The monoterpene perillyl alcohol (POH) is a naturally occurring compound derived from citrus fruits, mint and herbs. It exhibited chemotherapeutic potential against various malignant tumors in preclinical models and is currently being tested in clinical trials in patients with refractory advanced cancers. POH inhibits cellular proliferation at the G1 phase of the cell cycle in vitro. However, the molecular mechanisms responsible for this effect have not been sufficiently elucidated. Here we showed that 1.0 mM POH upregulates p15INK4b and p21WAF1/Cip1, resulting in hypophosphorylation of the retinoblastoma (RB) protein and subsequent G1 arrest in human immortalized keratinocyte HaCaT cells. The induction of p15INK4b was mediated through its promoter, but that of p21WAF1/Cip1 was not. The small interfering RNA (siRNA) of either p15INK4b or p21WAF1/Cip1 significantly attenuated the increase in the G1 cell population caused by POH. The induction of p15INK4b and p21WAF1/Cip1 and subsequent G1 arrest by POH was also observed in other cancer cell lines. These results suggest that the induction of p15INK4b as well as p21WAF1/Cip1 is associated with the antiproliferative effect of POH.
Regulation of the cell cycle is important in cellular proliferation, and therefore the loss of cell cycle control is involved in carcinogenesis (1). Cyclins and cyclin-dependent kinases (CDKs), in association with each other, play pivotal roles in promoting the transition of cells from the G1 phase to the S phase of the cell cycle by phosphorylating the tumor-suppressor retinoblastoma (RB) protein (2,3). Cyclin-CDK complex activation is negatively regulated by CDK inhibitors (CKIs). The first family of CKIs, referred to as the CIP/KIP family, consists of p21WAF1/Cip1, p27Kip1 and p57Kip2. Each member inhibits a broader spectrum of cyclin/CDK complexes including cyclin E/A-CDK2 and cyclin D-CDK4/6 (4,5). The second family of CKIs is called the INK4 family, which comprises p15INK4b, p16INK4a, p18INK4c and p19INK4d. These molecules are specific inhibitors of cyclin D-CDK4/6 complexes (5,6). Many studies have shown that RB is directly, or indirectly, inactivated in most human cancers. Abnormalities leading to malignancies frequently relate to loss or dysfunction of tumor-suppressor molecules including members of these two CKI families, which activate the RB pathway (5,7).
Perillyl alcohol (POH) is a naturally occurring monoterpene found in the essential oils of cherry, lemongrass, gingergrass, cranberry, perilla, mint, lavender, sage, wild bergamot, caraway and celery seeds (8,9). It has been shown that POH exerts antitumor activity against malignant tumor cells in vitro and in vivo. POH inhibits the growth of various types of malignant tumor cells in vitro through blockade of proliferation, angiogenesis and migration, and induction of differentiation and apoptosis (10–14). Regarding the antiproliferative activity of POH, this monoterpene is reported to cause cell cycle arrest at the G1 phase through downregulation of cyclin D1 and upregulation of p21WAF1/Cip1 in murine mammary transformed cells or through upregulation of both p21WAF1/Cip1 and p27Kip1 in human pancreatic adenocarcinoma cells (15,16). Moreover, POH significantly inhibits the growth of mammary and liver tumors in rodent models (12,17). Based on these preclinical data, clinical studies using POH have commenced in patients with advanced malignancies. However, two phase II studies in patients with refractory metastatic breast cancer and metastatic androgen-independent prostate cancer reported that no objective responses were observed (18,19). On the other hand, a recent clinical study showed that intranasal administration of POH increased the overall survival of patients with recurrent glioblastoma (20).
In this study, we demonstrated that POH caused G1 arrest in malignant tumor cells through p15INK4b and p21WAF1/Cip1 induction leading to the dephosphorylation of the RB protein. We suggest that not only p21WAF1/Cip1 but also p15INK4b could be important molecular targets that mediate the antitumor effects of POH.
