Involvement of the cell cycle inhibitor CIP1/WAF1 in lung alveolar epithelial cell growth arrest induced by glucocorticoids

Cyclophosphamide Attenuates Fibrosis in Lupus Nephritis by Regulating Mesangial Cell Cycle Progression

Objectives

Most patients with systemic lupus erythematosus (SLE) develop lupus nephritis (LN) with severe kidney manifestations. Renal fibrosis can be primarily attributed to overproliferation of mesangial cells (MCs), which are subject to drug treatment. Nevertheless, the detailed mechanisms remain elusive. We sought to identify the effect of cyclophosphamide (CTX), a drug commonly used for LN treatment, on MC proliferation and explore its underlying mechanisms. Material/Methods. Cell proliferation and fibrosis in mouse kidney tissues were determined by histopathology staining techniques. Flow cytometry was used for cell cycle analysis. Cell cycle regulators were examined in vitro following treatment of immortalized human MCs with platelet-derived growth factor subunit B (PDGF-B). Quantitative real-time PCR and western blot analyses were used to measure the mRNA and protein levels of candidate cell cycle regulators, respectively.

Results

CTX inhibited cell overproliferation induced by platelet-derived growth factor subunit B in vitro and in vivo. CTX (40 mg/l) was sufficient to induce G0/G1 phase cell cycle arrest. CTX treatment downregulated many critical cell cycle regulators including cyclins and cyclin-dependent kinases but upregulated cyclin-dependent kinase inhibitors. Additionally, CTX-treated samples showed significantly reduced fibrosis, as indicated by lower expression of interleukin-1β and α-smooth muscle actin.

Conclusion

CTX inhibits proliferation of MCs by modulating cell cycle regulator and therefore arresting them at G1 phase. CTX treatment significantly alleviates the severity of renal fibrosis. These findings provide novel insights into the mechanisms by which CTX affects LN.

Effect of immunosuppression on the human mesangial cell cycle

The present study investigated the effects of immunosuppressive agents [tacrolimus (Tac), cyclosporine A (CsA), mycophenolic acid (MMF) and methylprednisone (MP)] on the proliferation, cell cycle progression and apoptotic rate of human mesangial cells. Cultured human mesangial cells were treated with several concentrations of the immunosuppressive agents for 24, 48 or 72 h. Cell cycle progression, proliferation and apoptosis were analyzed using an MTT assay and flow cytometry. Tac and CsA significantly inhibited the proliferation of human mesangial cells in a dose- and time-dependent manner. Cell cycle analysis revealed that Tac and CsA arrested mesangial cells in the G₀/G₁ phase, preventing them from entering S phase. Similarly, MP inhibited human mesangial cell growth by causing cell cycle arrest in G₀/G₁ phase. MMF also inhibited mesangial cell proliferation, but accomplished this by preventing progression from S phase to the G₂/M phase. The combination of MP and MMF synergistically inhibited mesangial cell proliferation. Tac, CsA, MP and MMF inhibited proliferation of human mesangial cells by blocking progression of the cell cycle. In conclusion, these agents, sequentially or in combination, may be used to effectively treat mesangial proliferative glomerular disease.

Chondrocyte p21(WAF1/CIP1) expression is increased by dexamethasone but does not contribute to dexamethasone-induced growth retardation in vivo

It has been shown that cell cycle genes play an important role in the coordination of chondrocyte proliferation and differentiation. The inhibitory effects of glucocorticoids (GCs) on chondrocyte proliferation are consistent with GCs disrupting cell cycle progression and promoting cell cycle exit. Cyclin-dependent kinase inhibitors (CDKIs) force cells to exit the cell cycle and differentiate, and studies have shown that expression of the CDKI p21(CIP1/WAF1) is increased in terminally differentiated cells. In this study, p21 mRNA and protein expression was increased during chondrocyte differentiation and after exposure to dexamethasone (Dex, 10(-6 )M) in murine chondrogenic ATDC5 cells. In 4-week-old mice lacking a functional p21 gene, Dex caused a reduction in body weight compared to saline control null mice, but this was consistent with the reduction in body weight observed in Dex-treated wild-type littermates. In addition, p21 ablation had no effect on the reduction in width of the growth plate or reduced mineral apposition rate in Dex-treated mice. However, an alteration in growth rate and epiphyseal structure is evident when comparing p21(-/-) and wild-type mice. These findings suggest that p21 does not directly contribute to GC-induced growth retardation in vivo but is involved in the maintenance of the growth plate.

