Activating transcription factor 3 and early growth response 1 are the novel targets of LY294002 in a phosphatidylinositol 3-kinase-independent pathway

LY294002, a phosphatidylinositol 3-kinase (PI3K) inhibitor, has been widely used to study the function of PI3K in cellular responses. Based on its inhibitory effect on PI3K, LY294002 has been shown to exert antitumorigenic effect in vivo and in vitro. Here, we report that LY294002 alters early growth response 1 (EGR-1) phosphorylation and subsequently enhances activating transcription factor 3 (ATF3) expression independently of PI3K inhibition. This pathway may be, in part, responsible for the antitumorigenic effect of LY294002 in human colorectal cancer cells. ATF3 expression was increased by LY294002, followed by the induction of apoptosis in several colorectal cancer cell lines. This is consistent with results showing that the down-regulation of the ATF3 gene by small interfering RNA suppressed LY294002-induced apoptosis in HCT-116 cells. On the other hand, ATF3 expression was not affected by another PI3K inhibitor, wortmannin, as well as phosphatase and tensin homologue or dominant-negative Akt overexpression. We also found that LY294002 increases ATF3 promoter activity and the transactivation is partly mediated by a GC-rich sequence located in the promoter. EGR-1 binds to the ATF3 promoter as assessed by gel shift assay. Furthermore, phosphorylated EGR-1 was highly increased in LY294002-treated cells, indicating that EGR-1 phosphorylation induced by LY294002 may facilitate ATF3 transactivation. Our data suggest that EGR-1 acts as a mediator in LY294002-induced ATF3 expression via a PI3K-independent pathway. ATF3 and EGR-1 may provide a novel explanation for the antitumorigenic properties of LY294002 in human colorectal cancer cells.

A Splice Variant of Stress Response Gene ATF3 Counteracts NF-κB-dependent Anti-apoptosis through Inhibiting Recruitment of CREB-binding Protein/p300 Coactivator

Activating transcription factor (ATF) 3 plays a role in determining cell fate and generates a variety of alternatively spliced isoforms in stress response. We have reported previously that splice variant ATF3deltaZip2, which lacks the leucine zipper region, is induced in response to various stress stimuli. However, its biological function has not been elucidated. By using cells treated with tumor necrosis factor-alpha and actinomycin D or cells overexpressing ATF3deltaZip2, we showed that ATF3deltaZip2 sensitizes cells to apoptotic cell death in response to tumor necrosis factor-alpha, at least in part through suppressing nuclear factor (NF)-kappaB-dependent transcription of anti-apoptotic genes such as cIAP2 and XIAP. ATF3deltaZip2 interacts with a p65 (RelA)-cofactor complex containing CBP/p300 and HDAC1 at NF-kappaB sites of the proximal promoter region of the cIAP2 gene in vivo and down-regulates the recruitment of CBP/p300. Our study revealed that ATF3deltaZip2 counteracts anti-apoptotic activity of NF-kappaB, at least in part, by displacing positive cofactor CBP/p300 and provides insight into the mechanism by which ATF3 regulates cell fate through alternative splicing in stress response.

Stress response gene ATF3 is a target of c-myc in serum-induced cell proliferation

The c-myc proto-oncogene encodes a transcription factor that promotes cell cycle progression and cell proliferation, and its deficiency results in severely retarded proliferation rates. The ATF3 stress response gene encodes a transcription factor that plays a role in determining cell fate under stress conditions. Its biological significance in the control of cell proliferation and its crosstalk regulation, however, are not well understood. Here, we report that the serum response of the ATF3 gene expression depends on c-myc gene and that the c-Myc complex at ATF/CREB site of the gene promoter plays a role in mediating the serum response. Intriguingly, ectopic expression of ATF3 promotes proliferation of c-myc-deficient cells, mostly by alleviating the impeded G1-phase progression observed in these cells, whereas ATF3 knockdown significantly suppresses proliferation of wild-type cells. Our study demonstrates that ATF3 is downstream of the c-Myc signaling pathway and plays a role in mediating the cell proliferation function of c-Myc. Our results provide a novel insight into the functional link of the stress response gene ATF3 and the proto-oncogene c-myc.

