Coupled transcriptional and translational control of cyclin-dependent kinase inhibitor p18INK4c expression during myogenesis

Alternative Transcription Start Site Usage and Functional Implications in Pathogenic Fungi

Pathogenic fungi require delicate gene regulation mechanisms to adapt to diverse living environments and escape host immune systems. Recent advances in sequencing technology have exposed the complexity of the fungal genome, thus allowing the gradual disentanglement of multiple layers of gene expression control. Alternative transcription start site (aTSS) usage, previously reported to be prominent in mammals and to play important roles in physiopathology, is also present in fungi to fine-tune gene expression. Depending on the alteration in their sequences, RNA isoforms arising from aTSSs acquire different characteristics that significantly alter their stability and translational capacity as well as the properties and biologic functions of the resulting proteins. Disrupted control of aTSS usage has been reported to severely impair growth, virulence, and the infectious capacity of pathogenic fungi. Here, we discuss principle concepts, mechanisms, and the functional implication of aTSS usage in fungi.

Exon skipping in genes encoding lineage-defining myogenic transcription factors in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a childhood sarcoma composed of myoblast-like cells, which suggests a defect in terminal skeletal muscle differentiation. To explore potential defects in the differentiation program, we searched for mRNA splicing variants in genes encoding transcription factors driving skeletal muscle lineage commitment and differentiation. We studied two RMS cases and identified altered splicing resulting in "skipping" the second of three exons in MYOD1. RNA-Seq data from 42 tumors and additional RMS cell lines revealed exon 2 skipping in both MYOD1 and MYF5 but not in MYF6 or MYOG. Complementary molecular analysis of MYOD1 mRNA found evidence for exon skipping in 5 additional RMS cases. Functional studies showed that so-called MYODΔEx2 protein failed to robustly induce muscle-specific genes, and its ectopic expression conferred a selective advantage in cultured fibroblasts and an RMS xenograft. In summary, we present previously unrecognized exon skipping within MYOD1 and MYF5 in RMS, and we propose that alternative splicing can represent a mechanism to alter the function of these two transcription factors in RMS.

From cyclins to CDKIs: Cell cycle regulation of skeletal muscle stem cell quiescence and activation

After extensive proliferation during development, the adult skeletal muscle cells remain outside the cell cycle, either as post-mitotic myofibers or as quiescent muscle stem cells (MuSCs). Despite its terminally differentiated state, adult skeletal muscle has a remarkable regeneration potential, driven by MuSCs. Upon injury, MuSC quiescence is reversed to support tissue growth and repair and it is re-established after the completion of muscle regeneration. The distinct cell cycle states and transitions observed in the different myogenic populations are orchestrated by elements of the cell cycle machinery. This consists of i) complexes of cyclins and Cyclin-Dependent Kinases (CDKs) that ensure cell cycle progression and ii) their negative regulators, the Cyclin-Dependent Kinase Inhibitors (CDKIs). In this review we discuss the roles of these factors in developmental and adult myogenesis, with a focus on CDKIs that have emerging roles in stem cell functions.

The distinct effects of P18 overexpression on different stages of hematopoiesis involve TGF-β and NF-κB signaling

Deficiency of P18 can significantly improve the self-renewal potential of hematopoietic stem cells (HSC) and the success of long-term engraftment. However, the effects of P18 overexpression, which is involved in the inhibitory effects of RUNX1b at the early stage of hematopoiesis, have not been examined in detail. In this study, we established inducible P18/hESC lines and monitored the effects of P18 overexpression on hematopoietic differentiation. Induction of P18 from day 0 (D0) dramatically decreased production of CD34highCD43- cells and derivative populations, but not that of CD34lowCD43- cells, changed the cell cycle status and apoptosis of KDR+ cells and downregulated the key hematopoietic genes at D4, which might cause the severe blockage of hematopoietic differentiation at the early stage. By contrast, induction of P18 from D10 dramatically increased production of classic hematopoietic populations and changed the cell cycle status and apoptosis of CD45+ cells at D14. These effects can be counteracted by inhibition of TGF-β or NF-κB signaling respectively. This is the first evidence that P18 promotes hematopoiesis, a rare property among cyclin-dependent kinase inhibitors (CKIs).

RNA splicing alteration in the response to platinum chemotherapy in ovarian cancer: A possible biomarker and therapeutic target

Since its discovery, alternative splicing has been recognized as a powerful way for a cell to amplify the genetic information and for a living organism to adapt, evolve, and survive. We now know that a very high number of genes are regulated by alternative splicing and that alterations of splicing have been observed in different types of human diseases, including cancer. Here, we review the accumulating knowledge that links the regulation of alternative splicing to the response to chemotherapy, focusing our attention on ovarian cancer and platinum-based treatments. Moreover, we discuss how expanding information could be exploited to identify new possible biomarkers of platinum response, to better select patients, and/or to design new therapies able to overcome platinum resistance.

The Role of CDKs and CDKIs in Murine Development

Cyclin-dependent kinases (CDKs) and their inhibitors (CDKIs) play pivotal roles in the regulation of the cell cycle. As a result of these functions, it may be extrapolated that they are essential for appropriate embryonic development. The twenty known mouse CDKs and eight CDKIs have been studied to varying degrees in the developing mouse, but only a handful of CDKs and a single CDKI have been shown to be absolutely required for murine embryonic development. What has become apparent, as more studies have shone light on these family members, is that in addition to their primary functional role in regulating the cell cycle, many of these genes are also controlling specific cell fates by directing differentiation in various tissues. Here we review the extensive mouse models that have been generated to study the functions of CDKs and CDKIs, and discuss their varying roles in murine embryonic development, with a particular focus on the brain, pancreas and fertility.

CDK inhibitors for muscle stem cell differentiation and self-renewal

Regeneration of muscle is undertaken by muscle stem cell populations named satellite cells which are normally quiescent or at the G0 phase of the cell cycle. However, upon signals from damaged muscle, satellite cells lose their quiescence, and enter the G1 cell cycle phase to expand the population of satellite cell progenies termed myogenic precursor cells (MPCs). Eventually, MPCs stop their cell cycle and undergo terminal differentiation to form skeletal muscle fibers. Some MPCs retract to quiescent satellite cells as a self-renewal process. Therefore, cell cycle regulation, consisting of satellite cell activation, proliferation, differentiation and self-renewal, is the key event of muscle regeneration. In this review, we summarize up-to-date progress on research about cell cycle regulation of myogenic progenitor cells and muscle stem cells during embryonic myogenesis and adult muscle regeneration, aging, exercise and muscle diseases including muscular dystrophy and muscle fiber atrophy, especially focusing on cyclin-dependent kinase inhibitors (CDKIs).

