Caveolin-1 expression negatively regulates cell cycle progression by inducing G(0)/G(1) arrest via a p53/p21(WAF1/Cip1)-dependent mechanism

Caveolin-1 is a principal component of caveolae membranes in vivo. Caveolin-1 mRNA and protein expression are lost or reduced during cell transformation by activated oncogenes. Interestingly, the human caveolin-1 gene is localized to a suspected tumor suppressor locus (7q31.1). However, it remains unknown whether caveolin-1 plays any role in regulating cell cycle progression. Here, we directly demonstrate that caveolin-1 expression arrests cells in the G(0)/G(1) phase of the cell cycle. We show that serum starvation induces up-regulation of endogenous caveolin-1 and arrests cells in the G(0)/G(1) phase of the cell cycle. Moreover, targeted down-regulation of caveolin-1 induces cells to exit the G(0)/G(1) phase. Next, we constructed a green fluorescent protein-tagged caveolin-1 (Cav-1-GFP) to examine the effect of caveolin-1 expression on cell cycle regulation. We directly demonstrate that recombinant expression of Cav-1-GFP induces arrest in the G(0)/G(1) phase of the cell cycle. To examine whether caveolin-1 expression is important for modulating cell cycle progression in vivo, we expressed wild-type caveolin-1 as a transgene in mice. Analysis of primary cultures of mouse embryonic fibroblasts from caveolin-1 transgenic mice reveals that caveolin-1 induces 1) cells to exit the S phase of the cell cycle with a concomitant increase in the G(0)/G(1) population, 2) a reduction in cellular proliferation, and 3) a reduction in the DNA replication rate. Finally, we demonstrate that caveolin-1-mediated cell cycle arrest occurs through a p53/p21-dependent pathway. Taken together, our results provide the first evidence that caveolin-1 expression plays a critical role in the modulation of cell cycle progression in vivo.     

Growth inhibition induced by transforming growth factor-β1 in human oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) is a world-wide health problem and its incidence accounts for 1.9–3.5% of all malignant tumors. Transforming growth factor beta/Smads (TGF-β/Smads) signaling pathway plays an important role in oncogenesis, but its function and molecular mechanisms in OSCC remain unclear. Expression of transforming growth factor-β receptor type II (TβRII) and Smad4 was studied by immunohistochemistry in 108 OSCC patients and 10 normal controls. Function and molecular mechanisms of TGF-β/Smads signaling pathway was then investigated in two human tongue squamous carcinoma cell lines with high and low metastasis (Tb and Tca8113) by RT-PCR, Western Blot, immunofluorescence, cell growth curve and flow cytometry (FCM), respectively. TβRII and Smad4 were significantly down-regulated in tumor tissues (with or without lymph node metastasis) compared to normal oral epithelium tissues (P < 0.05). TGF-β1 induced arrest of the cell cycle rather than cell death in Tca8113 and Tb cells, and this influence was mediated by the increasing the expression and changing the location of its downstream components of TGF-β1/Smads signaling pathway. TGF-β1 rapidly increased the expression of p15 and p21 in both Tca8113 and Tb cells. TGF-β1 did not increase p27 expression in Tca8113 cells, but p27 expression was increased in Tb cells. These indicated that TGF-β1 induced G₁ arrest of cell cycle through a different regulating pathway in Tb cells compared with Tca8113 cells. Thus, we conclude that TGF-β/Smads signaling pathway play a important role on cell growth and metastasis potential in OSCC. Xiumei Wang, Wenjing Sun, and Jing Bai contributed equally to this paper.     

LYG-202 inhibits the proliferation of human colorectal carcinoma HCT-116 cells through induction of G1/S cell cycle arrest and apoptosis via p53 and p21(WAF1/Cip1) expression

