Dihydrotestosterone induces p27 degradation via direct binding with SKP2 in ovarian and breast cancer

The effect of androgens on the prevention or promotion of cell cycle progression in ovarian and breast cancer is controversial. This effect determines the basic rules of how to treat postmenopausal patients properly after surgery. In the present study, we investigated the effects of different androgens, namely dihydrotestosterone (DHT), testosterone and dehydroepiandrosterone (DHEA) in MCF-7 breast cancer and OVCAR-3 ovarian cancer cells. We found that DHT could down-regulate p27, but not p21, within 4 h. Further studies proved that DHT induced p27 degradation through the ubiquitin-proteasome proteolytic pathway. Inhibition of the androgen receptor and of phosphorylation did not hamper the degradation. However, proteolysis inhibitors effectively antagonized the DHT-induced p27 degradation. Knockdown of the S-phase kinase-associated protein 2 (SKP2), rather than of the Kip1 ubiquitination promoting complex 1 (KPC1), also prevented p27 down-regulation. DHT led to the direct binding of p27 to SKP2 independent of the phosphorylation status. Collectively, DHT, but not the other examined androgens regulated the degradation of p27 in MCF-7 and OVCAR-3 cells. Furthermore, SKP2 was shown to act as a DHT receptor.     

Steroid- and retinoid-mediated growth arrest and apoptosis in WEHI-231 cells: role of NF-kappaB, c-Myc and CKI p27(Kip1)

IgM cross-linking induces NF-kappaB inactivation, c-Myc down-regulation, and cyclin kinase inhibitor p27(Kip1) accumulation in WEHI-231 murine B lymphoma cells. p27(Kip1) up-regulation leads to a decreased cyclin-dependent kinase 2 activity, retinoblastoma protein hypophosphorylation, G1 arrest and apoptosis. Similar to membrane (m) IgM cross-linking in B lymphoma cells, steroids and retinoids down-regulate c-Myc (via NF-kappaB inactivation) and induce apoptosis in T cell hybridomas and thymocytes. In this study, we determined if steroids and retinoids have similar effects in WEHI-231 cells. Our results show that steroids and retinoids induce NF-kappaB inactivation, c-Myc down-regulation, p27(Kip1) up-regulation, G1 arrest, and apoptosis. Importantly, these hormones enhance anti-IgM-induced apoptosis in WEHI-231 cells. Similar to mIgM signaling, all these effects are prevented by treatment with CD40 ligand. Caspase inhibition, on the other hand, rescues cells from steroid/retinoid-induced apoptosis, but has no effect on growth arrest, p27(Kip1), and c-Myc. Together, these findings suggest that steroids/retinoids and mIgM cross-linking share a common signal transduction pathway leading to G1 arrest and cell death.     

Inhibition of cell cycle progression via p27Kip1 upregulation and apoptosis induction by an ethanol extract of Rhus verniciflua Stokes in AGS gastric cancer cells

Botanical preparations are widely used by patient with cancer in Korea, Japan and China. Rhus verniciflua Stokes (RVS) has traditionally been used as a medicinal ingredient for the therapy of stomach and uterine cancer. In this study, we showed that exposure to an ethanol extract of RVS (50 microg/ml) resulted in a synergistic inhibitory effect on cell growth in AGS cells. Growth inhibition was related with the inhibition of proliferation and induction of apoptosis. The extract induces G1-cell cycle arrest through the regulation of cyclins, the induction of p27Kip1, and decrease the CDK2 kinase activity. The upregulated p27Kip1 level is caused by protein stability increment by the reduction of Skp2, a key molecule related with p27Kip1 ubiquitination and degradation, and de novo protein synthesis. RVS extract induces apoptosis through the expression of Bax, poly(ADP-ribose) polymerase (PARP) and activation of caspase-3. RVS extract induces G1-cell cycle arrest via accumulation of p27Kip1 controlled by Skp2 reduction and apoptosis passing through an intrinsic pathway in human gastric cancer cells but not in normal cells, therefore we suggest that this extract could be a candidate medicine or compound for the development of novel class of anti-cancer drugs.     

Over-expression of p45(SKP2) in Kaposi's sarcoma correlates with higher tumor stage and extracutaneous involvement but is not directly related to p27(KIP1) down-regulation.

