Cooperative role between p21cip1/waf1 and p27kip1 in premature senescence in glandular proliferative lesions in mice.

Cellular senescence has been considered a novel target for cancer therapy. It has also been pointed out that p21(cip1/waf1) and p27(kip1) cyclin-dependent kinase inhibitors (CKIs) play a role in cellular senescence in some tumor types. Therefore, in order to address the possibility of a cooperative role between p21 and p27 proteins in senescence in vivo we analyzed cellular senescence in spontaneous glandular proliferative lesions (adrenal, thyroid and pituitary glands) in a double-KO mice model, using γH2AX, p53, p16, PTEN and Ki67 as senescence markers. The results obtained showed that p21p27 double-null mice had the lowest number of γH2AX positive cells in glandular hyperplasias and benign tumors. Also, in this group, Ki67 proliferation index correlated with a lower immunohistochemical expression of γH2AX and p53. The expression of p16 and PTEN do not seem to cause synergism of senescence in the benign lesions analyzed in p21p27 double-KO mice. These observations suggest an intrinsic cooperation between p21 and p27 CKIs in the activation of stress-induced cellular senescence and tumor progression in vivo, which would be a physiological mechanism to prevent tumor cell proliferation.

Identification of a novel cyclin required for the intrinsic apoptosis pathway in lymphoid cells.

We have identified an early step common to pathways activated by different forms of intrinsic apoptosis stimuli. It requires de novo synthesis of a novel cyclin, cyclin O, that forms active complexes primarily with Cdk2 upon apoptosis induction in lymphoid cells. Cyclin O expression precedes glucocorticoid and gamma-radiation-induced apoptosis in vivo in mouse thymus and spleen, and its overexpression induces caspase-dependent apoptosis in cultured cells. Knocking down the endogenous expression of cyclin O by shRNA leads to the inhibition of glucocorticoid and DNA damage-induced apoptosis due to a failure in the activation of apical caspases while leaving CD95 death receptor-mediated apoptosis intact. Our data demonstrate that apoptosis induction in lymphoid cells is one of the physiological roles of cyclin O and it does not act by perturbing a normal cellular process such as the cell cycle, the DNA damage checkpoints or transcriptional response to glucocorticoids.

Activation of Cdk2 is a requirement for antigen-mediated thymic negative selection.

Apoptosis plays a critical role in T cell development and thymic selection. Thymocytes which undergo antigen-induced negative selection have been demonstrated to die by apoptosis. Despite this, relatively little is known about the specific apoptotic pathway involved in negative selection. We have examined the role of cyclin-dependent kinase 2 (Cdk2), a key regulator of thymocyte apoptosis, in this process. Stimulation of thymocytes with cognate antigen leads to a large increase in Cdk2 kinase activity. We also show that pharmacological inhibitors of Cdk2 block thymocyte apoptosis in response to antigen. Our data show that Cdk2 activity is essential for the apoptotic pathway used in negative selection.

Bad can act as a key regulator of T cell apoptosis and T cell development.

Bad is a distant relative of Bcl-2 and acts to promote cell death. Here, we show that Bad expression levels are greatly increased in thymocytes during apoptosis. We generated bad transgenic mice to study the action of upregulated Bad expression on T cell apoptosis. The T cells from these mice are highly sensitive to apoptotic stimuli, including anti-CD95. The numbers of T cells are greatly depleted and the processes of T cell development and selection are perturbed. We show that the proapoptotic function of Bad in primary T cells is regulated by Akt kinase and that Bad overexpression enhances both cell cycle progression and interleukin 2 production after T cell activation. These data suggest that Bad can act as a key regulator of T cell apoptosis and that this is a consequence of its upregulation after exposure to death stimuli.

A link between cell cycle and cell death: Bax and Bcl-2 modulate Cdk2 activation during thymocyte apoptosis.

Resting thymocytes undergoing apoptosis in response to specific stimuli degrade the cdk inhibitor p27(Kip1) and upregulate Cdk2 kinase activity. Inhibition of Cdk2 kinase activity efficiently blocks cell death via certain apoptosis pathways whereas overexpression of Cdk2 accelerates such cell death, suggesting its involvement in the signal transduction pathways activated by certain apoptotic stimuli. We found that Cdk2 activation during thymocyte apoptosis can be regulated by p53, Bax and Bcl-2. The highly elevated Cdk2 kinase activity in the apoptosing thymocytes is not associated with its canonical cyclins, cyclin E and cyclin A, and requires de novo synthesis of proteins for activation to take place. We therefore propose Cdk2 activation to be a crucial event in distinct pathways of apoptosis and the point at which the cell cycle and cell death pathways interact.

