Oral gossypol in the treatment of patients with refractory metastatic breast cancer: a phase I/II clinical trial

Gossypol has demonstrated in vitro effects on cell cycle regulation and anti-tumor activity against mammary carcinoma cell lines. This Phase I/II study assesses both the effect of gossypol on cell cycle regulatory proteins in vivo and the clinical effect. Twenty women with refractory metastatic breast cancer received oral gossypol at daily doses between 30 and 50mg per day. Gossypol plasma levels were measured (n=8) and the modulation of the retinoblastoma (Rb) gene protein and Cyclin D1 was assessed by serial biopsies (n=4). Grade I–II toxicities with gossypol treatment included nausea in 30% of patients, fatigue 15%, emesis 15%, altered taste sensation 15% and diarrhea in 10% of patients. Two of the three patients receiving 50mg/day experienced dose limiting dermatologic toxicity (grade III). One patient had a minor response and two patients had stable disease with >50 decline in serial assessments of the serum tumor markers. Immunohistochemical analysis of cyclin D1 and Rb expression in serial biopsies of four patients revealed both a concurrent decrease in cyclin D1 expression and an increase in nuclear Rb expression in three patients. The maximal tolerated dose (MTD) of gossypol was 40mg/day. Gossypol appears to affect the expression of Rb protein and cyclin D1 in breast cancer metastases at doses achievable, yet had negligible antitumor activity against anthracycline and taxane refractory metastatic breast cancer. The cell cycle regulatory effects of gossypol suggest a potential role for gossypol as a modulating agent in conjunction with other cell cycle specific compounds.     

A lipid-soluble iron chelator alters cell cycle regulatory protein binding in breast cancer cells compared to normal breast cells

The lipid-soluble iron chelator desferri-exochelin (D-Exo) causes reversible cell cycle arrest in normal human mammary epithelial cells (NHMEC) but triggers apoptotic cell death in human breast cancer cells. We studied the effects of iron chelation with D-Exo on cell cycle regulatory proteins in cultures of NHMEC and MCF-7 breast cancer cells. In co-immunoprecipitation studies, D-Exo inhibited binding of cyclins A and E to cyclin dependent kinase 2 (CDK2) in NHMEC, but in MCF-7 cells binding of these cyclins to CDK2 was enhanced. D-Exo treatment markedly increased expression of p53 and increased binding of p21 to CDK2 in the MCF-7 cells but not in NHMEC. Therefore differences in effects of iron chelation on cell cycle protein binding in cancer cells compared to normal cells may trigger apoptosis in cancer cells while normal breast cells are spared.     

Effects of gossypol on the cell cycle phases in T-47D human breast cancer cells.

Since gossypol, a naturally occurring component of cottonseed oil, exhibits a broad spectrum of activities, we have examined it as an antitumor agent on breast cancer. The effects of different concentrations of gossypol on the T-47D human breast cancer cell cycle phases were studied using cytometric image processing on Feulgen stained nuclei. The proportion of cells at different cell cycle phases was determined by discriminate analysis of the image parameters and gave good classification ranging from 86 to 100%. Gossypol was found to increase the G0/G1 fraction of the T-47D cells. This cell kinetic alteration by gossypol was shown to be dose dependent and reversible. Complete reversal of the effect of gossypol was observed after four days with a simple change to gossypol-free medium. The cells then progressed into S and G2/M phase, thus indicating that gossypol-treated cells remain viable. Gossypol was shown to have a strong inhibitory effect on cellular proliferation in T-47D cells. It was also found that this agent is only toxic to cells at the highest dose tested (10 microM). The results of this study may be of clinical significance in the treatment of breast cancer, since gossypol shows strong antiproliferative properties.     