Elevated macrophage migration inhibitory factor (MIF) levels in the urine of patients with focal glomerular sclerosis

The pathogenesis of focal glomerular sclerosis (FGS) is poorly understood. Macrophage migration inhibitory factor (MIF) is a potent pro-inflammatory cytokine released from T cells and macrophages, and is a key molecule in inflammation. To examine further the possible role of MIF in FGS, we measured MIF levels in the urine. The purpose of the present study was to evaluate the involvement of MIF in FGS. Urine samples were obtained from 20 FGS patients. The disease controls included 40 patients with minimal-change nephrotic syndrome (MCNS) and membranous nephropathy (MN). A group of healthy subjects also served as controls. Biopsies were performed in all patients prior to entry to the study. The samples were assayed for MIF protein by a sandwich enzyme-linked immunosorbent assay (ELISA). The levels of MIF in the urine of FGS patients were significantly higher than those of the normal controls and patients with MCNS and MN. In contrast, the levels of urinary MIF (uMIF) in patients with MCNS and MN did not differ significantly from normal values. In the present study, attention also focused on the relationship between uMIF levels and pathological features. Among the patients with FGS, uMIF levels were significantly correlated with the grade of mesangial matrix increase and that of interstitial fibrosis. There was also a significant correlation between uMIF levels and the number of both intraglomerular and interstitial macrophages. Although the underlying mechanisms remain to be determined, our study presents evidence that urinary excretion of MIF is increased in FGS patients with active renal lesions.

Ruptured aneurysm at a duplicated middle cerebral artery with accessory middle cerebral artery

Although many cerebral vascular anomalies are widely recognized, others are less well known or unclassified. Accessory middle cerebral artery (MCA) and duplicated MCA are among uncommon anomalies. We present a very rare case of subarachnoid haemorrhage due to rupture of a saccular aneurysm arising from a duplicated middle cerebral artery which was associated with an accessory middle cerebral artery.

Mucin expression in pleomorphic adenoma of salivary gland: a potential role for MUC1 as a marker to predict recurrence

BACKGROUND

Pleomorphic adenoma of the salivary gland (PA) is essentially a benign neoplasm. However, patients with recurrent PA are difficult to manage. There are rare reports on useful immunohistochemical markers to detect a high risk of recurrence when the primary lesions are resected.

AIMS

To find a new marker to predict the recurrence of PA.

METHODS

Primary lesions of PA were collected from nine patients showing subsequent recurrence and from 40 patients without recurrence during at least 10 years of follow up of the disease. Paraffin wax embedded tumour samples of the two groups were examined for the expression profiles of MUC1 (differentially glycosylated forms), MUC2, MUC4, MUC5AC, and MUC6 using immunohistochemistry. Several clinicopathological factors were also examined.

RESULTS

In univariate analysis of the factors examined, MUC1/DF3 high expression (more than 30% of the neoplastic cells stained) in the primary lesions was seen more frequently in patients with recurrence (four of nine) than in those without recurrence (three of 40; p = 0.011). Larger tumour size (more than 3.0 cm) of the primary PA was also a significant (p = 0.035) risk factor for the recurrence of PA. In multivariate analysis, only high expression of MUC1/DF3 was found to be a significant independent risk factor for the recurrence of PA (p = 0.021).

CONCLUSIONS

Expression of MUC1/DF3 in PA is a useful marker to predict its recurrence. Those patients with PA showing positive MUC1/DF3 expression should be followed up carefully.

Antimonocyte chemoattractant protein-1 gene therapy reduces experimental in-stent restenosis in hypercholesterolemic rabbits and monkeys

In-stent restenosis results exclusively from neointimal hyperplasia due to mechanical injury and a foreign body response to the prosthesis. Inflammation mediated by monocyte chemoattractant protein-1 (MCP-1) might therefore underlie in-stent restenosis. We recently devised a new strategy for anti-MCP-1 gene therapy by transfecting an N-terminal deletion mutant of the MCP-1 gene into skeletal muscles. We used this strategy to investigate the role of MCP-1 in experimental in-stent restenosis in hypercholesterolemic rabbits and monkeys. Transfection of the mutant MCP-1 gene suppressed monocyte infiltration/activation in the stented arterial wall and markedly reduced the development of neointimal hyperplasia. This strategy also suppressed local expression of MCP-1 and inflammatory cytokines. Therefore, inhibition of MCP-1-mediated inflammation is effective in reducing experimental in-stent restenosis. This strategy might be a useful form of gene therapy against human in-stent restenosis.

