its sulfhydration and produces a huge misfolding of ER-associated proteins, triggering ER-dependent apoptosis. Sdox may represent the prototype of innovative anthracyclines, effective against doxorubicin-resistant/Pgp-expressing osteosarcoma cells by perturbing the ER functions.