We recently established that LYG-202, a new flavonoid with a piperazine substitution, exerts an anti-tumor effect in vivo and in vitro. In the present study, we demonstrate that LYG-202 induces G1/S phase arrest and apoptosis in human colorectal carcinoma HCT-116 cells. Data showed that the blockade of the cell cycle was associated with increased p21(WAF1/Cip1) and Rb levels and reduced expression of cyclin D1, cyclin E, and CDK4. Moreover, PARP cleavage, activation of caspase-3, caspase-8, and caspase-9, and an increased ratio of Bax/Bcl-2 were detected in LYG-202-induced apoptosis. Additionally, activation of p53 resulted in the up-regulation of its downstream targets PUMA and p21(WAF1/Cip1), as well as the down-regulation of its negative regulator MDM2, suggesting that the p53 pathway may play a crucial role in LYG-202-induced cell cycle arrest and apoptosis. Furthermore, siRNA knockdown of p53 attenuated the G1 cell cycle arrest and apoptosis induced by LYG-202, as the effects of LYG-202 on up-regulation of p21(WAF1/Cip1) and down-regulation of Bcl-2 and pro-caspase-3 were partly inhibited in p53 siRNA transfected cells compared with control siRNA transfected cells. Collectively, these data indicate that LYG-202 exerts its anti-tumor potency by activating the p53-p21 pathway for G1/S cell cycle arrest and apoptosis in colorectal cancer cells.     

Environmental chemical-induced pro/pre-B cell apoptosis: analysis of c-Myc,p27Kip1, and p21WAF1 reveals a death pathway distinct from clonal deletion

Polycyclic aromatic hydrocarbons (PAH) are common environmental pollutants that suppress the immune system in part by inducing pro/pre-B cell apoptosis. The PAH-induced death signaling pathway resembles the signaling cascade activated during clonal deletion and modeled by B cell receptor cross-linking or by dexamethasone exposure of immature surface Ig(+) B cells in that apoptosis is mediated by NF-kappa B down-regulation. Because a PAH-induced, clonally nonrestricted deletion of B cells would have important implications for B cell repertoire development, the nature of the PAH-induced intracellular death signal was studied further. Particular emphasis was placed on the roles of growth arrest and c-Myc, p27(Kip1), and p21(WAF1) expression, because all of these elements contribute to clonal deletion. As in clonal deletion models, and as predicted by the down-regulation of NF-kappa B, PAH-induced death of pro/pre-B cells was at least partially dependent on c-Myc down-regulation. Furthermore, whereas dexamethasone induced a G(0)/G(1) cell cycle arrest, PAH had no effect on pro/pre-B cell growth, indicating that growth arrest and apoptosis occur by separable signaling pathways in this early phase of B cell development. Finally, in contrast to clonal deletion, PAH-induced pro/pre-B cell death was not dependent on p27(Kip1) or p21(WAF1) up-regulation but did coincide with p53 induction. These results distinguish the PAH-induced apoptosis pathway from that activated during clonal deletion and indicate that signaling cascades leading to growth arrest and/or apoptosis in pro/pre-B cells differ from those active at later B cell developmental stages.     

Genistein induces G<Subscript>2</Subscript>/M cell cycle arrest via stable activation of ERK1/2 pathway in MDA-MB-231 breast cancer cells

Genistein is an isoflavonoid present in soybeans that exhibits anti-carcinogenic effect. Several studies have shown that genistein can trigger G2/M cell cycle arrest and inhibit cell growth in human breast cancer cells. In the present study, we assessed the role of MEK-ERK cascade in regulation of genistein-mediated G2/M cell cycle arrest in the hormone-independent cell line MDA-MB-231. Flow cytometric analysis showed that treatment of MDA-MB-231 cells with genistein induced a concentration-dependent accumulation of cells in the G2/M phase of the cell cycle, with a parallel depletion of the percentage of cells in G0/G1. Genistein-mediated G2/M arrest was associated with a decrease in the protein levels of Cdk1, cyclinB1, and Cdc25C as determined by Western blot analysis. Genistein induced a slow and stable activation of phosphorylated ERK1/2 in a concentration- and time-dependent manner in MDA-MB-231 cells. MEK1/2-specific inhibitor PD98059 blocked genistein-induced activation of ERK1/2 and markedly attenuated genistein-induced G2/M arrest. Furthermore, genistein induced the expression of Ras and Raf-1 protein. Genistein also up-regulated steady-state levels of both c-Jun and c-Fos. PD98059 did not depress genistein-induced up-regulation of Ras and Raf-1 protein. However, it markedly blocked genistein-induced up-regulation of c-Jun and c-Fos. These results suggest that the Ras/MAPK/AP-1 signal pathway may be involved in genistein-induced G2/M cell cycle arrest in MDA-MB-231 breast cancer cells.     

