Content uploaded by Nadine Darwiche
Author content
All content in this area was uploaded by Nadine Darwiche on Mar 11, 2014
Content may be subject to copyright.
A preview of the PDF is not available
Mouse skin tumor progression results in differential expression of retinoic acid and retinoid X receptors
Abstract and Figures
Retinoids are powerful regulators of epidermal cell growth and differentiation and are widely used in the prevention and treatment of skin disorders and cancers in humans. Since many of the effects of retinoids on cell growth and differentiation are mediated by nuclear retinoid receptors (RARs and RXRs), we were interested in determining RAR and RXR gene expression during mouse skin tumor progression. The two-stage system of mouse skin carcinogenesis was used to generate papillomas and carcinomas, and the different stages of malignant progression (papillomas, differentiated squamous cell carcinomas, undifferentiated squamous cell carcinomas, and spindle cell carcinomas) were characterized in each tumor by specific keratin expression prior to receptor characterization. Using in situ hybridization analysis, we show that the two major RAR isoforms (alpha 1 and gamma 1), which account for most of RARs in the skin, were expressed in both the basal and suprabasal layers in mouse epidermis. In contrast, RXR alpha transcripts were compartmentalized to the basal cell layers and concentrated in hair follicles. During skin tumor progression, RAR (alpha 1 and gamma 1) transcripts were down-modulated in malignant tumor cells, whereas RXR (alpha and beta) transcript expression was expanded in papillomas and carcinomas as the number of undifferentiated cells also increased. RXR gamma was not detected in the skin or at any stage during skin tumor progression. Spindle cell tumors lacked markers of the keratinocyte phenotype and lost RAR expression, yet retained expression of RXR alpha and beta. The increased abundance of transcripts for RXRs and decreased presence of RARs in skin tumor progression may favor other nuclear signal transduction pathways requiring RXR for heterodimer formation and contribute to phenotypic progression of cancer cells.
Figures - uploaded by Nadine Darwiche
Author content
All figure content in this area was uploaded by Nadine Darwiche
Content may be subject to copyright.
... On the other hand, the nonmelanome skin cancer include epithelial skin cancer, basal cell carcinoma (BCC), cutaneous squamous cell carcinoma (cSCC), dermatofibrosarcoma protuberans (DFSP), and Merkel cell carcinoma (MCC) [680][681][682][683]. Epithelial skin cancer is the most prevalent malignancy in humans [682]. ...
... On the other hand, the nonmelanome skin cancer include epithelial skin cancer, basal cell carcinoma (BCC), cutaneous squamous cell carcinoma (cSCC), dermatofibrosarcoma protuberans (DFSP), and Merkel cell carcinoma (MCC) [680][681][682][683]. Epithelial skin cancer is the most prevalent malignancy in humans [682]. cSCC and BCC are the most common malignant skin cancer presenting an aggressive biological behaviours [683,684]. ...
In recent years, the scientific community has seen an increasing interest in the use of natural compounds in medicines, food supplements, cosmetics and dermatological products. An important example is Sambucus nigra L. whose flowers (elderflowers) and berries (elderberries) have been widely used in traditional medicine.
The traditional use of elderflowers in the relief of early symptoms of common cold has been officially recognised by the European Union, where several products in the form of herbal tea, tincture or liquid extract, are available on the market. Elderberries have also been traditionally used in the form of herbal tea, syrup or juice. Nevertheless, no single herbal substance/herbal preparation is registered as medicine.
In line with the need for comprehensive pharmacological studies to validate the traditional use of S. nigra, namely its anti-inflammatory activity, one of the aims of this PhD thesis was to screen and characterize biological activities of S. nigra L. extracts obtained using different extraction methodologies. Then, to optimize the biological activity of the best extract (i.e. with high interest for therapeutic uses), different extract-loaded nanocarriers were prepared.
Different materials were applied in this task such as polymeric and lipid-based materials. In this specific context, the purpose of using nanotechnology as strategy was to increase the stability of bioactive compounds and to modulate their release, creating a potential topical formulation.
Due to the harvesting time of elderflowers and elderberries, in a first part of this PhD thesis, initial nanoencapsulation studies were performed using hyaluronic acid as model drug.