Bax. The pro-apoptotic Bcl-2 family member, Bax.

Bax is a pro-apoptotic member of the Bcl-2 family of genes which regulate programmed cell death. The Bax protein shares highly conserved domains with Bcl-2, some of which are required for the formation of Bax-Bcl-2 heterodimers. Bax expression is elevated in certain tissues after apoptotic stimuli and can be directly regulated by p53. Bax -/- mice have increased numbers of lymphoid cells and bax -/- neurons survive in culture following nerve growth factor deprivation. Bax can accelerate cell cycle entry in T-cells and has recently been shown to have a tumour suppressor function as well as carrying mutations in certain cancers. Bax can form ion-conducting channels in planar lipid bilayers which may be the biochemical mechanism through which it exerts its multiple effects. Pharmacological manipulation of Bax has implications for many diseases involving apoptosis such as cancer or neurodegenerative disorders.

The role of the tumor suppressor p53 in spermatogenesis.

The p53 protein appeared to be involved in both spermatogonial cell proliferation and radiation response. During normal spermatogenesis in the mouse, spermatogonia do not express p53, as analyzed by immunohistochemistry. However, after a dose of 4 Gy of X-rays, a distinct p53 staining was present in spermatogonia, suggesting that, in contrast to other reports, p53 does have a role in spermatogonia. To determine the possible role of p53 in spermatogonia, histological analysis was performed in testes of both p53 knock out C57BL/6 and FvB mice. The results indicate that p53 is an important factor in normal spermatogonial cell production as well as in the regulation of apoptosis after DNA damage. First, p53 knock out mouse testes contained about 50% higher numbers of A1 spermatogonia, indicating that the production of differentiating type spermatogonia by the undifferentiated spermatogonia is enhanced in these mice. Second, 10 days after a dose of 5 Gy of X-rays, in the p53 knock out testes, increased numbers of giant sized spermatogonial stem cells were found, indicating disturbance of the apoptotic process in these cells. Third, in the p53 knock out testis, the differentiating A2-B spermatogonia are more radioresistant compared to their wild-type controls, indicating that p53 is partly indispensable in the removal of lethally irradiated differentiating type spermatogonia. In accordance with our immunohistochemical data, Western analysis showed that levels of p53 are increased in total adult testis lysates after irradiation. These data show that p53 is important in the regulation of cell production during normal spermatogenesis either by regulation of cell proliferation or, more likely, by regulating the apoptotic process in spermatogonia. Furthermore, after irradiation, p53 is important in the removal of lethally damaged spermatogonia.

Transgenic mice in apoptosis research.

Transgenic mice have proved to be a valuable tool in various aspects of apoptosis research. They are particularly useful for studying apoptosis-related gene products in primary cells which may lead to different effects from similar experiments using immortalized cell lines. They allow the impact of these gene products on multi-faceted physiological processes to be identified. Transgenic mice have been generated expressing molecules ranging from Bcl-2 and Bcl-2 family members to CD95, superoxide dismutase and rhodopsin. This review details some of the insights revealed from such studies in diverse areas ranging from lymphoid development to neurodegeneration and effects on intracellular signalling.

Bax alpha perturbs T cell development and affects cell cycle entry of T cells.

Bax alpha can heterodimerize with Bcl-2 and Bcl-X(L), countering their effects, as well as promoting apoptosis on overexpression. We show that bax alpha transgenic mice have greatly reduced numbers of mature T cells, which results from an impaired positive selection in the thymus. This perturbation in positive selection is accompanied by an increase in the number of cycling thymocytes. Further to this, mature T cells overexpressing Bax alpha have lower levels of p27Kip1 and enter S phase more rapidly in response to interleukin-2 stimulation than do control T cells, while the converse is true of bcl-2 transgenic T cells. These data indicate that apoptotic regulatory proteins can modulate the level of cell cycle-controlling proteins and thereby directly impact on the cell cycle.

Immunolocalization of mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase in rat liver.

We report the preparation of specific polyclonal antibodies raised against two synthetic peptides deduced from the cDNA sequence for the rat liver mitochondrial 3-hydroxy-3-methylglutaryl Coenzyme A (HMG-CoA) synthase gene. Immunoelectron microscopy using these antibodies on hepatic cryoultrathin sections confirms the mitochondrial localization of this protein in hepatocytes. Immunofluorescence microscopy on frozen sections of adult rat liver revealed fluorescence inside all hepatocytes, with no evidence of zonation, indicating that ketogenesis may not be limited to specific regions of rat liver but is extended to all hepatocytes.