Sox17 promotes cell cycle progression and inhibits TGF-beta/Smad3 signaling to initiate progenitor cell behavior in the respiratory epithelium

The Sry-related high mobility group box transcription factor Sox17 is required for diverse developmental processes including endoderm formation, vascular development, and fetal hematopoietic stem cell maintenance. Expression of Sox17 in mature respiratory epithelial cells causes proliferation and lineage respecification, suggesting that Sox17 can alter adult lung progenitor cell fate. In this paper, we identify mechanisms by which Sox17 influences lung epithelial progenitor cell behavior and reprograms cell fate in the mature respiratory epithelium. Conditional expression of Sox17 in epithelial cells of the adult mouse lung demonstrated that cell cluster formation and respecification of alveolar progenitor cells toward proximal airway lineages were rapidly reversible processes. Prolonged expression of Sox17 caused the ectopic formation of bronchiolar-like structures with diverse respiratory epithelial cell characteristics in alveolar regions of lung. During initiation of progenitor cell behavior, Sox17 induced proliferation and increased the expression of the progenitor cell marker Sca-1 and genes involved in cell cycle progression. Notably, Sox17 enhanced cyclin D1 expression in vivo and activated cyclin D1 promoter activity in vitro. Sox17 decreased the expression of transforming growth factor-beta (TGF-beta)-responsive cell cycle inhibitors in the adult mouse lung, including p15, p21, and p57, and inhibited TGF-beta1-mediated transcriptional responses in vitro. Further, Sox17 interacted with Smad3 and blocked Smad3 DNA binding and transcriptional activity. Together, these data show that a subset of mature respiratory epithelial cells retains remarkable phenotypic plasticity and that Sox17, a gene required for early endoderm formation, activates the cell cycle and reinitiates multipotent progenitor cell behavior in mature lung cells.

Identification of glucocorticoid-regulated genes that control cell proliferation during murine respiratory development

Glucocorticoids play a vital role in fetal respiratory development and act via the intracellular glucocorticoid receptor (GR) to regulate transcription of key target genes. GR-null mice die at birth due to respiratory dysfunction associated with hypercellularity and atelectasis. To identify events associated with this lung phenotype we examined perinatal cellular proliferation rates and apoptotic indices. We demonstrate that compared to wild-type controls, day 18.5 postcoitum (p.c.) GR-null mouse lungs display significantly increased cell proliferation rates (1.8-fold P < 0.05) and no change in apoptosis. To examine underlying molecular mechanisms, we compared whole genome expression profiles by microarray analysis at 18.5 days p.c. Pathways relating to cell proliferation, division and cell cycle were significantly down-regulated while pathways relating to carbohydrate metabolism, kinase activities and immune responses were significantly up-regulated. Differential levels of gene expression were verified by quantitative-RT-PCR and/or Northern analysis. Key regulators of proliferation differentially expressed in the lung of 18.5 p.c. GR-null lungs included p21 CIP1 (decreased 2.9-fold, P < 0.05), a negative regulator of the cell cycle, and Mdk (increased 6.0-fold, P < 0.05), a lung growth factor. The more under-expressed genes in 18.5 p.c. GR-null lungs included Chi3l3 (11-fold, P < 0.05), a macrophage inflammatory response gene and Ela1 (9.4-fold, P < 0.05), an extracellular matrix remodeling enzyme. Our results demonstrate that GR affects the transcriptional status of a number of regulatory processes during late fetal lung development. Amongst these processes is cell proliferation whereby GR induces expression of cell cycle repressors while suppressing induction of a well characterized cell cycle stimulator.

