Uncovering novel targets for cancer chemoprevention

The pleiotropic regulation of cyclin D1 by newly identified sesaminol-binding protein ANT2

The expression of cyclin D1 is upregulated in various cancer cells by diverse mechanisms, such as increases in mRNA levels, the promotion of the translation by mammalian target of rapamycin complex 1 (mTORC1) signaling and the protein stabilization. We here show that sesaminol, a sesame lignan, reduces the expression of cyclin D1 with decreasing mRNA expression levels, inhibiting mTORC1 signaling and promoting proteasomal degradation. We subsequently generated sesaminol-immobilized FG beads to newly identify sesaminol-binding proteins. As a consequence, we found that adenine nucleotide translocase 2 (ANT2), the inner mitochondrial membrane protein, directly bound to sesaminol. Consistent with the effects of sesaminol, the depletion of ANT2 caused a reduction in cyclin D1 with decreases in its mRNA levels, mTORC1 inhibition and the proteasomal degradation of its protein, suggesting that sesaminol negatively regulates the function of ANT2. Furthermore, we screened other ANT2-binding compounds and found that the proliferator-activated receptor-γ agonist troglitazone also reduced cyclin D1 expression in a multifaceted manner, analogous to that of the sesaminol treatment and ANT2 depletion. Therefore, the chemical biology approach using magnetic FG beads employed in the present study revealed that sesaminol bound to ANT2, which may pleiotropically upregulate cyclin D1 expression at the mRNA level and protein level with mTORC1 activation and protein stabilization. These results suggest the potential of ANT2 as a target against cyclin D1-overexpressing cancers.

Effect of Quercetin on Cell Cycle and Cyclin Expression in Ovarian Carcinoma and Osteosarcoma Cell Lines

Resistance to chemotherapeutic drugs is a major problem in cancer treatment. The search for new interventions able to overcome this resistance may involve compounds of natural origin, such as flavonoids, ubiquitously present in many foods. In the present study, the cytotoxic effects and cell cycle modulation of the flavonoid quercetin were investigated in ovarian carcinoma (SKOV3) and osteosarcoma (U2OS) human cell lines and in their cisplatin (CDDP)-resistant counterparts (SKOV3/CDDP and U2OSPt cells, respectively). Quercetin (10–50 μM) caused evident changes in the distribution of cell cycle phases in the CDDP-resistant SKOV3/CDDP ovarian cell line. The levels of cyclin D1 and cyclin B1 were determined by means of Western blot in all cell lines incubated with quercetin (50 μM) for 48 hours. The cyclin D1 expression was significantly decreased following the treatment with quercetin in SKOV3 and U2OSPt cells, but not in SKOV3/CDDP and U2OS cells. The reduction of cyclin D1 level could be linked to the G1/S phase alteration found in quercetin-treated cells. Although cyclin B1 is required for G2/M phase, and despite our observation that quercetin influenced the G2/M phase of cell cycle, the flavonoid did not affect cyclin B1 levels in all cell lines, indicating the involvement of other possible mechanisms. These results suggest that quercetin, exceeding the resistance to CDDP, might become an interesting tool to evaluate cytotoxic activity in combination with chemotherapy drugs.

All-trans retinoic acid suppresses topoisomerase IIα through the proteasomal pathway

Topoisomerase IIα is a nuclear enzyme that alters DNA topology. It is a well-known anticancer target and related to cell differentiation status. All-trans retinoic acid (ATRA), an important active metabolite of vitamin A, is a promising anticancer agent in numerous malignancies. However, there are little data on the effect of retinoids on topoisomerase IIα regulation. In the present study, we investigated the relationship between ATRA and topoisomerase IIα, and the potential mechanisms of ATRA on topoisomerase IIα regulation. In several human carcinoma cell lines, ATRA was shown to suppress topoisomerase IIα protein, but not mRNA expression. ATRA induced the degradation of topoisomerase IIα through the proteasome pathway, but not the lysosome pathway. Ubiquitination was involved in this degradation. Western blot and immunocytochemistry proved that ATRA-induced topoisomerase IIα repression occurred only in the cell nuclei. ATRA not only influenced the cycle procession but also reduced the expression of cyclin D1. Cyclin D1, which is involved in cell differentiation, was regulated by topoisomerase IIα. Similar to cyclin D1, knockdown of topoisomerase IIα resulted in the increased differentiation of the cells, which was in contrast to the overexpression of topoisomerase IIα in the cells. Taken together, these data suggested that ATRA could target topoisomerase IIα and exert potential beneficial effects on cell differentiation.

