Cell biology. Cell cycle inhibitors may help brake growth as cells develop

Hnf4a Is Required for the Development of Cdh6-Expressing Progenitors into Proximal Tubules in the Mouse Kidney

BACKGROUND

Hepatocyte NF 4α (Hnf4a) is a major regulator of renal proximal tubule (PT) development. In humans, a mutation in HNF4A impairs PT functions and is associated with Fanconi renotubular syndrome (FRTS). In mice, mosaic deletion of Hnf4a in the developing kidney reduces the population of PT cells, leading to FRTS-like symptoms. The molecular mechanisms underlying the role of Hnf4a in PT development remain unclear.

METHODS

The gene deletion tool Osr2Cre removed Hnf4a in developing nephrons in mice, generating a novel model for FRTS. Immunofluorescence analysis characterized the mutant phenotype, and lineage analysis tested whether Cadherin-6 (Cdh6)-expressing cells are PT progenitors. Genome-wide mapping of Hnf4a binding sites and differential gene analysis of Hnf4a mutant kidneys identified direct target genes of Hnf4a.

RESULTS

Deletion of Hnf4a with Osr2Cre led to the complete loss of mature PT cells, lethal to the Hnf4a mutant mice. Cdh6^high, lotus tetragonolobus lectin-low (LTL^low) cells serve as PT progenitors and demonstrate higher proliferation than Cdh6^low, LTL^high differentiated PT cells. Additionally, Hnf4a is required for PT progenitors to differentiate into mature PT cells. Genomic analyses revealed that Hnf4a directly regulates the expression of genes involved in transmembrane transport and metabolism.

CONCLUSIONS

Hnf4a promotes the differentiation of PT progenitors into mature PT cells by regulating the expression of genes associated with reabsorption, the major function of PT cells.

Alcohol induced hepatic degeneration in a hepatitis C virus core protein transgenic mouse model

Hepatitis C virus (HCV) has become a major public health issue. It is prevalent in most countries. HCV infection frequently begins without clinical symptoms, before progressing to persistent viremia, chronic hepatitis, cirrhosis and hepatocellular carcinoma (HCC) in the majority of patients (70% to 80%). Alcohol is an independent cofactor that accelerates the development of HCC in chronic hepatitis C patients. The purpose of the current study was to evaluate ethanol-induced hepatic changes in HCV core-Tg mice and mutant core Tg mice. Wild type (NTG), core wild-Tg mice (TG-K), mutant core 116-Tg mice (TG-116) and mutant core 99-Tg mice (TG-99) were used in this investigation. All groups were given drinking water with 10% ethanol and 5% sucrose for 13 weeks. To observe liver morphological changes, we performed histopathological and immunohistochemical examinations. Histopathologically, NTG, TG-K and TG-116 mice showed moderate centrilobular necrosis, while severe centrilobular necrosis and hepatocyte dissociation were observed in TG-99 mice with increasing lymphocyte infiltration and piecemeal necrosis. In all groups, a small amount of collagen fiber was found, principally in portal areas. None of the mice were found to have myofibroblasts based on immunohistochemical staining specific for α-SMA. CYP2E1-positive cells were clearly detected in the centrilobular area in all groups. In the TG-99 mice, we also observed cells positive for CK8/18, TGF-β1 and phosphorylated (p)-Smad2/3 and p21 around the necrotic hepatocytes in the centrilobular area (p < 0.01). Based on our data, alcohol intake induced piecemeal necrosis and hepatocyte dissociation in the TG-99 mice. These phenomena involved activation of the TGF-β1/p-Smad2/3/p21 signaling pathway in hepatocytes. Data from this study will be useful for elucidating the association between alcohol intake and HCV infection.