Hyaluronic acid is commonly used through intraarticular administration for viscosupplementation in osteoarthritis and other inflammation disorders. Therefore, the first studies consisted of producing an characterising polymeric nanoparticles made of poly(lacticco-glycolic acid) (PLGA) were prepared with and without hyaluronic acid. The inclusion of hyaluronic acid was achieved with an efficiency higher than 70%, but resulted in a marked particle size increase. Particles revealed an in vitro sustained release profile and in vitro cell compatibility, as well as a risk of haemolysis less than 1%, ensuring their safety. In vivo antiinflammatory studies showed a higher inhibition for hyaluronic acid-loaded PLGA particles when compared to hyaluronic acid suspension (78% versus 60%). Results were not so different from the positive control, clearly suggesting that this formulation may be a promising alternative to the current hyaluronic acid injectable dosage form.
Having characterised the intended particulate carrier, different extraction methods were studied to obtain the S. nigra extracts from elderflowers and elderberries. For this purpose, fresh flowers, and fresh and dried berries were considered, resulting in eighteen S. nigra extracts.
Several parameters were considered for selecting the extraction method, i.e. yield of extraction, type of solvent, flavonoid content and biological activity of the resultant extracts (antioxidant activity, total polyphenol content, collagenase, elastase, tyrosinase and acetylcholinesterase inhibition). The most promising extracts were characterized for in vitro and in vivo antiinflammatory activity and cytotoxicity (skin and monocytic cells). The most promising extracts were those obtained from fresh flowers using ultrasounds method with methanol. Specifically, these extracts showed results similar to positive controls, particularly the antioxidant activity (75 ± 2%), collagenase inhibition (94 ± 1%) and in vitro anti-inflammatory activity (97 ± 3%).
Nevertheless, extracts of fresh flowers using ultrasounds/ethanol presented higher collagenase inhibition (88 ± 3%) and in vitro anti-inflammatory activity (102 ± 2%). Cytotoxicity testing confirmed their safety.
The second aim of the present PhD thesis was to optimize the activity of the resultant
methanolic extracts through encapsulation in different types of nanocarriers: polymeric
nanoparticles based on PLGA and poly-Ɛ-caprolactone (PCL) and lipid-based nanoparticles (ethosomes). The so obtained nanoparticulate formulations were analysed in terms of particle size and morphology, physicochemical stability over the time, extract encapsulation efficiency, release profile and biological activities (e.g., anti-inflammatory activity, collagenase inhibition, antioxidant activity). Small and well-defined polymeric nanoparticles and ethosomes were prepared. The highest encapsulation efficiency (76%) was found in PLGA nanoparticles. The same happened for the anti-inflammatory activity (60.7 ± 9.0%). On the other hand, ethosomes presented a very promising value of collagenase inhibition.
At the end, this thesis validates and supports the scientific evidence of potential uses of S. nigra as a therapeutic agent, in the case of polymeric nanoparticles, or as cosmetic ingredient, in the case of ethosomes. However, further studies should be carried out, hopefully attracting interest from pharmaceutical and cosmetic industries.
... RARg is the major RAR expressed in human and mouse epidermis, whereas very little expression of RARa and no expression of RARb is seen in skin (Duong and Rochette-Egly, 2011). RARg has been shown to be a tumor suppressor, as loss of RARg predisposed skin to tumors (Chen et al., 2004;Darwiche et al., 1995Darwiche et al., , 1996. In addition, there is evidence that UV radiation from the sun induces a dramatic decrease in RARg by proteasomal degradation resulting in premature skin aging and skin cancer (Chen et al., 2004;Wang et al., 1999). ...
... RARg was found to be highly expressed in keratinocytes and its expression is decreased in skin cancers (Darwiche et al., 1995;Finzi et al., 1992;Xu et al., 2001). Early studies also suggested that RARs may play a role in skin tumor development and that RARg may function as a tumor suppressor (Chen et al., 2004;Duong and Rochette-Egly, 2011). ...