Estrogenic regulation of a novel 34 kDa protein associated with cyclin B1 in MCF-7 breast cancer cells

Recent studies have revealed altered regulation of cyclins in breast cancer cells. To understand the role of aberrant cyclin B1 expression in the proliferation of breast cancer cells, we examined cyclin B1-associated proteins in estrogen-responsive MCF-7 cells in a cell cycle-dependent manner. Immunoprecipitation of cell lysate with a monoclonal anti-human cyclin B1 antibody, followed by Western blot probing with an anti-human cdc2 (PSTAIR) antibody revealed the presence of a 34 kDa protein in estradiol-treated cells at 16 h after initiation of cell cycle progression. Flow cytometry and [H-3]-thymidine (Thd) incorporation experiments showed a dramatic increase in the percentage of S phase cells at this time point. This protein was suppressed by an antiestrogen, 4-hydroxytamoxifen. It was not found in MCF-1OA, a normal breast epithelial cell line. The 34 kDa protein was not reactive with antibodies raised against other cyclin dependent kinases (CDKs), including p34(cdc2(Carboxy terminal)). This protein was functionally active as determined by histone H1 kinase activity. These data suggest that the induction of a cyclin B1-associated 34 kDa protein during the G1 --> S transition might be a positive regulator of cell cycle progression in estrogen-responsive breast cancer cells.     

Enhancing cisplatin sensitivity in MCF-7 human breast cancer cells by down-regulation of Bcl-2 and cyclin D1

MCF-7 human breast cancer cells are relatively resistant to cisplatin treatment compared to other breast cancer cell lines. In order to identify possible targets for sensitizing the breast cancer cells to cisplatin treatment protein expression levels and the phosphorylation status of 27 different signaling proteins were examined. MCF-7 cells expressed high levels of anti-apoptotic Bcl-2 protein relative to more cisplatin sensitive breast cancer cells. After cisplatin treatment a decrease in cyclin D1 was seen in all the breast cancer cells studied. Therefore, Bcl-2 and cyclin D1 were chosen as putative targets for increasing cell death and growth arrest induced by cisplatin, thereby enhancing the drug sensitivity in MCF-7. RNA interference, using Bcl-2- and cyclin D1- siRNAs sensitized MCF-7 cells to cisplatin treatment and by simultaneous knockdown of both Bcl-2 and cyclin D1 further sensitization was seen. This shows the potential of targeting both apoptotic- and cell cycle-regulating pathways to enhance the effect of chemotherapy.     

Sesamin, a lignan of sesame, down-regulates cyclin D1 protein expression in human tumor cells

Sesamin is a major lignan constituent of sesame and possesses multiple functions such as antihypertensive, cholesterol-lowering, lipid-lowering and anticancer activities. Several groups have previously reported that sesamin induces growth inhibition in human cancer cells. However, the nature of this growth inhibitory mechanism remains unknown. The authors here report that sesamin induces growth arrest at the G1 phase in cell cycle progression in the human breast cancer cell line MCF-7. Furthermore, sesamin dephosphorylates tumor-suppressor retinoblastoma protein (RB). It is also shown that inhibition of MCF-7 cell proliferation by sesamin is correlated with down-regulated cyclin D1 protein expression, a proto-oncogene that is overexpressed in many human cancer cells. It was found that sesamin-induced down-regulation of cyclin D1 was inhibited by proteasome inhibitors, suggesting that sesamin suppresses cyclin D1 protein expression by promoting proteasome degradation of cyclin D1 protein. Sesamin down-regulates cyclin D1 protein expression in various kinds of human tumor cells, including lung cancer, transformed renal cells, immortalized keratinocyte, melanoma and osteosarcoma. Furthermore, depletion of cyclin D1 protein using small interfering RNA rendered MCF-7 cells insensitive to the growth inhibitory effects of sesamin, implicating that cyclin D1 is at least partially related to the antiproliferative effects of sesamin. Taken together, these results suggest that the ability of sesamin to down-regulate cyclin D1 protein expression through the activation of proteasome degradation could be one of the mechanisms of the antiproliferative activity of this agent.     

Growth inhibition of carcinogen-transformed MCF-12F breast epithelial cells and hormone-sensitive BT-474 breast cancer cells by 1alpha-hydroxyvitamin D5

