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Vitamin C Inactivates the Proteasome Inhibitor PS-341 in Human Cancer Cells

Authors' Affiliations: ¹ Department of Hematology and Oncology, Winship Cancer Institute, Emory University School of Medicine and ² Department of Chemistry and Center for Biotechnology and Drug Design, Georgia State University, Atlanta, Georgia

Requests for reprints: Shi-Yong Sun, Winship Cancer Institute, Emory University School of Medicine, 1365-C Clifton Road NE, C3088, Atlanta, GA 30322. Phone: 404-778-2170; Fax: 404-778-5520; E-mail: shi-yong_sun{at}emoryhealthcare.org.

Purpose: PS-341 (bortezomib, Velcade), the first proteasome inhibitor approved by the Food and Drug Administration for the treatment of patients with relapsed multiple myeloma, induces apoptosis in human cancer cell lines. Vitamin C (ascorbic acid) is an essential water-soluble vitamin required for many normal physiologic functions and has to be obtained through diet or supplemental tablets in humans. Here we studied the potential effect of vitamin C on the anticancer activity of PS-341 in human cancer cell lines.

Experimental Design: The effects of vitamin C on apoptosis induction by PS-341 alone and by PS-341 combined with tumor necrosis factor–related apoptosis-inducing ligand were studied. In addition, the effects of vitamin C and other antioxidants on PS-341-mediated proteasome inhibition were also examined. Finally, the direct chemical interaction between vitamin C and PS-341 was determined.

Results: Vitamin C abrogated the ability of PS-341 to induce apoptosis in various human cancer cell lines, to induce G₂-M arrest, and to augment apoptosis induced by tumor necrosis factor–related apoptosis-inducing ligand. Moreover, vitamin C suppressed PS-341-mediated inhibition of proteasome activity. PS-341 itself did not induce generation of intracellular reactive oxygen species whereas other antioxidants failed to abrogate its biological activity. Importantly, we detected a direct chemical interaction between vitamin C and PS-341.

Conclusion: Vitamin C directly binds to PS-431, thus inactivating PS-341 independent of its antioxidant activity. Our findings suggest that vitamin C may have a negative effect on PS-341-mediated anticancer activity.

Commentary

Velcade and Vitamin C: Too Much of a Good Thing?

Laurence Catley and Kenneth C. Anderson
Clin. Cancer Res. 2006 12: 3-4. [Full Text] [PDF]

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