Materials and methodsCell culture and reagents
Human immortalized keratinocyte HaCaT cells were a kind gift from Dr N.E. Fusenig, German Cancer Research Center, Heidelberg, Germany. Human colon cancer cell lines HT-29 and SW620 were obtained as cell lines of NCI-60 from the NCI Developmental Therapeutics Program (NCI DTP). These cells were maintained in DMEM supplemented with 10% fetal bovine serum, 4 mM L-glutamine, 100 U/ml penicillin and 100 μg/ml streptomycin. Cell cultures were incubated at 37°C in a humidified atmosphere with 5% CO2. POH was purchased from Wako (320-52902; Osaka, Japan), dissolved in dimethyl sulfoxide (DMSO) and diluted to the final concentrations in each volume of culture medium used.
Growth inhibition assay
Cells were plated at 5×104 cells in 12-well plates. One day after inoculation of cells, various concentrations of POH were added to the culture medium. From the first to the second day after plating, the numbers of viable cells were counted using a trypan blue dye exclusion test.
Cell cycle analysis
For flow cytometry, 5×104 cells were plated in 12-well plates. One day later, unsynchronized cells were exposed to 1.0 mM POH for 24 h. The cells were then treated with Triton X-100 and RNase A, and their nuclei were stained with propidium iodide before DNA content was measured using a BD FACS Calibur flow cytometer (BD Biosciences, Franklin Lakes, NJ). At least 10,000 cells were counted and the ModFit LD V2.0 software package (BD Biosciences) was used to analyze the data.
Protein isolation and western blot analysis
Cells were lysed in SDS buffer [50 mM Tris-HCl (pH 7.5), 1% SDS]. The protein extract was then boiled for 5 min and loaded onto a 12% (for p15INK4b and p21WAF1/Cip1 detection), a 10% (for α-tubulin detection) or a 5% (for RB detection) polyacrylamide gel, subjected to electrophoresis and transferred to a nitrocellulose membrane. The following primary antibodies were used: anti-p15INK4b (sc-612; Santa Cruz Biotechnology, Inc., Santa Cruz, CA) anti-p21WAF1/Cip1 (sc-397; Santa Cruz Biotechnology, Inc.), anti-p27Kip1 (sc-528; Santa Cruz Biotechnology, Inc.), anti-pRB (554136; BD Pharmingen) and anti-α-tubulin (CP06; Calbiochem, San Diego, CA). The signal was then developed with Chemi-Lumi One (Nacalai Tesque, Kyoto, Japan) or Immobilon Western (EMD Millipore, MA).
RNA isolation and real-time reverse transcription (RT)-PCR
Real-time RT-PCR analysis was performed as previously described (21). The GeneAmp5700 (Applied Biosystems, CA) was used to quantify the expression level of p15INK4b and p21WAF1/Cip1 mRNAs and normalized to β2MG mRNA. Real-time RT-PCR primer probes for p15INK4b (Hs00394703), p21WAF1/Cip1 (Hs00355782) and β2MG (Hs99999907) were purchased from Applied Biosystems.
Transfection and luciferase assay
The p15INK4b-luciferase fusion plasmid was described previously (22). HaCaT cells were seeded at 1.6×105 cells/well in 6-well plates. One day later, cells were transfected with the plasmid or pGVB2 (a vacant control; 2.5 μg) using Lipofectamine LTX and Plus reagent (Invitrogen, Carlsbad, CA) according to the manufacturer’s instructions. After 24 h, transfected cells were treated with POH at various concentrations for 12 h and then harvested. Luciferase assays were then performed using luciferase assay reagents (Promega, Madison, WI) and a luminometer.
Small interfering RNA (siRNA)
The p15INK4b (CDKN2B HSS141533) and the negative control (Negative Universal Control High #3) siRNAs were purchased from Invitrogen. The p21WAF1/Cip1 siRNA (s415) was purchased from Ambion (Carlsbad, CA). One day before transfection, HaCaT cells were seeded at 9×104 cells/well in 6-well plates without antibiotics. The p15INK4b, p21WAF1/Cip1 or a negative control siRNA (20 nM) was transfected into cells using Lipofectamine RNAiMax (Invitrogen) according to the manufacturer’s instructions. Twenty-four hours after the transfection, cells were treated with 1.0 mM POH for 24 h and then harvested.
