Id2 gene-targeted crosstalk between Wnt and retinoid signaling regulates proliferation in human keratinocytes

Molecular networks in atopic mothers impact the risk of infant atopy

BACKGROUND

The prevalence of atopic diseases has increased with atopic dermatitis (AD) as the earliest manifestation. We assessed if molecular risk factors in atopic mothers influence their infants' susceptibility to an atopic disease.

METHODS

Pregnant women and their infants with (n = 174, high-risk) or without (n = 126, low-risk) parental atopy were enrolled in a prospective birth cohort. Global differentially methylated regions (DMRs) were determined in atopic (n = 92) and non-atopic (n = 82) mothers. Principal component analysis was used to predict atopy risk in children dependent on maternal atopy. Genome-wide transcriptomic analyses were performed in paired atopic (n = 20) and non-atopic (n = 15) mothers and cord blood. Integrative genomic analyses were conducted to define methylation-gene expression relationships.

RESULTS

Atopic dermatitis was more prevalent in high-risk compared to low-risk children by age 2. Differential methylation analyses identified 165 DMRs distinguishing atopic from non-atopic mothers. Inclusion of DMRs in addition to maternal atopy significantly increased the odds ratio to develop AD in children from 2.56 to 4.26. In atopic compared to non-atopic mothers, 139 differentially expressed genes (DEGs) were identified significantly enriched of genes within the interferon signaling pathway. Expression quantitative trait methylation analyses dependent on maternal atopy identified 29 DEGs controlled by 136 trans-acting methylation marks, some located near transcription factors. Differential expression for the same nine genes, including MX1 and IFI6 within the interferon pathway, was identified in atopic and non-atopic mothers and high-risk and low-risk children.

CONCLUSION

These data suggest that in utero epigenetic and transcriptomic mechanisms predominantly involving the interferon pathway may impact and predict the development of infant atopy.

The Use of Retinoids for the Prevention and Treatment of Skin Cancers: An Updated Review

Retinoids are natural and synthetic vitamin A derivatives that are effective for the prevention and the treatment of non-melanoma skin cancers (NMSC). NMSCs constitute a heterogenous group of non-melanocyte-derived skin cancers that impose substantial burdens on patients and healthcare systems. They include entities such as basal cell carcinoma and cutaneous squamous cell carcinoma (collectively called keratinocyte carcinomas), cutaneous lymphomas and Kaposi’s sarcoma among others. The retinoid signaling pathway plays influential roles in skin physiology and pathology. These compounds regulate diverse biological processes within the skin, including proliferation, differentiation, angiogenesis and immune regulation. Collectively, retinoids can suppress skin carcinogenesis. Both topical and systemic retinoids have been investigated in clinical trials as NMSC prophylactics and treatments. Desirable efficacy and tolerability in clinical trials have prompted health regulatory bodies to approve the use of retinoids for NMSC management. Acceptable off-label uses of these compounds as drugs for skin cancers are also described. This review is a comprehensive outline on the biochemistry of retinoids, their activities in the skin, their effects on cancer cells and their adoption in clinical practice.

Olfactory Receptor OR7A17 Expression Correlates with All-Trans Retinoic Acid (ATRA)-Induced Suppression of Proliferation in Human Keratinocyte Cells

Olfactory receptors (ORs), which belong to the G-protein-coupled receptor family, have been widely studied as ectopically expressed receptors in various human tissues, including the skin. However, the physiological functions of only a few OR types have been elucidated in skin cells. All-trans retinoic acid (ATRA) is a well-known medication for various skin diseases. However, many studies have shown that ATRA can have adverse effects, resulting from the suppression of cell proliferation. Here, we investigated the involvement of OR7A17 in the ATRA-induced suppression of human keratinocyte (HaCaT) proliferation. We demonstrated that OR7A17 is expressed in HaCaT keratinocytes, and its expression was downregulated by ATRA. The ATRA-induced downregulation of OR7A17 was attenuated via RAR α or RAR γ antagonist treatment, indicating that the effects of ATRA on OR7A17 expression were mediated through nuclear retinoic acid receptor signaling. Moreover, we found that the overexpression of OR7A17 induced the proliferation of HaCaT cells while counteracting the antiproliferative effect of ATRA. Mechanistically, OR7A17 overexpression reversed the ATRA-induced attenuation of Ca²⁺ entry. Our findings indicated that ATRA suppresses cell proliferation through the downregulation of OR7A17 via RAR α- and γ-mediated retinoid signaling. Taken together, OR7A17 is a potential therapeutic target for ameliorating the anti-proliferative effects of ATRA.