Phosphatidylinositol 3-Akt-kinase-dependent phosphorylation of p21(Waf1/Cip1) as a novel mechanism of neuroprotection by glucocorticoids

The role of glucocorticoids in the regulation of apoptosis remains incongruous. Here, we demonstrate that corticosterone protects neurons from apoptosis by a mechanism involving the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). In primary cortical neurons, corticosterone leads to a dose- and Akt-kinase-dependent upregulation with enhanced phosphorylation and cytoplasmic appearance of p21(Waf1/Cip1) at Thr 145. Exposure of neurons to the neurotoxin ethylcholine aziridinium (AF64A) results in activation of caspase-3 and a dramatic loss of p21(Waf1/Cip1) preceding apoptosis in neurons. These effects of AF64A are reversed by pretreatment with corticosterone. Corticosterone-mediated upregulation of p21(Waf1/Cip1) and neuroprotection are completely abolished by glucocorticoid and mineralocorticoid receptor antagonists as well as inhibitors of PI3- and Akt-kinase. Both germline and somatically induced p21(Waf1/Cip1) deficiency abrogate the neuroprotection by corticosterone, whereas overexpression of p21(Waf1/Cip1) suffices to protect neurons from apoptosis. We identify p21(Waf1/Cip1) as a novel antiapoptotic factor for postmitotic neurons and implicate p21(Waf1/Cip1) as the molecular target of neuroprotection by high-dose glucocorticoids.

Cell cycle proteins in normal and chemically induced abnormal secondary palate development: a review

Cell cycle progression and thus proper cell number is essential for normal development of organs and organisms. Craniofacial tissues including the secondary palate are vulnerable to disruption of cell cycle progression and proliferation by many chemicals including mycotoxin, secalonic acid D (SAD), glucocorticoids, retinoic acid and 2,3,7,8-tetrachlorodibenzodioxin. Induction of cleft palate (CP) by SAD in mice occurs from a reduction in the size of developing palatal shelves. This is associated with an inhibition of proliferation of murine and human embryonic palatal mesenchymal (MEPM and HEPM) cells as well as a G1/S block of cell cycle. In murine embryonic palates and HEPM cells, SAD inhibited G1/S-phase-specific cyclin-dependent kinase (CDK)2 activity, reduced the level of cyclin E and increased the level of the CDK2 inhibitor, p21. These results, together with those from other laboratories, suggest that common cell cycle protein targets (biomarkers), relevant to the pathogenesis of CP by multiple chemical exposures, that can form the basis for the diagnosis and the development of preventive strategies, are likely to exist.

Intracellular colocalization and interaction of IGF-binding protein-2 with the cyclin-dependent kinase inhibitor p21CIP1/WAF1 during growth inhibition

It is presently unknown whether any member of the IGFBP (insulin-like growth factor binding protein) family directly participates in the control of cell proliferation. We have previously documented that induction of IGFBP-2 was associated with inhibition of DNA synthesis in lung alveolar epithelial cells. In the present study, we investigated the relationship between IGFBP-2 and the cell cycle inhibitor p21CIP1/WAF1 further. We used serum deprivation to inhibit the proliferation of MLE (mouse lung epithelial)-12 cells, and characterized the spatial localization of IGFBP-2. We found that growth inhibition, which was supported by the strong induction of p21CIP1/WAF1, was correlated with increased secretion of IGFBP-2 and, unexpectedly, with its increased localization in the nucleus and particularly in the cytoplasm. By coimmunoprecipitation, we discovered that IGFBP-2 is capable of binding to p21CIP1/WAF1. Interaction between these two proteins was further supported by colocalization of the proteins within growth-arrested cells, as visualized by confocal microscopy. Furthermore, this interaction increased with the duration of the stress, but was suppressed when proliferation was restimulated by the addition of serum. The recombinant expression of GFP (green fluorescent protein)-tagged IGFBP-2 in transfected MLE-12 cells demonstrated its ability to bind specifically to p21CIP1/WAF1. Taken together, these results provide a link between IGFBP-2 and p21CIP1/WAF1 in the regulation of alveolar lung cell proliferation.