The CDK4/6 inhibitor PD0332991 reverses epithelial dysplasia associated with abnormal activation of the cyclin-CDK-Rb pathway

Loss of normal growth control is a hallmark of cancer progression. Therefore, understanding the early mechanisms of normal growth regulation and the changes that occur during preneoplasia may provide insights of both diagnostic and therapeutic importance. Models of dysplasia that help elucidate the mechanisms responsible for disease progression are useful in highlighting potential targets for prevention. An important strategy in cancer prevention treatment programs is to reduce hyperplasia and dysplasia. This study identified abnormal upregulation of cell cycle-related proteins cyclin D1, cyclin-dependent kinase (CDK)4, CDK6, and phosphorylated retinoblastoma protein (pRb) as mechanisms responsible for maintenance of hyperplasia and dysplasia following downregulation of the initiating viral oncoprotein Simian virus 40 (SV40) T antigen. Significantly, p53 was not required for successful reversal of hyperplasia and dysplasia. Ligand-induced activation of retinoid X receptor and PPARγ agonists attenuated cyclin D1 and CDK6 but not CDK4 or phosphorylated pRb upregulation with limited reversal of hyperplasia and dysplasia. PD0332991, an orally available CDK4/6 inhibitor, was able to prevent upregulation of cyclin D1 and CDK6 as well as CDK4 and phosphorylated pRb and this correlated with a more profound reversal of hyperplasia and dysplasia. In summary, the study distinguished CDK4 and phosphorylated pRb as targets for chemoprevention regimens targeting reversal of hyperplasia and dysplasia.

Gambogenic acid induces G1 arrest via GSK3β-dependent cyclin D1 degradation and triggers autophagy in lung cancer cells

Cyclin D1, an oncogenic G1 cyclin which can be induced by environmental carcinogens and whose over-expression may cause dysplasia and carcinoma, has been shown to be a target for cancer chemoprevention and therapy. In this study, we investigated the effects and underlying mechanisms of action of a polyprenylated xanthone, gambogenic acid (GEA) on gefitinib-sensitive and -resistant lung cancer cells. We found that GEA inhibited proliferation, caused G1 arrest and repressed colony-forming activity of lung cancer cells. GEA induced degradation of cyclin D1 via the proteasome pathway, and triggered dephosphorylation of GSK3β which was required for cyclin D1 turnover, because GSK3β inactivation by its inhibitor or specific siRNA markedly attenuated GEA-caused cyclin D1 catabolism. GEA induced autophagy of lung cancer cells, possibly due to activation of GSK3β and inactivation of AKT/mTOR signal pathway. These results indicate that GEA is a cyclin D1 inhibitor and a GSK3β activator which may have chemopreventive and therapeutic potential for lung cancer.

Rapid downregulation of cyclin D1 induced by geranylgeranoic acid in human hepatoma cells

Geranylgeranoic acid (GGA) and its derivatives are currently under development as chemopreventive agents against second primary hepatoma in Japan. We aimed to evaluate chemoprevention targets of GGA and a surrogate marker of chemopreventive response to clarify the molecular mechanism of hepatoma chemoprevention with GGA. Human hepatoma-derived cell lines such as HuH-7, PLC/PRF/5, and HepG-2, were treated with GGA and its derivatives. Cellular dynamics of several cell-cycle-related proteins were assessed by either immunoblotting or immunofluorescence method. The cellular expression of cyclin D1 protein was suppressed immediately after GGA treatment. This reduction was partially blocked by pretreatment with 26S proteasome inhibitor MG-132, indicating that proteasomal degradation was involved in GGA-induced disappearance of cyclin D1. A phosphorylation of retinoblastoma protein (RB) at serine 780, a target site of cyclin D1-dependent kinase 4, was rapidly decreased in GGA-treated HuH-7 cells. Furthermore, subcellular fractionation, Western blotting, and immunofluorescence revealed GGA-induced nuclear accumulation of RB. These results strongly suggest that cyclin D1 may be a target of chemopreventive GGA in human hepatoma cells. GGA-induced rapid repression of cyclin D1, and a consequent dephosphorylation and nuclear translocation of RB, may influence cell cycle progression and may be relevant to GGA-induced cell death mechanisms.

