Posttranscriptional stabilization underlies p53-independent induction of p21WAF1/CIP1/SDI1 in differentiating human leukemic cells

Antiproliferative effect of a vitamin D3 analog, EB1089, on HL-60 cells by the induction of TGF-beta receptor

EB1089 is a novel 1,25(OH)2D3 analog that has more potent antitumor properties with reduced hypercalcemic effects than 1,25(OH)2D3. We investigated the role of transforming growth factor-beta1 (TGF-beta1) in the growth inhibition of human acute myeloid leukemia cell line, HL-60, by EB1089. Clonal growth of HL-60 cells was inhibited in a dose-dependent manner by EB1089. Although TGF-beta1 alone slightly inhibited proliferation of HL-60 cells, the addition of TGF-beta1 into culture treated with 10(-8) M of EB1089 showed a significant synergistic antiproliferative effect in a dose-dependent manner. EB1089 up-regulated the expression of TGF-beta receptor type I (TGF-beta RI), type II (TGF-beta RII) and TGF-beta1. Antiproliferative effect of EB1089 was partially reversed by TGF-beta1 neutralizing antibody (anti-TGF-beta1). Vitamin D receptor (VDR) expression was increased by TGF-beta1, suggesting synergistic action of TGF-beta1 and EB1089. Combined treatment of EB1089 and TGF-beta1 resulted in an increased expression of cyclin-dependent kinase inhibitor (CDKI), p27 protein, compared to either ligand alone. Up-regulation of p27 protein expression by either TGF-beta1 or EB1089 was reduced by anti-TGF-beta1. These findings suggest that TGF-beta1 is involved in the antiproliferative effect of EB1089 on HL-60 cells.

Ras induces p21Cip1/Waf1 cyclin kinase inhibitor transcriptionally through Sp1-binding sites

p21Cip1/Waf1 cyclin-dependent kinase inhibitor (p21) is inducible by Raf and mitogen-activated protein kinase kinase (MAPKK), but the level of regulation is unknown. We show here by conditional and transient Ras-expression models that Ras induces p21. Induction of p21 in conditionally Ras-expressing cells is posttranscriptional utilizing mitogen-activated protein kinase (MAPK) pathway. Transient, high-level Ras-expression induces transcriptional activation of p21 mediated by a GC-rich region in p21 promoter -83-54 bp relative to the transcription initiation site containing binding sites for Sp1-family transcription factors. Mutation of either Sp1-binding site 2 or 4 in this region decreases the magnitude of induction of promoter activity by Ras, but only the simultaneous mutation of both sites abolishes fully the induction. Electrophoretic mobility shift assays using an oligonucleotide corresponding to Sp1-binding site 2 indicate that both Sp1 and Sp3 transcription factors bind to this region. The results demonstrate that the central cytosolic growth regulator Ras is a potent transcriptional and posttranscriptional inducer of the nuclear growth inhibitor p21.

Proliferation arrest and induction of CDK inhibitors p21 and p27 by depleting the calcium store in cultured C6 glioma cells

C6 glioma - Ca2+ depletion - proliferation arrest morphology change - CDK inhibitor In this study, we investigated the role of the intracellular calcium store in modulating the cellular proliferation and the expression of cell cycle regulatory proteins in cultured C6 glioma cells. By means of microspectrofluorimetry and Ca(2+)-sensitive indicator fura-2, we found that the intracellular Ca2+ pump inhibitors, thapsigargin (TG) irreversibly and 2,5-ditert-butyl-hydroquinone (DBHQ) reversibly depleted the Ca(2+)-store accompanied with the induction of G0/G1 arrest, an increase in glial fibrillary acidic protein (GFAP) expression and morphological changes from a round flat shape to a differentiated spindle-shaped cell. The machinery underlying these changes induced by Ca(2+)-store depletion was investigated. The results indicated that Ca(2+)-store depletion caused an increased expression of p21 and p27 proteins (cyclin-dependent kinase inhibitors), with unchanged mutant p53 protein of C6 cells but reduced amounts of the cell cycle regulators: cyclin-dependent kinase 2 (CDK2), cdc2, cyclin C, cyclin D1, cyclin D3 and proliferating cell nuclear antigen (PCNA) in a time-dependent manner. These findings indicate a new function of the endoplasmic reticulum (ER) Ca2+ store in regulating cellular proliferation rate through altering the expression of p21 and p27 proteins. Moreover, cellular differentiation as revealed by spindle-shaped morphology and induced GFAP expression were also modulated by the ER Ca2+ store. The implication of this finding is that the abnormal growth of cancer cells such as C6 glioma cells may be derived from a signalling of the ER which can be manipulated by depleting the Ca2+ store.

Intestinal adaptation and enterocyte apoptosis following small bowel resection is p53 independent

Adaptation following small bowel resection (SBR) signals enterocyte proliferation and apoptosis. Because p53-induced p21(waf1/cip1) may be important for apoptosis in many cells, we hypothesized that these genes are required for increased enterocyte apoptosis during adaptation. Male C57BL/6 (wild-type) or p53-null mice underwent 50% proximal SBR or sham operation (bowel transection-reanastomosis). Adaptation (DNA-protein content, villus height-crypt depth, enterocyte proliferation), appearance of apoptotic bodies, and p53 and p21(waf1/cip1) protein expression were measured in the ileum after 5 days. Adaptation was equivalent after SBR in both wild-type and p53-null mice as monitored by significantly increased ileal DNA-protein content, villus height, and enterocyte proliferation. The number of crypt apoptotic bodies increased significantly after SBR evenly in both wild-type and p53-null mice. In the p53-null mice, SBR substantially induced the expression of p21(waf1/cip1) protein in villus enterocytes. The p53-independent induction of p21(waf1/cip1) may account for the similar intestinal response to SBR between wild-type and p53-null mice. Intestinal adaptation and increased enterocyte apoptosis following intestinal resection occur via a p53-independent mechanism.

Novel 20-epi-vitamin D3 analog combined with 9-cis-retinoic acid markedly inhibits colony growth of prostate cancer cells

BACKGROUND

1,25 dihydroxyvitamin D3 (1,25D) and retinoids may play an important role in preventing progression of prostate cancer.

METHODS

We examined the ability of four novel 20-epi-vitamin D3 analogs (CB1093, KH1060, KH1266, and CB1267), either alone or in combination with 9-cis retinoic acid (RA) to inhibit colony growth of a human prostate cancer cell line, LNCaP, using soft agar as well as bone marrow stroma. Also, the effect of these analogs on the cell cycle and expression of Ki-67, p21(waf-1), and p27(kip1) in LNCaP cells was examined.

RESULTS

The analog CB1267 was the most potent, with 8 x 10(-10) M of the analog inhibiting 50% colony growth (ED50) of LNCaP. 9-cis-RA also inhibited colony growth of LNCaP (ED50, 5 x 10(-7) M). Combined, CB1267 and 9-cis-RA synergistically inhibited colony growth and significantly increased the number of LNCaP cells in G0/G1 phase. Cell cycle arrest was associated with increased levels of p21(waf-1) and p27(kip1) and decreased expression of Ki-67 protein. Pulse-exposure to this combination (5 x 10(-8) M) irreversibly inhibited colony growth, both in soft agar and on normal human bone marrow stroma.

CONCLUSIONS

Combination of a new vitamin D3 analog (CB1267) and a retinoid (9-cis-RA) potently inhibited colony formation of LNCaP prostate cancer cells in vitro, suggesting further studies in animal models. This combination may afford an interesting therapeutic approach to low-burden prostate cancer.

Differential apoptosis by indomethacin in gastric epithelial cells through the constitutive expression of wild-type p53 and/or up-regulation of c-myc

Nonsteroidal anti-inflammatory drug (NSAID)-induced apoptosis is considered to be an important mechanism in the antineoplastic effects and damage produced by the drugs in the gastrointestinal tract. In this study, two different gastric cancer cell lines, MKN28 (mutant-type p53) and AGS (wild-type p53), were compared as to growth inhibition, apoptosis, and cell cycle and apoptosis-related gene expression in response to indomethacin treatment. Cell growth was measured by MTT (3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyl tetrazolium bromide) assay. Apoptosis was characterized by acridine orange staining and DNA fragmentation, and cell cycle kinetics by flow cytometry. The mRNA and protein levels of p53, p21waf1/cip1, and c-myc were determined by Northern and Western blotting. The results showed that indomethacin initiated growth inhibition and apoptosis in both cell lines without cell cycle shifting. AGS cells were more sensitive to growth inhibitory activity and apoptosis of indomethacin than MKN28 cells. In MKN28 cells, the levels of p53, p21waf1/cip1, and c-myc mRNA remained unchanged over the 24-hr treatment with indomethacin, but the p53 protein level was elevated after 4 hr. There was no change in the p21waf1/cip1 and c-myc protein levels in the MKN28 cells. In AGS cells, a progressive increase in c-myc mRNA and protein levels was noted, while p53 and p21waf1/cip1 remained unchanged. It can be concluded that wild-type p53 and/or up-regulation of c-myc is associated with indomethacin-mediated differential apoptosis in gastric epithelial cells.

