Curcumin, or diferuloylmethane, is a hydrophobic polyphenol derived from the rhizome of the herb Curcuma longa
. It is better known as the yellow pigment in the widely used Asian spice turmeric. Recently, curcumin gained attention as an anti-cancer agent because of its chemopreventive and chemotherapeutic potential while having no discernable side effects. Curcumin induces apoptosis in various tumor cells and can prevent tumor initiation and growth in carcinogen-induced rodent models as well as in subcutaneous and orthotopic tumor xenografts [1
]. Although it is still not known why curcumin preferentially kills tumor cells, it has been identified as one of the major natural agents that inhibit tumor initiation and tumor promotion.
Curcumin inhibits the proliferation of a wide variety of cancer cells including breast, blood, colon, liver, pancreas, kidney, prostate, and skin [1
]. We and others have shown that it induces cell death in medulloblastoma, the most common pediatric brain tumor [3
], and inhibits tumor growth in in vivo
medulloblastoma models [3
]. Curcumin has been suggested to selectively target tumor cells by affecting signaling pathways that regulate cell growth and survival and thus preferably induces apoptosis in highly proliferating cells [6
]. Accumulating evidence suggests that curcumin-induced cell death is mediated both by the activation of cell death pathways and by the inhibition of growth/proliferation pathways [6
]. Cell cycle regulatory proteins and checkpoints are downstream elements of cellular signaling cascades crucial for cell proliferation. Curcumin exerts various effects on cell cycle proteins and checkpoints, including p53, cyclin D1, cyclin dependent kinases (CDK), and CDK inhibitors (CDKi) such as p16INK4a
, and p27KIP1
]. It most often induces G2/M arrest, although G0/G1 arrest has been found in some cells [7
]. It is well accepted that a prolonged arrest in G2/M phase leads to apoptotic cell death [9
]. However, how curcumin induces G2/M arrest is not well understood.
The mitotic checkpoint, also known as the spindle assembly checkpoint (SAC) is the major cell cycle control mechanism in mitosis and delays the onset of anaphase until each single kinetochore has become attached to the mitotic spindle [10
]. At the molecular level, the SAC is a signaling pathway consisting of multiple components that communicate between local spindle attachment and global cytoplasmic signaling to delay segregation. One of the key regulators of the SAC is the anaphase promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase. In humans, the APC/C is a multi-protein complex consisting of at least 12 different subunits that requires other cofactors for proper functioning; a ubiquitin-activating (E1) enzyme, a ubiquitin-conjugating (E2) enzyme and co-activator proteins Cdc20 or Cdh1 [11
]. Upon activation, APC/C ubiquitinates cyclin B and securin and targets them for destruction by proteolysis allowing for mitotic exit [11
]. However, APC/C is not only a major effector of the SAC that ensures cell cycle arrest upon spindle disruption but it also promotes cell death upon prolonged mitotic arrest [10
]. Thus, APC has become an attractive drug target to control the growth and proliferation of cancer cells and facilitate their apoptotic death.
Curcumin has a diverse range of molecular targets, including thioredoxin reductase, cyclooxygenase-2 (COX-2), protein kinase C, 5-lipoxygenase (5-LOX), and tubulin [6
], supporting the concept that it may act upon numerous biochemical and molecular cascades. One interesting feature of curcumin is its ability to crosslink proteins such as the cystic fibrosis chloride channel (CFTR) thereby activating the channel [13
]. In this study, we provide evidence that Cdc27, a component of the APC/C is a novel target for curcumin and that curcumin binds and crosslinks Cdc27. We also show that curcumin inhibits APC/C activity suggesting that curcumin binding to Cdc27 might play an important role in prolonged G2/M arrest induced apoptosis. In addition, curcumin preferentially induced apoptosis in cells progressing through G2/M and expressing phosphorylated Cdc27 usually found in highly proliferating cells. Thus, our studies reveal that the SAC is a molecular target of curcumin and, in addition, provide a possible explanation for why curcumin preferably induces cell death in cancer cells as previously reported [6