Transfection of the ketogenic mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme A synthase cDNA into Mev-1 cells corrects their auxotrophy for mevalonate.

A somatic cell mutant of the Chinese hamster ovary (CHO)-K1 (called Mev-1), auxotrophic for mevalonate by virtue of a complete lack of detectable cytosolic 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase activity, was transfected with a plasmid containing the cDNA for ketogenic mitochondrial HMG-CoA synthase under the control of SV40 early promoter. The resulting stable cell line (Mev-SM) was able to grow in the absence of mevalonate. Analysis of Western blot showed that the new cell line strongly expressed mitochondrial HMG-CoA synthase protein. Immunocytochemical studies using specific antibodies against mitochondrial HMG-CoA synthase showed that the protein was located exclusively inside the mitochondria. The prototroph cell line Mev-SM can incorporate labeled acetate into cholesterol in the absence of mevalonate. These results show that the new cell line may circumvent the lack of cytosolic HMG-CoA synthase activity by producing cholesterol-convertible HMG-CoA inside the mitochondria.

Peroxisome proliferator-activated receptor mediates induction of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene by fatty acids.

Fatty acids induce an increase in the transcription of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase gene, which encodes an enzyme that has been proposed as a control site of ketogenesis. We studied whether the peroxisome proliferator-activated receptor (PPAR) is involved in the mechanism of this transcriptional induction. We found that cotransfection of a rat mitochondrial HMG-CoA synthase promoter-chloramphenicol acetyltransferase reporter plasmid and a PPAR expression plasmid in the presence of the peroxisome proliferator clofibrate led to a more than 30-fold increase in chloramphenicol acetyltransferase activity, relative to the activity in the absence of both PPAR and inducer. Linoleic acid, a polyunsaturated fatty acid, increased this activity as potently as does clofibrate and more effectively than does monounsaturated oleic acid. We have identified, by deletional analysis, an element located 104 base pairs upstream of the mitochondrial HMG-CoA synthase gene, which confers PPAR responsiveness to homologous and heterologous promoters. This is the first example of a peroxisome proliferator-responsive element (PPRE) in a gene encoding a mitochondrial protein. This element contains an imperfect direct repeat that is similar to those described in the PPREs of other genes. Furthermore, gel retardation and cotransfection assays revealed that, as for other genes, PPAR heterodimerizes with retinoid X receptor and that both receptors cooperate for binding to the mitochondrial HMG-CoA synthase PPRE and subsequent activation of the gene. In conclusion, our data demonstrate that regulation of mitochondrial HMG-CoA synthase gene expression by fatty acids is mediated by PPAR, supporting the hypothesis that PPAR has an important role at the transcriptional level in the regulation of lipid metabolism.

Gene expression of enzymes regulating ketogenesis and fatty acid metabolism in regenerating rat liver.

Levels of mRNA for mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase, carnitine palmitoyltransferase I (CPT I) and carnitine palmitoyltransferase II (CPT II), fatty acid synthase (FAS) and actin were analysed during liver regeneration. mRNA levels for mitochondrial HMG-CoA synthase decreased rapidly, reaching a minimum 12 h after partial hepatectomy and returning to normal at 24-36 h. In contrast, CPT I, CPT II and FAS mRNAs increased throughout the period examined. Expression of actin increased significantly during regeneration. Levels of mRNA for mitochondrial HMG-CoA synthase also decreased as a result of surgical stress, although the effect of hepatectomy was much greater. We determined the levels of mitochondrial HMG-CoA synthase using specific antibodies. The amount of protein rapidly decreased, although less markedly than the corresponding mRNA levels. These results show that the decrease described in ketogenesis in partially hepatectomized rats correlated with the decrease in the expression of mitochondrial HMG-CoA synthase, suggesting that this enzyme may also be a control point in ketogenesis in the regenerating liver, as it is in normal and diabetic rats.

Methylation of the regulatory region of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene leads to its transcriptional inactivation.

The mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase gene is expressed in a limited set of tissues in the adult rat. Methylation of the 5' flanking region of the gene in vitro leads to its transcriptional inactivation when transfected in hepatoma-derived cell lines. In liver and kidney, expression of the gene correlates inversely with its degree of methylation, indicating that the methylation of the 5' flanking region and the first exon of the gene may be one of the factors responsible for the repression of its transcription. During the fetal/neonatal transition, a process of selective undermethylation of specific sites takes place in the 5' flanking region of the mitochondrial HMG-CoA synthase gene. Moreover, treatment with the hypomethylating agent 5-azacytidine of a hepatoma-derived cell line that presents barely detectable levels of mitochondrial HMG-CoA synthase mRNA leads to a significant increase in the mRNA levels. These results point to methylation as one of the regulatory mechanisms that operate on the mitochondrial HMG-CoA synthase gene.

Testis and ovary express the gene for the ketogenic mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase.

Ketogenesis has been thought to occur exclusively in the mitochondrial compartment of liver cells. After analysis of five different rat tissues, it was shown that the gene for mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, one of the major control points in the pathway (1992. Casals et al. Biochem. J. 283: 261-264) was expressed only in liver (1990. Ayté et al. Proc. Natl. Acad. Sci. USA. 87: 3874-3878). However, exhaustive analysis of organs and tissues has shown that, in addition to liver cells, testis and ovary express this committed gene in levels similar to those of liver, not only as mRNAs but also as immunodetectable mitochondrial HMG-CoA synthase protein. Immunocytochemical studies locate the mitochondrial HMG-CoA synthase protein in Leydig cells, theca interna cells of ovarian follicle, corpus luteum cells of ruptured ovarian follicle, and epidermal cells of the oviduct. The development of gonadal function appears to be accompanied by mitochondrial HMG-CoA synthase gene expression, as hypophysectomy reduces the expression pattern in gonads. Changes induced in mitochondrial HMG-CoA synthase levels after the depletion of lipoprotein levels in blood closely mimic those of the cholesterogenic cytosolic HMG-CoA synthase and HMG-CoA reductase. These results suggest that mitochondrial HMG-CoA synthase could perform a function similar to that of cytosolic HMG-CoA synthase in de novo cholesterogenesis in gonads, at variance with its ketogenic role in liver.

The rat mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme-A-synthase gene contains elements that mediate its multihormonal regulation and tissue specificity.

Mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme-A (HMG-CoA) synthase, a liver-specific enzyme, is a constituent of the HMG-CoA cycle responsible for ketone-body synthesis. We report the isolation and characterization of genomic clones that encompass the gene for rat mitochondrial HMG-CoA synthase. The gene spans at least 24 kbp and contains ten exons and nine introns. The 5' flanking region of the gene has also been cloned and characterized. Exon 1 contains the untranslated sequence of the transcript, extending downstream to enclose the coding region for the putative mitochondrial-targeting signal (35 amino acids). The 1149-bp proximal region of the transcription start point permits transcription of a reporter gene in transfected hepatoma cells but not in an extrahepatic cell line, confirming the function of the promoter. A truncated construct of 142 bp is still able to promote transcription in hepatoma cells, suggesting the presence of liver-specific enhancer elements in the proximal promoter region. The 5' flanking region contains typical promoter elements, including a TATA box and several putative recognition sequences for transcription factors involved in controlling both basal-level and hormone-modulated transcription rates. Furthermore, the presence in the mitochondrial HMG-CoA-synthase promoter of cis-elements, responsible for the multihormonal regulation of transcription, is supported by transient transfection experiments.

Structural characterization of the 3' noncoding region of the gene encoding rat mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase.

We report the structural analysis of the 3' noncoding region of the rat mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase-encoding gene. This region, encoded by a single exon of the gene, spans 410 nucleotides. By using S1-nuclease protection analysis, we have mapped two major alternative polyadenylation signals, in close proximity. Computer analysis of the 3' noncoding region has detected the existence of four inverted repeats that could form stem-and-loop structures, with the thermodynamic free energy of formation ranging from -12.2 kcal/mol to -20.4 kcal/mol. The putative regulatory role of these structural features in mRNA stability is discussed.

Regulation of the expression of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene. Its role in the control of ketogenesis.

We have explored the role of mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase in regulating ketogenesis. We had previously cloned the cDNA for mitochondrial HMG-CoA synthase and have now studied the regulation in vivo of the expression of this gene in rat liver. The amount of processed mitochondrial HMG-CoA synthase mRNA is rapidly changed in response to cyclic AMP, insulin, dexamethasone and refeeding, and is greatly increased by starvation, fat feeding and diabetes. We conclude that one point of ketogenic control is exercised at the level of genetic expression of mitochondrial HMG-CoA synthase.