DNA-damaging compounds, commonly used as chemotherapeutic drugs, are known to trigger cells to undergo programmed cell death such as apoptosis and necroptosis. However, the molecular mechanism of DNA damage-induced cell death is not fully understood. Here, we report that RARγ has a critical role in DNA damage-induced programmed cell death, specifically in necroptosis. The loss of RARγ abolishes the necroptosis induced by DNA damage. In addition, cells that lack RARγ are less susceptible to extrinsic apoptotic pathway activated by DNA-damaging agents whereas the intrinsic apoptotic pathway is not affected. We demonstrate that RARγ is essential for the formation of RIPK1/RIPK3 death complex, known as Ripoptosome, in response to DNA damage. Furthermore, we show that RARγ plays a role in skin cancer development by using RARγ1 knockout mice and human squamous cell carcinoma biopsies. Hence, our study reveals that RARγ is a critical component of DNA damage-induced cell death.
... RARg is the major RAR expressed in human and mouse epidermis, whereas very little expression of RARa and no expression of RARb is seen in skin (Duong and Rochette-Egly, 2011). RARg has been shown to be a tumor suppressor, as loss of RARg predisposed skin to tumors (Chen et al., 2004;Darwiche et al., 1995Darwiche et al., , 1996. In addition, there is evidence that UV radiation from the sun induces a dramatic decrease in RARg by proteasomal degradation resulting in premature skin aging and skin cancer (Chen et al., 2004;Wang et al., 1999). ...
... RARg was found to be highly expressed in keratinocytes and its expression is decreased in skin cancers (Darwiche et al., 1995;Finzi et al., 1992;Xu et al., 2001). Early studies also suggested that RARs may play a role in skin tumor development and that RARg may function as a tumor suppressor (Chen et al., 2004;Duong and Rochette-Egly, 2011). ...
DNA-damaging compounds, commonly used as chemotherapeutic drugs, are known to trigger cells to undergo programmed cell death such as apoptosis and necroptosis. However, the molecular mechanism of DNA damage-induced cell death is not fully understood. Here, we report that RARg has a critical role in DNA damage-induced programmed cell death, specifically in necroptosis. The loss of RARg
abolishes the necroptosis induced by DNA damage. In addition, cells that lack RARg are less susceptible to extrinsic apoptotic pathway activated by DNA-damaging agents whereas the intrinsic apoptotic pathway is not affected. We demonstrate that RARg is essential for the formation of RIPK1/RIPK3 death complex, known as Ripoptosome, in response to DNA damage. Furthermore, we show that RARg plays a role in skin cancer development by using RARg1 knockout mice and human squamous cell carcinoma biopsies. Hence, our study reveals that RARg is a critical component of DNA damage-induced cell death.
... On the other hand, the nonmelanome skin cancer include epithelial skin cancer, basal cell carcinoma (BCC), cutaneous squamous cell carcinoma (cSCC), dermatofibrosarcoma protuberans (DFSP) and Merkel cell carcinoma (MCC) (Amanda F. Nahhas, 2017;Darwiche et al., 1995;Lanssens and Ongenae, 2011;Misak et al., 2013). Epithelial skin cancer is the most prevalent malignancy in humans (Darwiche et al., 1995). ...
... On the other hand, the nonmelanome skin cancer include epithelial skin cancer, basal cell carcinoma (BCC), cutaneous squamous cell carcinoma (cSCC), dermatofibrosarcoma protuberans (DFSP) and Merkel cell carcinoma (MCC) (Amanda F. Nahhas, 2017;Darwiche et al., 1995;Lanssens and Ongenae, 2011;Misak et al., 2013). Epithelial skin cancer is the most prevalent malignancy in humans (Darwiche et al., 1995). cSCC and BCC are the most common malignant skin cancer presenting an agressive biological behaviours (Amanda F. Nahhas, 2017;Chen et al., 2012). ...
Nanotechnology involves the engineering of functional systems at nanoscale and it can be described as a collection of methods and techniques for processing materials to create products with special physicochemical properties. The rapid developments in nanotechnology have allowed the incorporation of therapeutic agents, actives for cosmetic, sensing agents into nanoparticles, for detection, prevention, and treatment of skin diseases. Nanoparticles promote the increase of penetration of drugs and many cosmetic chemicals across the skin. Nanoparticles offer many advantages as carrier systems since they can improve the solubility of poorly water-soluble drugs or actives such as phytocompounds, permeate the skin through different mechanisms, modify drug or actives pharmacokinetic and ultimately, improve their bioavailability. In this review, we discuss the recent advances of different types of nanoparticles for skin delivery over a period of 40 years. This review emphasizes approaches to overcome the drawbacks and limitations associated with the conventional systems and the advances and application that are poised to further enhance the efficacy of topical formulations with nanoparticles, offering the possibility of simplified dosing regimen that may improve treatment outcomes using novel delivery nanosystems.