Expression of p21Cip1/Waf1/Sdi1 and p27Kip1Cyclin-Dependent Kinase Inhibitors During Human Hematopoiesis

Expression of p21 and p27 cyclin-dependent kinase inhibitors is associated with induced differentiation and cell-cycle arrest in some hematopoietic cell lines. However, it is not clear how these inhibitors are expressed during normal hematopoiesis. We examined various human hematopoietic colonies derived from cord blood CD34+cells, bone marrow, and peripheral blood cells using a quantitative reverse transcription-polymerase chain reaction assay, immunochemistry, and/or Western blot analysis. p21 mRNA was expressed increasingly over time in all of the colonies examined (granulocytes, macrophages, megakaryocytes, and erythroblasts), whereas p27 mRNA levels remained low, except for erythroid bursts. Erythroid bursts expressed both p21 and p27 mRNAs with differentiation but expressed neither protein, whereas both proteins were expressed in megakaryocytes and peripheral blood monocytes. In bone marrow, p21 was immunostained almost exclusively in a subset of megakaryocytes and p27 protein was present in megakaryocytes, plasma cells, and endothelial cells. In megakaryocytes, reciprocal expression of p27 to Ki-67 was evident and an inverse relationship between p21 and Ki-67 positivities was also present, albeit less obvious. These observations suggest that a complex lineage-specific regulation is involved in p21 and p27 expression and that these inhibitors are involved in cell-cycle exit in megakaryocytes.

Expression of p21Cip1/Waf1/Sdi1 and p27Kip1Cyclin-Dependent Kinase Inhibitors During Human Hematopoiesis

Expression of p21 and p27 cyclin-dependent kinase inhibitors is associated with induced differentiation and cell-cycle arrest in some hematopoietic cell lines. However, it is not clear how these inhibitors are expressed during normal hematopoiesis. We examined various human hematopoietic colonies derived from cord blood CD34+cells, bone marrow, and peripheral blood cells using a quantitative reverse transcription-polymerase chain reaction assay, immunochemistry, and/or Western blot analysis. p21 mRNA was expressed increasingly over time in all of the colonies examined (granulocytes, macrophages, megakaryocytes, and erythroblasts), whereas p27 mRNA levels remained low, except for erythroid bursts. Erythroid bursts expressed both p21 and p27 mRNAs with differentiation but expressed neither protein, whereas both proteins were expressed in megakaryocytes and peripheral blood monocytes. In bone marrow, p21 was immunostained almost exclusively in a subset of megakaryocytes and p27 protein was present in megakaryocytes, plasma cells, and endothelial cells. In megakaryocytes, reciprocal expression of p27 to Ki-67 was evident and an inverse relationship between p21 and Ki-67 positivities was also present, albeit less obvious. These observations suggest that a complex lineage-specific regulation is involved in p21 and p27 expression and that these inhibitors are involved in cell-cycle exit in megakaryocytes.

1,25-Dihydroxyvitamin D3 Induces Differentiation of a Retinoic Acid–Resistant Acute Promyelocytic Leukemia Cell Line (UF-1) Associated With Expression of p21WAF1/CIP1 and p27KIP1

Retinoic acid (RA) resistance is a serious problem for patients with acute promyelocytic leukemia (APL) who are receiving all-transRA. However, the mechanisms and strategies to overcome RA resistance by APL cells are still unclear. The biologic effects of RA are mediated by two distinct families of transcriptional factors: RA receptors (RARs) and retinoid X receptors (RXRs). RXRs heterodimerize with 1,25-dihydroxyvitamin D3[1,25(OH)2D3] receptor (VDR), enabling their efficient transcriptional activation. The cyclin-dependent kinase (cdk) inhibitor p21WAF1/CIP1 has a vitamin D3–responsive element (VDRE) in its promoter, and 1,25(OH)2D3 enhances the expression of p21WAF1/CIP1 and induces differentiation of selected myeloid leukemic cell lines. We have recently established a novel APL cell line (UF-1) with features of RA resistance. 1,25(OH)2D3 can induce growth inhibition and G1 arrest of UF-1 cells, resulting in differentiation of these cells toward granulocytes. This 1,25(OH)2D3-induced G1 arrest is enhanced by all-trans RA. Also, 1,25(OH)2D3 (10−10 to 10−7 mol/L) in combination with RA markedly inhibits cellular proliferation in a dose- and time-dependent manner. Associated with these findings, the levels of p21WAF1/CIP1 and p27KIP1 mRNA and protein increased in these cells. Northern blot analysis showed that p21WAF1/CIP1 and p27KIP1 mRNA and protein increased in these cells. Northern blot analysis showed that p21WAF1/CIP1 and p27KIP1 transcripts were induced after 6 hours’ exposure to 1,25(OH)2D3 and then decreased to basal levels over 48 hours. Western blot experiments showed that p21WAF1/CIP1 protein levels increased and became detectable after 12 hours of 1,25(OH)2D3treatment and induction of p27KIP1 protein was much more gradual and sustained in UF-1 cells. Interestingly, the combination of 1,25(OH)2D3 and RA markedly enhanced the levels of p27KIP1 transcript and protein as compared with levels induced by 1,25(OH)2D3 alone. In addition, exogenous p27KIP1 expression can enhance the level of CD11b antigen in myeloid leukemic cells. In contrast, RA alone can induce G1 arrest of UF-1 cells; however, it did not result in an increase of p21WAF1/CIP1 and p27KIP1transcript and protein expression in RA-resistant cells. Taken together, we conclude that 1,25(OH)2D3 induces increased expression of cdk inhibitors, which mediates a G1 arrest, and this may be associated with differentiation of RA-resistant UF-1 cells toward mature granulocytes.

1,25-Dihydroxyvitamin D3 Induces Differentiation of a Retinoic Acid–Resistant Acute Promyelocytic Leukemia Cell Line (UF-1) Associated With Expression of p21WAF1/CIP1 and p27KIP1

Retinoic acid (RA) resistance is a serious problem for patients with acute promyelocytic leukemia (APL) who are receiving all-transRA. However, the mechanisms and strategies to overcome RA resistance by APL cells are still unclear. The biologic effects of RA are mediated by two distinct families of transcriptional factors: RA receptors (RARs) and retinoid X receptors (RXRs). RXRs heterodimerize with 1,25-dihydroxyvitamin D3[1,25(OH)2D3] receptor (VDR), enabling their efficient transcriptional activation. The cyclin-dependent kinase (cdk) inhibitor p21WAF1/CIP1 has a vitamin D3–responsive element (VDRE) in its promoter, and 1,25(OH)2D3 enhances the expression of p21WAF1/CIP1 and induces differentiation of selected myeloid leukemic cell lines. We have recently established a novel APL cell line (UF-1) with features of RA resistance. 1,25(OH)2D3 can induce growth inhibition and G1 arrest of UF-1 cells, resulting in differentiation of these cells toward granulocytes. This 1,25(OH)2D3-induced G1 arrest is enhanced by all-trans RA. Also, 1,25(OH)2D3 (10−10 to 10−7 mol/L) in combination with RA markedly inhibits cellular proliferation in a dose- and time-dependent manner. Associated with these findings, the levels of p21WAF1/CIP1 and p27KIP1 mRNA and protein increased in these cells. Northern blot analysis showed that p21WAF1/CIP1 and p27KIP1 mRNA and protein increased in these cells. Northern blot analysis showed that p21WAF1/CIP1 and p27KIP1 transcripts were induced after 6 hours’ exposure to 1,25(OH)2D3 and then decreased to basal levels over 48 hours. Western blot experiments showed that p21WAF1/CIP1 protein levels increased and became detectable after 12 hours of 1,25(OH)2D3treatment and induction of p27KIP1 protein was much more gradual and sustained in UF-1 cells. Interestingly, the combination of 1,25(OH)2D3 and RA markedly enhanced the levels of p27KIP1 transcript and protein as compared with levels induced by 1,25(OH)2D3 alone. In addition, exogenous p27KIP1 expression can enhance the level of CD11b antigen in myeloid leukemic cells. In contrast, RA alone can induce G1 arrest of UF-1 cells; however, it did not result in an increase of p21WAF1/CIP1 and p27KIP1transcript and protein expression in RA-resistant cells. Taken together, we conclude that 1,25(OH)2D3 induces increased expression of cdk inhibitors, which mediates a G1 arrest, and this may be associated with differentiation of RA-resistant UF-1 cells toward mature granulocytes.