Repression of CDKN2C caused by PML/RARα binding promotes the proliferation and differentiation block in acute promyelocytic leukemia

Inappropriate cell proliferation during oncogenesis is often accompanied by inactivation of components involved in the cell cycle machinery. Here, we report that cyclin-dependent kinase inhibitor 2C (CDKN2C) as a member of the cyclin-dependent kinase inhibitors is a target of the PML/RARα oncofusion protein in leukemogenesis of acute promyelocytic leukemia (APL).We found that CDKN2C was markedly downregulated in APL blasts compared with normal promyelocytes. Chromatin immunoprecipitation combined with quantitative polymerase chain reaction demonstrated that PML/RARα directly bound to the CDKN2C promoter in the APL patient-derived cell line NB4. Luciferase assays indicated that PML/RARα inhibited the CDKN2C promoter activity in a dose-dependent manner. Furthermore, all-trans retinoic acid treatment induced CDKN2C expression by releasing the PML/RARα binding on chromatin in NB4 cells. Functional studies showed that ectopic expression of CDKN2C induced a cell cycle arrest at the G₀/G₁ phase and a partial differentiation in NB4 cells. Finally, the transcriptional regulation of CDKN2C was validated in primary APL patient samples. Collectively, this study highlights the importance of CDKN2C inactivation in the abnormal cell cycle progression and differentiation block of APL cells and may provide new insights into the study of pathogenesis and targeted therapy of APL.

Identification and characterization of methylation-dependent/independent DNA regulatory elements in the human SLC9B1 gene

The human NHEDC1 (hNHEDC1) protein is thought to be essential for sperm motility and fertility however the mechanisms regulating its gene expression are largely unknown. In this study we have identified multiple DNA regulatory elements in the 5' end of the gene encoding hNHEDC1 (SLC9B1) and have explored the role that DNA methylation at these elements plays in the regulation of its expression. We first show that the full-length hNHEDC1 protein is testis-specific for the tissues that we tested and that it localizes to the cells of the seminiferous tubules. In silico analysis of the SLC9B1 gene locus identified two putative promoters (P1 and P2) and two CpG islands - CpGI (overlapping with P1) and CpGII (intragenic) - at the 5' end of the gene. By deletion analysis of P1, we show that the region from -23 bp to +200 bp relative to the transcription start site (TSS) is sufficient for optimal promoter activity in a germ cell line. Additionally, in vitro methylation of the P1 (the -500 bp to +200 bp region relative to the TSS) abolishes its activity in germ cells and somatic cells strongly suggesting that DNA methylation at this promoter could regulate SLC9B1 expression. Furthermore, bisulfite-sequencing analysis of the P1/CpGI uncovered reduced methylation in the testis vs. lung whereas CpGII displayed no differences in methylation between these two tissues. Additionally, treatment of HEK 293 cells with 5-aza-2-Deoxycytidine led to upregulation of NHEDC1 transcript and reduced methylation in the promoter CpGI. Finally, we have uncovered both enhancer and silencer functions of the intragenic SLC9B1 CpGII. In all, our data suggests that SLC9B1 gene expression could be regulated via a concerted action of DNA methylation-dependent and independent mechanisms mediated by these multiple DNA regulatory elements.

PGC-1α is associated with C2C12 Myoblast differentiation

PGC-1α has been implicated as an important mediator of functional capacity of skeletal muscle. However, the role of PGC-1α in myoblast differentiation remains unexplored. In the present study, we observed a significant up-regulation of PGC-1α expression during the differentiation of murine C2C12 myoblast. To understand the biological significance of PGC-1α up-regulation in myoblast differentiation, C2C12 cells were transfected with murine PGC-1α cDNA and siRNA targeting PGC-1α, respectively. PGC-1α over-expressing clones fused to form typical myotubes with higher mRNA level of myosin heavy chain isoform I (MyHCI) and lower MyHCIIX. No obvious differentiation was observed in PGC-1α-targeted siRNA-transfected cells with marked decrement of mRNA levels of MyHCI and MyHCIIX. Furthermore, PGC-1α increased the expression of MyoD and MyoG in C2C12 cells, which controlled the commitment of precursor cells to myotubes. These results indicate that PGC-1α is associated with myoblast differentiation and elevates MyoD and MyoG expression levels in C2C12 cells.

FoxO-dependent regulation of diacylglycerol kinase α gene expression

Diacylglycerol kinase α (DGKα) regulates diacylglycerol levels, catalyzing its conversion into phosphatidic acid. The α isoform is central to immune response regulation; it downmodulates Ras-dependent pathways and is necessary for establishment of the unresponsive state termed anergy. DGKα functions are regulated in part at the transcriptional level although the mechanisms involved remain poorly understood. Here, we analyzed the 5' end structure of the mouse DGKα gene and detected three binding sites for forkhead box O (FoxO) transcription factors, whose function was confirmed using luciferase reporter constructs. FoxO1 and FoxO3 bound to the 5' regulatory region of DGKα in quiescent T cells, as well as after interleukin-2 (IL-2) withdrawal in activated T cells. FoxO binding to this region was lost after complete T cell activation or IL-2 addition, events that correlated with FoxO phosphorylation and a sustained decrease in DGKα gene expression. These data strongly support a role for FoxO proteins in promoting high DGKα levels and indicate a mechanism by which DGKα function is downregulated during productive T cell responses. Our study establishes a basis for a causal relationship between DGKα downregulation, IL-2, and anergy avoidance.

Contrasting behavior of the p18INK4c and p16INK4a tumor suppressors in both replicative and oncogene-induced senescence

The cyclin-dependent kinase (CDK) inhibitors, p18(INK4c) and p16(INK4a), both have the credentials of tumor suppressors in human cancers and mouse models. For p16(INK4a), the underlying rationale is its role in senescence, but the selective force for inactivation of p18(INK4c) in incipient cancer cells is less clear. Here, we show that in human fibroblasts undergoing replicative or oncogene-induced senescence, there is a marked decline in the levels of p18(INK4c) protein and RNA, which mirrors the accumulation of p16(INK4a). Downregulation of INK4c is not dependent on p16(INK4a), and RAS can promote the loss of INK4c without cell-cycle arrest. Downregulation of p18(INK4c) correlates with reduced expression of menin and E2F1 but is unaffected by acute cell-cycle arrest or inactivation of the retinoblastoma protein (pRb). Collectively, our data question the idea that p18(INK4c) acts as a backup for loss of p16(INK4a) and suggest that the apparent activation of p18(INK4c) in some settings represents delayed senescence rather than increased expression. We propose that the contrasting behavior of the two very similar INK4 proteins could reflect their respective roles in senescence versus differentiation.

The myogenic kinome: protein kinases critical to mammalian skeletal myogenesis

Myogenesis is a complex and tightly regulated process, the end result of which is the formation of a multinucleated myofibre with contractile capability. Typically, this process is described as being regulated by a coordinated transcriptional hierarchy. However, like any cellular process, myogenesis is also controlled by members of the protein kinase family, which transmit and execute signals initiated by promyogenic stimuli. In this review, we describe the various kinases involved in mammalian skeletal myogenesis: which step of myogenesis a particular kinase regulates, how it is activated (if known) and what its downstream effects are. We present a scheme of protein kinase activity, similar to that which exists for the myogenic transcription factors, to better clarify the complex signalling that underlies muscle development.