Down-regulation of p27Kip1 promotes cell proliferation of rat neonatal cardiomyocytes induced by nuclear expression of cyclin D1 and CDK4. Evidence for impaired Skp2-dependent degradation of p27 in terminal differentiation

Mammalian cardiomyocytes lose their capacity to proliferate during terminal differentiation. We have previously reported that the expression of nuclear localization signal-tagged cyclin D1 (D1NLS) and its partner cyclin-dependent kinase 4 (CDK4) induces proliferation of rat neonatal cardiomyocytes. Here we show that the D1NLS/CDK4 cells, after their entry into the cell cycle, accumulated cyclin-dependent kinase inhibitor p27 in the nuclei and decreased the cyclin-dependent kinase 2 (CDK2) activity, leading to early cell cycle arrest. Biochemical analysis demonstrated that Skp2-dependent p27 ubiquitylation was remarkably suppressed in cardiomyocytes, whereas Skp2, a component of Skp1-Cullin-F-box protein ubiquitin ligase, was more actively ubiquitylated compared with proliferating rat fibroblasts. Specific degradation of p27 by co-expressing Skp2 or p27 small interfering RNA caused an increase of CDK2 activity and overrode the limited cell cycle. These data altogether indicate that the impaired Skp2-dependent p27 degradation is causally related to the loss of proliferation in cardiomyocytes. This provides a novel insight in understanding the molecular mechanism by which mammalian cardiomyocytes cease to proliferate during terminal differentiation.

Overexpression of lipoprotein lipase improves insulin resistance induced by a high-fat diet in transgenic rabbits

AIMS/HYPOTHESIS

Dysfunctions of lipoprotein lipase (LPL) have been found to be associated with dyslipidaemias, atherosclerosis, obesity and insulin resistance. There are two conflicting hypotheses regarding the roles of LPL in glucose metabolism and insulin resistance. Whether systemically increased LPL activity would be beneficial or detrimental to insulin sensitivity is yet to be resolved. To address this issue, we studied transgenic rabbits overexpressing human LPL transgene.

METHODS

LPL transgenic and control rabbits were fed a 10% high-fat diet (HFD) for 16 weeks. To evaluate glucose metabolism, we compared plasma levels of glucose and insulin in transgenic rabbits with control rabbits and performed an intravenous glucose tolerance test. In addition, we measured adipose tissue accumulation in HFD-fed rabbits.

RESULTS

Increased LPL activity in transgenic rabbits resulted in a significant reduction of plasma triglycerides and non-esterified fatty acids, but not in basal levels of glucose and insulin. HFD feeding induced an elevation of plasma glucose levels accompanied by hyperinsulinaemia in control rabbits, but was significantly inhibited in transgenic rabbits. The intravenous glucose tolerance test showed that transgenic rabbits had faster glucose clearance associated with lower levels of insulin secretion than control rabbits. In addition, there was a significant reduction of body adipose tissue in transgenic rabbits compared with in control rabbits fed an HFD. Scanning electron microscopic examination revealed that adipocytes in transgenic rabbits were predominately small cells.

CONCLUSIONS/INTERPRETATION

Our results showed that systemically increased LPL activity improves insulin resistance and reduces adipose accumulation in transgenic rabbits, indicating that systemic elevation of LPL may have potential benefits for the treatment of insulin resistance and obesity.

Blue light inhibits melanin synthesis in B16 melanoma 4A5 cells and skin pigmentation induced by ultraviolet B in guinea-pigs

BACKGROUND

Little has been known about the effects of visible light in mammalian cells. We recently found that blue light not only suppressed the growth of B16 melanoma cells in a time-dependent manner but also inhibited metastasis of the B16 melanoma cells to the lung. These findings suggest that exposure to blue light modifies the functions of B16 melanoma cells. The present study investigated the effects of blue light on B16 melanoma 4A5 cells and Weiser-Maple guinea-pigs to confirm the biological effect of blue light on melanin formation.