Production of cloned pigs by nuclear transfer of preadipocytes following cell cycle synchronization by differentiation induction

Four methods of cell cycle synchronization of porcine preadipocytes for use as nuclear donors in somatic cell cloning were compared: serum starvation, differentiation induction, contact inhibition and roscovitine treatment. After three days of differentiation induction, the percentage of nuclear donor cells synchronized at the G0/G1 phase reached a peak value of 91.8%, which was significantly higher (P<0.05) than the percentage attained by serum starvation (84.9-89.8%), contact inhibition (78.3-83.7%) or roscovitine treatment (67.8-80.3%). Cell cycle synchronization by serum starvation, contact inhibition and roscovitine treatment all increased the percentage of apoptotic cells, while no increase was observed when the donor-cell cycle was synchronized by differentiation induction (Annexin V-positive: 15.7% to 19.3% vs. 7.7%, P<0.05; TUNEL-positive: 12.8% to 14.0% vs. 8.3%, P<0.05). Additionally, comparison of the in vitro development of nuclear transfer (NT) embryos formed from the nuclei of differentiation-induced or serum-starved preadipocytes revealed that, in both cases, a high proportion of embryos developed to the blastocyst stage (39.0 and 33.7%, respectively). In this study, NT embryos reconstructed with preadipocytes synchronized by differentiation induction were transferred to four recipient pigs, three of which gave birth to a total of 17 piglets (4.2%, 17/403). These results demonstrate that donor-cell cycle synchronization by differentiation induction enables effective production of cloned pigs. The findings also indicate that differentiation induction of multipotent cells is an excellent method of cell cycle synchronization that permits highly efficient synchronization of cells at the G0/G1 phase.

Beyond the cell cycle: a new role for Cdk6 in differentiation

Over 10 years ago, cdk6 was identified as a new member in a family of vertebrate cdc-2 related kinases. This novel kinase was found to partner with the D-type cyclins and to possess pRb kinase activity in vitro and has since been understood to function solely as a pRb kinase in the regulation of the G(1) phase of the cell cycle. In the past 2 years, several independent studies in multiple cell types have indicated a novel role for cdk6 in differentiation. For example, cdk6 expression must be reduced to allow proper osteoblast and osteoclast differentiation, forced cdk6 expression blocked differentiation of mouse erythroid leukemia cells and cdk6 expression in primary astrocytes favors the expression of progenitor cell markers. Since exit from the cell cycle is a necessary step in terminal differentiation, down-regulation of a mitogenic factor may be expected in this process, however it is surprising that this association has not been previously uncovered and that it is apparently not shared with cdk4, long understood to be a functional homolog of cdk6. The mechanism of cdk6 function in differentiation is not understood, but it may extend beyond the established role of cdk6 as a pRb kinase. As this story unfolds it will be important to discover if the function of cdk6 in differentiation is pRb-dependent or pRb-independent, since pRb has long been established as a key factor in initiating and maintaining cell cycle exit during differentiation.

Inhibition of prostate cancer cell growth by an avocado extract: role of lipid-soluble bioactive substances

Although the avocado is known as a rich source of monounsaturated fatty acids, there has been far less attention given to its content of other bioactive substances including carotenoids, which might contribute to cancer preventive properties similar to those attributed to other fruits and vegetables. The yellow-green color of the avocado prompted us to study the carotenoid content of this fruit using established methods in our laboratory. The California Hass avocado (Persea americana Mill.) was selected for study, because it is the most commonly consumed variety in the southwest United States. These avocados were found to contain the highest content of lutein among commonly eaten fruits as well as measurable amounts of related carotenoids (zeaxanthin, alpha-carotene, and beta-carotene). Lutein accounted for 70% of the measured carotenoids, and the avocado also contained significant quantities of vitamin E. An acetone extract of avocado containing these carotenoids and tocopherols was shown to inhibit the growth of both androgen-dependent (LNCaP) and androgen-independent (PC-3) prostate cancer cell lines in vitro. Incubation of PC-3 cells with the avocado extract led to G(2)/M cell cycle arrest accompanied by an increase in p27 protein expression. Lutein alone did not reproduce the effects of the avocado extract on cancer cell proliferation. In common with other colorful fruits and vegetables, the avocado contains numerous bioactive carotenoids. Because the avocado also contains a significant amount of monounsaturated fat, these bioactive carotenoids are likely to be absorbed into the bloodstream, where in combination with other diet-derived phytochemicals they may contribute to the significant cancer risk reduction associated with a diet of fruits and vegetables.