Statistical analysis
Statistical evaluation of the data was performed using the Student’s t-test for simple comparison between treatments and controls. p<0.05 was considered to indicate a statistically significant difference.
ResultsCell growth inhibition and G1 arrest by POH in HaCaT cells
We first investigated the antiproliferative effects of POH in human immortalized keratinocyte HaCaT cells. The growth of HaCaT cells was measured in the presence or absence of various concentrations of POH (Fig. 1A). POH inhibited the growth of HaCaT cells in a dose-dependent manner. Notably, 1.0 mM POH had a cytostatic effect. To examine the effects of POH on cell cycle progression, the DNA content of cell nuclei was measured by flow cytometry. POH increased the percentage of cells in the G1 phase and decreased the percentage of cells in the S phase in a dose-dependent manner (Fig. 1B). These data demonstrate that POH arrests the HaCaT cell cycle at the G1 phase.
p15<sup>INK4b</sup> and p21<sup>WAF1/Cip1</sup> induction and hypophosphorylation of the RB protein by POH in HaCaT cells
We aimed to elucidate whether cell cycle-associated molecules are influenced by treatment with POH in HaCaT cells. We discovered that POH increased p15INK4b protein expression in a dose-dependent manner (Fig. 2). Additionally, POH increased p21WAF1/Cip1, which is consistent with previous studies (15,16). Of note, POH had no effect on the protein expression levels of p27Kip1. Both p15INK4b and p21WAF1/Cip1 are members of the CKI families and subsequently dephosphorylate the RB protein leading to G1 cell cycle arrest. We, therefore, examined whether POH alters the phosphorylation status of RB. A hyperphosphorylated form of the RB protein was converted into a hypophosphorylated form by POH treatment in a dose-dependent manner (Fig. 2). Taken together, these results indicate that POH elevates p15INK4b and p21WAF1/Cip1 protein levels, and subsequently converts a hyperphosphorylated form of the RB protein into a hypophosphorylated form in HaCaT cells.
Mechanisms of p15<sup>INK4b</sup> and p21<sup>WAF1/Cip1</sup> induction by POH in HaCaT cells
We next investigated whether POH affects p15INK4b and p21WAF1/Cip1 mRNA expression in HaCaT cells using real-time RT-PCR. Both mRNAs were significantly increased by POH in a dose-dependent manner (Fig. 3A). Since these mRNAs are induced by POH, we analyzed the effect of POH on the promoter activity using p15INK4b or p21WAF1/Cip1 promoter-luciferase fusion plasmids in a transient assay. POH upregulated the promoter activity of p15INK4b (Fig. 3B), however, POH did not elevate that of p21WAF1/Cip1 (data not shown). These results suggest that p15INK4b and p21WAF1/Cip1 are differentially regulated by POH.
p15<sup>INK4b</sup> and p21<sup>WAF1/Cip1</sup> are important targets of POH-induced G1 arrest
The present results raise the possibility that upregulation of p15INK4b and p21WAF1/Cip1 proteins by POH contributes to its induction of G1 arrest. If these molecules are key targets of POH-induced G1 arrest, p15INK4b or p21WAF1/Cip1-depleted cells should be insensitive to the effect of POH. Transfection of HaCaT cells with p15INK4b or p21WAF1/Cip1 siRNA impaired the induction of these proteins by POH (Fig. 4A). Additionally, these siRNAs significantly restored POH-altered percentages of the G1 and S cell populations when compared with the control siRNA (Fig. 4B). These results imply that both p15INK4b and p21WAF1/Cip1 play pivotal roles in POH-induced G1 arrest.
POH causes G1 arrest through induction of p15<sup>INK4b</sup> and p21<sup>WAF1/Cip1</sup> in other cancer cell lines
To investigate whether the effects of POH on G1 arrest could be observed more generally, other cancer cell lines, HT-29 and SW620, were similarly assayed. POH inhibited the proliferation and caused G1 arrest in these cells (Fig. 5A and B). Moreover, POH increased p15INK4b and p21WAF1/Cip1 protein expression and hypophosphorylated the RB protein in both cell lines (Fig. 5C). Taken together, these results suggest that POH has antitumor activity against various malignant tumor cells through induction of p15INK4b and p21WAF1/Cip1 and subsequent G1 arrest.