Antiproliferative mechanism of a cannabinoid agonist by cell cycle arrest in human gastric cancer cells

For gastric cancers, the antineoplastic activity of cannabinoids has been investigated in only a few reports and knowledge regarding the mechanisms involved is limited. We have reported previously that treatment of gastric cancer cells with a cannabinoid agonist significantly decreased cell proliferation and induced apoptosis. Here, we evaluated the effects of cannabinoids on various cellular mediators involved in cell cycle arrest in gastric cancer cells. AGS and MKN-1 cell lines were used as human gastric cancer cells and WIN 55,212-2 as a cannabinoid agonist. Cell cycles were analyzed by flow cytometry and western blotting. Treatment with WIN 55,212-2 arrested the cell cycle in the G0/G1 phase. WIN 55,212-2 also upregulated phospho-ERK1/2, induced Kip1/p27 and Cip1/WAF1/p21 expression, decreased cyclin D1 and cyclin E expression, decreased Cdk 2, Cdk 4, and Cdk 6 expression levels, and decreased phospho-Rb and E2F-1 expression. ERK inhibitor decreased the proportion of G0/G1 phase which was induced by WIN 55,212-2. Inhibition of pAKT led to cell cycle arrest in gastric cancer cells. Cell cycle arrest preceded apoptotic response. Thus, this cannabinoid agonist can reduce gastric cancer cell proliferation via G1 phase cell cycle arrest, which is mediated via activation of the MAPK pathway and inhibition of pAKT.     

β-Ionone-induced apoptosis in human osteosarcoma (U2os) cells occurs via a p53-dependent signaling pathway

β-Ionone is a constituent of vegetables and fruits, and can induce apoptosis in some types of malignant cells. However, the mechanism of apoptosis in osteosarcoma (U2os) cells is currently unclear. In this study, we determined whether β-ionone can induce apoptosis in U2os cells in vitro and which signal pathway(s) is involved. We found that β-ionone inhibited cell proliferation in U2os cells in a concentration- and time-dependent manner and caused cell cycle arrest at the G1-S phase. TUNEL assay, DNA ladder and assessment of Caspase 3 activity showed that apoptosis was the determinant in the effects of β-ionone. Furthermore, Expression of the p53 protein increased in a concentration-dependent and time-dependent manner according to immunocytochemistry and immunoblotting after β-ionone treatment. In addition, β-ionone upregulated Bax protein and downregulated Bcl2 protein which led to Bax translocation and cytochrome c release, subsequently activated Caspase 3, thus resulting in apoptosis. In summary, these data suggested that β-ionone induced apoptosis in a concentration-dependent manner in U2os cells via a p53-dependent mitochondrial pathway.     

Helicobacter pylori γ-glutamyltranspeptidase induces cell cycle arrest at the G1-S phase transition

In our previous study, we showed that Helicobacter pylori γ-glutamyltranspeptidase (GGT) is associated with H. pylori-induced apoptosis through a mitochondrial pathway. To better understand the role of GGT in apoptosis, we examined the effect of GGT on cell cycle regulation in AGS cells. To determine the effect of recombinant GGT (rGGT) on cell cycle distribution and apoptosis, rGGT-treated and untreated AGS cells were analyzed in parallel by flow cytometry using propidium iodide (PI). We found that rGGT inhibited the growth of AGS cells in a time-dependent manner, and that the pre-exposure of cells to a caspase-3 inhibitor (z-DEVD-fmk) effectively blocked GGT-induced apoptosis. Cell cycle analysis showed G1 phase arrest and apoptosis in AGS cells following rGGT treatment. The rGGT-mediated G1 phase arrest was found to be associated with down-regulation of cyclin E, cyclin A, Cdk 4, and Cdk 6, and the up-regulation of the cyclindependent kinase (Cdk) inhibitors p27 and p21. Our results suggest that H. pylori GGT induces cell cycle arrest at the G1-S phase transition.     