N-Myc is a downstream target of RET signaling and is required for transcriptional regulation of p18(Ink4c) by the transforming mutant RET(C634R)

Inherited activating mutations in RET predispose humans to Multiple Endocrine Neoplasia type-2 (MEN2). The MEN2A-specific mutation RET(C634R), RET2A, has been shown to simultaneously downregulate the CDKIs p18 and p27, and upregulate cyclin D1. Importantly, the loss of p18 is necessary and sufficient for RET2A-mediated hyperproliferation. The loss of N-Myc in mice results in embryonic lethality due to a lack of neuronal progenitor cells that fail to proliferate, correlate with accumulation of p18 and p27. Therefore, N-Myc may regulate expression of both CDKIs. Also, N-Myc is expressed predominantly in neuroendocrine cells that give rise to the primary cell types affected in MEN2A. Together these studies suggest that N-Myc is a downstream target of RET2A signaling that prevents accumulation of p18 and/or p27. We report that MAPK activation by RET2A leads to a transient induction of N-Myc mRNA and protein levels, and that N-Myc induction is required to maintain low p18 and p27 levels. Induced N-Myc levels correlate with increased binding of N-Myc to an initiator consensus binding site in the p18 promoter, and this binding is essential for RET2A-mediated transcriptional regulation of p18. Finally, loss of N-Myc induction prevents RET2A-mediated hyperproliferation. Our results demonstrate for the first time that N-Myc is a downstream target of RET2A signaling, and propose that induction of N-Myc by RET2A is a key step leading to lower p18 levels during MEN2A tumorigenesis.

Genomic organization and regulation of the human orexin (hypocretin) receptor 2 gene: identification of alternative promoters

Orexins (hypocretins), acting via their receptors, are involved in the control of feeding behaviour, sleep, arousal and energy homoeostasis. However, regulation of the human orexin receptor 2 (hOX2R) gene remains unknown. We have identified four transcripts arising from alternative splicing from three exons. These exon 1 variants were designated exons 1A, 1B and 1C on the basis of their 5′–3′ order. RT (reverse transcription)–PCR demonstrates the differential expression in various human tissues. The alternative 5′-UTRs (untranslated regions) possessed by these isoforms have different translational efficiencies, which regulate the level of protein expression. In the present study, we have demonstrated that the hOX2R gene is regulated by two promoters and the novel transcripts are regulated by the distal promoter located upstream of exon 1A. We have demonstrated that the AP-1 (activator protein 1) motif is critical for sustaining the basal activity of distal promoter. Analysis of the proximal promoter revealed the region regulating promoter activity contained putative binding elements including those for CREB (cAMP-response-element-binding protein), GATA-2 and Oct-1. Using the chromatin immunoprecipitation assay, we demonstrated that CREB, GATA-2 and Oct-1 transcription factors bind to these critical regulatory promoter elements. Mutational studies suggested that these motifs functioned independently, but have a compound effect regulating hOX2R gene transcription. Furthermore, proximal promoter activity is enhanced by both PKA (protein kinase A) and PKC (protein kinase C) pathway activation, via binding of CREB and GATA-2 transcription factors. In conclusion, we have demonstrated that expression of hOX2R is regulated by a complex involving a proximal PKA/PKC-regulated promoter and a distal promoter regulating tissue-specific expression of alternative transcripts which in turn post-transcriptionally regulate receptor levels.

Promoter choice and translational repression determine cell type-specific cell surface density of the inhibitory receptor CD85j expressed on different hematopoietic lineages

CD85j (ILT2/LILRB1/LIR-1) is an inhibitory receptor that recognizes major histocompatibility complex (MHC) class Ia and Ib alleles that are widely expressed on all cell types. On ligand recognition, CD85j diminishes kinase activity by recruiting phosphatases to motifs within its cytoplasmic domain. Within the hematopoietic system, CD85j is expressed with cell-specific patterns and cell surface densities that reflect the different roles of cell contact-mediated inhibition in these lineages. While monocytes ubiquitously have high cell surface expression, B lymphocytes start to express CD85j at intermediate levels during early B-cell maturation and natural killer (NK) cells and T cells exhibit a low level of expression on only a subset of cells. The cell-specific expression pattern is accomplished by 2 complementing but not independent mechanisms. Lymphocytes and monocytes use distinct promoters to drive CD85j expression. The lymphocyte promoter maps 13 kilobases (kb) upstream of the monocyte promoter; its use results in the inclusion of a distant exon into the 5'-untranslated region. A short sequence stretch within this exon has the unique function of repressing CD85j protein translation and is responsible for the subdued expression in lymphocytes. These cell-specific mechanisms allow tailoring of CD85j levels to the distinct roles it plays in different hematopoietic lineages.

Differential post-transcriptional regulation of two Ink4 proteins, p18 Ink4c and p19 Ink4d

Cyclin(-D-)-dependent kinase (Cdk) inhibitors of the Ink4 family specifically bind to Cdk4 and Cdk6, but not to other Cdks. Ink4c and Ink4d mRNAs are maximally and periodically expressed during the G(2)/M phase of the cell division cycle, but the abundance of their encoded proteins is regulated through distinct mechanisms. Both proteins undergo polyubiquitination, but the half life of p18(Ink4c) (approximately 10 hours) is much longer than that of p19(Ink4d) (approximately 2.5 hours). Lysines 46 and 112 are preferred sites of ubiquitin conjugation in p18(Ink4c), although substitution of these and other lysine residues with arginine, particularly in combination, triggers protein misfolding and accelerates p18(Ink4c) degradation. When tethered to either catalytically active or inactive Cdk4 or Cdk6, polyubiquitination of p18(Ink4c) is inhibited, and the protein is further stabilized. Conversely, in competing with p18(Ink4c) for binding to Cdks, cyclin D1 accelerates p18(Ink4c) turnover. In direct contrast, polyubiquitination of p19(Ink4d) is induced by its association with Cdks, whereas cyclin D1 overexpression retards p19(Ink4d) degradation. Although it has been generally assumed that p18(Ink4c) and p19(Ink4d) are biochemically similar Cdk inhibitors, the major differences in their stability and turnover are likely key to understanding their distinct biological functions.

Different gene-expression profiles for the poorly differentiated carcinoma and the highly differentiated papillary adenocarcinoma in mammary glands support distinct metabolic pathways

BACKGROUND

Deregulation of Stat5 in the mammary gland of transgenic mice causes tumorigenesis. Poorly differentiated carcinoma and highly differentiated papillary adenocarcinoma tumors evolve. To distinguish the genes and elucidate the cellular processes and metabolic pathways utilized to preserve these phenotypes, gene-expression profiles were analyzed.

METHODS

Mammary tumors were excised from transgenic mice carrying a constitutively active variant of Stat5, or a Stat5 variant lacking s transactivation domain. These tumors displayed either the carcinoma or the papillary adenocarcinoma phenotypes. cRNAs, prepared from each tumor were hybridized to an Affymetrix GeneChip(R) Mouse Genome 430A 2.0 array. Gene-ontology analysis, hierarchical clustering and biological-pathway analysis were performed to distinct the two types of tumors. Histopathology and immunofluorescence staining complemented the comparison between the tumor phenotypes.