Metabolism of 1alpha,25(OH)2D3 and its 20-epi analog integrates clonal expansion, maturation and apoptosis during HL-60 cell differentiation

Induction of growth arrest and monocyte differentiation of HL-60 leukemia cells by 1alpha,25 dihydroxyvitamin D3 (1alpha,25(OH)2D3) is well established. By contrast, we have observed, that 1alpha,25(OH)2D3 and its metabolites play separate roles in clonal expansion and survival of differentiating HL-60 cells. Cells that had differentiated by 48 h (CD14 positive) grew slower than control cells, whereas CD14 negative cells were growing faster at this time point. Inhibiting 1alpha,25(OH)2D3 or 1alpha,25(OH)2-20-epi-D3 metabolism, by the 25(OH)D3-24-hydroxylase inhibitor ketoconazole, abolished hyperproliferation of CD14 negative cells. Instead, both the onset of differentiation and subsequent apoptosis were enhanced. These events were associated with immediate up-regulation of the cyclin-dependent kinase inhibitor p21(waf1) and a lack of sustained expression, respectively. Stimulation and inhibition of growth by vitamin D3-related compounds was observed to be concentration and metabolite specific. Low amounts of 1alpha,25(OH)2-20-epi-D3 and 1alpha,24,25(OH)3-20-epi-D3 stimulated HL-60 cell growth. At higher concentrations, 1alpha,25(OH)2-20-epi-D3 was a more potent inducer than 1alpha,24,25(OH)3-20-epi-D3 of HL-60 differentiation; 1alpha,25(OH)2-20-epi-24-oxo-D3 was exclusively pro-differentiative at all concentrations. 1alpha,25(OH)2-20-epi-D3 and 1alpha,24,25(OH)3-20-epi-D3 stimulated proliferation of KG-1a leukemia cells, but neither of these compounds nor 1alpha,25(OH)3-20-epi-24-oxo-D3 exerted pro-differentiative effects on these cells. These findings shed new light on the pro- and anti-proliferative effects of 1alpha,25(OH)2D3 and lead to the postulate that metabolism of 1alpha,25(OH)2D3 and its 20-epi analog regulates different subsets of genes so as to co-ordinate population expansion and the differentiation process. Furthermore, 1alpha,25(OH)2D3 metabolism and/or sensitivity to the effects of metabolites may be altered in transformed cells to derive a clonal advantage.

Raf-1-induced cell cycle arrest in LNCaP human prostate cancer cells

Prostate cancer is the most commonly diagnosed neoplasm in men. LNCaP cells continue to possess many of the molecular characteristics of in situ prostate cancer. These cells lack ras mutations, and mitogen-activated protein kinase (MAPK) is not extensively phosphorylated in these cells. To determine the effects of ras/raf/MAPK pathway activation in these cells, we transfected LNCaP cells with an activatable form of c-raf-1(deltaRaf-1:ER). Activation of deltaRaf-1:ER, with resultant MAPK activation, reduced plating efficiency and soft agarose cloning efficiency 30-fold in LNCaP cells. Cell cycle distribution showed an accumulation of cells in G1 and was associated with the induction of CDK inhibitor p21WAF1/CIP1 at the protein and mRNA levels. p21WAF1/CIP1 mRNA stability was increased after deltaRaf-1:ER activation. In addition, activated deltaRaf-1:ER induced the senescence associated-beta-galactosidase in LNCaP cells. These data demonstrate that raf activation can activate growth inhibitory pathways leading to growth suppression in prostate carcinoma cells and also suggest that raf/MEK/MAPK pathway activation, rather than inhibition, may be a therapeutic target for some human prostate cancer cells.

Genistein-induced upregulation of p21WAF1, downregulation of cyclin B, and induction of apoptosis in prostate cancer cells

Increased soy consumption in Asian diets, resulting in increased serum isoflavone levels, has been associated with a decreased risk for prostate adenocarcinoma (PCa). The isoflavone genistein is believed to be the anticancer agent found in soy, and significant levels of genistein have been detected in human prostatic fluid, implicating the role of genistein in PCa prevention. Recent studies have demonstrated genistein's ability to inhibit cell growth and induce apoptosis in several cell lines; however, the molecular mechanisms of genistein's effect are not known. We have evaluated the mechanism by which genistein may inhibit PCa cell growth. Here we report that genistein inhibits PCa cell growth in culture in a dose-dependent manner, which is accompanied by a G2/M cell cycle arrest. Cell growth inhibition was observed with concomitant downregulation of cyclin B, upregulation of the p21WAF1 growth-inhibitory protein, and induction of apoptosis. Collectively, these results provide experimental evidence for a novel effect of genistein on cell cycle gene regulation, resulting in the inhibition of cell growth and ultimate demise of tumor cells.

Comparative effects of overexpression of p27Kip1 and p21Cip1/Waf1 on growth and differentiation in human colon carcinoma cells

Recent studies have shown that decreased expression of p27Kip1 is associated with high grade tumors and an unfavorable prognosis in several types of human cancer. To clarify the role of p27Kip1 in colon cancer, we have overexpressed this protein in the HT29 colon cancer cell line. The derivatives displayed an increase in the p27Kip1 protein in cyclin E/CDK2 immunoprecipitates and a decrease in cyclin E-associated kinase activity when compared to vector control clones, providing evidence that the overexpressed protein was functional. Clones with a high level of p27Kip1 displayed partial growth inhibition in monolayer culture and a decrease in plating efficiency, even though they expressed increased levels of the cyclin D1 protein. Using alkaline phosphatase expression as a marker, we found that the p27Kip1 overexpressor clones displayed a 2-3-fold increase in sensitivity to induction of differentiation by 2 mM sodium butyrate. In contrast to these results, derivatives of HT29 cells that stably overexpressed p21Cip1/Waf1 displayed decreased sensitivity to the induction of differentiation. These findings may explain why decreased levels of p27Kip1 in certain human cancers is associated with high grade (poorly differentiated) tumors, and suggest that strategies that increase the level of p27Kip1 may be useful in cancer therapy.

Inhibition of Human Umbilical Vein Endothelial Cell Proliferation by the CXC Chemokine, Platelet Factor 4 (PF4), Is Associated With Impaired Downregulation of p21Cip1/WAF1

Human PF4 is a heparin-binding chemokine known to be capable of inhibiting endothelial cell proliferation and angiogenesis. To explore the biological mechanisms responsible for this action, we investigated the effect of PF4 on epidermal growth factor (EGF)-stimulated human umbilical vein endothelial cells (HUVEC), a model system in which stimulation is essentially independent of interaction with cell-surface glycosaminoglycans. Based on previous findings that PF4 blocks endothelial cell cycle entry and progression into S phase, we studied the molecular mechanism(s) of PF4 interference with cell cycle machinery. PF4 treatment of EGF-stimulated HUVEC caused a decrease in cyclin E–cyclin-dependent kinase 2 (cdk2) activity with resulting attenuation of retinoblastoma protein phosphorylation. PF4-dependent downregulation of cyclin E-cdk2 activity was associated with increased binding of the cyclin-dependent kinase inhibitor, p21Cip1/WAF1, to the cyclin E-cdk2 complex. Analysis of total cellular p21Cip1/WAF1 showed that in the presence of PF4, p21Cip1/WAF1 levels were sustained at time points when p21Cip1/WAF1 was no longer detectable in cells stimulated by EGF in the absence of PF4. These findings indicate that PF4 inhibition of HUVEC proliferation in response to EGF is associated with impaired downregulation of p21Cip1/WAF1 and provide the first evidence for interference with cell cycle mechanisms by a chemokine.