Several studies have established the active form of vitamin D(3) as an effective tumor-suppressing agent; however, its antitumor activity is achieved at doses that are hypercalcemic in vivo. Therefore, less calcemic vitamin D(3) analog, 1alpha-hydroxy-24-ethyl-cholecalciferol (1alpha[OH]D5), was evaluated for its potential use in breast cancer chemoprevention. Previously, 1alpha(OH)D5 showed anticarcinogenic activity in several in vivo and in vitro models. However, its effects on growth of normal tissue were not known. The present study was conducted to determine the effects of 1alpha(OH)D5 on the growth of normal mouse mammary gland and normal-like human breast epithelial MCF-12F cells and to compare these effects with carcinogen-transformed MCF-12F and breast cancer cells. No significant difference was observed in the growth or morphology of cultured mouse mammary gland and MCF-12F cells in the presence of 1alpha(OH)D5. However, the transformed MCF-12F cells underwent growth inhibition (40-60%, P < 0.05) upon 1alpha(OH)D5 treatment as determined by cell viability assays. Cell cycle analysis showed marked increase (50%) in G-1 phase for cells treated with 1alpha(OH)D5 compared with the controls. Moreover, the percentage of cells in the synthesis (S) phase of cell cycle was decreased by 70% in transformed MCF-12F, BT-474 and MCF-7 cells. The growth arrest was preceded by an increase in expression of cell cycle regulatory proteins p21(Waf-1) and p27(Kip-1). In addition, differential expression studies of parent and transformed MCF-12F cell lines using microarrays showed that prohibitin mRNA was increased 4-fold in the transformed cells. These results indicate that the growth inhibitory effect of 1alpha(OH)D5 was achieved in both carcinogen-transformed MCF-12F and breast cancer cells at a dose that was non-inhibitory in normal-like breast epithelial cells.     

BRCA1 overexpression sensitizes cancer cells to lovastatin via regulation of cyclin D1-CDK4-p21WAF1/CIP1 pathway: analyses using a breast cancer cell line and tumoral xenograft model

It is well established that statins display potent anticancer activity in several types of proliferating tumor cells. However, how to promote the sensitivity of statins to mammary cancer is yet to be completely deciphered. The purpose of this study was to investigate whether breast cancer susceptibility gene 1 (BRCA1) overexpression sensitizes mammary cancer cells to statins. MCF-7 cells, which have only one wild-type BRCA1 allele, were transfected with pcDNA3-beta-HA-hsBRCA1 plasmids via liposomes to reconstitute BRCA1 overexpression human breast cancer cell line, and tumoral xenografts with BRCA1 overexpression were subsequently established in BALB/c nude mice. Then, the inhibitory activity of lovastatin on cancer cells and tumoral xenografts, and the underlying mechanism involving in cell-cycle regulatory proteins were analyzed. The proliferative ability of MCF-7 cells treated with lovastatin was reduced compared to normal, and further decreased in the presence of excess BRCA1, detected by methyl thiazolyl tetrazolium and flow cytometry techniques in vitro or by 5-bromodeoxyuridine incorporation in vivo. Additionally, the mRNA and protein expression of cyclin D1, cyclin-dependent kinase 4 (CDK4) and retinoblastoma protein (pRb), was further down-regulated under exposure to lovastatin in condition of BRCA1 overexpression, but the expression of p21WAF1/CIP1, a cyclin-dependent kinase inhibitor (CDKI), was further up-regulated, both in vitro and in vivo detected with quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot analysis. Moreover, we found the further reduced volume of tumoral xenografts treated with lovastatin in the presence of BRCA1 overexpression. Our results suggest that BRCA1 overexpression sensitizes cancer cells to lovastatin via regulation of cyclin D1-CDK4-p21WAF1/CIP1 pathway, which will provide an innovative experimental framework to study control of breast cancer cell proliferation.     

Peroxisome proliferator-activated receptor gamma agonists induce proteasome-dependent degradation of cyclin D1 and estrogen receptor alpha in MCF-7 breast cancer cells

Treatment of MCF-7 cells with the peroxisome proliferator-activated receptor (PPAR) gamma agonists ciglitazone or 15-deoxy-Delta 12,14-prostaglandin J2 resulted in a concentration- and time-dependent decrease of cyclin D1 and estrogen receptor (ER) alpha proteins, and this was accompanied by decreased cell proliferation and G(1)-G(0)-->S-phase progression. Down-regulation of cyclin D1 and ER alpha by PPARgamma agonists was inhibited in cells cotreated with the proteasome inhibitors MG132 and PSII, but not in cells cotreated with the protease inhibitors calpain II and calpeptin. Moreover, after treatment of MCF-7 cells with 15-deoxy-Delta 12,14-prostaglandin J2 and immunoprecipitation with cyclin D1 or ER alpha antibodies, there was enhanced formation of ubiquitinated cyclin D1 and ER alpha bands. Thus, PPARgamma-induced inhibition of breast cancer cell growth is due, in part, to proteasome-dependent degradation of cyclin D1 (and ER alpha), and this pathway may be important for other cancer cell lines.     