METHODS

The effect of red, green, and blue light on melanin synthesis in B16 melanoma 4A5 cells was measured. The back skin of brown Weiser-Maple guinea-pigs was exposed to ultraviolet B (UVB; 588 mJ/cm(2) (0.7 mW/cm(2)x 14 min) three times a week for 2 weeks to induce melanin deposition. Thirty minutes after each UVB exposure, blue light was applied for 30 min. Pigmentation of the exposed areas of skin was checked once a week, and photographs of the skin were taken by digital camera. Observation was continued for 18 days after the final UVB exposure.

RESULTS

Melanin synthesis in B16 melanoma 4A5 cells was selectively suppressed by blue light, but blue light did not induce decolorization of previously produced melanin. In the back skin of brown guinea-pigs, the brightness of the sites exposed to UVB began to decrease on the fifth day of the experiment, decreasing further from the 12th day to the 18th day after UVB exposure. The brightness of the sites exposed to UVB and blue light decreased in a manner similar during the UVB exposure, but remained relatively unchanged from the 12th day to the 30th day.

CONCLUSIONS

These results suggest that blue light suppresses melanin formation following repeated UVB exposure. Further investigation with various light such as blue light may lead to a new approach to the care of ultraviolet-affected skin such as hyperpigmentation.

All-trans retinoic acid directly up-regulates thrombopoietin transcription in human bone marrow stromal cells

OBJECTIVE

All-trans retinoic acid (ATRA) has been used as the first-line therapy for patients with acute promyelocytic leukemia (APL). We previously reported that ATRA increased serum thrombopoietin (TPO) levels accompanied by thrombocytosis during ATRA therapy for APL. In this study, we investigated the mechanism of transcriptional regulation of TPO gene by ATRA using human bone marrow stromal cells.

PATIENTS AND METHODS

Real time reverse transcriptase polymerase chain reaction and Western blotting were performed to quantify TPO mRNA and protein levels in cells from the human bone marrow stromal cell line KM101. Luciferase-based reporter assay, electrophoretic mobility shift assay (EMSA), and chromatin immunoprecipitation (ChIP) assay were performed to identify a retinoic acid responsive element in the promoter region of TPO gene (TPO-RARE).

RESULTS

TPO mRNA expression was up-regulated by approximately 2.9 times 8 hours after stimulation with 10(-6) M ATRA in KM101 cells. In contrast, ATRA did not alter TPO mRNA expression in cells from the human hepatoma cell line HepG2. Protein level of KM101 cells also was increased with 10(-6) M ATRA for 48 hours in KM101 cells. We found the synthesized RARalpha protein bound to [gamma-32P]-labeled TPO-RARE probe and its binding was competed by adding 200x amount of cold TPO-RARE probe by EMSA. In addition, [gamma-32P]-labeled TPO-RARE probe bound to KM101 nuclear protein extract was supershifted by anti-RARalpha antibody and modified by treatment with ATRA. The relative luciferase activity of TPO gene was increased by 2.2x and the histone H4 was acetylated through TPO-RARE after ATRA stimulation in KM101 cells by ChIP assay.

CONCLUSION

These data support the direct up-regulation of TPO transcription by ATRA stimulation in human bone marrow stromal cells and propose one of the mechanisms of thrombocytosis during ATRA therapy for APL.

Combined status of MUC1 mucin and surfactant apoprotein A expression can predict the outcome of patients with small-size lung adenocarcinoma

AIM

Lung cancer is still a disease of high mortality, despite advanced diagnostic techniques. Here, we aim to report a unique method to predict the recurrence and outcome of patients with pulmonary adenocarcinomas.

METHODS AND RESULTS

Immunohistochemical expression of MUC1 mucin and surfactant apoprotein A (SP-A) was examined in 185 cases of surgically removed lung adenocarcinomas of non-bronchioloalveolar type smaller than 30 mm. Staining results were evaluated semiquantitatively, and the expression of MUC1 and SP-A was compared in each case. There were 140/185 (76%) cases showing MUC1 expression higher than SP-A expression (MUC1>SP-A), and 45/185 (24%) cases showing MUC1 expression lower than or equal to SP-A expression (MUC1<or =SP-A). Recurrence was observed in 24% (34/140) of the patients with a MUC1>SP-A pattern, but in 7% (3/45) of the patients with a MUC1< or =SP-A pattern after the median observation period of 41 months (1-99 months) (P < 0.01). The MUC1>SP-A group showed higher recurrence and worse survival than the MUC1<or =SP-A group by Kaplan-Meier's method (P < 0.01 and P < 0.005). Even in the patients with apparently favourable prognostic factors (well-differentiated adenocarcinoma or no lymph node metastasis), recurrence and survival were different between the two groups.

