Cyclin D2 overexpression in transgenic mice induces thymic and epidermal hyperplasia whereas cyclin D3 expression results only in epidermal hyperplasia

Investigating Thymic Epithelial Cell Diversity Using Systems Biology

The thymus is an intricate organ consisting of a diverse population of thymic epithelial cells (TECs). Cortical and medullary TECs and their subpopulations have distinct roles in coordinating the development and selection of functionally competent and self-tolerant T cells. Recent advances made in technologies such as single-cell RNA sequencing have made it possible to investigate and resolve the heterogeneity in TECs. These findings have provided further understanding of the molecular mechanisms regulating TEC function and expression of tissue-restricted Ags. In this brief review, we focus on the newly characterized subsets of TECs and their diversity in relation to their functions in supporting T cell development. We also discuss recent discoveries in expression of self-antigens in the context of TEC development as well as the cellular and molecular changes occurring during embryonic development to thymic involution.

Cyclin-Dependent Kinase 4 expression alters the number of keratinocyte stem cells in the mouse hair follicle

Hair follicles regenerate periodically by spontaneously undergoing cycles of growth, regression, and relative quiescence. During the hair cycle, follicle stem cells residing in a specialized niche remain quiescent, and they are stimulated to proliferate throughout the growth phase of the hair follicle. Although cell cycle regulators play a prominent role during the activation of hair follicle stem cells, the identity and the role of these regulators have not been confirmed. Herein, we reported that stem cells located in the bulge region of the HF (BuSCs) express high levels of cyclin‐dependent kinase 4 (CDK4) through the quiescent phase of the hair cycle. Using gain‐ and loss‐of‐function studies, we have determined that the CDK4 protein level affects the number of BuSCs. Transgenic expression of CDK4 in the bulge region of the hair follicles reduces the number of BuSCs, whereas CDK4 ablation resulted in an increasing number of BuSCs. These results suggest that deregulation of CDK4 protein levels contributes to distorting the self‐renewal/proliferation balance and, in turn, altering the number of BuSCs.

Nonproliferative and Proliferative Lesions of the Rat and Mouse Hematolymphoid System

The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative changes in rats and mice. The purpose of this publication is to provide a standardized nomenclature for classifying changes observed in the hematolymphoid organs, including the bone marrow, thymus, spleen, lymph nodes, mucosa-associated lymphoid tissues, and other lymphoid tissues (serosa-associated lymphoid clusters and tertiary lymphoid structures) with color photomicrographs illustrating examples of the lesions. Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous lesions as well as lesions induced by exposure to test materials. The nomenclature for these organs is divided into 3 terminologies: descriptive, conventional, and enhanced. Three terms are listed for each diagnosis. The rationale for this approach and guidance for its application to toxicologic pathology are described in detail below.

Cyclin D3 deficiency inhibits skin tumor development, but does not affect normal keratinocyte proliferation

Rearrangement and amplification of the D-type cyclin genes have been reported in human cancer. Previous studies have demonstrated that Ras-mediated skin tumorigenesis depends on pathways that act through cyclin D1 and D2; however, the role of cyclin D3 remains unknown. The present study demonstrates that cyclin D3 ablation does not affect keratinocyte proliferation, but instead increases apoptosis levels in the bulge region of the hair follicle. Consequently, cyclin D3 ablation reduces skin papilloma development in a Ras-dependent carcinogenesis model. Previous results revealed that cyclin D3 preferentially binds to cyclin-dependent kinase 6 (CDK6) in mouse keratinocytes and transgenic expression of CDK6 (K5CDK6 mice) inhibits skin tumor development. Thus, we hypothesized that the inhibitory effect of CDK6 is dependent on cyclin D3 expression. To test this hypothesis, a mouse model that overexpresses CDK6 and does not express cyclin D3 (K5CDK6/cyclin D3-/− compound mouse) was developed. Biochemical analysis of the epidermis of K5CDK6/cyclin D3-/− mice revealed that cyclin D3 ablation resulted in increased expression of cyclin D1 protein, with a consequent elevation in the level of CDK6/cyclin D1 and CDK4/cyclin D1 complexes. These findings were concurrent with the increase skin papilloma malignant progression observed in K5CDK6/cyclin D3-/− mice. In summary the absence of cyclin D3 led to fewer number of papillomas in cyclin D3-ablated mice than in the wild-type owing to increased apoptosis, suggesting that alterations in cell survival are a crucial mechanism for crippling cellular defense against neoplasia. The results of the current study also suggest that although cyclin D3 expression does not alter the tumor suppressive role of CDK6 in skin carcinogenesis, the compensatory increase in cyclin D1 can shift the balance towards malignant progression.

RB inactivation in keratin 18 positive thymic epithelial cells promotes non-cell autonomous T cell hyperproliferation in genetically engineered mice

Thymic epithelial cells (TEC), as part of thymic stroma, provide essential growth factors/cytokines and self-antigens to support T cell development and selection. Deletion of Rb family proteins in adult thymic stroma leads to T cell hyperplasia in vivo. To determine whether deletion of Rb specifically in keratin (K) 18 positive TEC was sufficient for thymocyte hyperplasia, we conditionally inactivated Rb and its family members p107 and p130 in K18+ TEC in genetically engineered mice (TgK18GT₁₂₁; K18 mice). We found that thymocyte hyperproliferation was induced in mice with Rb inactivation in K18+ TEC, while normal T cell development was maintained; suggesting that inactivation of Rb specifically in K18+ TEC was sufficient and responsible for the phenotype. Transplantation of wild type bone marrow cells into mice with Rb inactivation in K18+ TEC resulted in donor T lymphocyte hyperplasia confirming the non-cell autonomous requirement for Rb proteins in K18+ TEC in regulating T cell proliferation. Our data suggests that thymic epithelial cells play an important role in regulating lymphoid proliferation and thymus size.