Antiestrogen regulation of cell cycle progression and cyclin D1 gene expression in MCF-7 human breast cancer cells

The molecular mechanisms by which antiestrogens inhibit breast cancer cell proliferation are not well understood. Using cultured breast cancer cell lines, we studied the effects of antiestrogens on proliferation and cell cycle progression and used this information to select candidate cell cycle regulatory genes that are potential targets for antiestrogens. Under estrogen- and serum-free conditions antiestrogens inhibited proliferation of MCF-7 cells stimulated with insulin. Cells were blocked at a point in G₁ phase. These effects are comparable with those in serum- and estrogen-containing medium and were also seen to a lesser degree in nude mice bearing MCF-7 tumors. Similar observations with other peptide mitogens suggest that the process inhibited by antiestrogens is common to estrogen and growth factor activated pathways. Other studies have identified G₁ cyclins as potential targets for growth factor and steroid hormone/steroid antagonist regulation of breast epithelial cell proliferation. In MCF-7 cells growing in the presence of fetal calf serum, cyclin D1 mRNA was rapidly down-regulated by steroidal and nonsteroidal antiestrogens by an apparently estrogen receptor mediated mechanism. Cyclin D1 gene expression was maximally inhibited before effects on entry into S phase and inhibition was therefore not merely a consequence of changes in cell cycle progression. Together with data on the effects of antiestrogens in serum-free conditions [1], these results suggest down-regulation of cyclin D1 by antiestrogens may be a general phenomenon in estrogen receptor-positive breast cancer cells, independent of culture conditions and class of antiestrogen. These observations are compatible with the hypothesis that reductions in cyclin D1 levels may mediate in part the action of antiestrogens in blocking entry of cells into S phase.     

Ectopic expression of cyclin D1 amplifies a retinoic acid-induced mitochondrial death pathway in breast cancer cells.

All-trans retinoic acid inhibits growth associated with downregulation of cyclin D1 and can cause low level apoptosis in estrogen receptor positive breast cancer cell lines. The cyclin D1 gene is amplified and/or the protein overexpressed in about one-third of breast cancers. Constitutive expression of cyclin D1 in estrogen receptor positive MCF-7 and ZR-75 breast cancer cells (MCF-7(cycD1) and ZR-75(cycD1)) Increased the fraction of cells in S phase and reduced the G1 accumulation following retinoic acid treatment compared with control cells. However, culture of MCF-7(cycD1) with 1 microM all-trans retinoic acid resulted in about threefold greater growth inhibition compared with vector-transfected cells. Hoechst staining of DNA and in situ DNA end-labeling analysis indicated that MCF-7(cycD1) and ZR-75(cycD1) cultures contained 4-6-fold more retinoic acid-induced apoptotic nuclei as vector-transfected cells. Retinoic acid treatment of vector-transfected clones resulted in Bax protein activation as assessed by exposure of the NH(2)-terminus of Bax but the proportion of cells containing activated Bax was increased in cyclin D-expressing cells treated with retinoic acid. The latter cells also displayed both immunocytochemical and biochemical evidence of translocation of cytochrome c into the cytosol following RA-treatment. Retinoic acid markedly decreased the Bcl-2 levels in MCF-7 and ZR-75 cells. Accordingly, coexpression of Bcl-2 and cyclin D1 rendered the cells resistant to retinoic acid-induced apoptosis. We conclude that constitutive expression of cyclin D1 sensitizes ER-positive breast cancer cells to a retinoic acid-induced mitochondrial death pathway involving Bax activation, cytochrome c release and caspase-9 cleavage.     

Effects of sulindac and its metabolites on growth and apoptosis in human mammary epithelial and breast carcinoma cell lines