... The clinical significance of CRABPII has been highlighted in several types of cancer, including non-small lung cancer (18), non-myeloma skin cancer (19) and pancreatic cancer cells (20), pinpointing that restoring the CRABPII signaling pathway may serve as a therapeutic intervention to ameliorate the efficacy of RA or sensitize cancer cells to this hormone. While CRABPII acts to deliver RA to its RAR receptors, each of the different isoforms of RAR exhibits specific function with each receptor regulating a subset of distinct genes (21)(22)(23)(24)(25)(26). RARβ has been implicated in inflammation and tumor suppression (21)(22)(23), while the loss of RARγ has been demonstrated in the progression of malignant squamous cell carcinoma (24,25). ...
... While CRABPII acts to deliver RA to its RAR receptors, each of the different isoforms of RAR exhibits specific function with each receptor regulating a subset of distinct genes (21)(22)(23)(24)(25)(26). RARβ has been implicated in inflammation and tumor suppression (21)(22)(23), while the loss of RARγ has been demonstrated in the progression of malignant squamous cell carcinoma (24,25). Selective activation of RARα by retinoids induces autophagy in ER-positive breast cancer cells (26). ...
Due to the anti-proliferative and anti-apoptotic effects of retinoic acid?(RA), this hormone has emerged as a target for several diseases, including cancer. However, development of retinoid resistance is a critical issue and efforts to understand the retinoid signaling pathway may identify useful biomarkers for future clinical trials. Apoptotic responses of RA are exhibited through the cellular RA-binding protein?II?(CRABPII)/retinoic acid receptor?(RAR) signaling cascade. Delivery of RA to RAR by CRABPII enhances the transcriptional activity of genes involved in cell death and cell cycle arrest. The purpose of this study was to investigate the role of curcumin in sensitizing RA-resistant triple-negative breast cancer?(TNBC) cells to RA-mediated apoptosis. We provide evidence that curcumin upregulates the expression of CRABPII, RAR? and RAR? in two different TNBC cell lines. Co-treatment of the cells with curcumin and RA results in increased apoptosis as demonstrated by elevated cleavage of poly(ADP-ribose) polymerase and cleaved caspase-9. Additionally, silencing CRABPII reverses curcumin sensitization of TNBC?cells to the apoptotic inducing effects of RA. These findings provide mechanistic insights into sensitizing TNBC?cells to RA-mediated cell death by curcumin-induced upregulation of the CRABPII/RAR pathway.
... Using a mouse skin tumor carcinogenesis model, it was shown that RAR-␥ expression decreases during tumor progression. 38 RA-induced growth inhibition is related to RA turnover rate among HNSCC cell lines, as was also observed by others and in our previous studies, 9,10,39 and therefore it was not surprising to find that the expression level of RAR-␥ was related to RA turnover rate. It has been suggested that the enzymes responsible for the turnover of RA are induced by RARs in the presence of RA. 40,41 After the discovery and cloning of a highly RA-specific cytochrome P450 enzyme, CYP26A1, 42,43 a more direct approach could be followed by measuring its mRNA expression levels. ...
Retinoids, analogues of vitamin A, can reverse premalignant lesions and prevent second primary tumors in patients with head and neck squamous cell carcinoma (HNSCC). The effects of retinoids are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which act as ligand‐activated transcription factors. The regulation of cell growth, differentiation and retinoid metabolism in normal, premalignant and malignant cells by retinoids is thought to be a result of their effects on gene expression. We investigated mRNA expression of RARs (α, β, and γ) and RXR‐β by means of RNase protection and related this to retinoic acid (RA)‐induced growth inhibition and RA turnover in four HNSCC cell lines (UM‐SCC‐14C, UM‐SCC‐22A, UM‐SCC‐35 and VU‐SCC‐OE). An RA‐resistant subline of UM‐SCC‐35 was generated by exposure to increasing concentrations of RA for 8 months (designated UM‐SCC‐35R). RA turnover was determined on the basis of decreasing RA levels in the cells and culture medium after exposure to 1 μM RA. We found that RAR‐γ mRNA expression was strongly correlated with RA‐induced growth inhibition (p = 0.016, R = 0.92) and RA turnover (p = 0.041, R = 0.86). RAR‐β transcript levels were reduced in three of five cell lines compared with normal mucosa, and these did not correlate with RA‐induced growth inhibition and RA turnover. Expression of RAR‐α and RXR‐β was not substantially altered in any of the cell lines. These findings suggest that in HNSCC cell lines RAR‐γ is the most important retinoid receptor for regulation of RA turnover rate and RA‐induced growth inhibition. © 2001 Wiley‐Liss, Inc.