Early lymphoid progenitors in mouse and man are highly sensitive to glucocorticoids

Glucocorticoids are extensively used in anti-inflammatory therapy and may contribute to the normal regulation of lymphopoiesis. This study utilized new information about the early stages of lymphopoiesis in mouse and man to determine precisely which cell types are hormone sensitive. Cycling B lineage precursors were depleted in dexamethasone-treated mice, while mature, non-dividing CD45R(Hi) CD19(Hi) lymphocytes, myeloid progenitors and stem cells with the potential for lymphocyte generation on transplantation were spared. Lineage marker-negative (Lin(-)) IL-7R(+) Flk-2(+) pro-lymphocytes also declined, but not as rapidly as the terminal deoxynucleotidyl transferase-positive cells within an early Lin(-) c-kit(Hi) Sca-1(Hi) fraction of bone marrow. Hormone-sensitive cells with additional properties of early lymphoid progenitors (ELP) were identified within the same Lin(-) c-kit(Hi) Sca-1(Hi) subset using human mu transgenic mice and recombination-activating gene 1 (RAG1)/green fluorescent protein knock-in animals. Furthermore, cells with a recent history of RAG1 expression were more glucocorticoid sensitive than mature lymphocytes in marrow and spleen. Lymphocyte progenitors in mice bearing a human bcl-2 transgene were protected from dexamethasone treatment. However, isolated progenitors from either wild-type or bcl-2 transgenic mice were directly sensitive to the hormone in stromal cell-free cultures, suggesting that additional factors must determine vulnerability to glucocorticoids. B lineage lymphocyte precursors were found to be abnormally elevated in the bone marrow of adrenalectomized or RU486-treated mice. This suggests that glucocorticoids may normally contribute to steady-state regulation of lymphopoiesis. Finally, parallel studies revealed that the earliest events in human lymphopoiesis are susceptible to injury during glucocorticoid therapy.

Glucocorticoids induce G1 cell cycle arrest in human neoplastic thymic epithelial cells

PURPOSE

Glucocorticoids exert anti-proliferative effects in various cell types and have long been known to induce apoptosis in thymocytes. Although a few reports have described the regression of human thymoma with glucocorticoid therapy, its effects on neoplastic thymic epithelial cells (TECs) have not been reported. In the present study, we investigated glucocorticoid receptor (GR) expression on neoplastic TECs and the effects of glucocorticoids in vitro on the cell cycle progression of tumor cells.

PATIENTS AND METHODS

Thymoma specimens were obtained during surgery from 21 patients. Three of the specimens with glucocorticoid therapy were examined using the TdT-mediated dUTP-biotin nick-end labeling method. Primary tumor specimens from ten untreated thymomas were examined for GR expression by immunohistochemistry. Isolated neoplastic TECs from the remaining eight untreated thymomas were examined using immunohistochemistry, flow cytometric and cell cycle analysis.

RESULTS

GR are expressed on neoplastic TECs as well as on non-neoplastic thymocytes in thymomas, regardless of WHO histological classification. Glucocorticoids caused an accumulation of TEC in G0/G1 phase in all cases examined (n = 6), and also induced apoptosis in the three with the lowest levels of Bcl-2 expression.

CONCLUSIONS

Our results indicate that neoplastic TECs express GR and that glucocorticoids directly suppress their in vitro proliferation.

Secalonic acid D blocks embryonic palatal mesenchymal cell-cycle by altering the activity of CDK2 and the expression of p21 and cyclin E

BACKGROUND

The mycotoxin, secalonic acid D (SAD), a known animal and potential human cleft palate (CP)-inducing agent, is produced by Pencillium oxalicum in corn. SAD selectively inhibits proliferation of murine embryonic palatal mesenchymal (MEPM) cells leading to a reduction in cell numbers. These effects can explain the reduction in shelf size and the resulting CP seen in the offspring of SAD-exposed mice. Ability of SAD to inhibit proliferation as well as to block the progression of cells from G1- to S-phase of the cell-cycle were also shown in the human embryonic palatal mesenchymal (HEPM) cells suggesting the potential CP-inducing effect of SAD in human beings

METHODS

Gestation day (GD) 12 mouse embryos and HEPM cells were used to test the hypothesis that the cell-cycle block induced by SAD results from a disruption of stage-specific regulatory components both in vivo and in vitro. The effects of SAD on the activity of various cyclin dependent kinases (CDK) and on the levels of various positive (cyclins and CDK) and negative (CDK inhibitors p15, 16, 18, 19, 21, 27, 57) cell-cycle regulators were assessed by performing kinase assays and immunoblots, respectively.