Arctiin attenuates high glucose-induced human retinal capillary endothelial cell proliferation by regulating ROCK1/PTEN/PI3K/Akt/VEGF pathway in vitro

Diabetic retinopathy (DR) is one of the most prominent microvascular complications of diabetes, which remains the leading cause of legal blindness in the world. Arctiin, a bioactive compound from Arctium lappa L., has been reported to have antidiabetic activity. In this study, we investigated the effect of arctiin on a human retinal capillary endothelial cell (HRCEC) line and how arctiin inhibits cell proliferation in high glucose (HG)-induced HRCECs. Results showed that arctiin decreased HG-induced HRCECs proliferation in a dose-dependent manner by inducing cell cycle arrest at the G0/G1 phase. Tube formation assay and immunofluorescence staining indicated that arctiin abrogated tube formation induced by HG-induced HRCECs in a dose-dependent manner via down-regulation of VEGF expression. Mechanistic study indicated that perturbation of the ROCK1/PTEN/PI3K/Akt signalling pathway plays a vital role in the arctiin-mediated anti-proliferative effect. Furthermore, pre-incubation of HRCECs with Y-27632 attenuated arctiin-induced cell cycle arrest, cell proliferation and tube formation inhibition. Y-27632 also reversed the activation of PTEN, the inactivation/dephosphorylation of PI3K/Akt and down-regulation of VEGF. Taken together, the results demonstrated that arctiin inhibits the proliferation of HG-induced HRCECs through the activation of ROCK1 and PTEN and inactivation of PI3K and Akt, resulting in down-regulation of VEGF, which inhibits endothelial cell proliferation.     

USP22 acts as an oncogene by the activation of BMI-1-mediated INK4a/ARF pathway and Akt pathway

Recent studies provided strong support for the view that ubiquitin-specific protease 22 (USP22) plays a central role in cell-cycle progression and also in pathological processes such as oncogenesis. We have recently shown that USP22 levels are elevated in colorectal carcinoma with associated increase in the expression of several cell-cycle-related genes. However, the precise mechanism for these functions of USP22 at molecular level has not been fully elucidated. Currently, we investigated the role of USP22 in human colorectal cancer (CRC). We observed that USP22 expression was statistically significantly correlated positively with that of BMI-1, c-Myc and both, pAkt (Ser473), and pAkt (Thr308), in primary tumor tissues from 43 CRC patients. Down-regulation of USP22 expression in HCT116 colorectal cancer cells by siRNA resulted in the accumulation of cells in the G1 phase of the cell cycle. RNAi-knockdown of USP22 in HCT16 cells also led to the repression of BMI-1 and was accompanied by the up-regulation of p16INK4a and p14ARF, with a consequent decrease in E2F1 and p53 levels. In addition, down-regulation of c-Myc-targeted cyclin D2 was also noticed in cells treated with USP22-siRNA. Furthermore, our results showed that USP22 deletion also caused down-regulation of Akt/GSK3β activity, which can also contribute to the reduction of cyclin D2. Collectively, our current results suggest that USP22 may act as an oncogene in CRC as it positively regulates cell cycle via both BMI-1-mediated INK4a/ARF pathway and Akt signaling pathway.     

卡铂通过抑制ERK1/2通路诱导人卵巢癌细胞S和G0/G1期阻滞

卡铂(carboplatin, CBP)是一种抗肿瘤活性较强的化疗药物, 通过诱导细胞周期阻滞抑制肿瘤细胞生长, 但其诱导细胞周期阻滞的报告不甚一致. 本研究探索卡铂对卵巢癌HO-8910细胞生长及细胞周期进程的影响. MTS结果显示, 卡铂以浓度和时间依赖方式抑制卵巢癌HO-8910细胞生长, 联合使用ERK1/2通路抑制剂PD98059可使卡铂抗卵巢癌细胞增殖作用增强. 采用Giemsa染色法观察到, 卡铂与PD98059单用或联用均能致卵巢癌细胞发生明显的形态学变化. 流式细胞术检测细胞周期发现, 随卡铂浓度的增高, S期阻滞作用增强; 抑制ERK1/2通路可拮抗卡铂对HO-8910细胞S期阻滞作用, 增加G1期阻滞作用, 而对G2/M期细胞影响不明显. Western印迹结果显示, 随卡铂浓度的增高, p-ERK1/2、Cdc2(Y15)和p Cdc2(T161)的表达逐渐升高, Cyclin E1和Cyclin B1的表达逐渐降低; 抑制ERK1/2通路可将卡铂上调,p-ERK1/2和p-Cdc2(T161)的作用反转为下调作用, 上调Cdc2(Y15)的表达受阻, 抑制Cyclin B1的下调作用, 促进Cyclin E1的下调作用. 本研究结果提示, 卡铂通过抑制ERK1/2激活, 诱导人卵巢癌HO-8910细胞S和G1期阻滞, 抑制卵巢癌细胞生长.     