Synergistic inhibition of prostate cancer cell lines by a 19-nor hexafluoride vitamin D3 analogue and anti-activator protein 1 retinoid

The secosteroid hormones, all-trans- and 9-cis-retinoic acid and vitamin D3, have demonstrated significant capacity to control proliferation in vitro of many solid tumour cell lines. Cooperative synergistic effects by these two ligands have been reported, and it is, therefore, possible that greater therapeutic effects could be achieved if these compounds were administered together. The role of retinoid-dependent anti-activator protein 1 (anti-AP-1) effects in controlling cancer cell proliferation appears significant. We have utilized an anti-AP-1 retinoid [2-(4,4-dimethyl-3,4-dihydro-2H-1 benzopyran-6-yl)carbonyl-2-(4-carboxyphenyl)-1,3,-dithiane; SR11238], which does not transactivate through a retinoic acid response element (RARE), and a potent vitamin D3 analogue [1alpha,25(OH)2-16-ene-23-yne-26,27-F6-19-nor-D3, code name LH] together at low, physiologically safer doses against a panel of prostate cancer cell lines that represent progressively more transformed phenotypes. The LNCaP (least transformed) and PC-3 (intermediately transformed) cell lines were synergistically inhibited in their clonal growth by the combination of LH and SR11238, whereas SR11238 alone was essentially inactive. DU-145 cells (most transformed) were completely insensitive to these analogues. LNCaP cells, but neither PC-3 nor DU-145, underwent apoptosis in the presence of LH and SR11238. Transactivation of the human osteocalcin vitamin D response element (VDRE) by LH was not enhanced in the presence of SR11238, although the expression of E-cadherin in these cells was additively up-regulated in the presence of both compounds. These data suggest the anti-AP-1 retinoid and the vitamin D3 analogue may naturally act synergistically to control cell proliferation, a process that is interrupted during transformation, and that this combination may form the basis for treatment of some androgen-independent prostate cancer.

Inhibition of Human Umbilical Vein Endothelial Cell Proliferation by the CXC Chemokine, Platelet Factor 4 (PF4), Is Associated With Impaired Downregulation of p21Cip1/WAF1

Human PF4 is a heparin-binding chemokine known to be capable of inhibiting endothelial cell proliferation and angiogenesis. To explore the biological mechanisms responsible for this action, we investigated the effect of PF4 on epidermal growth factor (EGF)-stimulated human umbilical vein endothelial cells (HUVEC), a model system in which stimulation is essentially independent of interaction with cell-surface glycosaminoglycans. Based on previous findings that PF4 blocks endothelial cell cycle entry and progression into S phase, we studied the molecular mechanism(s) of PF4 interference with cell cycle machinery. PF4 treatment of EGF-stimulated HUVEC caused a decrease in cyclin E–cyclin-dependent kinase 2 (cdk2) activity with resulting attenuation of retinoblastoma protein phosphorylation. PF4-dependent downregulation of cyclin E-cdk2 activity was associated with increased binding of the cyclin-dependent kinase inhibitor, p21Cip1/WAF1, to the cyclin E-cdk2 complex. Analysis of total cellular p21Cip1/WAF1 showed that in the presence of PF4, p21Cip1/WAF1 levels were sustained at time points when p21Cip1/WAF1 was no longer detectable in cells stimulated by EGF in the absence of PF4. These findings indicate that PF4 inhibition of HUVEC proliferation in response to EGF is associated with impaired downregulation of p21Cip1/WAF1 and provide the first evidence for interference with cell cycle mechanisms by a chemokine.

Cip/Kip cyclin-dependent kinase inhibitors: brakes of the cell cycle engine during development

Precise control of cell-cycle progression is believed to be critical for normal development, while oncogenesis may be a direct result of its disturbance. Cell-cycle progression is regulated predominantly by a series of serine/threonine kinases, the cyclin-dependent kinases (CDKs). The activities of the CDKs are controlled by a variety of mechanisms, and a group of molecules that inhibit CDK activity, CDK inhibitors (CKIs), has recently become the focus of interest, particularly in the fields of development and tumorigenesis. To date, seven CKIs have been identified in mammals and categorized into two families, the Cip/Kip and Ink4 families. The Cip/Kip family is well conserved phylogenetically, suggesting that it is biologically important. Despite the structural and biochemical similarities among the Cip/Kip members, the phenotypes of knockout mice of each Cip/Kip member are surprisingly different, which suggests that the Cip/Kip CKIs have a variety of physiological functions. In this review, the biological roles of Cip/Kip CKIs in development and tumor suppression are discussed.

DNA damage induces p21 protein expression by inhibiting ubiquitination in ML-1 cells

We previously reported that deferoxamine, an iron chelating agent, induced p53 and cell accumulation in the G1 phase of ML-1 cells in the same way as the DNA damaging agent, etoposide. Etoposide treatment increased expression of the p21 gene, a cyclin kinase inhibitor, at both the mRNA and protein levels. However, deferoxamine treatment only increased the p21 mRNA level without the appearance of a detectable protein product. A substrate for cyclin kinase, pRB, was unphosphorylated by etoposide treatment, but remained unaffected by deferoxamine, indicating that p21 was functional after etoposide, but not after deferoxamine treatment. Therefore, in the present study, we investigated the involvement of the ubiquitin proteasome pathway in post-transcriptional regulation of p21. By the addition of lactacystin, a proteasome inhibitor, to deferoxamine treatment, the level of unubiquitinated p21 protein product was similar to that induced by etoposide treatment, and the ubiquitinated p21 bands became apparent. After etoposide treatment, the level of ubiquitinated p21 was diminished and a high level of unubiquitinated p21 expression was observed. We concluded that (1) efficient expression of p21 protein requires inhibition of the ubiquitin-proteasome pathway, and (2) DNA damage inhibits the ubiquitination of p21.

Expression of p21 (Waf1/Cip1) in head and neck cancer in relation to proliferation, differentiation, p53 status and cyclin D1 expression

p21(Waf1/Cipl) is a critical downstream effector in the p53-dependent pathway of growth control and causes growth arrest through inhibition of cyclin-dependent kinases. In this study 67% of 43 head and neck squamous cell carcinoma (HNSCC) and 60% of 15 tumour-adjacent oral dysplasias overexpressed p21 by immunohistochemical staining. Overexpression of p21 in HNSCC was independent of the presence of functional p53, as assessed by analysis of mutations and loss of heterozygosity and by immunohistochemisty. Rather, the expression pattern of p21 was associated with differentiation. Furthermore, in most tumours, the p21 positive cells did not incorporate bromodeoxyuridine (BrdU), which indicates inhibition of proliferation by p21 in these cells. In some tumours, p21 was also expressed in proliferating cells. In these latter tumour cells, cyclin D1 was frequently expressed as well. Therefore, we suggest that expression of cyclin D1 might overcome the inhibitory effect of p21 in these cells.

Expression of cyclin-dependent kinase inhibitor p21 in human liver

The p21 protein is a universal inhibitor of cyclin-dependent kinases and of cell-cycle progression and is involved in numerous growth-inhibitory pathways in cell culture systems. Recent studies suggest that p21 regulates hepatocyte cell cycle progression in models of liver regeneration. The present study was designed to investigate the possible involvement of p21 in the control of hepatocyte proliferation in human liver diseases. To examine that, the expression of p21 in clinical liver biopsy specimens was determined by immunohistochemistry. This was correlated with hepatocyte Ki-67 immunostaining (a marker of hepatocyte proliferation in vivo) as well as histologic features. Little p21 or Ki-67 expression was detected in normal human liver or in specimens of nonalcoholic steatohepatitis. In patients with alcoholic hepatitis, increased expression of p21, but not of Ki-67, was observed. In specimens with chronic hepatitis C, hepatocyte p21 expression was significantly correlated with Ki-67 immunostaining, as well as with the degree of inflammation and fibrosis. These results indicate that hepatocyte p21 expression is upregulated in response to hepatic injury and correlates with histologic markers of proliferation and disease activity. This study provides evidence that p21 plays a role in the regulation of hepatocyte proliferation in human liver diseases.

p21(waf1) mRNA contains a conserved element in its 3'-untranslated region that is bound by the Elav-like mRNA-stabilizing proteins

The Elav-like proteins are specific mRNA-binding proteins that regulate mRNA stability. The neuronal members of this family (HuD, HuC, and Hel-N1) are required for neuronal differentiation. In this report, using purified HuD protein we have localized a high affinity HuD binding site to a 42-nucleotide region within a U-rich tract in the 3'-untranslated region p21(waf1) mRNA. The binding of HuD to this site is readily displaced by an RNA oligonucleotide encoding the HuD binding site of c-fos. The sequence of this binding site is well conserved in human, mouse, and rat p21(waf1) mRNA. p21(waf1) is an inhibitor of cyclin-dependent kinases and proliferating cell nuclear antigen and induces cell cycle arrest at G1/S, a requisite early step in cell differentiation. The identification of an Elav-like protein binding site in the 3'-untranslated region of p21(waf1) provides a novel link between the induction of differentiation, mRNA stability, and the termination of the cell cycle.