RESULTS

In the murine embryonic palates, SAD specifically inhibited G1/S-phase-specific CDK2 activity, reduced the level of cyclin E and tended to increase the level of the CIP/kip CDK inhibitor, p21. In the HEPM cell cultures, exposure to IC50 of SAD significantly affected all of the above targets. In addition, a reduction in the levels/activity of CDK 4/6, a reduction in the levels of cyclins D1, D2, D3, E, A, and all INK4 family proteins, and an increase in the level of the CIP/kip CDK inhibitor, p57, were also seen.

CONCLUSIONS

These results suggest that the S-phase-specific cell-cycle proteins CDK2, cyclin E and possibly p21 are the common targets of SAD in murine palatal shelves in vivo and in human embryonic palatal mesenchymal cells in vitro and may be relevant to the pathogenesis of SAD-induced CP.

Identification of target genes involved in the antiproliferative effect of glucocorticoids reveals a role for nuclear factor-(kappa)B repression

Glucocorticoid hormones (GCs) exert an antiproliferative effect on most cells. However, the molecular mechanism is still largely unclear. We investigated the antiproliferative mechanism by GCs in human embryonic kidney 293 cells with stably introduced glucocorticoid receptor (GR) mutants that discriminate between cross-talk with nuclear factor-(kappa)B (NF-(kappa)B) and activator protein-1 signaling, transactivation and transrepression, and antiproliferative vs. non-antiproliferative responses. Using the GR mutants, we here demonstrate a correlation between repression of NF-(kappa)B signaling and antiproliferative response. Gene expression profiling of endogenous genes in cells containing mutant GRs identified a limited number of genes that correlated with the antiproliferative response. This included a GC-mediated up-regulation of the NF-(kappa)B-inhibitory protein I(kappa)B(alpha), in line with repression of NF-(kappa)B signaling being important in the GC-mediated antiproliferative response. Interestingly, the GC-stimulated expression of I(kappa)B(alpha) was a direct effect despite the inability of the GR mutant to transactivate through a GC-responsive element. Selective expression of I(kappa)B(alpha) in human embryonic kidney 293 cells resulted in a decreased percentage of cells in the S/G2/M phase and impaired cell proliferation. These results demonstrate that GC-mediated inhibition of NF-(kappa)B is an important mechanism in the antiproliferative response to GCs.

Hyperoxia induces macrophage cell cycle arrest by adhesion-dependent induction of p21Cip1 and activation of the retinoblastoma protein

Hyperoxia induces growth arrest, apoptosis, necrosis, and morphological changes (spreading and adhesion) in various types of cells. The mechanism of hyperoxia-induced cell growth arrest has not been well elucidated, especially in macrophages. One possible mechanism is a role of cell adhesion in hyperoxia-induced cell cycle arrest. To evaluate this finding, macrophages were cultured in normoxia (21% O2) or hyperoxia (95% O2) in adhesion or low adhesion conditions. Incubation of macrophages in hyperoxia induced cell cycle arrest. The hyperoxia-induced cell cycle arrest was prevented by low adhesion conditions. To evaluate pathways potentially involved in hyperoxia-induced growth arrest, we measured extracellular regulated kinase and retinoblastoma protein activation and p21Cip1 and p53 accumulation. Hyperoxia strongly induced activation of extracellular regulated kinase and retinoblastoma protein as well as up-regulation of p21Cip1. These effects of hyperoxia were attenuated under low adhesion conditions, suggesting a role for integrin-dependent signaling. The induction of p21Cip1 and activation of retinoblastoma protein occurred via a p53-independent mechanism. These results suggest that adhesion-dependent pathways are required for hyperoxia-induced cell cycle arrest in macrophages.