A valid model of initiated epidermis requires that normal keratinocytes should suppress the growth of initiated keratinocytes and that this suppression should be overcome on exposure to tumour promoters. Subgenomic SV40 transformed human keratinocytes (HK-4), which display the initiated phenotype, were cultured within a confluent monolayer of normal human keratinocytes and exposed to various tumour promoters and anti-promoters. This system appeared to be successful in identifying both phorbol ester and non-phorbol ester type tumour promoters in a dose-dependent manner as determined by the size of the colonies of the HK-4 cells. The ranking of three phorbol ester-type tumour promoters matched the order of potency of these compounds as tumour promoters in mouse skin. In this model tumour promoters appear to induce the clonal expansion of HK-4 cells indirectly by selectively stimulating the differentiation of the normal cells. The anti-promoters retinoic acid and fluocinolone acetonide were unsuccessful in inhibiting TPA induced colonies in this human model. A major reason for the absence of anti-promoter activity in the co-culture has been attributed to the abnormal conditions of the cells (i.e grown on plastic and submerged in medium). The presence of a dermis and an air-liquid interface is important in cellular functions such as differentiation and proliferation. Cells grown as organotypic cultures better represent the in vivo situation compared with those in submerged cultures. Therefore the co-culture of HK-4 and normal human keratinocytes was incorporated within a skin equivalent at the air-liquid interface to produce a three dimensional model of initiated human epidermis. Mimicking the situation in vivo may give rise to a more accurate interpretation of tumour promotion in human skin. In addition, this system allows test samples to be added either topically (placed on the surface of the skin equivalent) or systemically (placed in the medium below the dermis).
Retinoids are essential for the maintenance of epithelial differentiation. As such, they play a fundamental role in chemoprevention of epithelial carcinogenesis and in differentiation therapy. Physiological retinoic acid is obtained through two oxidation steps from dietary retinol, i.e. retinol→retinal→retinoic acid. The latter retinal→retinoic acid step is irreversible and eventually marks disposal of this essential nutrient, through cytochrome P450-dependent oxidative steps. Mutant mice deficient in aryl hydrocarbon receptor (AHR) accumulate retinyl palmitate, retinol and retinoic acid. This suggests a direct connection between the AHR and retinoid homeostasis. Retinoids control gene expression through the nuclear retinoic acid receptors (RARs) α, β and γ and 9-cis-retinoic acid receptors α, β and γ, which bind with high affinity the natural ligands all-trans-retinoic acid and 9-cis-retinoic acid, respectively. Retinoids are effective chemopreventive agents against skin, head and neck, breast, liver and other forms of cancer. Differentiation therapy of acute promyelocytic leukemia (APL) is based on the ability of retinoic acid to induce differentiation of leukemic promyelocytes. Patients with relapsed, retinoid-resistant APL are now being treated with arsenic oxide, which results in apoptosis of the leukemic cells. Interestingly, induction of differentiation in promyelocytes and consequent remission of APL following retinoid therapy depends on expression of a chimeric PML–RARα fusion protein resulting from a t(15;17) chromosomal translocation. This protein functions as a dominant negative against the function of both PML and RARs and its overexpression is able to recreate the phenotypes of the disease in transgenic mice. The development of new, more effective and less toxic retinoids, alone or in combination with other drugs, may provide additional avenues for cancer chemoprevention and differentiation therapy.