F-Box protein p45(SKP2) is the substrate-specific receptor of ubiquitin-protein ligase SCF/p45(SKP2) and is involved in the degradation of p27(Kip1) through the ubiquitin/proteasome pathway. In addition, p45(SKP2) facilitates proteolysis of other molecules related to the cell cycle, is frequently over-expressed in transformed cells, and induces S phase in quiescent cells. The aim of this study was to determine whether p45(SKP2) expression is altered in aggressive lesions of Kaposi's sarcoma and its relation to p27(KIP1)down-regulation. We performed immunohistochemistry using antibodies directed to p45(SKP2), p27(KIP1), and Ki67 on paraffin blocks corresponding to 47 cases of Kaposi's sarcoma (8 macules, 10 plaques, 12 tumors, and 15 extracutaneous lesions). p45(SKP2) nuclear over-expression was present in all Kaposi's sarcoma stages, being significantly increased in skin tumors (mean +/- 95% confidence interval: 39.2 +/- 18.8) and extracutaneous lesions (25.8 +/- 17.3) as compared with macules (18.9 +/- 8.2) and plaques (29.2 +/- 12.0; P =.0199). On the other hand, Kaposi's sarcoma progression was associated with a decrease in p27(KIP1) expression and Ki67 immunoreactivity was independent of disease stage. No statistically significant differences were found in regard to patients' sex and human immunodeficiency virus status and regression analysis failed to show a correlation among p45(SKP2), p27(KIP1) and Ki67 immunostaining scores. These findings suggest that p45(SKP2) is involved in Kaposi's sarcoma progression, not only by promoting the degradation of p27(KIP1) but also through other mechanisms still unknown.     

p27 Kip1 protein expression in Hashimoto's thyroiditis

AIMS: Hashimoto's thyroiditis (HT) is an autoimmune disease in which both proliferation and apoptosis are enhanced. p27(Kip1) protein protects tissues from disease mechanisms that involve excessive cell proliferation and apoptosis. This study investigated whether there is loss of p27(Kip1) expression in HT and whether p27(Kip1) immunoreactivity has any relation to the proliferative indicator Ki-67. Because p27(Kip1) is regulated through either degradation, mediated by the S phase kinase associated protein 2 (Skp2), or sequestration, via D3 cyclin, the expression of these proteins was also investigated. METHODS: Immunohistochemistry was used to assess p27(Kip1), Ki-67, Skp2, and cyclin D3 expression in 19 cases of HT and in 10 normal thyroids. The results were evaluated by image analysis and reported as labelling indices (LIs) in both groups. RESULTS: The p27(Kip1) LI was lower in HT than in normal thyroid (28% v 75%; p < 0.001), whereas Ki-67 (1.13% v 0.13%), Skp2 (0.74% v 0.15%), and cyclin D3 (1.56% v 0.00%) LIs were higher in HT than in normal thyroids (p < 0.001). There was no correlation between p27(Kip1) and the expression of Ki-67, Skp2, and cyclin D3. CONCLUSIONS: p27(Kip1) downregulation is not exclusive to tumours but occurs also in HT, independently of the proliferative status and of changes in Skp2 and cyclin D3 expression. Further investigation is required to understand the mechanisms leading to p27 deregulation because these observations suggest that the regulation of p27(Kip1) expression in epithelial thyroid cells may play a role in HT pathogenesis.     

Adapter protein connections: The MRL and Grb7 protein families

Insulin-like growth factor-1 receptor (IGF-1R) has been shown to be important for melanoma cell growth and survival. In this study we first show, using immunohistochemistry, that progression from benign nevi to malignant melanoma is paralleled by an increased expression of IGF-1R and a down-regulation of the cyclin-dependent kinase inhibitor p27^Kip1. Even though the expression of p27^Kip1 was drastically reduced compared to benign tumors, detectable amounts of it could be assayed by Western blotting in cultured melanoma cells. To analyze whether there is a causative relationship between the IGF-1 pathway and p27^Kip1 expression, melanoma cells were treated with αIR-3, an antibody blocking the IGF-1 binding to IGF-1R, or Tunicamycin, which inhibits the translocation of IGF-1R to the cell surface. From these studies we could conclude that the overall expression of p27^Kip1 is independent of the IGF-1 pathway. In contrast, the association of p27^Kip1 with the different cyclins was drastically affected. Both TM and αIR-3 decreased the binding of p27^Kip1 to cyclin D1, whose expression was drastically reduced. On the other hand there was an increased binding of p27^Kip1 to cyclin E and cyclin A. This redistribution of p27^Kip1 may be a mechanism for growth arrest and induction of apoptosis following interruption of the IGF-1 pathway in melanoma cells.     