CDK2 is involved in the S-phase lengthening induced by glucocorticoids in normal human lymphocytes

Involvement of CDK2 in glucocorticoid-mediated S-phase lengthening was analyzed in this work. Dexamethasone (DXM) treatment of PHA-stimulated lymphocytes induced a decrease in CDK2 mRNA expression without any change in mRNA stability. This glucocorticoid-induced decrease in CDK2 mRNA expression could be suppressed by cycloheximide treatment. These results support the hypothesis of an indirect effect of DXM at the transcriptional level. Furthermore, CDK2 protein expression also decreased while the rate of protein synthesis and stability remained unchanged, suggesting a second post-translational level of regulation with the preferential degradation of a CDK2 protein stock fraction. The analysis of co-precipitated proteins showed that glucocorticoids induced modifications of protein complex composition. We found i) a preservation of the linkage capability of CDK2 for cyclin E and A, ii) a relative increase in p27kip1 linkage, and iii) a decrease in p21waf1 complexed with CDK2. As a consequence of CDK2 decrease and modifications of protein complex composition, pRb and histone H1 kinase activity of CDK2 was profoundly decreased. All these results pinpoint the role of CDK2 in glucocorticoid-induced S-phase lengthening and the potential activator role of p21waf1 for CDK2 in human lymphocytes.

P21Waf1 control of epithelial cell cycle and cell fate

As a broad-acting cyclin-dependent kinase inhibitor, p21(WAF1) occupies a central position in the cell cycle regulation of self-renewing tissues such as oral mucosa and skin. In addition to regulating normal cell cycle progression decisions, p21(WAF1) integrates genotoxic insults into growth arrest and apoptotic signaling pathways that ultimately determine cell fate. As a result of its complex interactions with cell cycle machinery and response to mutagenic agents, p21(WAF1) also has stage-specific roles in epithelial carcinogenesis. Finally, a view is emerging of p21(WAF1) as not merely a cyclin-dependent kinase inhibitor, but also as a direct participant in regulating genes involved in growth arrest, senescence, and aging, thus providing an additional layer of control over matters of the cell cycle. This review discusses these various roles played by p21(WAF1) in cell cycle control, and attempts to relate these to epithelial cell biology, with special emphasis on keratinocytes.

Regulation of macrophage migration inhibitory factor expression by glucocorticoids in vivo

Glucocorticoid hormones are important anti-inflammatory agents because of their anti-inflammatory and proapoptotic action within the immune system. Their clinical usefulness remains limited however by side effects that result in part from their growth inhibitory action on sensitive target tissues. The protein mediator, macrophage migration inhibitory factor (MIF), is an important regulator of the host immune response and exhibits both glucocorticoid-antagonistic and growth-regulatory properties. MIF has been shown to contribute significantly to the development of immunopathology in several models of inflammatory disease. Although there is emerging evidence for a functional interaction between MIF and glucocorticoids in vitro, little is known about their reciprocal influence in vivo. We investigated the expression of MIF in rat tissues after ablation of the hypothalamic-pituitary-adrenal axis and after high-dose glucocorticoid administration. MIF expression is constitutive and independent of the influence of adrenal hormones. Hypophysectomy and the attendent loss of pituitary hormones, by contrast, decreased MIF protein content in the adrenal gland. Administration of dexamethasone was found to increase MIF protein expression in those organs that are considered to be sensitive to the growth inhibitory effects of glucocorticoids (immune and endocrine tissues, skin, and muscle). This increase was most likely because of a posttranscriptional regulatory effect because tissue MIF mRNA levels were not influenced by dexamethasone treatment. Finally, MIF immunoneutralization enhanced lymphocyte egress from blood during stress-induced lymphocyte redistribution, consistent with a functional interaction between MIF and glucocorticoids on immune cell trafficking in vivo. These findings suggest a role for MIF in both the homeostatic and physiological action of glucocorticoids in vivo.

Tumor growth-promoting properties of macrophage migration inhibitory factor (MIF)

First identified nearly 40 years ago, macrophage migration inhibitory factor (MIF) is a potent pro-inflammatory cytokine and is an essential component of immune and inflammatory responses. Recent studies suggest that MIF may also contribute to multiple aspects of tumor progression and neoplasia. This review will attempt to summarize these findings focusing on MIF's ability to modulate cell proliferation, tumor angiogenesis and tumor suppressor activity.