Discussion
Numerous studies have shown that dysfunction of the RB pathway is the most frequent event in human malignant tumors (6,7). Therefore, we focused our studies on agents that reactivate RB function through induction of the two CKI families. As a result, we previously demonstrated that p15INK4b is upregulated by a histone deacetylase inhibitor trichostatin A, a naturally occurring compound indole-3-carbinol, an epidermal growth factor receptor inhibitor gefitinib (ZD1839) and a novel MEK inhibitor JTP-70902 (22–25). Additionally, we found that p21WAF1/Cip1 is increased by trichostatin A, a dietary flavonoid apigenin and a plant alkaloid cryptolepine (26–28).
p16INK4a and p15INK4b are encoded within the INK4a/ARF/INK4b locus on chromosome 9p21. Deletion of this locus is the most frequent cytogenetic abnormality of the RB pathway in human hematopoietic malignancies (6). On the other hand, in many malignant solid tumors, p16INK4a is inactivated through not only gene deletions, but also point mutations or transcriptional silencing by methylation of the promoter. In contrast to p16INK4a, however, alteration of the p15INK4b gene is a rare event in solid tumors (6,29). Moreover, among the INK4 family, p15INK4b has a function similar to that of p16INK4a. These studies suggest that p15INK4b may act as a replacement for p16INK4a when p16INK4a is inactivated. Krimpenfort et al indicated that p15INK4b can fulfill a critical backup function for p16INK4a in human tumors with p16INK4a deficiency (30). In the present study, we showed that depletion of p15INK4b protein using siRNA suppressed the G1-arresting activity of POH. These findings suggest that induction of p15INK4b by POH could be, at least partially, involved in its antiproliferative activity. Taken together, the ability of POH to induce p15INK4b might be useful for inhibiting the growth of solid tumors where the p16INK4a-RB pathway is inactivated.
p21WAF1/Cip1 is known to be a major effector of the tumor suppressor p53. Therefore, p21WAF1/Cip1 is regarded as a tumor-suppressor gene (31). On the other hand, it has been shown that p21WAF1/Cip1 plays oncogenic roles in certain cellular circumstances through its ability to suppress apoptosis and promote the assembly of cyclin D with CDK4 and CDK6 (32–34). Thus, these data indicate that p21WAF1/Cip1 induction confers a growth advantage in tumor development in certain type of cancers, while it has the opposite effect in others. We revealed that POH upregulated p21WAF1/Cip1 as well as p15INK4b proteins and subsequently caused G1 arrest in three malignant tumor cell lines. Additionally, we showed that depletion of p21WAF1/Cip1 protein using siRNA rendered HaCaT cells insensitive to POH-induced G1 arrest. These data suggest that the induction of p21WAF1/Cip1 by POH could be at least partially involved in its antiproliferative activity.
POH is readily metabolized to perillic acid (PA) and dihydroperillic acid (DHPA) in animals, whereas in humans PA is the major circulating metabolite (17,35). Haag et al(17) reported that in a rat mammary cancer model administration of a 2.5% POH diet for 3 weeks caused complete regression in 22 out of 27 (81%) primary tumors, while the plasma levels of POH metabolites were approximately 800 μM in rats given a 2% POH diet for 10 weeks. Based on the data from preclinical models, POH has been tested in phase I and II clinical trials in patients with refractory solid malignancies. The mean peak PA plasma levels ranged between 415 and 630 μM and minimal toxicities were observed in patients when doses of POH at 1600 or 2100 mg/m2 were administered orally (36–38). Recently, an encouraging clinical study carried out by da Fonseca et al(20), showed that intranasal administration of 440 mg POH increased the overall survival of patients with recurrent GBM when compared with untreated controls.