BRAF provides proliferation and survival signals in MSI colorectal carcinoma cells displaying BRAF(V600E) but not KRAS mutations

BRAF kinase is a downstream target of KRAS and activates the MAPK pathway. These two molecules are prone to mutations in sporadic microsatellite unstable (MSI) colorectal carcinomas (CRC) and BRAF V600E mutations are inversely associated with oncogenic KRAS mutations. The biological significance of BRAF V600E oncogenic activation is not well established in this type of tumour. We aimed to study proliferation and survival effects induced by BRAF inhibition in MSI CRC cell lines harbouring distinct genetic backgrounds (BRAF V600E or KRAS G13D). Suppression of BRAF in BRAF V600E MSI CRC cell lines by RNA interference significantly inhibited proliferation and induced apoptosis, as demonstrated by BrdU incorporation and TUNEL assay, respectively. No significant differences were seen in proliferation and apoptosis, in cell lines harbouring KRAS G13D, after BRAF inhibition. We further analysed proliferation-associated molecules (pERK1/2, cyclin D1, p27 Kip1) and apoptosis-associated molecules (Bcl-2, Bax, pAkt, pBad, XIAP) in all cell lines. After BRAF down-regulation, we found a more pronounced decrease in ERK1/2 phosphorylation and cyclin D1 expression levels in BRAF-mutated cell lines in comparison to KRAS mutated cells. Upon BRAF inhibition, we also found an increase in p27(Kip1) levels and a more pronounced decrease in the levels of anti-apoptotic protein Bcl-2, specifically in cell lines with BRAF V600E. In conclusion, we have shown that MSI KRAS and BRAF mutant CRC cell lines respond differently to BRAF knockdown. This report provides evidence supporting BRAF as a good target for therapeutic intervention in patients with sporadic MSI CRC harbouring activating mutations in BRAF but not in KRAS.     

p27Kip1 modulates cell migration through the regulation of RhoA activation

The tumor suppressor p27(Kip1) is an inhibitor of cyclin/cyclin-dependent kinase (CDK) complexes and plays a crucial role in cell cycle regulation. However, p27(Kip1) also has cell cycle-independent functions. Indeed, we find that p27(Kip1) regulates cell migration, as p27(Kip1)-null fibroblasts exhibit a dramatic decrease in motility compared with wild-type cells. The regulation of motility by p27(Kip1) is independent of its cell-cycle regulatory functions, as re-expression of both wild-type p27(Kip1) and a mutant p27(Kip1) (p27CK(-)) that cannot bind to cyclins and CDKs rescues migration of p27(-/-) cells. p27(-/-) cells have increased numbers of actin stress fibers and focal adhesions. This is reminiscent of cells in which the Rho pathway is activated. Indeed, active RhoA levels were increased in cells lacking p27(Kip1). Moreover, inhibition of ROCK, a downstream effector of Rho, was able to rescue the migration defect of p27(-/-) cells in response to growth factors. Finally, we found that p27(Kip1) binds to RhoA, thereby inhibiting RhoA activation by interfering with the interaction between RhoA and its activators, the guanine-nucleotide exchange factors (GEFs). Together, the data suggest a novel role for p27(Kip1) in regulating cell migration via modulation of the Rho pathway.     

High glucose-induced hypertrophy of mesangial cells requires p27(Kip1), an inhibitor of cyclin-dependent kinases