RESULTS

The nucleus-cytoskeleton-plasma membrane axis is a major target for differential gene expression between phenotypes. In the carcinoma, stronger expression of genes coding for specific integrins, cytoskeletal proteins and calcium-binding proteins highlight cell-adhesion and motility features of the tumor cells. This is supported by the higher expression of genes involved in O-glycan synthesis, TGF-beta, activin, their receptors and Smad3, as well as the Notch ligands and members of the gamma-secretase complex that enable Notch nuclear localization. The Wnt pathway was also a target for differential gene expression. Higher expression of genes encoding the degradation complex of the canonical pathway and limited TCF expression in the papillary adenocarcinoma result in membranal accumulation of beta-catenin, in contrast to its nuclear translocation in the carcinoma. Genes involved in cell-cycle arrest at G1 and response to DNA damage were more highly expressed in the papillary adenocarcinomas, as opposed to favored G2/M regulation in the carcinoma tumors.

CONCLUSION

At least six metabolic pathways support the morphological and functional differences between carcinomas and papillary adenocarcinomas. Differential gene-expression profiles favor cell adhesion, motility and proliferation in the carcinoma. Cell-cell contact, polarity, earlier cell-cycle arrest and DNA damage control are better displayed in the papillary adenocarcinoma.

Molecular mechanisms underlying the MiT translocation subgroup of renal cell carcinomas

Renal cell carcinomas (RCCs) represent a heterogeneous group of neoplasms, which differ in histological, pathologic and clinical characteristics. The tumors originate from different locations within the nephron and are accompanied by different recurrent (cyto)genetic anomalies. Recently, a novel subgroup of RCCs has been defined, i.e., the MiT translocation subgroup of RCCs. These tumors originate from the proximal tubule of the nephron, exhibit pleomorphic histological features including clear cell morphologies and papillary structures, and are found predominantly in children and young adults. In addition, these tumors are characterized by the occurrence of recurrent chromosomal translocations, which result in disruption and fusion of either the TFE3 or TFEB genes, both members of the MiT family of basic helix-loop-helix/leucine-zipper transcription factor genes. Hence the name MiT translocation subgroup of RCCs. In this review several features of this RCC subgroup will be discussed, including the molecular mechanisms that may underlie their development.

p19Ink4d and p18Ink4c cyclin-dependent kinase inhibitors in the male reproductive axis

The loss of the cyclin-dependent kinase inhibitors (CKIs) p18(Ink4c) and p19(Ink4d) leads to male reproductive defects (Franklin et al., 1998. Genes Dev 12: 2899-2911; Zindy et al., 2000. Mol Cell Biol 20: 372-378; Zindy et al., 2001. Mol Cell Biol 21: 3244-3255). In order to assess whether these inhibitors directly or indirectly affect male germ cell differentiation, we examined the expression of p18(Ink4c) and p19(Ink4d) in spermatogenic and supporting cells in the testis and in pituitary gonadotropes. Both p18(Ink4c) and p19(Ink4d) are most abundant in the testis after 18 days of age and are expressed in purified populations of spermatogenic and testicular somatic cells. Different p18(Ink4c) mRNAs are expressed in isolated spermatogenic and Leydig cells. Spermatogenic cells also express a novel p19(Ink4d) transcript that is distinct from the smaller transcript expressed in Sertoli cells, Leydig cells and in other tissues. Immunohistochemistry detected significant levels of p19(Ink4d) in preleptotene spermatocytes, pachytene spermatocytes, condensing spermatids, and Sertoli cells. Immunoprecipitation-Western analysis detected both CKI proteins in isolated pachytene spermatocytes and round spermatids. CDK4/6-CKI complexes were detected in germ cells by co-immunoprecipitation, although the composition differed by cell type. p19(Ink4d) was also identified in FSH+ gonadotrophs, suggesting that this CKI may be independently required in the pituitary. Possible cell autonomous and paracrine mechanisms for the spermatogenic defects in mice lacking p18(Ink4c) or p19(Ink4d) are supported by expression of these CKIs in spermatogenic cells and in somatic cells of the testis and pituitary.

Expression of p16Ink4a Compensates for p18Ink4c Loss in Cyclin-Dependent Kinase 4/6–Dependent Tumors and Tissues

Cell cycle progression from G(1) to S phase depends on phosphorylation of pRb by complexes containing a cyclin (D type or E type) and cyclin-dependent kinase (e.g., cdk2, cdk4, or cdk6). Ink4 proteins function to oppose the action of cdk4/6-cyclin D complexes by inhibiting cdk4/6. We employed genetic and pharmacologic approaches to study the interplay among Ink4 proteins and cdk4/6 activity in vivo. Mouse embryo fibroblasts (MEF) lacking p16(Ink4a) and p18(Ink4c) showed similar growth kinetics as wild-type MEFs despite increased cdk4 activity. In vivo, germline deficiency of p16(Ink4a) and p18(Ink4c) resulted in increased proliferation in the intermediate pituitary and pancreatic islets of adult mice, and survival of p16(Ink4a-/-);p18(Ink4c-/-) mice was significantly reduced due to aggressive pituitary tumors. Compensation among the Ink4 proteins was observed both in vivo in p18(Ink4c-/-) mice and in MEFs from p16(Ink4a-/-), p18(Ink4c-/-), or p16(Ink4a-/-);p18(Ink4c-/-) mice. Treatment with PD 0332991, a specific cdk4/6 kinase inhibitor, abrogated proliferation in those compartments where Ink4 deficiency was associated with enhanced proliferation (i.e., islets, pituitary, and B lymphocytes) but had no effect on proliferation in other tissues such as the small bowel. These data suggest that p16(Ink4a) and p18(Ink4c) coordinately regulate the in vivo catalytic activity of cdk4/6 in specific compartments of adult mice.

Transcription of rat TRPV1 utilizes a dual promoter system that is positively regulated by nerve growth factor

The capsaicin receptor, also known as 'transient receptor potential vanilloid receptor subtype 1' (TRPV1, VR1), is an ion channel subunit expressed in primary afferent nociceptors, which plays a critical role in pain transduction and thermal hyperalgesia. Increases in nociceptor TRPV1 mRNA and protein are associated with tissue injury-inflammation. As little is understood about what controls TRPV1 RNA transcription in nociceptors, we functionally characterized the upstream portion of the rat TRPV1 gene. Two functional rTRPV1 promoter regions and their transcription initiation sites were identified. Although both promoter regions directed transcriptional activity in nerve growth factor (NGF) treated rat sensory neurons, the upstream Core promoter was the most active in cultures enriched in sensory neurons. Because NGF is a key modulator of inflammatory pain, we examined the effect of NGF on rTRPV1 transcription in PC12 cells. NGF positively regulated transcriptional activity of both rTRPV1 promoter regions in PC12 cells. We propose that the upstream regulatory region of the rTRPV1 gene is composed of a dual promoter system that is regulated by NGF. These findings support the hypothesis that NGF produced under conditions of tissue injury and/or inflammation directs an increase of TRPV1 expression in nociceptors in part through a transcription-dependent mechanism.

Prolactin in man: a tale of two promoters

The pituitary hormone prolactin (PRL) is best known for its role in the regulation of lactation. Recent evidence furthermore indicates PRL is required for normal reproduction in rodents. Here, we report on the insertion of two transposon-like DNA sequences in the human prolactin gene, which together function as an alternative promoter directing extrapituitary PRL expression. Indeed, the transposable elements contain transcription factor binding sites that have been shown to mediate PRL transcription in human uterine decidualised endometrial cells and lymphocytes. We hypothesize that the transposon insertion event has resulted in divergent (pituitary versus extrapituitary) expression of prolactin in primates, and in differential actions of pituitary versus extrapituitary prolactin in lactation versus pregnancy respectively. Importantly, the TE insertion might provide a context for some of the conflicting results obtained in studies of PRL function in mice and man.