The modulation of radiation-induced cell death by genistein in K562 cells: activation of thymidine kinase 1

Ionizing radiation is one of the most effective tools in cancer therapy. In a previous study, we reported that protein tyrosine kinase (PTK) inhibitors modulate the radiation responses in the human chronic myelogenous leukemia (CML) cell line K562. The receptor tyrosine kinase inhibitor, genistein, delayed radiation-induced cell death, while non-recepter tyrosine kinase inhibitor, herbimycin A (HMA) enhances radiation-induced apoptosis. In this study, we focused on the modulation of radiation-induced cell death by genistein and performed PCR-select suppression subtractive hybridization (SSH) to understand its molecular mechanism. We identified human thymidine kinase 1 (TK1), which is cell cycle regulatory gene and confirmed expression of TK1 mRNA by Northern blot analysis. Expression of TK1 mRNA and TK1 enzymatic activity were parallel in their increase and decrease. TK1 is involved in G1-S phase transition of cell cycle progression. In cell cycle analysis, we showed that radiation induced G2 arrest in K562 cells but it was not able to sustain. However, the addition of genistein to irradiated cells sustained a prolonged G2 arrest up to 120 h. In addition, the expression of cell cycle-related proteins, cyclin A and cyclin B1, provided the evidences of G1/S progression and G2-arrest, and their relationship with TK1 in cells treated with radiation and genistein. These results suggest that the activation of TK1 may be critical to modulate the radiation-induced cell death and cell cycle progression in irradiated K562 cells.

Gene expression profile analysis of regenerating liver after portal vein ligation in rats by a cDNA microarray system

AIMS

We assessed changes in gene expression of hypertrophied liver after portal vein ligation (PL) in a test group of rats compared to a control group, which had the same size liver but no PL.

METHODS

The portal veins of the left and median lobes in the test group were ligated in an initial operation. Four days after the PL, the liver volume of the posterior caudate lobe (5%) increased two-fold and comprised 10% of the liver. A 90% hepatectomy was then performed, leaving only the hypertrophied posterior caudate lobe, and leaving the normal anterior and posterior caudate lobes (10%) in the control (sham) group. A comparison of the expression profiles between two groups was performed using cDNA microarrays and the hepatic ATP level was measured.

RESULTS

The survival rate for the PL group was significantly higher than for the sham group at 4 days after the hepatectomy (56.3% and 26.7%, P < 0.05). Gene expression of cyclin D1, proliferating cell nuclear antigen, cyclin A and B was upregulated, and the cyclin-dependent kinase inhibitor was downregulated. Increases were observed in: (i) pyruvate dehydrogenase, the tricarboxylic acid cycle cycle regulator, (ii) acyl-CoA dehydrogenase, the oxidation regulator, and (iii) cytochrome oxidases, the oxidative phosphorylation regulator. Hepatic ATP concentration after hepatectomy was better maintained in the PL group than in the sham group (0.48 +/- 0.01 micromol/ml vs. 0.33 +/- 0.01 micromol/ml, P < 0.05).

CONCLUSION

The regenerating liver increased tolerance for extended hepatectomy compared to normal liver. It is believed that this is because the induced rapid regeneration of the remaining liver after hepatectomy increases ATP metabolism.

High expression of p21WAF1/CIP1 is correlated with human hepatocellular carcinoma in patients with hepatitis C virus-associated chronic liver diseases

p21(WAF1/CIP1) (p21) protein is a universal inhibitor of cyclin-dependent kinases and is regulated transcriptionally by p53, which is activated by DNA stress. Hepatocytes in chronic hepatitis receive several DNA stresses by lymphocytes and Kupffer cells. Therefore, we analyzed p21 expression of hepatocytes in hepatitis C virus (HCV)-associated chronic liver diseases and investigated the possible involvement of p21 in hepatocarcinogenesis. We examined p21 expression in 35 cases of HCV-associated chronic hepatitis and 25 cases of HCV-associated liver cirrhosis by immunohistochemical analysis. The p21 labeling index (LI) was calculated as the ratio of positive cells to total cells. p21-positive hepatocytes were more numerous in areas of intense inflammation and spotty necrosis and areas close to fibrosis, and were increased according to the degrees of grading and staging. The p21 LI with liver cirrhosis was significantly higher than that with chronic hepatitis (14.4 +/- 5.9 versus 11.1 +/- 4.2, P = 0.014). The cumulative incidence of hepatocellular carcinoma (HCC) was significantly higher in the p21 LI >or=14% group than in the p21 LI <14% group (P = 0.0079). Multivariate analysis demonstrated that p21 expression can be recognized as an independent significant factor for HCC development (relative risk 5.00, P = 0.039). p21 LI decreased significantly after interferon therapy. These results suggested that p21 is up-regulated by the stress of inflammation and fibrosis in HCV-associated chronic liver diseases and that high p21 expression might be related to hepatocarcinogenesis in cirrhotic patients.