Effect of cAMP Signaling Regulation in Osteogenic Differentiation of Adipose-Derived Mesenchymal Stem Cells

The successful implementation of adipose-derived mesenchymal stem cells (ADSCs) in bone regeneration depends on efficient osteogenic differentiation. However, a literature survey and our own experience demonstrated that current differentiation methods are not effective enough. Since the differentiation of mesenchymal stem cells (MSCs) into osteoblasts and adipocytes can be regulated by cyclic adenosine monophosphate (cAMP) signaling, we investigated the effects of cAMP activator, forskolin, and inhibitor, SQ 22,536, on the early and late osteogenic differentiation of ADSCs cultured in spheroids or in a monolayer. Intracellular cAMP concentration, protein kinase A (PKA) activity, and inhibitor of DNA binding 2 (ID2) expression examination confirmed cAMP up- and downregulation. cAMP upregulation inhibited the cell cycle and protected ADSCs from osteogenic medium (OM)-induced apoptosis. Surprisingly, the upregulation of cAMP level at the early stages of osteogenic differentiation downregulated the expression of osteogenic markers RUNX2, Osterix, and IBSP, which was more significant in spheroids, and it is used for the more efficient commitment of ADSCs into preosteoblasts, according to the previously reported protocol. However, cAMP upregulation in a culture of ADSCs in spheroids resulted in significantly increased osteocalcin production and mineralization. Thus, undifferentiated and predifferentiated ADSCs respond differently to cAMP pathway stimulation in terms of osteogenesis, which might explain the ambiguous results from the literature.

Could the inhibitor of DNA binding 2 and 4 play a role in white matter injury?

White matter injury (WMI) prevents the normal development of myelination, leading to central nervous system myelination disorders and the production of chronic sequelae associated with WMI, such as chronic dyskinesia, cognitive impairment and cerebral palsy. This results in a large emotional and socioeconomic burden. Decreased myelination in preterm infant WMI is associated with the delayed development or destruction of oligodendrocyte (OL) lineage cells, particularly oligodendrocyte precursor cells (OPCs). The development of cells from the OL lineage involves the migration, proliferation and different stages of OL differentiation, finally leading to myelination. A series of complex intrinsic, extrinsic and epigenetic factors regulate the OPC cell cycle withdrawal, OL lineage progression and myelination. We focus on the inhibitor of DNA binding 2 (ID2), because it is widely involved in the different stages of OL differentiation and genesis. ID2 is a key transcription factor for the normal development of OL lineage cells, and the pathogenesis of WMI is closely linked with OL developmental disorders. ID4, another family member of the IDs protein, also plays a similar role in OL differentiation and genesis. ID2 and ID4 belong to the helix-loop-helix family; they lack the DNA-binding sequences and inhibit oligodendrogenesis and OPC differentiation. In this review, we mainly discuss the roles of ID2 in OL development, especially during OPC differentiation, and summarize the ID2-mediated intracellular and extracellular signaling pathways that regulate these processes. We also discuss ID4 in relation to bone morphogenetic protein signaling and oligodendrogenesis. It is likely that these developmental mechanisms are also involved in the myelin repair or remyelination in human neurological diseases.