We revealed that POH upregulated p15INK4b as well as p21WAF1/Cip1 protein and subsequently caused G1 arrest in three malignant tumor cell lines. Additionally, we showed that depletion of the p15INK4b or p21WAF1/Cip1 protein rendered HaCaT cells resistant to POH-induced G1 arrest. These results indicate that induction of both p15INK4b and p21WAF1/Cip1 is at least partially associated with sensitivity to the antiproliferative effect of POH. POH driven activation of RB function through induction of CKIs may contribute to new strategies which have been termed ‘gene-regulating chemotherapy’ for the treatment of malignancies. (39,40). In short, POH is promising as a molecular-targeted anticancer drug against a variety of malignant tumors.
Acknowledgements
We thank Dr Y. Matsuzaki for his helpful discussion and useful advice. We were supported by a Grant-in-Aid from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
AbbreviationsPOH
perillyl alcohol
CDK
cyclin-dependent kinase
CKI
CDK inhibitor
RB
retinoblastoma
siRNA
small interfering RNA
RT-PCR
reverse transcription-PCR
ReferencesMassagueJG1 cell-cycle control and cancerNature4322983062004SherrCJMammalian G1 cyclinsCell73105910651993DowdySFHindsPWLouieKReedSIArnoldAWeinbergRAPhysical interaction of the retinoblastoma protein with human D cyclinsCell734995111993XiongYHannonGJZhangHCassoDKobayashiRBeachDp21 is a universal inhibitor of cyclin kinasesNature3667017041993SherrCJRobertsJMCDK inhibitors: positive and negative regulators of G1-phase progressionGenes Dev13150115121999RousselMFThe INK4 family of cell cycle inhibitors in cancerOncogene18531153171999BurkhartDLSageJCellular mechanisms of tumour suppression by the retinoblastoma geneNat Rev Cancer86716822008KelloffGJCrowellJAHawkETSteeleVELubetRABooneCWCoveyJMDoodyLAOmennGSGreenwaldPHongWKParkinsonDRBagheriDBaxterGTBlundenMDoeltzMKEisenhauerKMJohnsonKKnappGGLongfellowDGMaloneWFNayfieldSGSeifriedHESwallLMSigmanCCNew agents for cancer chemopreventionJ Cellular Biochem (Suppl)261281996CrowellPLPrevention and therapy of cancer by dietary monoterpenesJ Nutr129S775S7781999StarkMJBurkeYDMcKinzieJHAyoubiASCrowellPLChemotherapy of pancreatic cancer with the monoterpene perillyl alcoholCancer Lett9615211995ShiWGouldMNInduction of differentiation in neuro-2A cells by the monoterpene perillyl alcoholCancer Lett95161995MillsJJChariRSBoyerIJGouldMNJirtleRLInduction of apoptosis in liver tumors by the monoterpene perillyl alcoholCancer Res559799831995WagnerJEHuffJLRustWLKingsleyKPlopperGEPerillyl alcohol inhibits breast cell migration without affecting cell adhesionJ Biomed Biotechnol21361402002LoutrariHHatziapostolouMSkouridouVPapadimitriouERoussosCKolisisFNPapapetropoulosAPerillyl alcohol is an angiogenesis inhibitorJ Pharmacol Exp Ther3115685752004ShiWGouldMNInduction of cytostasis in mammary carcinoma cells treated with the anticancer agent perillyl alcoholCarcinogenesis231311422002WisemanDAWernerSRCrowellPLCell cycle arrest by the isoprenoids perillyl alcohol, geraniol, and farnesol is mediated by p21Cip1 and p27Kip1 in human pancreatic adenocarcinoma cellsJ Pharmacol Exp Ther320116311702007HaagJDGouldMNMammary carcinoma regression induced by perillyl alcohol, a hydroxylated analog of limoneneCancer Chemother Pharmacol344774831994LiuGOettelKBaileyHUmmersenLVTutschKStaabMJHorvathDAlbertiDArzoomanianRRezazadehHMcGovernJRobinsonEDeMetsDWildingGPhase II trial of perillyl alcohol (NSC 641066) administered daily in patients with metastatic androgen-independent prostate cancerInvest New