Downregulation of hemidesmosomal proteins in nasopharyngeal carcinoma cells

Hemidesmosome (HD) is a transmembrane complex that mediates attachment of epithelial cells to the basement membrane. Abnormal expression of HD components has been reported in several types of human cancers and is believed to play a role in tumor invasion and metastasis. Using differential gene display, we have identified downregulation of BPAG1 expression in nasopharyngeal carcinoma cells. BPAG1 is a major component of hemidesmosome. In the present study, we have extended our work to investigate the expression pattern of other components in the HD complex, namely, BPAG2, ITGalpha6 and ITGbeta4 in three distinct biological groups of nasopharyngeal epithelial cells: (a) non-malignant nasopharyngeal epithelial cells established from primary culture of nasopharyngeal explants, (b) non-malignant nasopharyngeal epithelial cells immortalized by viral oncogenes, SV40 or HPV16E6E7, and (c) nasopharyngeal carcinoma (NPC) cells. Both non-malignant primary cultured nasopharyngeal epithelial cells and immortalized nasopharyngeal epithelial cell lines expressed all the HD components examined, although the immortalized cells expressed a lower level of HD components compared with the non-malignant nasopharyngeal cells established from primary culture. In contrast, downregulation of HD components is commonly observed in nasopharyngeal carcinoma cells. Loss of HD expression in NPC may be associated with the undifferentiated properties of NPC cells and may have prognostic significance.

Early increase in cyclin-D1 expression and accelerated entry of mouse hepatocytes into S phase after administration of the mitogen 1, 4-Bis[2-(3,5-Dichloropyridyloxy)] benzene

We have previously demonstrated that hepatocyte proliferation induced by the mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP) is independent of changes in cytokines, immediate early genes, and transcription factors that are considered to be necessary for regeneration of the liver after partial hepatectomy (PH) or necrosis. To further investigate the differences between mitogen-induced mouse hepatocyte proliferation and liver regeneration after PH, we have measured the expression of cyclin D1, cyclin D3, cyclin E, and cyclin A and of the cyclin-dependent kinases CDK2, CDK4, and CDK6. The involvement of the cyclin-dependent kinase inhibitors p21 and p27 and of the oncosuppressor gene p53 was also examined at different times after stimulation of hepatocyte proliferation. Results showed that a single administration of TCPOBOP caused a very rapid increase in the levels of cyclin D1, a G1 protein, when compared with two thirds PH (8 hours versus 30 hours). The early increase in cyclin D1 protein levels was associated with a faster onset of increased expression of S-phase-associated cyclin A (24 hours versus 36 hours with PH mice). Accordingly, measurement of bromodeoxyuridine (BrdU) incorporation revealed that, although approximately 8% of hepatocytes were BrdU-positive as early as 24 hours after TCPOBOP, no significant changes in BrdU incorporation were observed at the same time point after two thirds PH. The expression of other proteins involved in cell cycle control, such as cyclin-dependent kinases (CDK4, CDK2, CDK6), was also analyzed. Results showed that expression of CDK2 was induced much more rapidly in TCPOBOP-treated mice (2 hours) than in mice subjected to PH (36 hours). A different pattern of expression in the two models of hepatocyte proliferation, although less dramatic, was also observed for CDK4 and CDK6. Expression of the CDK inhibitors p21 and p27 and the oncosuppressor gene p53 variably increased after two thirds PH, whereas basically no change in protein levels was found in TCPOBOP-treated mice. The results demonstrate that profound differences in many cell cycle-regulatory proteins exist between direct hyperplasia and compensatory regeneration. Cyclin D1 induction is one of the earlier events in hepatocyte proliferation induced by the primary mitogen TCPOBOP and suggests that a direct effect of the mitogen on this cyclin may be responsible for the rapid onset of DNA synthesis observed in TCPOBOP-induced hyperplasia.