Retinoids are a group of structural and functional analogs of vitamin A. They regulate a number of fundamental physiological processes including vision, reproduction, metabolism, differentiation, bone development, and pattern formation during embryogenesis (De Luca 1991; Gudas et al. 1994; Lotan 1995a). Certain retinoids are capable of modulating cell growth, differentiation, and apoptosis and suppressing carcinogenesis in a variety of tissue types (e.g., lung, skin, mammary gland, prostate, bladder) in animal models (Moon et al. 1994; Lotan 1996) and in clinical trials with patients with premalignant or malignant lesions of the oral cavity, cervix, bronchial epithelium, skin, and other organs (Kraemer et al. 1992; Hong and Itri 1994; Lotan 1996; Hong and Sporn 1997; Moon et al. 1997). Some retinoids exhibit antitumor activity against fully malignant cells in vitro as reflected by suppression of proliferation and induction of differentiation or apoptosis (Amos and Lotan 1991; Lotan et al. 1991; Gudas et al. 1994; Lotan 1995a). Consequently, retinoids are considered as potential therapeutic agents as well (Smith et al. 1992). The ability of all-trans-retinoic acid (ATRA) to induce differentiation of acute promyelocytic leukemia cells into granulocytes is the basis for its therapeutic activity in patients and ATRA is currently used for therapy of this type of cancer (Degos 1997; Suooignet et al. 1997; Tallman et al. 1997; see Chap. 11, Sect. III).
I. Introduction NUCLEAR receptors mediate the signals of a broad variety of fat-soluble hormones, including the steroid and vitamin D3 hormones, thyroid hormones, and vitamin A-derived hormones and analogs (retinoids). The receptors represent one of the largest transcription factor families known (reviewed in Refs. 1–3). Only some of the members are ligand-binding receptors, while others are “orphan receptors” for which specific ligands have not yet been identified and may not exist. These proteins may therefore function as constitutive or negative transcription factors, or coregulators of ligand-binding receptors. The receptors regulate gene transcription by interacting with specific DNA sequences (called response elements) that are usually located in the promoter regions of the responsive genes. All receptors contain a highly conserved cysteine-rich region, the DNA- binding domain (DBD) that forms two “zinc-finger” structures that allow protein-DNA as well as proteinprotein interaction (4). A second but...
Methods to isolate and culture intact mouse hair follicles and interfollicular epidermal cells provide a model to test the potential of each to form tumors as a consequence of ras(Ha) gene activation and to determine the risk for progression in the resultant tumors. The v-ras(Ha) oncogene was introduced into intact or dissociated hair follicle cells or interfollicular epidermal cells from newborn mouse skin via a defective retroviral vector. Either immediately after infection or after an additional 6 days of culture, the v-ras(Ha) cells were transferred to nude mice as a skin graft. Both cell populations formed squamous papillomas which were indistinguishable based on morphology and immunocytochemistry. All papillomas expressed epidermal specific markers whether derived from hair follicle or interfollicular cells, and many regressed. After 16 weeks in vivo, 20-30% of the benign skin tumors in all groups converted to malignancy. In addition to papillomas, hair follicle derived populations produced hemangiomas in many animals. None of the groups formed basal cell carcinomas, keratoacanthomas or tumors with characteristics of differentiating hair follicle cells. These studies indicate that ras gene activation can contribute to benign squamous neoplasia originating from several skin-derived cell types. The underlying factors which determine the variable risk for neoplastic progression of skin papillomas after ras gene activation is not simply the origin of the tumor cell from hair follicle or interfollicular epidermis. The activated ras oncogene can also transform skin endothelial cells but does not appear to directly contribute to transformation of the more differentiated cells of the hair follicle.