Drugs213673722003BaileyHHAttiaSLoveRRFassTChappellRTutschKHarrisLJumonvilleAHansenRShapiroGRStewartJAPhase II trial of daily oral perillyl alcohol (NSC 641066) in treatment-refractory metastatic breast cancerCancer Chemother Pharmacol621491572008da FonsecaCOSimãoMLinsIRCaetanoROFuturoDQuirico-SantosTEfficacy of monoterpene perillyl alcohol upon the survival rate of patients with recurrent glioblastomaJ Cancer Res Clin Oncol1372872932011KoyamaMIzutaniYGodaAEMatsuiTAHorinakaMTomosugiMFujiwaraJNakamuraYWakadaMYogosawaSSowaYSakaiTHistone deacetylase inhibitors and 15-deoxy-Delta12,14-prostaglandin J2 synergistically induce apoptosisClin Cancer Res16232023322010HitomiTMatsuzakiYYokotaTTakaokaYSakaiTp15INK4b in HDAC inhibitor-induced growth arrestFEBS Lett5543473502003MatsuzakiYKoyamaMHitomiTKawanakaMSakaiTIndole-3-carbinol activates the cyclin-dependent kinase inhibitor p15INK4b geneFEBS Lett5761371402004KoyamaMMatsuzakiYYogosawaSHitomiTKawanakaMSakaiTZD1839 induces p15INK4b and causes G1 arrest by inhibiting the mitogen-activated protein kinase/extracellular signal-regulated kinase pathwayMol Cancer Ther6157915872007YamaguchiTYoshidaTKurachiRKakegawaJHoriYNanayamaTHayakawaKAbeHTakagiKMatsuzakiYKoyamaMYogosawaSSowaYYamoriTTajimaNSakaiTIdentification of JTP-70902, a p15INK4b-inductive compound, as a novel MEK1/2 inhibitorCancer Sci98180918162007SowaYOritaTMinamikawaSNakanoKMizunoTNomuraHSakaiTHistone deacetylase inhibitor activates the WAF1/Cip1 gene promoter through the Sp1 sitesBiochem Biophys Res Commun2411421501997TakagakiNSowaYOkiTNakanishiRYogosawaSSakaiTApigenin induces cell cycle arrest and p21/WAF1 expression in a p53-independent pathwayInt J Oncol261851892005MatsuiTASowaYMurataHTakagiKNakanishiRAokiSYoshikawaMKobayashiMSakabeTKuboTSakaiTThe plant alkaloid cryptolepine induces p21WAF1/Cip1 and cell cycle arrest in a human osteosarcoma cell lineInt J Oncol319159222007EstellerMCornPGBaylinSBHermanJGA gene hypermethylation profile of human cancerCancer Res61322532292001KrimpenfortPIjpenbergASongJYvan der ValkMNawijnMZevenhovenJBernsAp15INK4b is a critical tumour suppressor in the absence of p16Ink4aNature4489439472007AbbasTDuttaAp21 in cancer: intricate networks and multiple activitiesNat Rev Cancer94004142009LaBaerJGarrettMDStevensonLFSlingerlandJMSandhuCChouHSFattaeyAHarlowENew functional activities for the p21 family of CDK inhibitorsGenes Dev118478621997CanmanCEGilmerTMCouttsSBKastanMBGrowth factor modulation of p53-mediated growth arrest versus apoptosisGenes Dev96006111995AbukhdeirAMParkBHp21 and p27: roles in carcinogenesis and drug resistanceExpert Rev Mol Med10e192008PhillipsLRMalspeisLSupkoJGPharmacokinetics of active drug metabolites after oral administration of perillyl alcohol, an investigational antineoplastic agent, to the dogDrug Metab Dispos236766801995RippleGHGouldMNStewartJATutschKDArzoomanianRZAlbertiDFeierabendCPomplunMWildingGBaileyHHPhase I clinical trial of perillyl alcohol administered dailyClin Cancer Res4115911641998RippleGHGouldMNArzoomanianRZAlbertiDFeierabendCSimonKBingerKTutschKDPomplunMWahamakiAMarnochaRWildingGBaileyHHPhase I clinical and pharmacokinetic study of perillyl alcohol administered four times a dayClin Cancer Res63903962000HudesGRSzarkaCEAdamsARanganathanSMcCauleyRAWeinerLMLangerCJLitwinSYeslowGHalberrTQianMGalloJMPhase I pharmacokinetic trial of perillyl alcohol (NSC 641066) in patients with refractory solid malignanciesClin Cancer Res6307130802000SakaiTMolecular cancer epidemiology - the present status and future possibilitiesJpn J Hygiene50103610461996SowaYSakaiTButyrate as a model for ‘Gene-regulating chemoprevention and chemotherapy’Biofactors22832872000