CONCLUSIONS

A high MUC1/SP-A ratio is strongly associated with a poor outcome in patients with small-size lung adenocarcinoma.

Mammalian elongin A is not essential for cell viability but is required for proper cell cycle progression with limited alteration of gene expression

Elongin A is a transcription elongation factor that increases the overall rate of mRNA chain elongation by RNA polymerase II. To investigate the function of Elongin A in vivo, the two alleles of the Elongin A gene have been disrupted by homologous recombination in murine embryonic stem (ES) cells. The Elongin A-deficient ES cells are viable, but show a slow growth phenotype because they undergo a delayed mitosis. The cDNA microarray and RNase protection assay using the wild-type and Elongin A-deficient ES cells indicate that the expression of only a small subset of genes is affected in the mutant cells. Taken together, our results suggest that Elongin A regulates transcription of a subset but not all of genes and reveal a linkage between Elongin A function and cell cycle progression.

Critical role of cyclin D1 nuclear import in cardiomyocyte proliferation

Mammalian cardiomyocytes irreversibly lose their capacity to proliferate soon after birth, yet the underlying mechanisms have been unclear. Cyclin D1 and its partner, cyclin-dependent kinase 4 (CDK4), are important for promoting the G1-to-S phase progression via phosphorylation of the retinoblastoma (Rb) protein. Mitogenic stimulation induces hypertrophic cell growth and upregulates expression of cyclin D1 in postmitotic cardiomyocytes. In the present study, we show that, in neonatal rat cardiomyocytes, D-type cyclins and CDK4 were predominantly cytoplasmic, whereas Rb remained in an underphosphorylated state. Ectopically expressed cyclin D1 localized in the nucleus of fetal but not neonatal cardiomyocytes. To target cyclin D1 to the nucleus efficiently, we constructed a variant of cyclin D1 (D1NLS), which directly linked to nuclear localization signals (NLSs). Coinfection of recombinant adenoviruses expressing D1NLS and CDK4 induced Rb phosphorylation and CDK2 kinase activity. Furthermore, D1NLS/CDK4 was sufficient to promote the reentry into the cell cycle, leading to cell division. The number of cardiomyocytes coinfected with these viruses increased 3-fold 5 days after infection. Finally, D1NLS/CDK4 promoted cell cycle reentry of cardiomyocytes in adult hearts injected with these viruses, evaluated by the expression of Ki-67, which is expressed in proliferating cells in all phases of the cell cycle, and BrdU incorporation. Thus, postmitotic cardiomyocytes have the potential to proliferate provided that cyclin D1/CDK4 accumulate in the nucleus, and the prevention of their nuclear import plays a critical role as a physical barrier to prevent cardiomyocyte proliferation. Our results provide new insights into the development of therapeutics strategies to induce regeneration of cardiomyocytes. The full text of this article is available at http://www.circresaha.org.

Expression of mucins (MUC1, MUC2, MUC5AC and MUC6) in mucinous carcinoma of the breast: comparison with invasive ductal carcinoma

AIMS

Mucinous carcinoma of the breast usually shows less frequent lymph node metastasis and more favourable outcome compared with invasive ductal carcinoma. The aim of this study is to compare the expression profiles of several mucins in mucinous carcinomas and invasive ductal carcinomas to gain insight into the relationship between the less aggressive biological nature of mucinous carcinoma and the role of mucins.

METHODS AND RESULTS

We examined the expression profiles of MUC1 (membrane-bound mucin) of different glycoforms (from non-glycosylated form to fully glycosylated form), MUC2 (intestinal type secretory mucin), MUC5AC (gastric surface type secretory mucin) and MUC6 (gastric pyloric gland type secretory mucin) in 17 mucinous carcinomas and 46 invasive ductal carcinomas using immunohistochemistry. Various glycoforms of MUC1 were expressed frequently in both mucinous carcinomas (65-100%) and invasive ductal carcinomas (92-100%), although non-glycosylated MUC1 (MUC1/CORE) and fully glycosylated MUC1 (MUC1/HMFG-1) showed significantly lower expression rates in mucinous carcinomas compared with those in invasive ductal carcinomas. The expression rates of MUC2 (94%) and MUC6 (71%) in mucinous carcinomas were significantly higher than those of MUC2 (15%) and MUC6 (15%) in invasive ductal carcinomas. There was no significant difference in the expression rate of MUC5AC in mucinous carcinomas (12%) and that in invasive ductal carcinomas (4%).