Hypertrophy of mesangial cells is one of the earliest morphological alterations in the kidney after the onset of diabetes mellitus. We have previously shown that cultured mesangial cells exposed to high ambient glucose arrest in the G1 phase of the cell cycle and that this is associated with an increased expression of inhibitors of the cyclin-dependent kinase (CDK)-inhibitors p21(Cip) and p27(Kip1). To further investigate a potential role of p27Kip1 in the development of glucose-induced hypertrophy, mesangial cells from p27Kip1 wild-type (+/+) and knockout (-/-) mice were established. High glucose medium (450 mg/dl) increased p21(Cip1) protein in p27Kip1+/+ and -/- mesangial cells, and increased p27Kip1 protein levels in p27Kip1+/+ cells. In contrast to high glucose increasing de novo protein synthesis in p27Kip1+/+ cells, high glucose did not increase protein synthesis in p27Kip1-/- cells. High glucose also reduced DNA synthesis and caused cell cycle arrest in p27Kip1+/+ cells. In contrast, despite an increase in transforming growth factor (TGF)-beta mRNA and protein expression, DNA synthesis and cell cycle progression were increased by high glucose in p27Kip1-/- cells. Exogenous TGF-beta comparably induced fibronectin mRNA in p27Kip1+/+ and -/- cells suggesting intact TGF-beta receptor transduction. In addition, high glucose failed to increase the total protein/cell number ratio in p27Kip1-/- cells. However, in the presence of high glucose, reconstituting p27Kip1 expression by transient or stable transfection in p27Kip1-/- cells, using an inducible expression system, increased the de novo protein synthesis and restored G1-phase arrest. These results show that p27Kip1 is required for glucose-induced mesangial cell hypertrophy and cell cycle arrest.     

Regulation of p27Kip1 during gentamicin mediated hair cell death

The INK4 and Kip/Cip families of Cyclin Dependent Kinase inhibitors (CKIs) are regulators of the cell cycle. In addition, CKIS including p27(Kip1) can protect cells from apoptosis in vitro. However, little is known about protective effect of p27(Kip1) in vivo. We used systemic treatment with aminoglycosides to induce hair-cell death in the basilar papilla (BP), the auditory organ of the avian inner ear, and characterised the expression of p27(Kip1) with confocal and immunofluorescence microscopy. In contrast to the adult mammalian cochlea where p27(Kip1) is expressed only in supporting cells, p27(Kip1) is found in the nuclei of both hair cells and supporting cells in the BP of the normal, mature bird. Forty-eight hours after gentamicin treatment, hair cells with TUNEL positive nuclei and hair cells with pyknotic nuclei were both detected, suggesting many hair cells die by apoptosis. When the BP was double labelled for p27(Kip1) and myosin VIIa, a hair-cell specific protein, all dying hair cells that had been ejected from the epithelium were found to be myosin VIIa positive but negative for p27(Kip1) even though nuclear remnants were still visible. In the transition zone where partial hair-cell loss occurs, freshly ejected hair cells lying immediately above the surface of the BP no longer expressed p27(Kip1). Damaged hair cells within the epithelium in the transition zone contained p27(Kip1) in their cytoplasm but not in their nuclei. These data support recent in vitro findings suggesting that p27(Kip1) protects cells from apoptosis and that its downregulation may be a general feature of programmed cell death.     

A novel target gene,SKP2, within the 5p13 amplicon that is frequently detected in small cell lung cancers

We investigated DNA copy-number aberrations in 22 cell lines derived from small cell lung cancers (SCLCs) using comparative genomic hybridization. A minimal common region at 5p13, within the 5p11-p13 amplicon that was most frequently involved, harbored the CDH6, PC4, and SKP2 genes. These three genes showed amplification and consequent overexpression in the SCLC cell lines. SKP2 positively regulates progression of cell cycle by targeting several regulators, such as the cell-cycle inhibitor p27(KIP1), for ubiquitin-mediated degradation. SKP2 was amplified in 7 (44%) of 16 primary SCLC tumors, and consequently overexpressed in 10 (83%) of the 12 of those tumors we examined. Expression levels of SKP2 protein were cell cycle-dependent in SCLC cells as well as in normal cells, and were correlated with the DNA copy-number of the gene. There was an inverse correlation between the expression of SKP2 and p27(KIP1) proteins. Down-regulation of SKP2 using an anti-sense oligonucleotide remarkably suppressed the growth of SCLC cells. Our results indicate that SKP2 is likely to be a target of the 5p13 amplification and to play an important role in the growth of SCLC cells.     