Effects of POH on cell growth in HaCaT cells. (A) One day after the inoculation of HaCaT cells, POH at 0.5 (black circle), 0.75 (white triangle) and 1.0 mM (black diamond) was added. After 24 h, cell growth was compared with a DMSO-treated control (white square). The number of viable cells was counted using a trypan blue dye exclusion test. Data represent means ± SD of triplicate experiments; *p<0.05, significantly different compared with DMSO-treated control. (B) Unsynchronized HaCaT cells were incubated for 24 h in the presence of either DMSO or POH at various concentrations as indicated, and cell cycle analysis was performed by measuring the DNA content of cells using flow cytometry. The percentage of cells at G1, S and G2/M phases are indicated. Data represent means ± SD of triplicate experiments; *p<0.05, significantly different compared with DMSO-treated control.
Effects of POH on the expression of cell cycle-related molecules in HaCaT cells. HaCaT cells were exposed for 24 h to either DMSO (0 mM) or to POH at various concentrations as indicated. The expression levels of CKIs, p15INK4b (p15), p21WAF1/Cip1 (p21) and p27Kip1 (p27), and RB were analyzed using western blotting. Hyperphosphorylated (ppRB) and hypophosphorylated (pRB) forms of RB are indicated. α-tubulin was used as the loading control.
Mechanisms of p15INK4b and p21WAF1/Cip1 induction by POH in HaCaT cells. (A) HaCaT cells were exposed to DMSO (0 mM) or POH at the indicated concentrations for 18 h, then the expression of p15INK4b and p21WAF1/Cip1 mRNAs was examined using real-time RT-PCR. Data represent means ± SD of triplicate experiments; *p<0.05, significantly different compared with DMSO-treated control. (B) HaCaT cells transiently transfected with a vacant control (PVGB2) or the p15INK4b-luciferase fusion plasmid (p15) were treated with or without POH at the indicated concentrations for 12 h, and luciferase activity was then measured. Data represent means of triplicate experiments ± SD; *p<0.05, significantly different compared with DMSO-treated control.
p15INK4b and p21WAF1/Cip1 induction is associated with POH-mediated G1 arrest. HaCaT cells were transfected with siRNAs as indicated, then treated with or without 1.0 mM POH for 24 h. (A) The expression levels of p15INK4b (p15), p21WAF1/Cip1 (p21) and α-tubulin (a loading control) proteins was assessed using western blotting. (B) Cell cycle analysis was performed by measuring the DNA content of cells using flow cytometry. The percentage of cells in the G1, S and G2/M phases are indicated. Data represent means ± SD of triplicate experiments. *p<0.05.
Effects of POH on cancer cells HT-29 and SW620. HT-29 and SW620 cells were treated with or without 1.0 mM POH for 24 h. (A) The number of viable cells exposed (black circle) or unexposed (white square) to 1.0 mM POH was counted using a trypan blue dye exclusion test. Data represent means ± SD of triplicate experiments; *p<0.05, significantly different compared with DMSO-treated control. (B) Cell cycle analysis was performed by measuring the DNA content of HT-29 and SW620 cells using flow cytometry. The percentages of cells in the G1, S and G2/M phases are indicated. Data represent means ± SD of triplicate experiments; *p<0.05, significantly different compared with DMSO-treated control. (C) The expression of p15INK4b (p15), p21WAF1/Cip1 (p21), RB and α-tubulin (a loading control) proteins was assessed using western blotting. Hyperphosphorylated (ppRB) and hypophosphorylated (pRB) forms of RB are indicated.