CONCLUSIONS

The expression rate of MUC1/CORE and MUC1/HMFG-1, which is related to poor prognosis in the gastric and colorectal cancers, is low in mucinous carcinomas. The high expression rate of gel-forming secretory mucins (MUC2 and MUC6) in mucinous carcinoma suggests that high production of these types of mucins may act as a barrier to cancerous extension resulting in their less aggressive biological behaviour.

Transcriptional activation of the human stress-inducible transcriptional repressor ATF3 gene promoter by p53

Activating transcription factor 3 (ATF3) is an immediate early response gene that is induced in cells exposed to a variety of stress stimuli. In this report, upon exposure of cells to ultraviolet (UV) or proteasome inhibitor MG132, ATF3 protein was induced more efficiently in cells with intact p53 allele than in those with null mutant p53 allele. In Saos-2 cells harboring the temperature-sensitive mutant p53(Val-138), the expression of ATF3 gene was more significant at permissive temperature of 32.5 degrees C than at non-permissive 37.5 degrees C. Reporter assay of the human ATF3 gene promoter identified two p53-responsive elements at -379 to -370 and -351 to -342 from the transcriptional start site. These elements were capable of conferring p53 responsiveness to a heterologous promoter and specifically bound p53 protein in electrophoretic mobility shift assay. Furthermore, ATF3 gene promoter was more significantly activated by UV in cells with wild p53 allele. These results clearly show that the human ATF3 gene is one of the target genes directly activated by p53 and may suggest a functional link between stress-inducible transcriptional repressor ATF3 and p53.

Transcriptional repressor activating transcription factor 3 protects human umbilical vein endothelial cells from tumor necrosis factor-alpha-induced apoptosis through down-regulation of p53 transcription

Activating transcription factor 3 (ATF3) is a transcriptional repressor that is rapidly induced in cells exposed to a wide range of stress stimuli. To clarify the role of ATF3 in determining cell fate, we overexpressed it in human umbilical vein endothelial cells (HUVECs) by adenovirus-mediated gene transfer. ATF3 protected these cells from tumor necrosis factor (TNF)-alpha-induced apoptosis, as measured by flow cytometric analysis, trypan blue exclusion assay, and cleavage of procaspase 3 and poly(ADP-ribose) polymerase. Northern blot and nuclear run on assay showed that the transcription of tumor suppressor gene p53 was down-regulated in the ATF3-overexpressing cells. In the transient expression assay, ATF3 suppressed the p53 gene promoter activity through its specific binding to an atypical AP-1 element, PF-1 site, in the p53 gene promoter. Furthermore, the cell-protecting effect of ATF3 was remarkably reduced in p53-deficient cells. These results demonstrate that overexpression of ATF3 suppresses TNF-alpha-induced cell death of HUVECs, at least in part, through down-regulating the transcription of p53 gene. ATF3 may function as a cell survival factor of endothelial cells during vascular inflammation and atherogenesis.

ATF3 inhibits doxorubicin-induced apoptosis in cardiac myocytes: a novel cardioprotective role of ATF3

Activating transcription factor (ATF) 3, a member of the ATF/cyclic adenosine monophosphate (cAMP)-responsive element binding protein (ATF/CREB) family of transcription factors, is induced by a wide range of stress stimuli. Although the ATF3 homodimer is known to repress transcription of several genes, its precise biological roles are still unclear. In this study, we investigated the functional role of ATF3 in doxorubicin (DOX=adriamycin)-treated neonatal rat cardiac myocytes. DOX rapidly activated JNK and c-Jun and induced ATF3 at both mRNA and protein level. Adenovirus-mediated expression of ATF3 protected cardiomyocytes from DOX-induced apoptosis, as determined by flow cytometry, cell viability, and TUNEL assay. It was further shown that p53, one of the apoptosis-inducing transcription factors, was downregulated in the ATF3-overexpressing cardiomyocytes. These results strongly suggest that ATF3 may function as a cytoprotective transcription factor in DOX-treated cardiac myocytes, at least in part, owing to downregulation of p53. ATF3 may be a novel therapeutic target that protects cardiac myocytes from DOX-induced apoptosis.