Regulation of p21(WAF1) Expression During Normal Myeloid Differentiation

The G1-phase cell-cycle inhibitor p21 has been proposed to mediate growth arrest during differentiation. Upregulation of p21 has been shown in multiple cell lines induced to differentiate; however, the mechanism of p21 induction during normal differentiation is largely unknown. In this report, we use normal hematopoietic precursor cells obtained from umbilical cord to model p21 regulation during differentiation. Myeloid maturation of CD34+ precursor cells is associated with a marked increase in p21 expression at the RNA and protein level. The upregulation of p21 transcripts during differentiation is associated with decreased binding to a highly conserved 44-bp fragment within the p21 promoter. This 44-bp regulatory element binds a novel modulator of p21 expression. It is of considerable interest that, although the binding activity is expressed in p53-negative as well as in p53-positive cells, the DNA sequence recognized by this protein overlaps a PuPuPuC(A/T)(T/A)GPyPyPy consensus sequence for p53.

Regulation of p21(WAF1) Expression During Normal Myeloid Differentiation

The G1-phase cell-cycle inhibitor p21 has been proposed to mediate growth arrest during differentiation. Upregulation of p21 has been shown in multiple cell lines induced to differentiate; however, the mechanism of p21 induction during normal differentiation is largely unknown. In this report, we use normal hematopoietic precursor cells obtained from umbilical cord to model p21 regulation during differentiation. Myeloid maturation of CD34+ precursor cells is associated with a marked increase in p21 expression at the RNA and protein level. The upregulation of p21 transcripts during differentiation is associated with decreased binding to a highly conserved 44-bp fragment within the p21 promoter. This 44-bp regulatory element binds a novel modulator of p21 expression. It is of considerable interest that, although the binding activity is expressed in p53-negative as well as in p53-positive cells, the DNA sequence recognized by this protein overlaps a PuPuPuC(A/T)(T/A)GPyPyPy consensus sequence for p53.

Growth arrest associated with 12-o-tetradecanoylphorbol-13-acetate-induced hematopoietic differentiation with a defective retinoblastoma tumor suppressor-mediated pathway

The retinoblastoma tumor suppressor (Rb) gene product plays an essential role in cell-cycle regulation. However, its role in terminal differentiation of hematopoietic cells is speculative. Here we show a model of 12-o-tetradecanoylphorbol-13-acetate (TPA)-induced hematopoietic differentiation and growth arrest with a defective Rb-mediated pathway. TPA treatment arrested the cell cycle of a human hematopoietic cell line, MEG-01s, at the G1-S boundary and induced expression of p21/SDI1/WAF1/CIP1 and p27/KIP1. Both of these proteins were present in cyclin E-associated complexes, the histone H1 and Rb kinase activities of which were then inactivated. However, MEG-01s cells lacked the intact Rb protein and the Rb-mediated pathway was defective. This model raises a question about the role for Rb in terminal differentiation of hematopoietic cells.

Post-transcriptional induction of p21cip1 protein in condylomata and dysplasias is inversely related to human papillomavirus activities

Infections of the genital and oral epithelia by human papillomaviruses cause condylomata, papillomas, and squamous intraepithelial neoplasms, some of which can progress to invasive cancers. We describe an induction of p21cip1/WAF1/sdi1 protein in a fraction of the spinous cells in benign lesions and in cervical intraepithelial neoplasia grades I and II. The induction appears to be post-transcriptional and independent of p53. p21cip1 antigen-positive cells were sporadic in cervical intraepithelial neoplasia III and rare and focal in carcinomas. In contrast, p21cip1 protein was below or at the threshold of detection in the differentiated cells of normal squamous epithelia from different body sites despite an up-regulation of p21cip1 RNA. In cervical intraepithelial neoplasias from patients who were also positive for the human immunodeficiency virus, there was an additional increase in p21cip1 RNA in the upper spinous cells without concomitant p21cip1 protein induction. A consistent inverse relationship was observed between the p21cip1 protein induction and abundant human papillomavirus DNA and RNAs. We propose that p21cip1 protein induction is a novel host response that inhibits viral DNA replication and thus prevents elevated viral transcription. This hypothesis can partly account for the heterogeneity and the differentiation-dependent viral activities commonly observed in benign human papillomavirus lesions.

Protein kinase C alpha modulates growth and differentiation in Caco-2 cells

BACKGROUND & AIMS

Caco-2 cells have been used extensively to elucidate events involved in intestinal cell proliferation and differentiation. Because individual isoforms of protein kinase C (PKC) and p21waf1, a cyclin-dependent kinase inhibitor, may regulate these processes, their role(s) on the growth and differentiation of Caco-2 cells were assessed.

METHODS

Protein abundance and subcellular distribution of several PKC isoforms, as well as the expression of p21waf1, were examined in preconfluent and postconfluent cells.

RESULTS

In cells at confluence (approximately 7 days postplating) and during their postconfluent phase (up to 20 days postplating), both total protein expression of PKC-alpha and its particulate distribution increased compared with their 3-day postplated counterparts. These findings were in agreement with those obtained by immunocytochemistry of PKC-alpha. In contrast, neither the total expression nor the subcellular distribution of PKC-betaI, -betaII, -delta, or -zeta changed significantly during these time periods. In addition, the expression of p21waf1, which can be induced by PKC-alpha, increased in postconfluent cells.

CONCLUSIONS

PKC-alpha, but not other isoforms of PKC, may modulate the proliferation and differentiation of Caco-2 cells. This regulation appears to be mediated, at least in part, via a mechanism involving p21waf1.

p21WAF1/Cip1 is associated with cyclin D1CCND1 expression and tubular differentiation but is independent of p53 overexpression in human breast carcinoma

p21WAF1/Cip1 is an inhibitor of cdk/cyclin complexes, and thus regulates the cell cycle. p21 is also related to cell differentiation and is regulated by wild-type p53, although p53-independent regulatory pathways have been proposed. In order to analyse p21 expression as well as its relationship with p53 in human breast cancer, an immunohistochemical analysis was undertaken of 77 breast carcinomas, 16 of them with an in situ component; 30 adjacent normal tissue samples; and five non-neoplastic specimens. Forty-four infiltrating carcinomas (57 per cent) were p21-positive. Expression of p21 was also observed in pre-invasive lesions, whereas normal ducts were negative or focally and weakly positive. p21 expression was associated with high histological grade (II + III) (P = 0.017) and poor tubule formation (P = 0.002), and was significantly less frequent in lobular carcinomas (P = 0.0001). p21 positivity also correlated with increased proliferation, but this seemed to be dependent on the histological grade. Twenty carcinomas (26 per cent) showed p53 overexpression, but this was not associated with p21 negativity, suggesting the existence of p53-independent mechanisms for p21 regulation in vivo. Cyclin D1CCND1 expression was analysed in the same series and an association between p21 and cyclin D1 expression was found, since 23 of 26 cyclin D1-positive carcinomas were p21-positive (P < 0.001 ...). In conclusion, p21 is frequently overexpressed in breast carcinomas and this occurs in the early stages of neoplastic progression. This overexpression seems to be independent of p53 status and might be involved in cyclin D1 modulation.

Adenovirus E1A-regulated transcription factor p120E4F inhibits cell growth and induces the stabilization of the cdk inhibitor p21WAF1

Adenovirus E1A proteins influence cell growth and phenotype through physical interactions with cellular proteins that regulate basic processes such as cell cycle progression, DNA synthesis, and differentiation. p120E4F is a low-abundance cellular transcription factor that represses the adenovirus E4 promoter and is regulated by E1A, through a phosphorylation-induced reduction of its DNA binding activity, to permit activation of the E4 promoter during early infection. To determine the normal biological role of p120E4F, we assessed its ability to influence fibroblast cell growth and transformation. p120E4F suppressed NIH 3T3 fibroblast colony formation but had little effect when coexpressed with E1A and/or activated ras. Cells that overexpressed p120E4F were inhibited in their ability to enter S phase, had elevated levels of the cdk inhibitor p21WAF1, and reduced cyclin D-cdk4/6 kinase activity. The increase of p21WAF1 levels occurred through a p53-independent posttranscriptional mechanism that included a three- to fourfold increase in the half-life of p21WAF1 protein. Coexpression of activated ras with p120E4F stimulated cyclin D1 expression, elevated cyclin D-cdk4/6 kinase activity, and accelerated cell growth. These data suggest an important role for p120E4F in normal cell division and demonstrate that p21WAF1 can be regulated by protein turnover.