Cyclin-dependent kinase inhibitors in yeast, animals, and plants: a functional comparison

The cell cycle is remarkably conserved in yeast, animals, and plants and is controlled by cyclin-dependent kinases (CDKs). CDK activity can be inhibited by binding of CDK inhibitory proteins, designated CKIs. Numerous studies show that CKIs are essential in orchestrating eukaryotic cell proliferation and differentiation. In yeast, animals, and plants, CKIs act as regulators of the G1 checkpoint in response to environmental and developmental cues and assist during mitotic cell cycles by inhibiting CDK activity required to arrest mitosis. Furthermore, CKIs play an important role in regulating cell cycle exit that precedes differentiation and in promoting differentiation in cooperation with transcription factors. Moreover, CKIs are essential to control CDK activity in endocycling cells. So, in yeast, animals, and plants, CKIs share many functional similarities, but their functions are adapted toward the specific needs of the eukaryote.

Simultaneous downregulation of CDK inhibitors p18(Ink4c) and p27(Kip1) is required for MEN2A-RET-mediated mitogenesis

Multiple endocrine neoplasia type 2A (MEN2A) is predisposed by mutations in the RET proto-oncogene. Low expression of the cyclin-dependent kinase inhibitor (CDKI) p27(Kip1) is present in thyroid tumors, and recent evidence demonstrates p27 downregulation by the active RET mutant, RET/PTC1, found in papillary thyroid carcinoma. This implicates decreased p27 activity as an important event during thyroid tumorigenesis. However, p27(-/-) mice develop MEN-like tumors only in combination with loss of another CDKI, p18(Ink4c). This suggests that p18 and p27 functionally collaborate in suppression of tumorigenesis, that loss of both is critical in the development of MEN tumors and that both p18 and p27 are regulated by RET. We report that induction of the constitutively active MEN2A-specific RET mutant, RET2A(C634R), correlates with reduced p18/p27, and elevated cyclin D protein levels, leading to increased CDK activity, increased pRb phosphorylation and proliferation under growth arrest conditions. Mechanistically, RET2A represses p18/p27 mRNA levels while elevating cyclin D1 mRNA levels. RET2A expression also correlates with decreased p27 protein stability. RET2A-mediated regulation of p18 and p27, but not of cyclins D1 and D2, requires functional mitogen-activated protein kinase signaling. Additionally, RET2A-dependent p18 repression is required and sufficient to increase cell proliferation. Perhaps most significantly, MEN2A adrenal tumors also display these changes in cell cycle expression profile, demonstrating the biological relevance of our cell culture studies. Our results demonstrate for the first time that RET2A regulates p18, and suggest that loss of not only p27 but also of p18 expression is a key step in MEN tumorigenesis.

Multiple alternate p21 transcripts are regulated by p53 in human cells

The p53 tumor-suppressor is a transcription factor that is stabilized in response to cellular stress leading to growth arrest or apoptosis. p21(WAF1/CIP1) is a major transcriptional target of p53 and it plays a critical role in p53-dependent cell cycle arrest. In this study, we identified multiple alternate human p21 transcripts that have their transcriptional start sites in the direct proximity of the distal p53 response element. These transcripts are upregulated as a result of DNA damage-induced p53 activation. Furthermore, the basal expression of these alternate transcripts is strongly regulated by p53 and they are undetectable in p53-knocked down cells. This is in contrast to classical p21 transcripts, which have reduced, albeit detectable expression levels in the absence of p53. The existence of the alternate transcripts underscores the complexity of the human p21 genomic locus and opens up new avenues for further investigation.

Alternative promoter usage of the membrane glycoprotein CD36

BACKGROUND

CD36 is a membrane glycoprotein involved in a variety of cellular processes such as lipid transport, immune regulation, hemostasis, adhesion, angiogenesis and atherosclerosis. It is expressed in many tissues and cell types, with a tissue specific expression pattern that is a result of a complex regulation for which the molecular mechanisms are not yet fully understood. There are several alternative mRNA isoforms described for the gene. We have investigated the expression patterns of five alternative first exons of the CD36 gene in several human tissues and cell types, to better understand the molecular details behind its regulation.

RESULTS

We have identified one novel alternative first exon of the CD36 gene, and confirmed the expression of four previously known alternative first exons of the gene. The alternative transcripts are all expressed in more than one human tissue and their expression patterns vary highly in skeletal muscle, heart, liver, adipose tissue, placenta, spinal cord, cerebrum and monocytes. All alternative first exons are upregulated in THP-1 macrophages in response to oxidized low density lipoproteins. The alternative promoters lack TATA-boxes and CpG islands. The upstream region of exon 1b contains several features common for house keeping gene and monocyte specific gene promoters.

CONCLUSION

Tissue-specific expression patterns of the alternative first exons of CD36 suggest that the alternative first exons of the gene are regulated individually and tissue specifically. At the same time, the fact that all first exons are upregulated in THP-1 macrophages in response to oxidized low density lipoproteins may suggest that the alternative first exons are coregulated in this cell type and environmental condition. The molecular mechanisms regulating CD36 thus appear to be unusually complex, which might reflect the multifunctional role of the gene in different tissues and cellular conditions.

Loss of the hSNF5 gene concomitantly inactivates p21CIP/WAF1 and p16INK4a activity associated with replicative senescence in A204 rhabdoid tumor cells

hSNF5, the smallest member of the SWI/SNF chromatin remodeling complex, is lost in most malignant rhabdoid tumors (MRT). In MRT cell lines, reexpression of hSNF5 induces G1 cell cycle arrest, elevated p16INK4a, and activated replicative senescence markers, such as beta-galactosidase (beta-Gal) and plasminogen activator inhibitor-1. To compare the replicative senescence caused by hSNF5 in A204 cells to normal cellular senescence, we examined the activation of both p16INK4a and p21CIP/WAF1. Analogous to normal cellular senescence, both p16INK4a and p21CIP/WAF1 were up-regulated following hSNF5 restoration. Furthermore, we found that hSNF5 bound the p16INK4a and p21CIP/WAF1 promoters, suggesting that it directly regulates transcription of these genes. Using p16INK4a RNA interference, we showed its requirement for the replicative senescence caused by hSNF5 but not the growth arrest. Instead, p21CIP/WAF1 remained activated by hSNF5 in the absence of high p16INK4a expression, apparently causing the growth arrest in A204. Interestingly, we also found that, in the absence of p16INK4a, reexpression of hSNF5 also increased protein levels of a second cyclin-dependent kinase (CDK) inhibitor, p18INK4c. However, our data show that lack of hSNF5 does not abrogate cellular responsiveness to DNA damage or growth-inhibitory factors. In summary, our studies suggest that hSNF5 loss may influence the regulation of multiple CDK inhibitors involved in replicative senescence.

