Abstract

Jab1 (Jun activation domain binding protein 1), integrated into COP9 signalosome complex (CSN), induces protein instability of many tumor suppressors and cell cycle regulators and is therefore a novel target in cancer therapy. Curcumin, an inhibitor of Jab1/CSN-associated kinase(s), has been reported to suppress tumor growth; however, curcumin is highly hydrophobic, and this feature prevents its usage as an antitumor drug. To increase the solubility and targeted delivery, we generated a water-soluble polyethylene glycol (PEG)-conjugated curcumin system, in which curcumin is covalently linked to PEG_35kD. PEGylated curcumin showed much greater reduction of cell growth than free curcumin in pancreatic cancer cells. Cells treated with PEGylated curcumin had increased arrest at the mitotic phase with the formation of abnormal multinucleated cells, indicating that this compound affects cell cycle progression, which may contribute to cell growth inhibition. The stabilities of Jab1 target proteins were also examined. PEGylated curcumin increased protein stability of these proteins in pancreatic cancer cells and directly inhibited the activity of Jab1/CSN-associated kinases. Moreover, the inhibitory effect of PEGylated curcumin on cell proliferation was blunted in pancreatic cancer cells with Jab1 knockdown. The results suggest that PEGylated curcumin inhibits cell proliferation through suppression of Jab1/CSN activity. More importantly, the new compound sensitized pancreatic cancer cells to gemcitabine-induced apoptosis and cell proliferation inhibitory effects. Collectively, the PEGylated curcumin conjugate has much more potent effects on pancreatic cancer cell growth inhibition than free curcumin. The current study provides a biologic rationale to treat patients with pancreatic adenocarcinoma with the nontoxic phytochemical conjugated to PEG for systemic delivery.