Glucocorticoids inhibit developmental stage-specific osteoblast cell cycle. Dissociation of cyclin A-cyclin-dependent kinase 2 from E2F4-p130 complexes

Unique cell cycle control is instituted in confluent osteoblast cultures, driving growth to high density. The postconfluent dividing cells share features with cells that normally exit the cell cycle; p27(kip1) is increased, p21(waf1/cip1) is decreased, free E2F DNA binding activity is reduced, and E2F4 is primarily nuclear. E2F4-p130 becomes the predominant E2F-pocket complex formed on E2F sites, but, unlike the complex that typifies resting cells, cyclin A and CDK2 are also present. Administration of dexamethasone at this, but not earlier stages, results in reduction of cyclin A and CDK2 levels with a parallel decrease in the associated kinase activity, dissociation of cyclin A-CDK2 from the E2F4-p130 complexes, and inhibition of G(1)/S transition. The glucocorticoid-mediated cell cycle attenuation is also accompanied by, but not attributable to, increased p27(kip1) and decreased p21(waf1/cip1) levels. The attenuation of osteoblast growth to high density by dexamethasone is associated with severe impairment of mineralized extracellular matrix formation, unless treatment commences in cultures that have already grown to high density. Both the antimitotic and the antiphenotypic effects are reversible, and both are antagonized by RU486. Thus, glucocorticoids induce premature attenuation of the osteoblast cell cycle, possibly contributing to the osteoporosis induced by these drugs in vivo.

Role of BCL-2 and cell cycle regulatory proteins for corticosensitivity assessment in childhood acute lymphoblastic leukaemia

Results of treatment in childhood acute lymphoblastic leukaemia (ALL) remain unsatisfactory because relapses occur even after high-dose chemotherapy. Corticosensitivity is used in numerous therapeutic trials as a prognostic factor for treatment choice. The aim of this study was to evaluate the role of cell cycle regulatory protein expression before and during the first 48 h of corticotherapy for predicting corticosensitivity. Fifty-two children presenting with ALL were studied at diagnosis and during the first 48 h of treatment for cell proliferation and apoptosis level by measurement of DNA content, and for expression of several cell proliferation regulatory proteins by means of Western blot. Glucocorticoids induced a significant decrease in the percentage of cells in S-phase and in CDK1, CDK4 and CDK6 expression and an increase in the percentage of cells in subG1 peak. Two criteria for corticosensitivity were used: (i) the number of blast cells after 7 d of treatment with a threshold at 1 x 109/l (usual criterion), (ii) the J8/J1 blast cell ratio, which is independent from initial leucocytosis. Bcl-2 expression at diagnosis was the best predictive variable for the usual corticosensitivity criterion in B- and T-cell ALL. For the second criterion, in B-cell ALL, p21waf1 expression at diagnosis was the sole (albeit poorly) predictive variable, whereas bcl-2 remained of high interest in T-cell ALL. Interestingly, these proteins, bcl-2 and p21waf1, are associated with prolonged cell lifespan and their increased expression is often linked to poor response to cytotoxic drugs. Such preliminary results call for subsequent studies on large independent sets of T-cell and B-cell lineage ALL in order to confirm the J8/J1 blast cell ratio value as well as the role of bcl-2 and p21waf1 expression in predicting corticosensitivity.

Retinoic acid-induced proliferation of lung alveolar epithelial cells is linked to p21(CIP1) downregulation

Retinoids, including retinol and retinoic acid (RA) derivatives, have been shown to be involved in the processes of lung development as well as of lung repair after injury. Recently, we have provided evidence that RA could stimulate proliferation of lung alveolar type 2 epithelial cells (E. Nabeyrat, V. Besnard, S. Corroyer, V. Cazals, and A. Clement. Am. J. Physiol. Lung Cell. Mol. Physiol. 275: L71-L79, 1998). To gain some insight into the mechanisms involved in the mitogenic action of RA, we focused in the present study on the effects of RA on the expression of G(1) phase cyclins and their cell cycle-dependent kinases (Cdks). Experiments were performed with serum-deprived cells cultured in the absence and presence of RA. The results showed no effects of RA on the expression of either cyclins or Cdks. In contrast, RA treatment was found to prevent the decrease in cyclin E-Cdk2 activity observed when cells were growth arrested by serum deprivation. The observation that changes in cyclin E-Cdk2 activity were not associated with modifications in the amount of complexes formed led to the suggestion that the Cdk inhibitory protein (CKI) was involved. Study of the CKI p21(CIP1) revealed marked differences in its expression in the absence and presence of RA, with a dramatic downregulation observed in RA-treated cells. Interestingly, immunoprecipitation experiments provided evidence that the decreased levels of p21(CIP1) were associated with a reduced interaction of this CKI with cyclin E-Cdk2 complexes. These data together with previous results obtained in various situations of type 2 cell growth arrest emphasize the role of p21(CIP1) in the control of lung alveolar epithelial cell proliferation.