On the role of skin in the regulation of local and systemic steroidogenic activities

The mammalian skin is a heterogeneous organ/tissue covering our body, showing regional variations and endowed with neuroendocrine activities. The latter is represented by its ability to produce and respond to neurotransmitters, neuropeptides, hormones and neurohormones, of which expression and phenotypic activities can be modified by ultraviolet radiation, chemical and physical factors, as well as by cytokines. The neuroendocrine contribution to the responses of skin to stress is served, in part, by local synthesis of all elements of the hypothalamo-pituitary-adrenal axis. Skin with subcutis can also be classified as a steroidogenic tissue because it expresses the enzyme, CYP11A1, which initiates steroid synthesis by converting cholesterol to pregnenolone, as in other steroidogenic tissues. Pregnenolone, or steroidal precursors from the circulation, are further transformed in the skin to corticosteroids or sex hormones. Furthermore, in the skin CYP11A1 acts on 7-dehydrocholesterol with production of 7-dehydropregnolone, which can be further metabolized to other Δ7steroids, which after exposure to UVB undergo photochemical transformation to vitamin D like compounds with a short side chain. Vitamin D and lumisterol, produced in the skin after exposure to UVB, are also metabolized by CYP11A1 to several hydroxyderivatives. Vitamin D hydroxyderivatives generated by action of CYP11A1 are biologically active and are subject to further hydroxylations by CYP27B1, CYP27A1 and CP24A. Establishment of which intermediates are produced in the epidermis in vivo and whether they circulate on the systemic level represent a future research challenge. In summary, skin is a neuroendocrine organ endowed with steroid/secosteroidogenic activities.

ZEB1 and TCF4 reciprocally modulate their transcriptional activities to regulate Wnt target gene expression

The canonical Wnt pathway (TCF4/β-catenin) has important roles during normal differentiation and in disease. Some Wnt functions depend on signaling gradients requiring the pathway to be tightly regulated. A key Wnt target is the transcription factor ZEB1 whose expression by cancer cells promotes tumor invasiveness by repressing the expression of epithelial specification markers and activating mesenchymal genes, including a number of Wnt targets such as LAMC2 and uPA. The ability of ZEB1 to activate/repress its target genes depends on its recruitment of corepressors (CtBP, BRG1) or coactivators (p300) although conditions under which ZEB1 binds these cofactors are not elucidated. Here, we show that TCF4 and ZEB1 reciprocally modulate each other's transcriptional activity: ZEB1 enhances TCF4/β-catenin-mediated transcription and, in turn, Wnt signaling switches ZEB1 from a repressor into an activator. In colorectal cancer (CRC) cells with active Wnt signaling, ZEB1 enhances transcriptional activation of LAMC2 and uPA by TCF4/β-catenin. However, in CRC cells with inactive Wnt, ZEB1 represses both genes. Reciprocal modulation of ZEB1 and TCF4 activities involves their binding to DNA and mutual interaction. Wnt signaling turns ZEB1 into an activator by replacing binding of CtBP/BRG1 in favor of p300. Using a mouse model of Wnt-induced intestinal tumorigenesis, we found that downregulation of ZEB1 reduces the expression of LAMC2 in vivo. These results identify a mechanism through which Wnt and ZEB1 transcriptional activities are modulated, offering new approaches in cancer therapy.

Cutaneous glucocorticosteroidogenesis: securing local homeostasis and the skin integrity

Human skin has the ability to synthesize glucocorticoids de novo from cholesterol or from steroid intermediates of systemic origin. By interacting with glucocorticoid receptors, they regulate skin immune functions as well as functions and phenotype of the epidermal, dermal and adnexal compartments. Most of the biochemical (enzyme and transporter activities) and regulatory (neuropeptides mediated activation of cAMP and protein kinase A dependent pathways) principles of steroidogenesis in the skin are similar to those operating in classical steroidogenic organs. However, there are also significant differences determined by the close proximity of synthesis and action (even within the same cells) allowing para-, auto- or intracrine modes of regulation. We also propose that ultraviolet light B (UVB) can regulate the availability of 7-dehydrocholesterol for transformation to cholesterol with its further metabolism to steroids, oxysterols or ∆7 steroids, because of its transformation to vitamin D3. In addition, UVB can rearrange locally produced ∆7 steroids to the corresponding secosteroids with a short- or no-side chain. Thus, different mechanisms of regulation occur in the skin that can be either stochastic or structuralized. We propose that local glucocorticosteroidogenic systems and their regulators, in concert with cognate receptors operate to stabilize skin homeostasis and prevent or attenuate skin pathology.