Regulation of translation via mRNA structure in prokaryotes and eukaryotes

The mechanism of initiation of translation differs between prokaryotes and eukaryotes, and the strategies used for regulation differ accordingly. Translation in prokaryotes is usually regulated by blocking access to the initiation site. This is accomplished via base-paired structures (within the mRNA itself, or between the mRNA and a small trans-acting RNA) or via mRNA-binding proteins. Classic examples of each mechanism are described. The polycistronic structure of mRNAs is an important aspect of translational control in prokaryotes, but polycistronic mRNAs are not usable (and usually not produced) in eukaryotes. Four structural elements in eukaryotic mRNAs are important for regulating translation: (i) the m7G cap; (ii) sequences flanking the AUG start codon; (iii) the position of the AUG codon relative to the 5' end of the mRNA; and (iv) secondary structure within the mRNA leader sequence. The scanning model provides a framework for understanding these effects. The scanning mechanism also explains how small open reading frames near the 5' end of the mRNA can down-regulate translation. This constraint is sometimes abrogated by changing the structure of the mRNA, sometimes with clinical consequences. Examples are described. Some mistaken ideas about regulation of translation that have found their way into textbooks are pointed out and corrected.

Menin regulates pancreatic islet growth by promoting histone methylation and expression of genes encoding p27Kip1 and p18INK4c

Menin, the product of the Men1 gene mutated in familial multiple endocrine neoplasia type 1 (MEN1), regulates transcription in differentiated cells. Menin associates with and modulates the histone methyltransferase activity of a nuclear protein complex to activate gene expression. However, menin-dependent histone methyltransferase activity in endocrine cells has not been demonstrated, and the mechanism of endocrine tumor suppression by menin remains unclear. Here, we show that menin-dependent histone methylation maintains the in vivo expression of cyclin-dependent kinase (CDK) inhibitors to prevent pancreatic islet tumors. In vivo expression of CDK inhibitors, including p27 and p18, and other cell cycle regulators is disrupted in mouse islet tumors lacking menin. Chromatin immunoprecipitation studies reveal that menin directly associates with regions of the p27 and p18 promoters and increases methylation of lysine 4 (Lys-4) in histone H3 associated with these promoters. Moreover, H3 Lys-4 methylation associated with p27 and p18 is reduced in islet tumors from Men1 mutant mice. Thus, H3 Lys-4 methylation is a crucial function of menin in islet tumor suppression. These studies suggest an epigenetic mechanism of tumor suppression: by promoting histone modifications, menin maintains transcription at multiple loci encoding cell cycle regulators essential for endocrine growth control.

Total body irradiation selectively induces murine hematopoietic stem cell senescence

Exposure to ionizing radiation (IR) and certain chemotherapeutic agents not only causes acute bone marrow (BM) suppression but also leads to long-term residual hematopoietic injury. This latter effect has been attributed to damage to hematopoietic stem cell (HSC) self-renewal. Using a mouse model, we investigated whether IR induces senescence in HSCs, as induction of HSC senescence can lead to the defect in HSC self-renewal. It was found that exposure of C57BL/6 mice to a sublethal dose (6.5 Gy) of total body irradiation (TBI) resulted in a sustained quantitative and qualitative reduction of LKS+ HSCs. In addition, LKS+ HSCs from irradiated mice exhibited an increased expression of the 2 commonly used biomarkers of cellular senescence, p16(Ink4a) and SA-beta-gal. In contrast, no such changes were observed in irradiated LKS- hematopoietic progenitor cells. These results provide the first direct evidence demonstrating that IR exposure can selectively induce HSC senescence. Of interest, the induction of HSC senescence was associated with a prolonged elevation of p21(Cip1/Waf1), p19(Arf), and p16(Ink4a) mRNA expression, while the expression of p27(Kip1) and p18(Ink4c) mRNA was not increased following TBI. This suggests that p21(Cip1/Waf1), p19(Arf), and p16(Ink4a) may play an important role in IR-induced senescence in HSCs.

Genomic organization of human arylamine N-acetyltransferase Type I reveals alternative promoters that generate different 5'-UTR splice variants with altered translational activities

In humans, a polymorphic gene encodes the drug-metabolizing enzyme NAT1 (arylamine N-acetyltransferase Type 1), which is widely expressed throughout the body. While the protein-coding region of NAT1 is contained within a single exon, examination of the human EST (expressed sequence tag) database at the NCBI revealed the presence of nine separate exons, eight of which were located in the 5' non-coding region of NAT1. Differential splicing produced at least eight unique mRNA isoforms that could be grouped according to the location of the first exon, which suggested that NAT1 expression occurs from three alternative promoters. Using RT (reverse transcriptase)-PCR, we identified one major transcript in various epithelial cells derived from different tissues. In contrast, multiple transcripts were observed in blood-derived cell lines (CEM, THP-1 and Jurkat), with a novel variant, not identified in the EST database, found in CEM cells only. The major splice variant increased gene expression 9-11-fold in a luciferase reporter assay, while the other isoforms were similar or slightly greater than the control. We examined the upstream region of the most active splice variant in a promoter-reporter assay, and isolated a 257 bp sequence that produced maximal promoter activity. This sequence lacked a TATA box, but contained a consensus Sp1 site and a CAAT box, as well as several other putative transcription-factor-binding sites. Cell-specific expression of the different NAT1 transcripts may contribute to the variation in NAT1 activity in vivo.

The human AKNA gene expresses multiple transcripts and protein isoforms as a result of alternative promoter usage, splicing, and polyadenylation

We previously showed that the human AKNA gene encodes an AT-hook transcription factor that regulates the expression of costimulatory cell surface molecules on lymphocytes. However, AKNA cDNA probes hybridize with multiple transcripts, suggesting either the existence of other homologous genes or a complex regulation operating on a single gene. Here we report evidence for the latter, as we find that AKNA is encoded by a single gene that spans a 61-kb locus of 24 exons on the fragile FRA9E region of human chromosome 9q32. This gene gives rise to at least nine distinct transcripts, most of which are expressed in a tissue-specific manner in lymphoid organs. Many of the AKNA transcripts originate from alternative splicing; others appear to derive from differential polyadenylation and promoter usage. The alternative AKNA transcripts are predicted to encode overlapping protein isoforms, some of which (p70 and p100) are readily detectable using a polyclonal anti-AKNA antisera that we generated. We also find that AKNA PEST-dependent cleavage into p50 polypeptides is targeted to mature B cells and appears to be required for CD40 upregulation. The unusual capacity of the AKNA gene to generate multiple transcripts and proteins may reflect its functional diversity, and it may also provide a fail-safe mechanism that preserves AKNA expression.

Biochemical and cellular mechanisms of mammalian CDK inhibitors: a few unresolved issues

p21 and p16, first identified as two small molecular weight proteins in CDK and cyclin immunocomplexes, represent two distinct families constituting a total of seven CDK inhibitors in mammalian cells. The physiological functions of these genes are believed to be broadly involved in connecting various cellular pathways to cell cycle control. Extensive studies over the past 10 years have led to a fairly clear understanding of their biochemical and cellular mechanisms and have also left some unresolved and controversial issues.