p27Kip1反义寡核苷酸对B淋巴瘤细胞WEHI-231细胞周期的影响

目的　探讨p2 7Kip1在抗原受体介导的B淋巴瘤细胞WEHI- 2 31细胞周期停止信号中的作用。方法　用抗IgM抗体诱导WEHI- 2 31细胞细胞周期停止。用合成的p2 7Kip1反义寡核苷酸抑制p2 7Kip1基因的表达。采用流式细胞仪 ,分析细胞核的DNA含量和细胞周期的变化。用体外激酶实验检测Cdk2的活性 ,Westernblot检测Rb蛋白的磷酸化水平。结果　合成的p2 7Kip1反义寡核苷酸能阻断抗IgM抗体诱导的p2 7Kip1基因表达的上调。用p2 7Kip1反义寡核苷酸处理WEHI-2 31细胞 ,可恢复抗原受体交联引起的周期素依赖性激酶Cdk2活性的降低 ,以及Rb蛋白磷酸化水平的下降 ,并使细胞周期恢复运转。结论　p2 7Kip1可能在抗原受体信号介导的WEHI-2 31细胞的细胞周期停止中发挥主要作用     

Reduced levels of the cell-cycle inhibitor p27Kip1 in epithelial dysplasia and carcinoma of the oral cavity.

Recent studies have shown that the cyclin-dependent kinase (cdk) inhibitors play important roles in cell cycle progression in normal cells. Alterations in the cdk inhibitors also appear to be important in cancer development in a number of human tumors. p27Kip1 is a member of the CIP/KIP family of cdk inhibitors that negatively regulates cyclin-cdk complexes. Reduced levels of p27Kip1 protein have been identified in a number of human cancers, and in some cases reduced p27Kip1 is associated with an increase in proliferative fraction. In the present study, we examined p27Kip1 protein by immunohistochemistry in 10 normal and 36 dysplastic epithelia and in 8 squamous cell carcinomas from one anatomical site within the oral cavity, the floor of the mouth. Proliferative activity was assessed in serial sections by determining the expression of the cell cycle proteins Ki-67 and cyclin A. p27kip1 protein was significantly reduced in oral dysplasias and carcinomas compared with that in normal epithelial controls. In addition, there was a significant reduction in p27Kip1 protein between low- and high-grade dysplasias, suggesting that changes in p27Kip1 expression may be an early event in oral carcinogenesis. There was increasing expression of Ki-67 and cyclin A proteins with increasingly severe grades of dysplasia compared with normal controls. Although there was a strong correlation between Ki-67 and cyclin A scores (r2= 0.61) for all categories of disease, there was a weak negative correlation between Ki-67 and p27Kip1 levels (r2 = 0.29) and between cyclin A and p27Kip1 levels (r2 = 0.25). In conclusion, this study has found that a reduction in the proportion of cells expressing p27Kip1 protein is frequently associated with oral dysplasia and carcinoma from the floor of the mouth. Furthermore, reductions in p27Kip1 levels are associated with increased cell proliferation, although other changes likely contribute to altered cell kinetics during carcinogenesis at this site.     

p27kip1 independently promotes neuronal differentiation and migration in the cerebral cortex

The generation of neurons by progenitor cells involves the tight coordination of multiple cellular activities, including cell cycle exit, initiation of neuronal differentiation, and cell migration. The mechanisms that integrate these different events into a coherent developmental program are not well understood. Here we show that the cyclin-dependent kinase inhibitor p27(Kip1) plays an important role in neurogenesis in the mouse cerebral cortex by promoting the differentiation and radial migration of cortical projection neurons. Importantly, these two functions of p27(Kip1) involve distinct activities, which are independent of its role in cell cycle regulation. p27(Kip1) promotes neuronal differentiation by stabilizing Neurogenin2 protein, an activity carried by the N-terminal half of the protein. p27(Kip1) promotes neuronal migration by blocking RhoA signaling, an activity that resides in its C-terminal half. Thus, p27(Kip1) plays a key role in cortical development, acting as a modular protein that independently regulates and couples multiple cellular pathways contributing to neurogenesis.     

Angiotensin II induces p27(Kip1) expression in renal tubules in vivo: role of reactive oxygen species