Id transcriptional regulators in adipogenesis and adipose tissue metabolism

Id proteins (Id1-Id4) are helix-loop-helix (HLH) transcriptional regulators that lack a basic DNA binding domain. They act as negative regulators of basic helix-loop-helix (bHLH) transcription factors by forming heterodimers and inhibit their DNA binding and transcriptional activity. Id proteins are implicated in the regulation of various cellular mechanisms such as cell proliferation, cellular differentiation, cell fate determination, angiogenesis and tumorigenesis. A handful of recent studies also disclosed that Id proteins have critical functions in adipocyte differentiation and adipose tissue metabolism. Here, we reviewed the progress made thus far in understanding the specific functions of Id proteins in adipose tissue differentiation and metabolism. In addition to reviewing the known mechanisms of action, we also discuss possible additional mechanisms in which Id proteins might participate in regulating adipogenic and metabolic pathways.

Wnt signaling in remyelination in multiple sclerosis: friend or foe?

Myelination is critical to normal functioning of the vertebrate nervous system. In demyelinating diseases such as multiple sclerosis, oligodendrocytes, the myelinating cells in the central nervous system, are targeted, resulting in myelin loss, axonal damage, and severe functional impairment. While spontaneous remyelination has been proven a failure in multiple sclerosis, understanding the molecular mechanism underlying oligodendrocyte biology, myelination, and remyelination becomes crucial. To date, a series of signaling pathways in regulating oligodendrocyte development and remyelination have been suggested and, among them, the Wnt/β-catenin/Tcf pathway has been considered a negative factor in the myelinating process. However, this notion has been challenged by recent studies, which showed a pro-myelinating effect of this pathway. This review summarizes the current contradictory concepts concerning the role of the Wnt pathway in the oligodendrocyte development and remyelination process, attempts to address the potential mechanism underlying this controversy, and recommends caution in targeting the Wnt pathway as a potential demyelinating therapy.

A small molecule inhibitor of SRC family kinases promotes simple epithelial differentiation of human pluripotent stem cells

Human pluripotent stem cells (hPSCs) provide unprecedented opportunities to study the earliest stages of human development in vitro and have the potential to provide unlimited new sources of cells for regenerative medicine. Although previous studies have reported cytokeratin 14+/p63+ keratinocyte generation from hPSCs, the multipotent progenitors of epithelial lineages have not been described and the developmental pathways regulating epithelial commitment remain largely unknown. Here we report membrane localization of β-catenin during retinoic acid (RA)--induced epithelial differentiation. In addition hPSC treatment with the Src family kinase inhibitor SU6656 modulated β-catenin localization and produced an enriched population of simple epithelial cells under defined culture conditions. SU6656 strongly upregulated expression of cytokeratins 18 and 8 (K18/K8), which are expressed in simple epithelial cells, while repressing expression of the pluripotency gene Oct4. This homogeneous population of K18+K8+Oct4- simple epithelial precursor cells can further differentiate into cells expressing keratinocyte or corneal-specific markers. These enriched hPSC-derived simple epithelial cells may provide a ready source for development and toxicology cell models and may serve as a progenitor for epithelial cell transplantation applications.

Coordinated control of oligodendrocyte development by extrinsic and intrinsic signaling cues

Oligodendrocytes, the myelin-forming cells for axon ensheathment in the central nervous system, are critical for maximizing and maintaining the conduction velocity of nerve impulses and proper brain function. Demyelination caused by injury or disease together with failure of myelin regeneration disrupts the rapid propagation of action potentials along nerve fibers, and is associated with acquired and inherited disorders, including devastating multiple sclerosis and leukodystrophies. The molecular mechanisms of oligodendrocyte myelination and remyelination remain poorly understood. Recently, a series of signaling pathways including Shh, Notch, BMP and Wnt signaling and their intracellular effectors such as Olig1/2, Hes1/5, Smads and TCFs, have been shown to play important roles in regulating oligodendrocyte development and myelination. In this review, we summarize our recent understanding of how these signaling pathways modulate the progression of oligodendrocyte specification and differentiation in a spatiotemporally-specific manner. A better understanding of the complex but coordinated function of extracellular signals and intracellular determinants during oligodendrocyte development will help to devise effective strategies to promote myelin repair for patients with demyelinating diseases.

