Sulforaphane, a bioactive compound from broccoli and broccoli sprouts, possess potent cancer chemopreventive activity. In the current studies, we have revealed a novel molecular target of sulforaphane in pancreatic cancer, evaluated the effect of sulforaphane on breast cancer stem cells, and compared different broccoli sprout preparations for delivery of sulforaphane for future chemoprevention studies.
We showed that heat shock protein 90 (Hsp90), a molecular chaperone regulating the maturation of a wide range of oncogenic proteins, as a novel target of sulforaphane. Different from traditional Hsp90 inhibitors that block ATP binding to Hsp90, sulforaphane disrupted Hsp90-p50Cdc37 interaction, induced Hsp90 client degradation, and inhibited pancreatic cancer in vitro and in vivo. We traced its activity to a novel interaction site of Hsp90. Proteolytic fingerprinting and LC-MS revealed sulforaphane interaction with Hsp90 N-terminus and p50Cdc37central domain. LC-MS tryptic peptide mapping and NMR spectra of full-length Hsp90 identified a covalent sulforaphane adduct in sheet 2 and the adjacent loop in Hsp90 N-terminal domain. Furthermore, we investigated the combination efficacy of sulforaphane and 17-allylamino 17-demethoxygeldanamycin (17-AAG) in pancreatic cancer. 17-AAG, an Hsp90 inhibitor that blocks ATP binding to Hsp90, has been evaluated in clinical trials; however, hepatotoxicity limits its application as a single agent. Our data indicated that sulforaphane potentiated the efficacy of 17-AAG through enhanced abrogation of Hsp90 function, while lowered the dose-limiting toxicity of 17-AAG. Concomitant use of sulforaphane and 17-AAG synergistically down-regulated Hsp90 client proteins.
New evidence has shown the existence of cancer stem cells (CSCs) in breast cancer. Targeting CSCs may reduce cancer recurrence. Our data showed that sulforaphane inhibited breast CSCs and down-regulated Wnt/β-catenin self-renewal pathway. Sulforaphane (1-5 µM) decreased aldehyde dehydrogenase-positive cell population by 65%-80% in human breast cancer cells, and reduced the size and number of mammospheres by 8-125-fold and 45%-75%, respectively, as evidenced by Aldefluor and mammosphere formation assays. Daily injection with 50 mg/kg sulforaphane for two weeks eliminated breast CSCs in nonobese diabetic/severe combined immune-deficient (NOD/SCID) xenograft mice, thereby abrogating tumor growth after re-implantation of primary tumor cells into the secondary mice. Western blotting and reporter assay showed that sulforaphane down-regulated Wnt/β-catenin pathway.
All these studies support the development of broccoli sprout preparations for chemoprevention studies. Therefore, we developed three preparations, compared their ability to deliver sulforaphane in vivo, and evaluated the pharmacokinetics and tissue distribution after oral administration. The sulforaphane-rich preparation generated by two-step procedure contained the highest amount of sulforaphane, 11 and 5 times higher than the freeze-dried sprouts with and without plant enzymes, respectively; and produced the greatest plasma response among all the three preparations, with the peak plasma concentration of sulforaphane 6 and 2.3 times higher, and the AUC 7.9 and 2.2 times higher, compared to the other two preparations. Consumption of 2.5 mg/g body weight of the sulforaphane-rich preparation resulted in rapid absorption and distribution, achieving high levels of sulforaphane and its glutathione conjugate in plasma and tissues. This study provides a broccoli sprout preparation that can serve as a good source of sulforaphane for further evaluation of chemopreventive efficacy.