Epithelial to mesenchymal transition in Madin-Darby canine kidney cells is accompanied by down-regulation of Smad3 expression,leading to resistance to transforming growth factor-beta-induced growth arrest

In normal epithelial cells, transforming growth factor-beta (TGF-beta) typically causes growth arrest in the G(1) phase of the cell cycle and may eventually lead to apoptosis. However, transformed cells lose these inhibitory responses and often instead show an increase in malignant character following TGF-beta treatment. In the canine kidney-derived epithelial cell line, MDCK, synergism between activation of the Raf/MAPK pathway and the resulting autocrine production of TGF-beta triggers transition from an epithelial to a mesenchymal phenotype. During this process, these cells become refractive to TGF-beta-induced cell cycle arrest and apoptosis. TGF-beta signals are primarily transduced to the nucleus through complexes of receptor-regulated Smads, Smad2 and Smad3 with the common mediator Smad, Smad4. Here we show that the transition from an epithelial to mesenchymal phenotype is accompanied by gradual down-regulation of expression of Smad3. Restoration of Smad3 to previous levels of expression restores the cell cycle arrest induced by TGF-beta without reverting the cells to an epithelial phenotype or impacting on the MAPK pathway. Regulation of apoptosis is not affected by Smad3 levels. These data attribute to Smad3 a critical role in the control of cell proliferation by TGF-beta, which is lost following an epithelial to mesenchymal transition.     

Notch1 signaling contributes to the oncogenic effect of HBx on human hepatic cells

Hepatocellular carcinoma (HCC) is a malignant tumor and hepatitis B virus X protein (HBx) plays a crucial role in its pathogenesis. The Notch1 signaling pathway is involved in various malignant tumors including liver cancers and down-regulation of Notch-1 may exert anti-tumor effects. Here, we demonstrate that inhibition of Notch1 by plasmid-based shRNA suppresses growth of human hepatic cells transfected with HBx through G0/G1 cell cycle arrest and apoptosis inhibition, possibly linked to the promoted expression of cyclin-dependent kinase inhibitor, P16, and decreased expression of apoptosis inhibitor, Bcl-2. The anti-proliferative and pro-apoptotic effects of Notch1 shRNA in HBx-transformed L02 cell may be partly mediated by down-regulation of nuclear factor-kappaB (NF-κB) binding activities, demonstrating possible cross-talk between Notch-1 and NF-κB signaling pathways. The oncogene HBx may therefore induce malignant transformation of human hepatic cells via Notch1 pathway, indicating that Notch1 plays a crucial role in HBx-related liver cancer and could be an effective therapeutic target for HCC.     

Growth suppression induced by Notch1 activation involves Wnt-beta-catenin down-regulation in human tongue carcinoma cells

BACKGROUND INFORMATION: Involvement of Notch1 signalling in several cancers is well known, but its role in human tongue squamous cell carcinoma, one of the most common carcinomas of the human oral cavity, remains poorly characterized. RESULTS: Our studies demonstrated that constitutively over-expressed active Notch1, via stable transfection of exogenous ICN (intracellular fragment of Notch), resulted in growth suppression of the human tongue cancer cell line Tca8113 in vitro and in vivo, accompanied by G(0)-G(1) cell cycle arrest and apoptosis. Moreover, down-regulation of beta-catenin protein expression was observed in Tca8113 cells stably expressing active Notch1. Activated Notch1 also led to dramatic increase in p21(WAF1/CIP1) and p53 expression with decreases in Skp2 (S-phase kinase-associated protein 2) and Bcl-2 (B-cell lymphocytic-leukaemia proto-oncogene 2) expression, which may participate in the induction of apoptosis and cell cycle arrest. CONCLUSIONS: Since the effects of the Notch1 pathway are cell-type specific and context-dependent in cell types where Notch1 has an anti-proliferative effect, down-regulation of Wnt/beta-catenin signalling may be one of the mechanisms which induces apoptosis and cell cycle arrest.     

Transient down-regulation of PKC-zeta RNA following crosslinking of membrane IgM on WEHI-231 B lymphoma cells.