Cyclin dependent kinase inhibitors

Progression through the eukaryotic cell cycle is regulated by the activities of a family of cyclin dependent kinases (CDKs). These kinases are negatively regulated by phosphorylation and by the action of cyclin kinase inhibitors (CKIs). In mammalian cells, two classes of CKIs have been identified, the INK4 class and the CIP/KIP class. These CKIs are versatile negative regulators of CDK function and have potential roles in development, checkpoint control and tumour suppression. Analysis of CKI knockout indicates that although these inhibitors are not generally required for survival, the phenotypes observed span the gamut of what might be expected for loss of a cell cycle inhibitor. This chapter summarizes our current understanding of the roles of CKIs in growth control.

Complex Regulation of CDK2 During Phorbol Ester-Induced Hematopoietic Differentiation

Phorbol myristate acetate (PMA) treatment of U937 human leukemic cells results in late G1 cell cycle arrest and terminal monocyte/macrophage-like differentiation. The PMA-induced G1 arrest involves a marked decrease in cdk2 activity, which correlates with total cdk2 dephosphorylation. Here, we show that the levels of cyclin A mRNA and protein markedly decrease during PMA-induced differentiation of U937 cells. In contrast, the level of cyclin E protein remains unchanged and in a complex with cdk2 during the entire course of PMA treatment. During the PMA-induced differentiation, cyclin E-associated cdk2 activity drops markedly. Furthermore, the amount of p27Kip1 protein associated with cyclin E/cdk2 greatly increases 24 to 72 hours after PMA treatment. The absence of changes in p27Kip1 mRNA levels by Northern blot suggest that the levels of this protein are controlled by posttranscriptional or posttranslational mechanism(s). These results show that the mechanisms mediating PMA-induced G1 arrest are complex. The inhibition of cdk2 activity is associated with (1) a decrease in cyclin A protein levels, (2) inactivation of cdk2 complexes, and (3) upregulation of p27Kip1 protein.

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

Glucocorticoids are known to impair the postnatal development of lung parenchyma by altering the formation of alveoli, and from the current understanding of the processes controlling the growth of the alveolar structure, it is likely that this impairment relies in large part on alteration of alveolar epithelial cell replication. From recent studies on the modulation of cell proliferation by glucocorticoids, it appears that events associated with the G1 phase of the cell cycle are a major target for the actions of these hormones. To gain some insights into the mechanisms involved in the growth arrest of lung alveolar epithelial cells by glucocorticoids, we focused in the present study on the effects of these hormones on the expression of the G1 cyclins and their cell cycle-dependent kinases (CDKs). We observed that when cells were blocked in their proliferation by dexamethasone treatment, no changes in the expression of the various G1 cyclins, D1, D2, D3, or E, could be documented. Also, the levels of CDK2 and CDK4 in glucocorticoid-treated cells did not exhibit significant modifications compared with the levels in proliferating cells. Evaluation of the activity of cyclin-CDK complexes showed that activation of cyclin D-CDK4 was not modified by dexamethasone. By contrast, differences in the activity of cyclin E-CDK2 complexes were found, with a profound decrease in the extracts of cells growth arrested by dexamethasone. Studies of the factors potentially implicated in the inactivation of these complexes strongly suggested a role for p21CIP1, as a dramatic accumulation of this protein was observed in cells treated with dexamethasone. Moreover, changes in p21CIP1 expression appeared to be controlled mostly at the posttranscriptional level. Interestingly, a decrease in the levels of p27KIP1 could be observed. These results indicate that glucocorticoids block entry of alveolar epithelial cells into S phase by specifically altering the activation of cyclin E-CDK2 complexes through induction of the CDK inhibitor p21CIP1.

Analysis of p21Waf1/Cip1 expression in normal, premalignant, and malignant cells during the development of human lung adenocarcinoma

Our studies suggested that adenocarcinoma of the peripheral lung mostly develops by several steps from atypical adenomatous hyperplasia through early adenocarcinoma to overt adenocarcinoma, and that some p53 abnormalities play an important role in this progression. In the present study, we examined by immunohistochemistry the expression of p53-inducible cyclin-dependent kinase inhibitor p21Waf1/Cip1 (p21) in the cells at various developmental stages of lung adenocarcinoma (32 lesions of adenomatous hyperplasia, 14 of early adenocarcinoma, 23 of well differentiated adenocarcinoma, and 17 of moderately or poorly differentiated adenocarcinoma) in comparison with 19 reactive proliferative lesions and analyzed the relationship between p53 and p21 expression. Bronchioalveolar cells in the normal lung expressed very little or no p21 and no p53 expression. In not only reactive but also neoplastic lesions regardless of their developmental stage, the cells expressed p21 at various frequencies. The average labeling indices ranged from 5.4 to 13.8%, and there was no significant difference between any of these categories. The expression of p21, however, tended to be relatively low in moderately and poorly differentiated adenocarcinomas (5.5%) compared to well differentiated adenocarcinomas (12.2%), and high-level p21 expressors (10% < or = positive cells) were more frequent in the latter group (1 of 17 (6%) versus 3 of 23 (35%), P < 0.05), suggesting that p21 expression is affected by the degree of differentiation of the neoplastic cells. Although the correlation was positive between the expression of p21 and p53 in reactive lesions (r = 0.88; P < 0.001), none was found in neoplastic lesions at any step or grade (-0.12 < or = r < or = 0.26). These results indicated that p21 expression depends upon p53 expression in reactive lung cells, whereas p21 expression is at least in part independent of that of p53 from the earliest to the most fully developed step of lung adenocarcinoma tumorigenesis. We concluded that disruption of the p53-dependent cell cycle regulation is a very early event in the tumorigenesis of lung adenocarcinoma.

Ectopic expression of decorin protein core causes a generalized growth suppression in neoplastic cells of various histogenetic origin and requires endogenous p21, an inhibitor of cyclin-dependent kinases

Decorin belongs to a family of secreted, small, leucine-rich proteoglycans that affect matrix assembly and cellular growth. Ectopic expression of decorin proteoglycan or protein core as a mutated form lacking any glycosaminoglycan side chains induced growth suppression in neoplastic cells of various histogenetic origins, including tumor cells derived from gastrointestinal, genital, skeletal, cutaneous, or bone marrow tissues. Exogenously added recombinant decorin also suppressed overall growth of the parental cell lines. In all stably-transfected clones, growth retardation was specifically associated with induction of the potent cyclin-dependent kinase inhibitor p21, but not p27, and subsequent translocation of p21 protein into the nuclei of decorin-expressing cells. This led to a greater proportion of the cells arrested in G1 phase of the cell cycle. These changes were independent of functional p53 or retinoblastoma protein. De novo expression of decorin in HCT116 human colon carcinoma cells harboring a disrupted p21 gene failed to induce growth suppression, in contrast to the wild-type cells in which p21 and growth arrest could be induced. These findings indicate that ectopic production of decorin protein core can retard the growth of a variety of tumor cells and that endogenous p21 is a required downstream effector of this biological axis.

p21WAF1 mutations and human malignancies

During the past few years, several categories of cyclin-dependent kinase inhibitors (CDKIs), which negatively regulate cyclin/cyclin-dependent kinase (CDK) activities, were cloned. The p21WAF1, also known as CIP1 or SDI1, was the first reported CDKI: it's expression is induced by wild-type p53. The p21WAF1 is a potent inhibitor of most cyclin/CDK complexes and also inhibits the ability of the proliferating cell nuclear antigen (PCNA) to activate DNA polymerase d. Alterations of the cell-cycle can cause cellular transformation. We analysed 471 primary samples from 15 types of human malignancies and 36 cell lines for structural alterations of the p21WAF1 gene. No changes were found in the coding region of p21WAF1 gene by polymerase-chain reaction-single-strand conformation polymorphism (PCR-SSCP) analysis. Many of these tumors had a normal p53 gene. Other investigators showed that p21WAF1 knockout mice did not have an increased incidence of cancer, while p53 knock-out mice did. Taken together, the absence of alterations of p21WAF1 in a series of malignancies suggests that p21WAF1 may not have a role in either onset or progression of most human cancers. Furthermore, p53 probably activates additional, critical tumor suppressor pathways.