Mesenchymal cells regulate retinoic acid receptor-dependent cortical thymic epithelial cell homeostasis

The vitamin A metabolite and transcriptional modulator retinoic acid (RA) is recognized as an important regulator of epithelial cell homeostasis in several tissues. Despite the known importance of the epithelial compartment of the thymus in T cell development and selection, the potential role of RA in the regulation of thymic cortical and medullary epithelial cell homeostasis has yet to be addressed. In this study, using fetal thymus organ cultures, we demonstrate that endogenous RA signaling promotes thymic epithelial cell (TEC) cell-cycle exit and restricts TEC cellularity preferentially in the cortical TEC compartment. Combined gene expression, biochemical, and functional analyses identified mesenchymal cells as the major source of RA in the embryonic thymus. In reaggregate culture experiments, thymic mesenchyme was required for RA-dependent regulation of TEC expansion, highlighting the importance of mesenchyme-derived RA in modulating TEC turnover. The RA-generating potential of mesenchymal cells was selectively maintained within a discrete Ly51(int)gp38(+) subset of Ly51(+) mesenchyme in the adult thymus, suggesting a continual role for mesenchymal cell-derived RA in postnatal TEC homeostasis. These findings identify RA signaling as a novel mechanism by which thymic mesenchyme influences TEC development.

ID2: A negative transcription factor regulating oligodendroglia differentiation

Remyelination of the central nervous system in multiple sclerosis patients is often incomplete. Remyelination depends on normal oligodendrogenesis and the differentiation of oligodendrocyte precursor cells (OPC) into mature oligodendrocytes (OL). Inhibitor of DNA binding (ID), a transcription factor, is thought to inhibit oligodendrogenesis and the differentiation of OPC. This Mini-Review aims to reveal the roles of and mechanisms used by IDs (mainly ID2) in this process. An interaction between ID2 and retinoblastoma tumor suppressor is responsible for the cell cycle transition from G1 to S. The translocation of ID2 between the nucleus and cytoplasm is regulated by E47 and OLIG. An interaction between ID2 and OLIG mediates the inhibitory effects of bone morphogenic proteins and G protein-coupled receptor 17 on oligodendroglia differentiation. ID2 expression is regulated by Wnt and histone deacetylases during the differentiation of OPC. ID4, another member of the ID family, functions similarly to ID2 in regulating the differentiation of OPC. The main difference is that ID4 is essential for oligodendrogenesis, whereas ID2 is nonessential. This could have important implications for demyelinating diseases, and interfering with these pathways might represent a viable therapeutic approach for these diseases.

Large scale RNAi screen reveals that the inhibitor of DNA binding 2 (ID2) protein is repressed by p53 family member p63 and functions in human keratinocyte differentiation

The inhibitor of DNA binding 2, dominant negative helix-loop-helix protein, ID2, acts as an oncogene and elevated levels of ID2 have been reported in several malignancies. Whereas some inducers of the ID2 gene have been characterized, little is known regarding the proteins capable to repress its expression. We developed siRNA microarrays to perform a large scale loss-of-function screen in human adult keratinocytes engineered to express GFP under the control of the upstream region of ID2 gene. We screened the effect of siRNA-dependent inhibition of 220 cancer-associated genes on the expression of the ID2::GFP reporter construct. Three genes NBN, RAD21, and p63 lead to a repression of ID2 promoter activity. Strikingly NBN and RAD21 are playing on major role in cell cycle progression and mitosis arrest. These results underline the pregnant need to silence ID2 expression at transcript level to promote cell cycle exit. Central to this inhibitory mechanism we find p63, a key transcription factor in epithelial development and differentiation, which binds specific cis-acting sequence within the ID2 gene promoter both in vitro and in vivo. P63 would not suppress ID2 expression, but would rather prevent excessive expression of that protein to enable the onset of keratinocyte differentiation.