Regulation of protein kinase C (PKC) isoform mRNAs has been studied in the immature, murine B lymphoma WEHI-231 by the MAPPing protocol and by slot blot analysis of unamplified mRNA. This membrane IgM (mIgM)-positive cell line has been previously used as a model to study signal transduction by mIgM in immature B lymphocytes and the role of those signals in the induction of immune tolerance in the B cell compartment. Stimulation of the cells by anti-mu antibodies, phorbol ester, or Ca2+ ionophore caused growth arrest and death of the cells. IL 4 and IL 5 slowed the growth of the cells. Of these stimuli, only anti-mu stimulation affected PKC mRNA levels. Anti-mu treatment caused a transient decrease in the amount of PKC-zeta isoform mRNA within 3 hr. Within 24 hr levels returned toward normal. Anti-mu had little or no effect on the expression of mRNA for the alpha, beta, delta, or epsilon isoforms of PKC. WEHI-231 cells do not express PKC-gamma. Although anti-mu treatment blocked progression of the cells from the G0/G1 stage into the S phase of cell cycle, viable sort selected cells in either the G0/G1 or the S/G2/M phases showed no clear difference in the expression of PKC-zeta message. Thus, there is not preferential regulation of expression of PKC-zeta during stages of the cell cycle. The results show that mIgM on WEHI-231 cells can transduce a signal that is not mediated by PKC or Ca2+ mobilization alone. The signal causes transient, selective down-regulation of mRNA encoding the zeta PKC isoform.     

Piperine Causes G1 Phase Cell Cycle Arrest and Apoptosis in Melanoma Cells through Checkpoint Kinase-1 Activation

In this study, we determined the cytotoxic effects of piperine, a major constituent of black and long pepper in melanoma cells. Piperine treatment inhibited the growth of SK MEL 28 and B16 F0 cells in a dose and time-dependent manner. The growth inhibitory effects of piperine were mediated by cell cycle arrest of both the cell lines in G1 phase. The G1 arrest by piperine correlated with the down-regulation of cyclin D1 and induction of p21. Furthermore, this growth arrest by piperine treatment was associated with DNA damage as indicated by phosphorylation of H2AX at Ser139, activation of ataxia telangiectasia and rad3-related protein (ATR) and checkpoint kinase 1 (Chk1). Pretreatment with AZD 7762, a Chk1 inhibitor not only abrogated the activation of Chk1 but also piperine mediated G1 arrest. Similarly, transfection of cells with Chk1 siRNA completely protected the cells from G1 arrest induced by piperine. Piperine treatment caused down-regulation of E2F1 and phosphorylation of retinoblastoma protein (Rb). Apoptosis induced by piperine was associated with down-regulation of XIAP, Bid (full length) and cleavage of Caspase-3 and PARP. Furthermore, our results showed that piperine treatment generated ROS in melanoma cells. Blocking ROS by tiron protected the cells from piperine mediated cell cycle arrest and apoptosis. These results suggest that piperine mediated ROS played a critical role in inducing DNA damage and activation of Chk1 leading to G1 cell cycle arrest and apoptosis.     

Differing responses of G2-related genes during differentiation of HL60 cells induced by TPA or DMSO

Differentiation leads to the cessation of cellular proliferation, but little is known about the molecular mechanisms of growth arrest. We compared the effect of two differentiation inducers, 12-o-tetradecanoyl 13-acetate (TPA) and dimethyl sulfoxide (DMSO) on both the cell-cycle and the modulation of G2-related genes in synchronized HL60 cells. TPA treatment of HL60 cells resulted in G1 arrest within 24 h. In contrast, the cell cycling of DMSO-treated cells was initially accelerated and they progressed to the second cycle before accumulating in the G1 phase. Expression of cyclin B, cdc25, wee1 and cdc2 was studied during cell cycle arrest by Northern blot hybridization. Expression of cyclin B, cdc25 and cdc2 fluctuated in association with cell cycle progression towards the G2/M phase, while wee1 expression remained constant in untreated cells. These four genes were highly expressed in TPA-treated cells for the first 12 h, but drastic down-regulation was seen at 18 h and expression became undetectable after 24 h. In contrast, no remarked changes of gene expression were seen in DMSO-treated cells. These findings suggest that cell cycle progression along with the initial process of differentiation in response to TPA differs from the response to DMSO and that the down-regulation of cdc2 expression by TPA-treated HL60 cells contributes to endorsement of G1 arrest.