Redox-mediated regulation of p21(waf1/cip1) expression involves a post-transcriptional mechanism and activation of the mitogen-activated protein kinase pathway

p21(waf1/cip1) gene expression is induced by DNA damage in cells with wild-type p53 and contributes to the arrest of cell growth. It was demonstrated that under many experimental conditions, including oxidative stress, p21(waf1/cip1) expression can be induced through p53-independent pathways. Since most of these experimental conditions induce the phosphorylation of mitogen-activated protein kinase (MAPK) and thus its activation, we evaluated p21(waf1/cip1) mRNA levels in cells exposed to an oxidative stress, induced by diethylmaleate (Et2Mal), and in which the MAPK pathway was blocked. The expression of a dominant-negative mutant of MEK, the MAPK kinase that phosphorylates and activates MAPK, and of a dominant-negative [Asn17]Ras mutant prevented the Et2Mal-induced accumulation of p21(waf1/cip1) mRNA. Similarly, the expression of MEK- and of [Asn17]Ras mutants decreased the 12-O-tetradecanoyl-phorbol 13-acetate (TPA)-mediated p21(waf1/cip1) induction. Furthermore, TPA-induced and serum-induced p21(waf1/cip1) mRNA accumulation was blocked by pretreating the cells with the antioxidant compound N-acetylcysteine, suggesting that oxidative stress is involved in these responses. p21(waf1/cip1) mRNA levels reached a maximum within 2 h of adding Et2Mal or TPA; however, the rate of transcription from a p21(waf1/cip1)-promoter construct did not increase during this period. In contrast, cells treated with actinomycin D show an increase of p21(waf1/cip1) mRNA stability after Et2Mal treatment. This result suggests that the increase in p21(waf1/cip1) mRNA at early times results from post-transcriptional regulatory events. Longer exposure to TPA may activate p21(waf1/cip1) gene transcription through an Sp1-dependent mechanism, while Et2Mal treatment gradually inhibits p21(waf1/cip1) gene transcription through oxidative changes that affect Sp1 binding to DNA.

Thrombopoietin-induced differentiation of a human megakaryoblastic leukemia cell line, CMK, involves transcriptional activation of p21(WAF1/Cip1) by STAT5

Although thrombopoietin (TPO) is known to play a fundamental role in both megakaryopoiesis and thrombopoiesis, the molecular mechanism of TPO-induced megakaryocytic differentiation is not known. In a human megakaryoblastic leukemia cell line, CMK, that showed some degree of megakaryocytic differentiation after culture with TPO, the cyclin-dependent kinase (Cdk) inhibitor p21(WAF1/Cip1), but not p27(Kip1), p16(INK4A), p15(INK4B), or p18(INK4C), was found to be upregulated in an immediately early response to TPO. The expression of p21 was found to be sustained over a period of 5 days by treatment with TPO in large polyploid cells that developed in response to TPO, but not in small undifferentiated cells, indicating a close correlation between the ligand-induced differentiation and p21 induction in CMK cells. To examine potential roles of Cdk inhibitors in megakaryocytic differentiation, CMK cells were transfected with the p21, p27, or p16 gene, together with a marker gene, beta-galactosidase, and were cultured with medium alone for 5 days. The ectopic expression of p21 or p27 but not of p16 led to induction of megakaryocytic differentiation of CMK cells. Overexpression of the N-terminal domain (amino acids [aa] 1 to 75) of p21 was sufficient to induce megakaryocytic differentiation, whereas that of the C-terminal domain (aa 76 to 164) had little or no effect on morphological features. Furthermore, we found that although TPO induced tyrosine phosphorylation of both STAT3 and STAT5 in CMK cells, only STAT5 showed binding activities to potential STAT-binding sites that locate in the promoter region of p21 gene (p21-SIE sites), thereby leading to transactivation of p21. These results suggested that p21 induction, possibly mediated through activated STAT5, could play an important role in TPO-induced megakaryocytic differentiation.

Induction of cyclin-dependent kinase inhibitor, p21WAF1, by treatment with 3,4-dihydro-6-[4-(3,4)-dimethoxybenzoyl)-1-piperazinyl]-2(1H)-quinoline (vesnarinone) in a human salivary cancer cell line with mutant p53 gene

It has been found by PCR-SSCP analysis and direct DNA sequencing that a human salivary adenosquamous carcinoma-forming cell line, TYS, has a mutant p53 gene at codon 281Asp-->His. When TYS cells were treated with a differentiation-inducing agent, vesnarinone, cellular proliferation was significantly inhibited on the basis of MTT assay. In addition, it has been found by Northern blotting and/or immunoblotting that expression of p21WAF1 and transforming growth factor-beta (TGF-beta) is up-regulated by treating TYS cells with vesnarinone. TGF-beta 1 alone also induced p21WAF1 expression in TYS cells. Moreover, it has been shown by ELISA that the treatment of TYS cells with vesnarinone results in the enhanced generation of latent TGF-beta 1. The expression of TGF-beta receptor (T beta R), including T beta R-I, T beta R-II and T beta R-III, on TYS cells was detected by affinity cross-linking using 125I-TGF-beta 1 and addition of active TGF-beta 1 into serum-free culture medium inhibited the growth of TYS cells in a concentration-dependent manner. These findings suggest that vesnarinone might directly induce expression of p21WAF1 gene in TYS cells, the product of which may be associated with the inhibition of cell growth and induce differentiation.

Nuclear accumulation of p53 correlates significantly with clinical features and inversely with the expression of the cyclin-dependent kinase inhibitor p21(WAF1/CIP1) in pancreatic cancer

Recent studies have suggested a p53-independent expression of p21(WAF1/CIP1). We investigated the correlation between p53 overexpression and the expression of p21(WAF1/CIP1) in 57 patients with pancreatic adenocarcinoma. By means of reverse transcription and polymerase chain reaction (RT-PCR), we examined the mRNA levels of WAF1/CIP1 and compared them with the p53 status in 20 patients and in a further six pancreatic tumour cell lines. In pancreatic cancer tissues, immunohistological evaluation revealed a significant correlation between active p53 and p21(WAF1/CIP1) (P < 0.005) as well as WAF1/CIP1 mRNA expression (P < 0.005). This coherence was also evident in human pancreatic carcinoma cell lines. The analysis of p53 and p21(WAF1/CIP1) expression in relation to clinicopathological features revealed a significant correlation between p53 overexpression and tumour stage, tumour size, grading and lymph node metastases, whereas p21(WAF1/CIP1) expression correlated only with tumour size. We conclude that the expression of p21(WAF1/CIP1) normally depends on active p53, but that there may also exist p53-independent pathways of induction that reduce the correlation of p21(WAF1/CIP1) to clinicopathological features.

Up-regulation of p21WAF1 expression in myeloid cells is activated by the protein kinase C pathway

Phorbol-12-myristate-13-acetate (PMA) induces p21WAF-1 expression in human myeloid leukaemic HL-60 cells. We show that this induction is specifically mediated by protein kinase C (PKC). In addition, the PKC inhibitor Ro 31-8220 with predominant PKC-alpha isoform specificity almost completely inhibited PMA-induced up-regulation of p21WAF1 in HL-60 cells as well as in the myelomonocytic leukaemic U937 cells. Pretreatment of HL-60 cells with Ro 31-8220 also inhibited PMA-induced activation of c-raf-1, a known PKC alpha target. In the phorbol ester-tolerant HL-60 subline (PET) with PKC-beta isoform deficiency PMA or bryostatin-1 induced p21WAF1 expression, but to a lesser extent than in wild-type HL-60 cells. In PET cells, Ro 31-8220 also inhibited PMA and bryostatin-1-induced up-regulation of p21WAF1 expression. Our findings indicate that at least in HL-60 cells up-regulation of p21WAF-1 is specifically activated by PKC. We suggest that PKC isoforms other than beta, presumably the PKC-alpha isoform, are involved in this process.

Novel form of p21(WAF1/CIP1/SDI1) protein in phorbol ester-induced G2/M arrest

Cell cycle progression requires activation of different cyclin-dependent kinases (CDKs) which are positively regulated by cyclins and negatively regulated by CDK inhibitors. Growth inhibition of the Calu-1 lung carcinoma cells induced with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent activator of protein kinase C, is associated with G2/M arrest and induction of expression of a novel, faster-migrating form of p21(WAF1/CIP1/SDI1) (p21) protein, an inhibitor of cyclin-dependent kinases. This faster-migrating p21 protein was also expressed in TPA-treated A549 lung carcinoma cells which also exhibited G2/M arrest but not in TPA-treated U937 leukemia cells, which only expressed a slower-migrating form of p21 protein. However, reverse transcriptase-polymerase chain reaction and Southern analysis demonstrated no evidence of novel splice in TPA-treated Calu-1 cells. On the other hand, immunoblotting analysis demonstrated that the faster-migrating p21 protein could be detected only by peptide antibody directed against the N terminus but not the C terminus, suggestive of truncation of the latter or protein modification that results in the loss of the C-terminal epitope. Correlation of G2/M arrest with expression of the faster-migrating p21 protein suggests that this novel form of p21 protein may be a mediator of G2/M arrest and growth inhibition.