Mitogen-activated protein kinase activated protein kinase 2 (MK2) participates in p38 MAPK regulated control of oligodendrocyte differentiation

The p38 mitogen-activated protein kinases (p38 MAPKs) are a family of kinases that regulate a number of cellular functions including cell migration, proliferation, and differentiation. We have previously reported a role for p38 MAPK in the regulation of oligodendrocyte (OLG) differentiation and Schwann cell myelination. Here, we extend our previous findings by showing that a p38 substrate, mitogen-activated protein kinase activated protein kinase 2 (MK2) is a downstream element of the p38 signaling pathway responsible for effecting OLG differentiation. Inhibition of MK2 activity in oligodendrocyte progenitors (OLPs) using CMPD1 [4-(2'-fluorobiphenyl-4-yl)-N-(4-hydroxyphenyl)-butyramide] blocked the activation of MK2 and resulted in decreased accumulation of myelin-differentiation markers, including myelin-associated glycoprotein (MAG) and myelin basic protein (MBP). We corroborated these findings using a small-interfering RNA to MK2, which decreased the myelin-specific lipid galactosylceramide and MAG. Treatment of cultures with CMPD1 decreased the steady state levels of mRNA encoding myelin transcription factor 1 (Myt1), MAG, MBP, and Opalin, a transmembrane sialylglycoprotein expressed in oligodendrocytes. In contrast, increases were observed in the mRNA levels of OLG transcriptional repressors, including transcription factor 4 (Tcf4), Notch1, and inhibitor of differentiation 2 (Id2). Furthermore, we found that the predominantly expressed isoform of p38 in OLGs, p38alpha, and MK2 can form coimmunoprecipitable complexes in OLPs and OLGs. Our results demonstrate that the p38-MK2 pathway is a component of the signaling cascade regulating OLG differentiation.

The role of R-spondin2 in keratinocyte proliferation and epidermal thickening in keloid scarring

Keloids are found only in humans and the underlying biochemical mechanisms of their pathogenesis remain unknown. R-spondins (Rspos) are a relatively new group of secreted proteins known to be Wnt/β-catenin signaling agonists, but their role in keloids has yet to be elucidated. We investigated the expression levels of R-spondin2 (Rspo2) in cell lysates and conditioned media of monocultures and co-cultures of fibroblasts and keratinocytes derived from keloids and normal skin. In this study we found increased protein expression and secretion of Rspo2 in respective monocultures of keloid fibroblasts and keratinocytes when compared with their normal counterparts. Double-chamber co-culture experiments implicated the role of keloid keratinocytes (KKs) in the induction of Rspo2 secretion from fibroblasts because of epithelial-mesenchymal interactions. Addition of recombinant human Rspo2 in culture increased the proliferation of keratinocytes and it acted synergistically with Wnt3a through the canonical Wnt/β-catenin pathway. Overexpression of Rspo2 in normal fibroblasts brought about thicker epidermis when compared with control fibroblasts in a skin organotypic culture model. This observation coincides with the hyperproliferative phenotype of thickened epidermis seen in keloids. Taken together, the results suggest the possible double paracrine action of KKs in inducing higher expression of Rspo2 in fibroblasts that promotes keratinocyte proliferation and epidermal thickening.

ID3 mediates X-ray-induced apoptosis of keratinocytes through the regulation of β-catenin

BACKGROUND

Ionizing radiation is used to treat many of cancers, however, it also produces unwanted side effect on normal tissues, such as radiodermatitis. We previously established an animal model for radiodermatitis, and found that X-ray irradiation induced the expression of ID3 in hairless mouse skin by cDNA microarray.

OBJECTIVE

The aim of this study is to investigate the functional role of ID3 in X-ray irradiated keratinocytes.

METHODS

Immunohistochemistry, RT-PCR and Western blot were performed to demonstrate the ID3 induction by X-ray irradiation. HaCaT keratinocytes were transduced with the recombinant adenovirus expressing HA-ID3, and then effects on apoptosis were analyzed.