p57Kip2, a glucocorticoid-induced inhibitor of cell cycle progression in HeLa cells

Glucocorticoids exert antiproliferative effects on a number of cell types, including the HeLa cervical carcinoma cell line. However, the mechanism responsible for the antiproliferative effect is poorly understood. In this report we have investigated the role of the recently identified cyclin-dependent kinase inhibitor (CDI) p57Kip2 in the antiproliferative effect conferred by glucocorticoids. When HeLa cells were treated with the synthetic glucocorticoid dexamethasone (DEX), the doubling time of exponentially growing cells increased 2-fold. Within 11 h of DEX treatment, this was accompanied by an accumulation of cells in the G1 phase of the cell cycle with a corresponding decreased proportion of cells in the S phase and decreased CDK2 activity. DEX treatment of the HeLa cells dramatically induced the protein and mRNA expression of the CDI p57Kip2. This induction was seen within 4 h of DEX treatment, preceding a major DEX-induced accumulation of cells in the G1 phase. DEX-induced mRNA expression of p57Kip2 did not require de novo protein synthesis, and the transcription of the p57Kip2 gene was increased as determined by a run-on transcription assay. Furthermore, DEX induction of p57Kip2 was not a consequence of the cell cycle arrest, since other growth inhibition signals did not result in strong p57Kip2 induction. Overexpression of p57Kip2 using HeLa cells stably transfected with a tetracycline-inducible vector showed that p57Kip2 is sufficient to reconstitute an antiproliferative effect similar to that seen in DEX-treated cells. Selective p57Kip2 expression by the tetracycline analog doxycycline to levels comparable to those observed on DEX induction resulted in a 1.7-fold increase in the doubling time and a shift of HeLa cells to the G1 phase as well as a decrease in CDK2 activity. Taken together, these results suggest that glucocorticoid treatment directly induces transcription of the p57Kip2 gene and that the p57Kip2 protein is involved in the glucocorticoid-induced antiproliferative effect.

Glucocorticoid inhibits oxidized LDL-induced macrophage growth by suppressing the expression of granulocyte/macrophage colony-stimulating factor

Glucocorticoid, an anti-inflammatory agent, inhibits the development of atherosclerosis in various experimental animal models. This is partially explained by its ability to inhibit smooth muscle cell migration and proliferation in the intima and to reduce chemotaxis of circulating monocytes and leukocytes into the subendothelial spaces. We have recently demonstrated that oxidized LDL (Ox-LDL) has a mitogenic activity for macrophages in vitro in which Ox-LDL-induced granulocyte/macrophage colony-stimulating factor (GM-CSF) production plays an important role. Proliferation of cellular components is one of the characteristic events in the development and progression of atherosclerotic lesions. In the present study, we investigated the effects of glucocorticoids on Ox-LDL-induced macrophage growth. Dexamethasone, prednisolone, and cortisol inhibited Ox-LDL-induced thymidine incorporation into macrophages by 85%, 70%, and 50%, respectively. Ox-LDL induced a significant production of GM-CSF by macrophages, which was effectively inhibited by dexamethasone, prednisolone, and cortisol by 80%, 65%, and 50%, respectively. Dexamethasone-mediated inhibition of Ox-LDL-induced GM-CSF mRNA expression and macrophage growth was significantly abrogated by RU-486, a glucocorticoid receptor antagonist. Our results suggest that the inhibitory effects of glucocorticoids on macrophage growth may be due to the inhibition of Ox-LDL-induced GM-CSF production through transactivation of the glucocorticoid receptor.