Elevated cyclin E levels, inactive retinoblastoma protein, and suppression of the p27(KIP1) inhibitor characterize early development of promyeloid cells into macrophages

Cyclin-dependent kinase inhibitors such as p27(KIP1) have recently been shown to lead to cellular differentiation by causing cell cycle arrest, but it is unknown whether similar events occur in differentiating promyeloid cells. Hematopoietic progenitor cells undergo lineage-restricted differentiation, which is accompanied by expression of distinct maturation markers. Here we show that the classical growth factor insulin-like growth factor I (IGF-I) potently promotes vitamin D(3)-induced macrophage differentiation of promyeloid cells, as assessed by measurement of a coordinate increase in expression of the integrin alpha subunit CD11b, the CD14 lipopolysaccharide receptor, and the macrophage-specific esterase, alpha-naphthyl acetate esterase, as early as 24 h following initiation of terminal differentiation. Addition of IGF-I to cells undergoing vitamin D(3)-induced differentiation also leads to an early increase in expression of cyclin E, phosphorylation of the retinoblastoma tumor suppressor protein, and a doubling of the cell number. Early expression of CD11b (24 h) is simultaneously accompanied by inhibition in the expression of p27(KIP1). Cell cycle analysis with propidium iodide revealed that CD11b expression at 24 h following initiation of differentiation occurs at all phases of the cell cycle instead of only those cells arrested in G(0)/G(1). Similarly, development of a novel double-labeling intra- and extracellular flow-cytometric technique demonstrated that single cells expressing the mature leukocyte differentiation antigen CD11b can also incorporate the thymidine analog bromodeoxyuridine. Likewise, expression of the intracellular DNA polymerase delta cofactor/proliferating-cell nuclear antigen at 24 h is also simultaneously expressed with the surface marker CD11b, indicating that these cells continue to proliferate early in their differentiation program. Finally, at 24 h following induction of differentiation, IGF-I promoted a fourfold increase in the uptake of [(3)H]thymidine by purified populations of CD11b-expressing cells. Taken together, these data demonstrate that the initial steps associated with terminal macrophage differentiation occur concomitantly with progression through the cell cycle and that these very early differentiation events do not require the accumulation of p27(KIP1).

Myofibrillar myopathy: no evidence of apoptosis by TUNEL

The pathogenesis of myofibrillar myopathy (MFM) is not known. Muscle biopsy specimens demonstrate increased expression of cell cycle regulatory proteins as well as the ectopic expression of lamin B and nuclear matrix protein in the cytoplasm, suggesting the possibility of apoptosis. The authors investigated for apoptosis using the TUNEL method in six muscle biopsy specimens from patients with MFM. There was no evidence of apoptotic myonuclei in any of the MFM muscle biopsies. Further studies regarding the pathogenesis of MFM and the possible role of mitotic catastrophe are needed.

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.

Activation of the p21 pathway of growth arrest and apoptosis by the beta 4 integrin cytoplasmic domain

The integrin alpha 6 beta 4, a receptor for members of the laminin family of basement membrane components, contributes to the function of epithelial cells and their oncogenically transformed derivatives. In our efforts to study alpha 6 beta 4-mediated functions in more detail and to assess the contribution of the beta 4 cytoplasmic domain in such functions, we identified a rectal carcinoma cell line that lacks expression of the beta 4 integrin subunit. This cell line, termed RKO, expresses alpha 6 beta 1 but not alpha 6 beta 4, and it interacts with laminin-1 less avidly than similar cell lines that express alpha 6 beta 4. We expressed a full-length beta 4 cDNA, as well as a mutant cDNA that lacks the beta 4 cytoplasmic domain, in RKO cells and isolated stable subclones of these transfectants. In this study, we report that subclones that expressed the full-length beta 4 cDNA in association with endogenous alpha 6 exhibited partial G1 arrest and apoptosis, properties that were not evident in RKO cells transfected with either the cytoplasmic domain mutant or the expression vector alone. In an effort to define a mechanism for these observed changes in growth, we observed that expression of the alpha 6 beta 4 integrin induced expression of the p21 (WAF1; CiP1) protein, an inhibitor of cyclin-dependent kinases. These data suggest that the beta 4 integrin cytoplasmic domain is linked to a signaling pathway involved in cell cycle regulation in the beta 4 transfected RKO cells.