Natural products and colon cancer: current status and future prospects

Carcinogenesis is a multistage process consisting of initiation, promotion and progression phases. Thus, the multistage sequence of events has many phases for prevention and intervention. Chemoprevention, a novel approach for controlling cancer, involves the use of specific natural products or synthetic chemical agents to reverse, suppress or prevent premalignancy before the development of invasive cancer. Several natural products, such as, grains, nuts, cereals, spices, fruits, vegetables, beverages, medicinal plants and herbs and their various phytochemical constituents including, phenolics, flavonoids, carotenoids, alkaloids, nitrogen containing as well as organosulfur compounds confer protective effects against wide range of cancers including colon cancer. Since diet has an important role in the etiology of colon cancer, dietary chemoprevention received attention for colon cancer prevention. However, identification of an agent with chemopreventive potential requires in vitro studies, efficacy and toxicity studies in animal models before embarking on human clinical trials. A brief introduction about colon cancer and the role of some recent natural products in colon cancer chemoprevention with respect to multiple molecular mechanisms in various in vitro, in vivo and clinical studies are described in this review.

Deficiency of a potential 3p21.3 tumor suppressor gene UBE1L (UBA7) does not accelerate lung cancer development in K-rasLA2 mice

Genetic lesions in chromosomal region 3p21.3 marks one of the earliest events in human lung cancer development. It is hypothesized that one or more tumor suppressor genes reside in this region. Identification and characterization of these genes are important for the understanding of lung cancer initiation. UBE1L (UBA7) is a long-suspected 3p21.3 residing tumor suppressor gene. It encodes the key enzyme that activates ISGylation, a novel, ubiquitination-like, post-translational protein modification system that is inducible by interferon. It has been implicated that ISGylation plays a variety of biological roles ranging from viral defense to tumor surveillance. Here we tested the possible function of ISGylation during lung cancer development by using the Ube1l-deficient mice and the K-ras(LA2) lung cancer mice. Protein ISGylation levels were largely unchanged during lung cancer progression. Ube1l deficiency neither altered the lung cancer progression nor affected the overall survival of K-ras(LA2) lung cancer mice. Our study suggests that Ube1l is not a tumor suppressor gene in K-ras(LA2) lung cancer mouse model. However, as described in the discussion, additional studies with other lung cancer mouse models will be necessary to elucidate the potential tumor suppressor function of UBE1L in K-RAS mutation independent human lung cancers.

Cyclin degradation for cancer therapy and chemoprevention

Cancer is characterized by uncontrolled cell division resulting from multiple mutagenic events. Cancer chemoprevention strategies aim to inhibit or reverse these events using natural or synthetic pharmacologic agents. Ideally, this restores normal growth control mechanisms. Diverse classes of compounds have been identified with chemopreventive activity. What unites many of them is an ability to inhibit the cell cycle by specifically modulating key components. This delays division long enough for cells to respond to mutagenic damage. In some cases, damage is repaired and in others cellular damage is sufficient to trigger apoptosis. It is now known that pathways responsible for targeting G1 cyclins for proteasomal degradation can be engaged pharmacologically. Emergence of induced cyclin degradation as a target for cancer therapy and chemoprevention in pre-clinical models is discussed in this article. Evidence for cyclin D1 as a molecular pharmacologic target and biological marker for clinical response is based on experience of proof of principle trials.

The regulation of cyclin D1 degradation: roles in cancer development and the potential for therapeutic invention

Cyclin D1 is an important regulator of cell cycle progression and can function as a transcriptionl co-regulator. The overexpression of cyclin D1 has been linked to the development and progression of cancer. Deregulated cyclin D1 degradation appears to be responsible for the increased levels of cyclin D1 in several cancers. Recent findings have identified novel mechanisms involved in the regulation of cyclin D1 stability. A number of therapeutic agents have been shown to induce cyclin D1 degradation. The therapeutic ablation of cyclin D1 may be useful for the prevention and treatment of cancer. In this review, current knowledge on the regulation of cyclin D1 degradation is discussed. Novel insights into cyclin D1 degradation are also discussed in the context of ablative therapy. A number of unresolved questions regarding the regulation of cellular cyclin D1 levels are also addressed.