Ink4a/Arf expression is a biomarker of aging

The Ink4a/Arf locus encodes 2 tumor suppressor molecules, p16INK4a and Arf, which are principal mediators of cellular senescence. To study the links between senescence and aging in vivo, we examined Ink4a/Arf expression in rodent models of aging. We show that expression of p16INK4a and Arf markedly increases in almost all rodent tissues with advancing age, while there is little or no change in the expression of other related cell cycle inhibitors. The increase in expression is restricted to well-defined compartments within each organ studied and occurs in both epithelial and stromal cells of diverse lineages. The age-associated increase in expression of p16INK4a and Arf is attenuated in the kidney, ovary, and heart by caloric restriction, and this decrease correlates with diminished expression of an in vivo marker of senescence, as well as decreased pathology of those organs. Last, the age-related increase in Ink4a/Arf expression can be independently attributed to the expression of Ets-1, a known p16INK4a transcriptional activator, as well as unknown Ink4a/Arf coregulatory molecules. These data suggest that expression of the Ink4a/Arf tumor suppressor locus is a robust biomarker, and possible effector, of mammalian aging.

A novel p21WAF1/CIP1 transcript is highly dependent on p53 for its basal expression in mouse tissues

p21WAF1/CIP1 is an important transcriptional target of p53 and it plays a critical role in growth arrest after DNA damage. Here, we report the identification of a novel alternate mouse p21 transcript that is conserved in evolution. It differs from the classical p21WAF1/CIP1 transcript in the first exon, which is located at approximately 2.8 kb upstream of transcriptional start site of p21WAF1/CIP1 and is sandwiched between two p53 binding sites. This novel p21 transcript is present in most mouse tissues with highest levels of expression in the spleen. In contrast to the classical p21WAF1/CIP1 transcript, this new transcript is highly dependent on p53 for its basal expression, as evidenced by its absence in nearly all of p53-/- mouse tissues. This transcript is also absent at nonpermissive temperature in a 10-1 mouse cell line lacking endogenous p53 and harboring temperature-sensitive p53 mutant. However, this novel transcript is induced to appreciable levels in the presence of high p53 activity at the permissive temperature. Our data suggest that p53-dependent induction of p21 may be an additive effect conferred by individual increases in the alternate and classical p21 transcripts.

Cell cycle inhibitors in normal and tumor stem cells

Emerging data suggest that stem cells may be one of the key elements in normal tissue regeneration and cancer development, although they are not necessarily the same entity in both scenarios. As extensively demonstrated in the hematopoietic system, stem cell repopulation is hierarchically organized and is intrinsically limited by the intracellular cell cycle inhibitors. Their inhibitory effects appear to be highly associated with the differentiation stage in stem/progenitor pools. While this negative regulation is important for maintaining homeostasis, especially at the stem cell level under physiological cues or pathological insults, it constrains the therapeutic use of adult stem cells in vitro and restricts endogenous tissue repair after injury. On the other hand, disruption of cell cycle inhibition may contribute to the formation of the so-called 'tumor stem cells' (TSCs) that are currently hypothesized to be partially responsible for tumorigenesis and recurrence of cancer after conventional therapies. Therefore, understanding how cell cycle inhibitors control stem cells may offer new strategies not only for therapeutic manipulations of normal stem cells but also for novel therapies selectively targeting TSCs.

Genetic evidence for functional dependency of p18Ink4c on Cdk4

The INK4 family of cyclin-dependent kinase (CDK) inhibitors negatively regulates cyclin D-dependent CDK4 and CDK6 and induces the growth-suppressive function of Rb family proteins. Mutations in the Cdk4 gene conferring INK4 resistance are associated with familial and sporadic melanoma in humans and result in a wide spectrum of tumors in mice, suggesting that INK4 is a major regulator of CDK4. Mice lacking the Cdk4 gene exhibit various defects in many organs associated with hypocellularity, whereas loss of the p18(Ink4c) gene results in widespread hyperplasia and organomegaly. To genetically test the notion that the function of INK4 is dependent on CDK4, we generated p18; Cdk4 double-mutant mice and examined the organs and tissues which developed abnormalities when either gene is deleted. We show here that, in all organs we have examined, including pituitary, testis, pancreas, kidney, and adrenal gland, hyperproliferative phenotypes associated with p18 loss were canceled. The double-mutant mice exhibited phenotypes very close to or indistinguishable from that of Cdk4 single-mutant mice. Mice lacking p27(Kip1) develop widespread hyperplasia and organomegaly similar to those developed by p18-deficient mice. The p27; Cdk4 double-mutant mice, however, displayed phenotypes intermediate between those of p27 and Cdk4 single-mutant mice. These results provide genetic evidence that in mice p18(Ink4c) and p27(Kip1) mediate the transduction of different cell growth and proliferation signals to CDK4 and that p18(Ink4c) is functionally dependent on CDK4.

In vivo self-renewing divisions of haematopoietic stem cells are increased in the absence of the early G1-phase inhibitor, p18INK4C

Self-renewal of stem cells is critical for tissue repair and maintenance of organ integrity in most mammalian systems. The relative asymmetry between self-renewal and differentiation in balance with apoptosis determines the size and durability of a stem-cell pool. Regulation of the cell cycle is one of the fundamental mechanisms underlying determination of cell fate. Absence of p21(Cip1/Waf1), a late G1-phase cyclin-dependent kinase inhibitor (CKI), has previously been shown to enable cell-cycle entry of haematopoietic stem cells, but leads to premature exhaustion of the stem cells under conditions of stress. We show here that deletion of an early G1-phase CKI, p18(INK4C), results in strikingly improved long-term engraftment, largely by increasing self-renewing divisions of the primitive cells in murine transplant models. Therefore, different CKIs have highly distinct effects on the kinetics of stem cells, possibly because of their active position in the cell cycle, and p18(INK4C) appears to be a strong inhibitor limiting the potential of stem-cell self-renewal in vivo.

Regulation of the MiTF/TFE bHLH-LZ transcription factors through restricted spatial expression and alternative splicing of functional domains

The MiTF/TFE (MiT) family of basic helix-loop-helix leucine zipper transcription factors is composed of four closely related members, MiTF, TFE3, TFEB and TFEC, which can bind target DNA both as homo- or heterodimers. Using real-time RT-PCR, we have analyzed the relative expression levels of the four members in a broad range of human tissues, and found that their ratio of expression is tissue-dependent. We found that, similar to the MiTF gene, the genes for TFEB and TFEC contain multiple alternative first exons with restricted and differential tissue distributions. Seven alternative 5' exons were identified in the TFEB gene, of which three displayed specific expression in placenta and brain, respectively. A novel TFEC transcript (TFEC-C) encodes an N-terminally truncated TFEC isoform lacking the acidic activation domain (AAD), and is exclusively expressed in kidney and small intestine. Furthermore, we observed that a considerable proportion of the TFEC transcripts splice out protein-coding exons, resulting in transcription factor isoforms lacking one or more functional domains, primarily the basic region and/or the AAD. These isoforms were always co-expressed with the intact transcription factors and may act as negative regulators of MiTF/TFE proteins. Our data reveal that multiple levels of regulation exist for the MiTF/TFE family of transcription factors, which indicates how these transcription factors may participate in various cellular processes in different tissues.