Nonsteriodal anti-inflammatory drugs (NSAIDs) are among the most commonly used medications in the United States and elsewhere, mainly for the treatment of arthritis. The NSAID sulindac causes regression and prevents the recurrence of premalignant colonic polyps in patients with familial adenomatous polyposis and inhibits colon carcinogenesis in rodents. Sulindac and sulindac sulfone, a metabolite of sulindac that lacks cyclooxygenase (cox) inhibitory activity, also inhibit mammary carcinogenesis in rats. To obtain insights into the relevance of these findings to human breast cancer, we examined the mechanism of action of sulindac and its sulfide and sulfone metabolites on the normal human mammary epithelial cell line MCF-10F and the human breast cancer cell line MCF-7. Of the three compounds, the sulfide was the most potent inhibitor of cell growth, although the sulfone and sulfoxide were also active at higher concentrations. Treatment of MCF-10F and MCF-7 cells with 100 µM sulindac sulfide resulted in accumulation of cells in the G1 phase of the cell cycle and induction of apoptosis. Apoptosis occurred within 24 h as determined by the TUNEL assay and DNA laddering was observed at 72 h. The accumulation of cells in G1 was associated with decreased levels of expression of cyclin D1 but no effect was seen on the expression of CDK4 or the immediate early response gene c-jun. Treatment with sulindac sulfide caused a striking induction of the CDK inhibitor p21^WAF1 in MCF-10F cells. The MCF-7 cell line expressed a high basal level of p21^WAF1 which did not change significantly after drug treatment. The pro-apoptotic gene BAX was not induced in either MCF-10F or MCF-7 cells by sulindac sulfide. Stable overexpression of cyclin D1, which frequently occurs in breast cancers, did not protect mammary epithelial cells from inhibition by the sulfide. These studies suggest that this class of compounds warrants further study with respect to breast cancer prevention and treatment.     

Novel retinoic acid metabolism blocking agents have potent inhibitory activities on human breast cancer cells and tumour growth

Antitumour effects of retinoids are attributed to their influence on cell proliferation, differentiation, apoptosis and angiogenesis. In our effort to develop useful agents for breast cancer therapy, we evaluated the effects of four representative retinoic acid metabolism blocking agents (RAMBAs, VN/14-1, VN/50-1, VN/66-1 and VN/69-1) on growth inhibition of oestrogen receptor positive (ER +ve, MCF-7 and T-47D) and oestrogen receptor negative (ER -ve, MDA-MB-231) human breast cancer cells. Additionally, we investigated the biological effects/molecular mechanism(s) underlying their growth inhibitory properties as well as their antitumour efficacies against MCF-7 and MCF-7Ca tumour xenografts in nude mice. We also assessed the effect of combining VN/14-1 and all-trans-retinoic acid (ATRA) on MCF-7 tumour xenografts. The ER +ve cell lines were more sensitive (IC(50) values between 3.0 and 609 nM) to the RAMBAs than the ER -ve MDA-MB-231 cell line (IC(50)=5.6-24.0 microM). Retinoic acid metabolism blocking agents induced cell differentiation as determined by increased expression of cytokeratin 8/18 and oestrogen receptor-alpha (ER-alpha). Similar to ATRA, they also induced apoptosis via activation of caspase 9. Cell cycle analysis indicated that RAMBAs arrested cells in the G1 and G2/M phases and caused significant downregulation (>80%) of cyclin D1 protein. In vivo, the growth of MCF-7 mammary tumours was dose-dependently and significantly inhibited (92.6%, P<0.0005) by VN/14-1. The combination of VN/14-1 and ATRA also inhibited MCF-7 breast tumour growth in vivo (up to 120%) as compared with single agents (P<0.025). VN/14-1 was also very effective in preventing the formation of MCF-7Ca tumours and it significantly inhibited the growth of established MCF-7Ca tumours, being as effective as the clinically used aromatase inhibitors, anastrozole and letrozole. Decrease in cyclin D1 and upregulation of cytokeratins, Bad and Bax with VN/14-1 may be responsible for the efficacy of this compound in inhibiting breast cancer cell growth in vitro and in vivo. Our results suggest that our RAMBAs, especially VN/14-1 may be useful novel therapy for breast cancer.     

Inhibition of breast cancer cell growth and induction of cell death by 1,1-bis(3'-indolyl)methane (DIM) and 5,5'-dibromoDIM