RESULTS

X-ray irradiation increased markedly the ID3 protein level in epidermis of mouse skin. X-ray irradiation also induced the expression of ID3 in HaCaT keratinocytes cultured in vitro, at both mRNA and protein levels. When ID3 was overexpressed by recombinant adenovirus, apoptosis of keratinocytes were induced even in the absence of X-ray irradiation. Furthermore, overexpression of ID3 sensitized X-ray-induced apoptosis. Interestingly, X-ray irradiation significantly reduced the endogenous β-catenin level, which was related with induction of apoptosis. Similarly, overexpression of ID3 led to remarkable reduction in β-catenin level.

CONCLUSION

These results suggest that ID3 plays a role as an apoptosis inducer in response to X-ray irradiation via the regulation of endogenous β-catenin level.

Mad3 negatively regulates B cell differentiation in the spleen by inducing Id2 expression

Immature B cells migrate to the spleen where they differentiate into mature cells. This final maturation step is crucial to enable B cells to become responsive to antigens and to participate in the immune response. Previously, we showed that Id2 acts as a negative regulator of the differentiation of immature B cells occurring in the spleen. Id2 expression has been found to depend on Myc-Max-Mad transcriptional complexes in mammary epithelial cells. Nearly all studies to date have shown that Mad proteins inhibit proliferation, presumably by antagonizing the function of Myc proteins. In the current study, we followed the Mad family members during peripheral B cell differentiation. We show that Mad3 actively regulates B cell differentiation. Our results demonstrate that high expression levels of Mad3 in immature B cells induce Id2 expression, which inhibits transcription of genes essential for B cell differentiation. During their differentiation to mature cells, B cells reduce their Mad3 expression, enabling the maturation process to occur.

HDAC1 and HDAC2 regulate oligodendrocyte differentiation by disrupting the beta-catenin-TCF interaction

Oligodendrocyte development is regulated by the interaction of repressors and activators in a complex transcriptional network. We found that two histone-modifying enzymes, HDAC1 and HDAC2, were required for oligodendrocyte formation. Genetic deletion of both Hdac1 and Hdac2 in oligodendrocyte lineage cells resulted in stabilization and nuclear translocation of beta-catenin, which negatively regulates oligodendrocyte development by repressing Olig2 expression. We further identified the oligodendrocyte-restricted transcription factor TCF7L2/TCF4 as a bipartite co-effector of beta-catenin for regulating oligodendrocyte differentiation. Targeted disruption of Tcf7l2 in mice led to severe defects in oligodendrocyte maturation, whereas expression of its dominant-repressive form promoted precocious oligodendrocyte specification in developing chick neural tube. Transcriptional co-repressors HDAC1 and HDAC2 compete with beta-catenin for TCF7L2 interaction to regulate downstream genes involved in oligodendrocyte differentiation. Thus, crosstalk between HDAC1/2 and the canonical Wnt signaling pathway mediated by TCF7L2 serves as a regulatory mechanism for oligodendrocyte differentiation.

Sorting out the functional role(s) of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) in cell proliferation and cancer

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) has many beneficial physiological functions ranging from enhancing fatty acid catabolism, improving insulin sensitivity, inhibiting inflammation and increasing oxidative myofibers allowing for improved athletic performance. Thus, given the potential for targeting PPARbeta/delta for the prevention and/or treatment of diseases including diabetes, dyslipidemias, metabolic syndrome and cancer, it is critical to clarify the functional role of PPARbeta/delta in cell proliferation and associated disorders such as cancer. However, there is considerable controversy whether PPARbeta/delta stimulates or inhibits cell proliferation. This review summarizes the literature describing the influence of PPARbeta/delta on cell proliferation, with an emphasis toward dissecting the data that give rise to opposing hypotheses. Suggestions are offered to standardize measurements associated with these studies so that interlaboratory comparisons can be accurately assessed.