Cranial neural crest-derived mesenchymal proliferation is regulated by Msx1-mediated p19(INK4d) expression during odontogenesis

Neural crest cells are multipotential progenitors that contribute to various cell and tissue types during embryogenesis. Here, we have investigated the molecular and cellular mechanism by which the fate of neural crest cell is regulated during tooth development. Using a two- component genetic system for indelibly marking the progeny of neural crest cells, we provide in vivo evidence of a deficiency of CNC-derived dental mesenchyme in Msx1 null mutant mouse embryos. The deficiency of the CNC results from an elevated CDK inhibitor p19(INK4d) activity and the disruption of cell proliferation. Interestingly, in the absence of Msx1, the CNC-derived dental mesenchyme misdifferentiates and possesses properties consistent with a neuronal fate, possibly through a default mechanism. Attenuation of p19(INK4d) in Msx1 null mutant mandibular explants restores mitotic activity in the dental mesenchyme, demonstrating the functional significance of Msx1-mediated p19(INK4d) expression in regulating CNC cell proliferation during odontogenesis. Collectively, our results demonstrate that homeobox gene Msx1 regulates the fate of CNC cells by controlling the progression of the cell cycle. Genetic mutation of Msx1 may alternatively instruct the fate of these progenitor cells during craniofacial development.

GATA-1-mediated proliferation arrest during erythroid maturation

Transcription factor GATA-1 is essential for erythroid and megakaryocytic maturation. GATA-1 mutations are associated with hematopoietic precursor proliferation and leukemogenesis, suggesting a role in cell cycle control. While numerous GATA-1 target genes specifying mature hematopoietic phenotypes have been identified, how GATA-1 regulates proliferation remains unknown. We used a complementation assay based on synchronous inducible rescue of GATA-1(-) erythroblasts to show that GATA-1 promotes both erythroid maturation and G(1) cell cycle arrest. Molecular studies combined with microarray transcriptome analysis revealed an extensive GATA-1-regulated program of cell cycle control in which numerous growth inhibitors were upregulated and mitogenic genes were repressed. GATA-1 inhibited expression of cyclin-dependent kinase (Cdk) 6 and cyclin D2 and induced the Cdk inhibitors p18(INK4C) and p27(Kip1) with associated inactivation of all G(1) Cdks. These effects were dependent on GATA-1-mediated repression of the c-myc (Myc) proto-oncogene. GATA-1 inhibited Myc expression within 3 h, and chromatin immunoprecipitation studies indicated that GATA-1 occupies the Myc promoter in vivo, suggesting a direct mechanism for gene repression. Surprisingly, enforced expression of Myc prevented GATA-1-induced cell cycle arrest but had minimal effects on erythroid maturation. Our results illustrate how GATA-1, a lineage-determining transcription factor, coordinates proliferation arrest with cellular maturation through distinct, interrelated genetic programs.

Suppression of ARG kinase activity by STI571 induces cell cycle arrest through up-regulation of CDK inhibitor p18/INK4c

ARG is a tyrosine kinase closely related to ABL, which is oncogenic when fused to the transcriptional repressor ETV6 (ETS translocation variant 6). In this study, we investigated the growth-inhibitory effect of STI571 (signal transduction inhibitor number 571) on ETV6/ARG-expressing cells and its molecular mechanisms using HT93A, a cell line derived from a patient with AML-M3 carrying t(1;12). STI571 effectively suppressed overall tyrosyl phosphorylation of intracellular proteins including ETV6/ARG fusion protein, as well as the growth of HT93A cells with an IC(50) of 200 nM. The growth inhibition was primarily because of cell cycle arrest at G1 phase when cells were treated with 100 nM STI571 for 48 h, and apoptosis was induced after longer exposure (72 h) or by a higher dose (1000 nM). STI571 increased the amount of p18/INK4c after 2 h of culture, when the cell cycle pattern was not yet affected, but not that of other CDK inhibitors (CKI). p18/INK4c was more abundant in G1-enriched fractions than in S- and G2/M-enriched fractions of STI571-treated HT93A cells, suggesting that the upregulation of p18/INK4c expression correlates with the cell cycle arrest. Treatment of HT93A cells with antisense oligonucleotides against the Ink4c gene abrogated the growth inhibition by STI571. These results suggest that leukemogenesis by an aberrant ARG kinase involves the suppression of p18/INK4c, which is ubiquitously expressed and considered the major CKI in hematopoietic stem cells. STI571 can be an effective drug for the treatment of leukemias with deregulated ARG kinase activity.

p18(INK4c) collaborates with other CDK-inhibitory proteins in the regenerating liver

p18(INK4c) belongs to the family of cyclin-dependent kinase inhibitory proteins that target the cyclin-dependent kinases and inhibit their catalytic activity. The role of p18(INK4c) for cell cycle progression in vivo is characterized poorly. Therefore, we studied the expression and physiologic relevance of p18 in quiescent and proliferating hepatocytes during liver regeneration. For our analysis we used single- (p18[INK4c], p27[KIP1], p21[CIP1/WAF1]), and double-mutant (p18/p21, p18/p27) mice. p18 expression was found in quiescent hepatocytes and a slight up-regulation was evident after partial hepatectomy (PH). p18 knockout animals showed normal cell cycle progression after PH. However, when p18/p21 and p18/p27 double-mutant mice were used, differences in cell cycle progression were evident compared with wild-type (wt) and single knockout animals. In p18/p21 knockout animals, the G1 phase was shortened as evidenced by an earlier onset of cyclin D and proliferating cell nuclear antigen (PCNA) expression and cyclin-dependent kinase (CDK) activation after PH. In contrast, in p18/p27 knockout animals, the G1 phase was unchanged, but the amount of proliferating hepatocytes (5-bromo-2'-deoxyuridine [BrdU] and PCNA positive) 48 hours after PH was elevated. In conclusion, our results suggest that p18 is involved in cell cycle progression after PH. Additionally we provide evidence that timing and strength of DNA synthesis in hepatocytes after PH is regulated tightly through the collaboration of different cell cycle inhibitors.

Highly coordinated gene regulation in mouse skeletal muscle regeneration

Mammalian skeletal muscles are capable of regeneration after injury. Quiescent satellite cells are activated to reenter the cell cycle and to differentiate for repair, recapitulating features of myogenesis during embryonic development. To understand better the molecular mechanism involved in this process in vivo, we employed high density cDNA microarrays for gene expression profiling in mouse tibialis anterior muscles after a cardiotoxin injection. Among 16,267 gene elements surveyed, 3,532 elements showed at least a 2.5-fold change at one or more time points during a 14-day time course. Hierarchical cluster analysis and semiquantitative reverse transcription-PCR showed induction of genes important for cell cycle control and DNA replication during the early phase of muscle regeneration. Subsequently, genes for myogenic regulatory factors, a group of imprinted genes and genes with functions to inhibit cell cycle progression and promote myogenic differentiation, were induced when myogenic stem cells started to differentiate. Induction of a majority of these genes, including E2f1 and E2f2, was abolished in muscles lacking satellite cell activity after gamma radiation. Regeneration was severely compromised in E2f1 null mice but not affected in E2f2 null mice. This study identifies novel genes potentially important for muscle regeneration and reveals highly coordinated myogenic cell proliferation and differentiation programs in adult skeletal muscle regeneration in vivo.