Expression of cyclin D1 and CDK4 causes hypertrophic growth of cardiomyocytes in culture: a possible implication for cardiac hypertrophy

Differentiated cardiomyocytes have little capacity to proliferate and show the hypertrophic growth in response to alpha1-adrenergic stimuli via the Ras/MEK pathway. In this study, we investigated a role of cyclin D1 and CDK4, a positive regulator of cell cycle, in cultured neonatal rat cardiomyocyte hypertrophy. D-type cyclins including cyclin D1 were induced in cells stimulated by phenylephrine. This induction was inhibited by MEK inhibitor PD98059 and the dominant negative RasN17, but mimicked by expression of the constitutive active Ras61L. Over-expression of cyclin D1 and CDK4 using adenovirus gene transfer caused the hypertrophic growth of cardiomyocytes, as evidenced by an increase of the cell size as well as the amount of cellular protein and its rate of synthesis. However, the cyclin D1/CDK4 kinase activity was not up-regulated in cells treated by hypertrophic stimuli or in cells over-expressing the cyclin D1 and CDK4. Furthermore, a CDK inhibitor, p16, did not inhibit the hypertrophic growth of cardiomyocytes. These results clearly indicated that cyclin D1 and CDK4 have a role in hypertrophic growth of cardiomyocytes through a novel mechanism(s) which appears not to be related to its activity required for cell cycle progression.

Identification and biochemical characterization of a novel transcription elongation factor, Elongin A3

The Elongin complex stimulates the rate of transcription elongation by RNA polymerase II by suppressing the transient pausing of the polymerase at many sites along the DNA template. Elongin is composed of a transcriptionally active A subunit and two small regulatory B and C subunits, the latter binding stably to each other to form a binary complex that interacts with Elongin A and strongly induces its transcriptional activity. To further understand the role of Elongin A in transcriptional regulation by RNA polymerase II, we are attempting to identify Elongin A-related proteins. Here, we report on the molecular cloning, expression, and biochemical characterization of human Elongin A3, a novel transcription elongation factor that exhibits 49 and 81% identity to Elongin A and the recently identified Elongin A2, respectively. The mRNA of Elongin A3 is ubiquitously expressed, and the protein is localized to the nucleus of cells. Mechanistic studies have demonstrated that Elongin A3 possesses similar biochemical features to Elongin A2. Both stimulate the rate of transcription elongation by RNA polymerase II and are capable of forming a stable complex with Elongin BC. In contrast to Elongin A, however, their transcriptional activities are not activated by Elongin BC. Structure-function analyses using fusion proteins composed of Elongin A3 and Elongin A revealed that the COOH-terminal region of Elongin A is important for the activation by Elongin BC.

An alternatively spliced isoform of transcriptional repressor ATF3 and its induction by stress stimuli

Activating transcription factor 3 (ATF3) is a member of the ATF/CREB family of transcription factors and its expression is increased by various pathophysiological conditions and in several cancer cells. In this study, we describe two alternatively spliced ATF3DeltaZip mRNAs: ATF3DeltaZip2a and ATF3DeltaZip2b. Both variants encoded the same truncated protein of 135 amino acids, which lacked the leucine zipper domain and was incapable of binding to the ATF/CRE motif. The ATF3DeltaZip2 protein was shown to be localized in the nuclei and counteracted the transcriptional repression by the full-length ATF3. Western blot analysis showed that ATF3DeltaZip2 was expressed in cells exposed to A23187. Further study showed that, similar to the full-length ATF3, the expression of ATF3DeltaZip2 was induced by a wide range of stress stimuli. However, its expression was not detectable in cancer cells that constitutively over-expressed ATF3. Taken together, our results suggest that ATF3DeltaZip2, a protein derived from alternatively spliced mRNAs, is induced by various stress signals and may modulate the activity of the full-length ATF3 protein during stress response.