Mutational analysis of the p21/WAF1/CIP1/SDI1 coding region in human tumor cell lines

p21/WAF1/CIP1/SDI1 is an important cell-cycle mediator with tumor suppressor gene capabilities, and its inactivation could potentially lead to tumor progression. Because tumor suppressor genes are commonly inactivated by somatic and germline mutations, we analyzed a variety of human tumor cell lines for p21 mutations. We used single-strand conformational analysis and direct sequencing to identify possible mutations in the p21 coding region. Two base-alterations were observed in 41 immortalized human tumor cell lines. A previously reported polymorphism that results in a serine-to-arginine amino-acid substitution at codon 31 was found in 24% (10 of 41) of the tumor cell lines but was also found in 10% (six of 62) of normal parental DNAs tested and 7% (three of 43) of normal DNAs from patients with primary endometrial tumors. Another nucleotide substitution found at codon 80 resulted in the replacement of threonine with methionine. Codon 80 changes were found in 7% (three of 41) of the tumor cell lines (all endometrial) and in 2% (one of 62) of the normal parental DNAs. This change was not found in any of the primary endometrial tumors examined. The biological activity of these base changes was analyzed by using in vitro cyclin-dependent kinase 2-cyclin A kinase assays and calcium phosphate transfections. We observed that wild-type p21 and the p21 variants had similar growth-inhibitory abilities. Thus, our results suggest that mutation of the p21 gene is not prevalent in human tumor cell lines and is not a probable mechanism of inactivation of this gene.

The WAF1-mediated p53 growth-suppressor pathway is intact in the coronary arteries of heart transplant recipients

It has been suggested that the interaction of cytomegalovirus (CMV) with the p53 tumor suppressor gene product plays a role in the development of coronary artery restenosis after angioplasty. CMV nucleic acids have been observed in the coronary arteries of allografted hearts, suggesting a possible role for the interaction of CMV with p53 in the development of accelerated graft arteriosclerosis in transplant recipients. Formalin-fixed, paraffin-embedded sections of coronary arteries from 19 transplanted hearts were immunostained for the p53 gene product using Target Unmasking Fluid (TUF)-mediated immunohistochemistry and the anti-p53 antibodies CM1 and DO7. Fresh-frozen sections of coronary arteries were also available from six of the 19 hearts, and these fresh-frozen sections were immunostained for the p53 gene product with the DO7 antibody and for WAF1 using the anti-WAF1 antibody EA10. Focal and weak staining for p53 was observed in smooth muscle and endothelial cells in two of 19 vessels, whereas the remaining 17 did not stain. CMV nucleic acids were previously shown in six of 13 of these hearts by in situ hybridization. The fresh-frozen sections of coronary arteries also did not stain for p53, but the smooth muscle cells in these vessels did stain intensely for WAF1. These results suggest three possibilities: (1) CMV-p53 interactions are not important in the development of accelerated graft arteriosclerosis; or (2) there is an interaction, but it is transient and not detectable at the time points examined in this study; or (3) there is an interaction, but binding of CMV to p53 leads to accelerated degradation of p53, as occurs with HPV-E6. The expression of WAF1 further suggests that the WAF1-mediated antiproliferative signal is intact in these vessels.

Tumor necrosis factor alpha: posttranscriptional stabilization of WAF1 mRNA in p53-deficient human leukemic cells

The p53 protein directly regulates the expression of the WAF1 (wild-type p53-activated fragment 1) protein which is a cyclin-dependent kinase inhibitor (CDK1). DNA damaging agents such as ionizing or UV radiation, and some chemical agents induce WAF1 in wild-type p53 containing cells, thereby halting cell cycle progression. WAF1 expression is also induced through a p53-independent pathway. Tumor necrosis factor alpha (TNF alpha) is a cytotoxic/cytostatic compound for some human cancer cells. We examined a series of myeloid leukemic cell lines that expressed either no p53 (HL-60, K562) or mutant inactive p53 (KG-1, KCL22,THP-1, U937). The KG-1, HL-60, K562, and KCL22 myeloid leukemic cells increased their levels of WAF1 mRNA in the presence of TNF alpha. We focused on KG-1 cells to determine how TNF alpha modulated WAF1 expression. WAF1 mRNA increased in a dose-dependent manner in the cells after exposure to increasing concentrations of TNF alpha, and this increase occurred in the absence of new protein synthesis. An increase of WAF1 protein and a concominant decrease of cyclin-dependent kinase 2 activity also was found in KG-1 cells. Flow cytometry using 5-bromo-2'-deoxyuridine showed an increase in the proportion of TNF alpha- treated KG-1 cells in the G0/G1 phase of the cell cycle. TNF alpha enhanced the rate of WAF1 transcription only 1.4 fold in TNF alpha-treated KG-1 cells as compared to untreated cells. Notably, however, the half-life (t 1/2) of WAF1 mRNA in TNF alpha-treated cells was 2.5 hours as compared to 0.5 hours in untreated cells. These results indicate that TNF alpha increases WAF1 levels at least in part via a postttranscriptional stabilization of the mRNA; and TNF alpha may mediate its cytostatic effects through WAF1 in some cell types.

p53-independent expression of the cyclin-dependent kinase inhibitor p21 in pancreatic carcinoma

The p53 tumor suppressor gene is mutated in the majority of pancreatic adenocarcinomas, and several studies have suggested that loss of p53 function may contribute to the aggressive clinical behavior of pancreas cancer. Although immunocytochemical accumulation of the p53 gene product has previously been assessed as a marker for p53 mutations in cancers of the pancreas and other organ systems, the relationship between p53 mutations and p53 protein accumulation is variable. The cyclin-dependent kinase inhibitor, p21 (also known as WAF1 and CIP1), is induced by wild-type but not mutant p53, and recent work has implicated p21 as a downstream mediator of the growth-suppressing and apoptosis-promoting functions of wild-type p53. In the present work, we sought to determine whether loss of p21 expression could more precisely identify those tumors with p53 mutations and/or loss, compared with immunocytochemical assessment of p53 protein accumulation. We evaluated p53 and p21 expression immunohistochemically in a series of 21 ductal adenocarcinomas of the pancreas with known p53 mutational status. Diffuse overexpression of p53 was found in 3 of 8 cases (38%) with wild-type p53 and 7 of 13 cases (54%) with p53 mutations with or without loss of heterozygosity at 17p. Surprisingly, expression of p21 correlated neither with p53 mutational status nor with p53 protein expression. In particular, strong p21 expression was seen even in carcinomas in which molecular analysis revealed a frameshift mutation in one allele of p53 and loss of the second. These data suggest that p21 expression in pancreatic adenocarcinoma may also be induced by a p53-independent pathway and that p21 expression, as assessed immunocytochemically, does not reflect the functional status of p53 in these carcinomas.

Structural integrity of the cyclin-dependent kinase inhibitor genes, p15, p16 and p18 in myeloid leukaemias

The cyclin-dependent kinase inhibitors known as p15, p16 and p18 have been suggested as candidates for tumour suppressor genes. We examined these genes for their alterations in 46 myeloid leukaemias and 15 myeloid leukaemia cell lines. p16 mRNA expression was studied in 41 myeloid leukaemias. The p15 and p16 genes were either deleted or mutated in myeloid leukaemia lines at a high frequency [6/15 (40%) for p15; 8/15 (53%) for p16] but alterations in primary myeloid leukaemias are much less frequent [2/46 (4%) for p15; 3/46 (6%) for p16]. Alterations of p18 were not found in any of the samples. 13 primary myeloid leukaemia samples had negligible levels of p16 mRNA. In summary, the deletions of p15 and p16 genes identified in the myeloid leukaemia cell lines probably occurred during their in vitro immortalization. Alterations of the p16 or p15 gene only occurred in primary acute myeloid leukaemia samples that were of mixed myeloid/lymphoid lineage (CD19/CD20-positive acute myeloid leukaemia [AML], CD2/CD19-positive AML, and lymphoid blastic crisis of chronic myeloid leukaemia). Further studies are required to determine if the absence of mRNA expression results from inactivation of the p16 gene.