1,1-Bis(3'-indolyl)methane (DIM) and the 5,5'-dibromo ring substituted DIM (5,5'-diBrDIM) inhibited growth of MCF-7 and MDA-MB-231 breast cancer cells, and IC50 values were 10-20 and 1-5 microM, respectively, in both cell lines. DIM and 5,5'-diBrDIM did not induce p21 or p27 protein levels or alter expression of Sp1 or Sp3 proteins in either cell line. In contrast, 10 microM 5,5'-diBrDIM downregulated cyclin D1 protein in MCF-7 and MDA-MB-231 cells 12 and 24 h after treatment. DIM (20 microM) also decreased cyclin D1 in MCF-7 (24 h) and MDA-MB-231 (12 h), and the DIM/5,5'-diBrDIM-induced degradation of cyclin D1 was blocked by the proteasome inhibitor MG132. Both DIM and 5,5'-diBrDIM induced apoptosis in MCF-7 cells and this was accompanied by decreased Bcl-2, release of mitochondrial cytochrome c, and decreased mitochondrial membrane potential as determined by the red/green fluorescence of JC-1. DIM and 5,5'-diBrDIM induced extensive necrosis in MDA-MB-231 cells; however, this was accompanied by decreased mitochondrial membrane potential primarily in cells treated with 5,5'-diBrDIM but not DIM. Thus, DIM and 5,5'-diBrDIM induce cell death in MCF-7 and MDA-MB-231 cells by overlapping and different pathways, and the ring-substituted DIM represents a novel class of uncharged mitochondrial poisons that inhibit breast cancer cell and tumor growth.     

ErbB-2 induces the cyclin D1 gene in prostate epithelial cells in vitro and in vivo

The receptor tyrosine kinase ErbB-2 plays an important role in the regulation of growth factor-induced signal transduction cascades in the epithelium, and ErbB-2 is frequently overexpressed in epithelial tumors. Our previous studies on clinical prostate cancer specimens indicated that ErbB-2 expression was increased in patients undergoing hormone ablation therapy. We had also shown that the critical cell cycle regulatory gene cyclin D1 and its promoter were targets of proliferative signaling in prostate cancer cell lines, and that cyclin D1 was required for ErbB-2-induced mammary tumorigenesis. In the current studies, we found that increased ErbB-2 membrane expression correlated with increased nuclear cyclin D1 staining in clinical prostate cancer specimens, and that expression of ErbB-2 was capable of inducing cell cycle progression in human prostate cancer cell lines. We further showed that ErbB-2 induced the cyclin D1 promoter in DU145 cells, and that small interfering RNA knockdown of cyclin D1 protein levels blocked a significant proportion of the heregulin-induced cell cycle progression in LNCaP cells. Probasin promoter-targeted expression of an activated ErbB-2 isoform induced cyclin D1 expression in the mouse prostate, commensurate with prostate intraepithelial neoplasia. Together, these in vitro and in vivo studies identify cyclin D1 as a critical downstream target of ErbB-2 in the prostate epithelium, both of which are possible therapeutic targets for cancer intervention. Furthermore, our novel mouse model provides a useful platform for ongoing in vivo investigations of ErbB-2 signaling in the prostate epithelium.     

Inhibition of cyclin-dependent kinase 1 (CDK1) by indirubin derivatives in human tumour cells

The bisindole indirubin has been described, more than 30 years ago, as being clinically active in the treatment of human chronic myelocytic leukaemia. However, the underlying mechanism of action has remained unclear. We have reported previously that indirubin and its analogues are potent and selective inhibitors of cyclin-dependent kinases (CDK). In this study, we investigated the influence of indirubin and derivatives on CDK1/cyclin B kinase in human tumour cells at concentrations known to induce growth inhibition. Cells of the mammary carcinoma cell line MCF-7, synchronized by serum deprivation, after serum repletion stay arrested in the G(1)/G(0)phase of the cell cycle in the presence of 2 microM indirubin-3'-monoxime. At higher drug concentrations (> or = 5 microM) an increase of the cell population in the G(2)/M phase is additionally observed. Cells synchronized in G(2)/M phase by nocodazole remain arrested in the G(2)/M phase after release, in the presence of indirubin-3'-monoxime (> or =5 microM). After 24 h treatment with 10 microM indirubin-3'-monoxime a sub-G(2)peak appears, indicative for the onset of apoptotic cell death. Treatment of MCF-7 cells with growth inhibitory concentrations of indirubin-3'-monoxime induces dose-dependent inhibition of the CDK1 activity in the cell. After 24 h treatment, a strong decrease of the CDK1 protein level along with a reduction of cyclin B in complex with CDK1 is observed. Taken together, the results of this study strongly suggest that inhibition of CDK activity in human tumour cells is a major mechanism by which indirubin derivatives exert their potent antitumour efficacy.