Ligand activation of peroxisome proliferator-activated receptor-beta/delta inhibits cell proliferation in human HaCaT keratinocytes

Although there is strong evidence that ligand activation of peroxisome proliferator-activated receptor (PPAR)-beta/delta induces terminal differentiation and attenuates cell growth, some studies suggest that PPARbeta/delta actually enhances cell proliferation. For example, it was suggested recently that retinoic acid (RA) is a ligand for PPARbeta/delta and potentiates cell proliferation by activating PPARbeta/delta. The present study examined the effect of ligand activation of PPARbeta/delta on cell proliferation, cell cycle kinetics, and target gene expression in human HaCaT keratinocytes using two highly specific PPARbeta/delta ligands [4-[[[2-[3-fluoro-4-(trifluoromethyl)phenyl]-4-methyl-5-thiazolyl]methyl]thio]-2-methylphenoxy acetic acid (GW0742) and 2-methyl-4-((4-methyl-2-(4-trifluoromethylphenyl)-1,3-thiazol-5-yl)-methylsulfanyl)phenoxy-acetic acid (GW501516)] and RA. Both PPARbeta/delta ligands and RA inhibited cell proliferation of HaCaT keratinocytes. GW0742 and GW501516 increased expression of known PPARbeta/delta target genes, whereas RA did not; RA increased the expression of known retinoic acid receptor/retinoid X receptor target genes, whereas GW0742 did not affect these genes. GW0742, GW501516, and RA did not modulate the expression of 3-phosphoinositide-dependent protein kinase or alter protein kinase B phosphorylation. GW0742 and RA increased annexin V staining as quantitatively determined by flow cytometry. The effects of GW0742 and RA were also examined in wild-type and PPARbeta/delta-null primary mouse keratinocytes to determine the specific role of PPARbeta/delta in modulating cell growth. Although inhibition of keratinocyte proliferation by GW0742 was PPARbeta/delta-dependent, inhibition of cell proliferation by RA occurred in both genotypes. Results from these studies demonstrate that ligand activation of PPARbeta/delta inhibits keratinocyte proliferation through PPARbeta/delta-dependent mechanisms. In contrast, the observed inhibition of cell proliferation in mouse and human keratinocytes by RA is mediated by PPARbeta/delta-independent mechanisms and is inconsistent with the notion that RA potentiates cell proliferation by activating PPARbeta/delta.

Truncation mutations abolish chromatin-associated activities of adenomatous polyposis coli

The adenomatous polyposis coli (APC) is a tumor suppressor whose loss of function leads to colon cancer. APC shuttles between the nucleus and cytoplasm, however its role in the nucleus remains elusive. We have found that nuclear APC specifically associates with transcriptionally active chromatin through structural elements located downstream to the region of frequent truncation mutations found in colorectal tumors. We show that a recombinant APC fragment comprising such elements associates in vivo with euchromatin and preferentially binds in vitro to acetylated histone H3. Induction of DNA double-strand breaks (DSB) stimulates accumulation of APC at the damaged DNA chromatin marked by histone H2AX and S139-phosphorylated histone H2AX. A nuclear complex containing the DNA-dependent protein kinase catalytic subunit (DNAPKcs) and APC associates with chromatin in response to DNA DSB. APC knockdown with siRNA decreased the rate of DNA DSB-induced S139 histone H2AX phosphorylation in cells expressing endogenous full-length APC, but not in colon cancer cells with its truncation mutants, whereas ectopic APC expression stimulated the H2AX phosphorylation regardless of the type of endogenous APC. Our data suggest that APC involves in the DSB DNA repair and that truncation mutations impair chromatin-associated functions of APC.

Wnt7b stimulates embryonic lung growth by coordinately increasing the replication of epithelium and mesenchyme

The effects of Wnt7b on lung development were examined using a conditional Wnt7b-null mouse. Wnt7b-null lungs are markedly hypoplastic, yet display largely normal patterning and cell differentiation. In contrast to findings in prior hypomorphic Wnt7b models, we find decreased replication of both developing epithelium and mesenchyme, without abnormalities of vascular smooth muscle development. We further demonstrate that Wnt7b signals to neighboring cells to activate both autocrine and paracrine canonical Wnt signaling cascades. In contrast to results from hypomorphic models, we show that Wnt7b modulates several important signaling pathways in the lung. Together, these cascades result in the coordinated proliferation of adjacent epithelial and mesenchymal cells to stimulate organ growth with few alterations in differentiation and patterning.