Primary mouse keratinocytes expressing the v.ras@I* oncogene keratinocytes) produce squamous papillomas when grafted onto nude mice and respond abnormally to signals for terminal differentiation both in vivo and in vitro. Since protein kinase C (PKC) activators and v@ras@I* induce similar phenotypicchangesin culturedkeratinocytes,and cellular diacylglycerol levels are constitutively elevated in ras-transformed kern tinocytes, we tested whether PKC is a downstream target for oncogenicma in this cell type. Ca2'-dependent PKC activity was increased in lysates from cultured v@rasHakeratinocytes when compared to control cells; in contrast, Ca2@-independentactivity decreased. Similar to PKC activators, v-ras5@ blockedCa2@-mediated expressionof the earlyepidermaldiffer entiation markers keratins Ki and K1Owhile Inducing aberrant expres slon of K8 Pretreatmentof v-ms'@keratinocyteswith bryostatinto block PKC function restored Ca2'-mediated expression of K! and K1Oand blocked abnormal expression of KS, suggesting that these responses are mediated by the PKC pathway. Furthermore, expression of Kl is restored at bryostatin doses which specifically down-modulate PKC-a, the only Ca2@-dependent PKC isozyme detected in cultured keratinocytes. In con treat to the inhibition of K! and K1O, Ca2@-induced expression of the late epidermal differentiation markers loricrin, filaggrin, and keratinocyte transglutaminase was accelerated by v@ras@@M, @ previously reported in
The BJC is owned by Cancer Research UK, a charity dedicated to understanding the causes, prevention and treatment of cancer and to making sure that the best new treatments reach patients in the clinic as quickly as possible. The journal reflects these aims. It was founded more than fifty years ago and, from the start, its far-sighted mission was to encourage communication of the very best cancer research from laboratories and clinics in all countries. The breadth of its coverage, its editorial independence and it consistent high standards, have made BJC one of the world's premier general cancer journals. Its increasing popularity is reflected by a steadily rising impact factor.
The suggestion that elevated γ-glutamyltranspeptidase (GGT) activity could be a marker for hepatic neoplasia has prompted us to investigate the role of this enzyme in normal and neoplastic mouse skin. In normal skin, as determined histochemically with N-(γ-L-glutamyl)-4-methoxy-2-naphthylamide as substrate, GGT was found only in the lower epithelium of growing hair follicles. GGT activity was never detected in the interfollicular epidermis of 1-day-old prenatal, newborn, or adult skin. Whole-skin enzyme activity measured biochemically, with L-γ-glutamyl p-nitroanilide as substrate, was elevated about 9-fold during periods of hair growth. GGT activity rose and then fell abruptly as the mice progressed through the growing phase and then the resting phase of the hair growth cycle. Skin tumors were induced by initiation with 7,12-dimethylbenz[a]anthracene followed by promotion with 12-O-tetradecanoylphorbol-13-acetate. Histochemically, GGT was not present in eight benign papillomas examined. However, single epithelial cells, islands of epithelial cells, and/or follicle-like structures were intensely GGT positive in seven of eight malignant squamous carcinomas examined. The association of GGT activity with normal hair growth and its presence in squamous carcinomas raises the following interesting possibilities: (a) aberrant gene activation in malignant interfollicular epidermis; (b) a recruitment of follicular cells by invasive malignant interfollicular epidermis;
Cellular responsiveness to retinoic acid and its metabolites is conferred through two structurally and pharmacologically distinct families of receptors: the retinoic acid receptors (RAR) and the retinoid X receptors (RXR). Here we report that the transcriptional activity of RAR and RXR can be reciprocally modulated by direct interactions between the two proteins. RAR and RXR have a high degree of cooperativity in binding to target DNA, consistent with previous reports indicating that the binding of either RAR or RXR to their cognate response elements is enhanced by factors present in nuclear extracts. RXR also interacts directly with and enhances the binding of nuclear receptors conferring responsiveness to vitamin D3 and thyroid hormone T3; the DNA-binding activities of these receptors are also stimulated by the presence of nuclear extracts. Together these data indicate that RXR has a central role in multiple hormonal signalling pathways.
Thyroid hormones and retinoic acid function through nuclear receptors that belong to the steroid/thyroid-hormone receptor superfamily. Thyroid hormone receptors (TRs) and retinoic acid receptors (RARs) require auxiliary nuclear proteins for efficient DNA binding. Here we report that retinoid X receptors RXR alpha is one of these nuclear proteins. RXR alpha interacts both with TRs and with RARs, forming heterodimers in solution that strongly interact with a variety of T3/retinoic acid response elements. Transfection experiments show that RXR alpha can greatly enhance the transcriptional activity of TR and RAR at low retinoic acid concentrations that do not significantly activate RXR alpha itself. Thus, RXR alpha enhances the transcriptional activity of other receptors and its own ligand sensitivity by heterodimer formation. Our studies reveal a new subclass of receptors and a regulatory pathway controlling nuclear receptor activities by heterodimer formation.