Previous studies have demonstrated that angiotensin II (ANG II) mediates cell cycle arrest of cultured renal tubular cells by induction of p27(Kip1), an inhibitor of cyclin-dependent kinases. However, it is not known whether ANG II exerts similar effects in vivo. Infusion of ANG II into naive rats for 7 days increased formation of reactive oxygen species in tubular cells of the kidney. Furthermore, ANG II infusion stimulated protein expression of p27(Kip1) as detected by western blotting of tubular lysates and immunohistochemistry. Infusion of ANG II reduced tubular proliferation as detected by proliferating-cell nuclear antigen (PCNA) immunohistochemistry. The increase in p27(Kip1) expression was not due to an increase in mRNA. Immunoprecipitation experiments revealed that the increased p27(Kip1) protein associates with cyclin-dependent kinase 2. Coadministration of the radical scavenger dimethylthiourea abolished this ANG II mediated p27(Kip1) expression without reducing systemic blood pressure. Furthermore, dimethylthiourea infusion attenuates the ANG II mediated G(1)-phase arrest of tubular cells. However, infusion of norepinephrine did not induce reactive oxygen species or p27(Kip1) expression, despite a significant increase in blood pressure. Thus ANG II induces p27(Kip1) expression in renal tubular cells in vivo. This effect is mediated by reactive oxygen species. Since tubular hypertrophy depends on G(1)-phase arrest and may promote subsequent development of interstitial fibrosis, administering oxygen radical scavenger may be a therapeutic tool to counteract ANG II dependent remodeling of renal tubular cells.     

A pathway regulated by cell cycle inhibitor p27Kip1 and checkpoint inhibitor Smad3 is involved in the induction of T cell tolerance

Peripheral tolerance is essential for immunological homeostasis. Tolerant T cells are thought to arise after T cell receptor ligation in conditions that are nonpermissive for replication. Here we have investigated the function of the cell cycle inhibitor p27(Kip1) in tolerance induction in vivo using naive T cell receptor-transgenic cells lacking the cyclin-dependent kinase (Cdk)-binding domain of p27(Kip1)(p27delta). Wild-type but not p27delta cells underwent tolerization. Tolerized wild-type cells had impaired Cdk2 and Cdc2 kinase activity and failed to phosphorylate the checkpoint inhibitor Smad3, leading to enhanced expression of the Cdk inhibitor p15. In contrast, p27delta cells proliferated in tolerizing conditions because of Cdk kinase activation and phosphorylation of Smad3, which resulted in no upregulation of p15. Smad3 'knockdown' prevented tolerance induction, whereas expression of a Smad3 mutant resistant to Cdk-mediated phosphorylation recapitulated molecular and functional events of tolerance. Thus, p27(Kip1) is required during induction of tolerance and Smad3 regulates T cell responses 'downstream' of p27(Kip1).     

p27~(Kip1)在永生化人神经前体细胞分化中的作用研究

目的:利用全反式视黄酸(all-trans retinoic acid,RA)诱导永生化的人神经前体细胞(immortalized human neural progenitor cells,hSN12W-TERTcells)分化,研究分化过程中细胞周期调节蛋白p27Kip1,p21Cip1,细胞周期蛋白激酶2(cyclin-dependent kinase2,cdk2)及细胞周期蛋白E(cyclinE)的变化,探讨永生化人神经前体细胞分化的相关分子机制。方法:取本课题组已经建立的永生化人神经前体细胞系hSN12W-TERT细胞(第12代)培养并给予1μmol/L RA诱导。在RA诱导的第3,7d观察细胞形态变化,用RT-PCR方法检测RA诱导前后hSN12W-TERT细胞p27Kip1,p21Cip1,cdk2及cyclinE mRNA的变化,用免疫细胞化学染色方法比较RA诱导前后p27Kip1蛋白表达的变化。结果:RA诱导第3d,hSN12W-TERT细胞比未经RA诱导的正常hSN12W-TERT细胞生长缓慢且形态发生改变,表现为胞体变小,突起延长增多。至RA诱导第7d时,hSN12W-TERT细胞形态变化更加明显,接近成熟神经元形态。RT-PCR结果表明,hSN12W-TERT细胞中p27Kip1mRNA的表达在RA诱导后明显增加,而p21Cip1mRNA的表达在RA诱导后略呈下降趋势。hSN12W-TERT细胞中cdk2、cyclinE的mRNA水平在RA诱导前后没有明显变化。免疫细胞化学染色结果显示,RA诱导第3d,hSN12W-TERT细胞p27Kip1的表达比未经RA诱导的正常hSN12W-TERT细胞明显增加(P0.05),RA诱导第7d,p27Kip1的表达进一步增加(P0.05)。结论:p27Kip1参与RA诱导的永生化人神经前体细胞生长阻滞和分化过程,p27Kip1可能在永生化人神经前体细胞的神经元分化过程中发挥重要作用;且RA诱导后p27Kip1蛋白含量的增加是通过转录水平调节的。