Expression of cell cycle and apoptosis regulators in thymus and thymic epithelial tumors

The human thymus supports the production of self-tolerant T cells with competent and regulatory functions. Various cellular components of the thymic microenvironment such as thymic epithelial cells (TEC) and dendritic cells play essential roles in thymic T cell differentiation. The multiple cellular events occurring during thymic T cell and TEC differentiation involve proteins regulating cell cycle and apoptosis. Dysregulation of the cell cycle and apoptosis networks is involved in the pathogenesis of thymic epithelial tumors (TET) which are divided into two broad categories, thymomas and thymic carcinomas. The present review focuses on the usefulness of the analysis of the expression patterns of major cell cycle and apoptosis regulators in order to gain insight in the histophysiology of thymus and the histopathology, the clinical behavior and the biology of TET.

Development of oral epithelial cell line ROE2 with differentiation potential from transgenic rats harboring temperature-sensitive simian virus40 large T-antigen gene

We have developed an immortalized oral epithelial cell line, ROE2, from fetal transgenic rats harboring temperature-sensitive simian virus 40 large T-antigen gene. The cells grew continuously at either a permissive temperature of 33°C or an intermediate temperature of 37°C. At the nonpermissive temperature of 39°C, on the other hand, growth decreased significantly, and the Sub-G1 phase of the cell cycle increased, indicating that the cells undergo apoptosis at a nonpermissive temperature. Histological and immunocytochemical analyses revealed that ROE2 cells at 37°C had a stratified epithelial-like morphology and expressed cytokeratins Krt4 and Krt13, marker proteins for oral nonkeratinized epithelial cells. Global-scale comprehensive microarray analysis, coupled with bioinformatics tools, demonstrated a significant gene network that was obtained from the upregulated genes. The gene network contained 16 genes, including Cdkn1a, Fos, Krt13, and Prdm1, and was associated mainly with the biological process of skin development in the category of biological functions, organ development. These four genes were validated by quantitative real-time polymerase chain reaction, and the results were nearly consistent with the microarray data. It is therefore anticipated that this cell line will be useful as an in vitro model for studies such as physiological functions, as well as for gene expression in oral epithelial cells.

Cycling up the epidermis: reconciling 100 years of debate

There is likely general consensus within the skin research community that cell cycle control is critical to epidermal homeostasis and disease. The current predominant model proposes that keratinocytes switch off DNA replication and undergo cell cycle and cell growth arrest as they initiate terminal differentiation. However, this model cannot explain key physiological features of the skin, mainly why squamous differentiation prevails over proliferation in benign hyperproliferative disorders. In recent years, we have proposed an alternative model that involves mitotic slippage and endoreplication. This new model is controversial and has encountered resistance within the field. However, looking back at history, the epidermal cell cycle has been a matter of controversy and debate for around 100 years now. The accumulated data are confusing and contradictory. Our present model can explain and reconcile both old and new paradoxical observations. Here, we explain and discuss the endoreplicative cell cycle, the evidence for and against its existence in human epidermis and the important implications for skin homeostasis and disease. We show that regardless of the strengths or weaknesses of the Endoreplication Model, the existing evidence in support of the Cell Cycle Arrest Model is very weak.

Transcriptome sequencing of neonatal thymic epithelial cells

In order to gain novel insights into thymus biology, we analysed the whole transcriptome of cortical and medullary thymic epithelial cells (cTECs and mTECs) and of skin epithelial cells (ECs). Consistent with their ability to express ectopic genes, mTECs expressed more genes than other cell populations. Out of a total of 15,069 genes expressed in TECs, 25% were differentially expressed by at least 5-fold in cTECs vs. mTECs. Genes expressed at higher levels in cTECs than mTECs regulate numerous cell functions including cell differentiation, cell movement and microtubule dynamics. Many positive regulators of the cell cycle were overexpressed in skin ECs relative to TECs. Our RNA-seq data provide novel systems-level insights into the transcriptional landscape of TECs, highlight substantial divergences in the transcriptome of TEC subsets and suggest that cell cycle progression is differentially regulated in TECs and skin ECs.

Inactivation of the RB family prevents thymus involution and promotes thymic function by direct control of Foxn1 expression

RB family genes control T cell production and promote thymic involution through reducing Foxn1 expression in thymic epithelial cells.

Thymic involution during aging is a major cause of decreased production of T cells and reduced immunity. Here we show that inactivation of Rb family genes in young mice prevents thymic involution and results in an enlarged thymus competent for increased production of naive T cells. This phenotype originates from the expansion of functional thymic epithelial cells (TECs). In RB family mutant TECs, increased activity of E2F transcription factors drives increased expression of Foxn1, a central regulator of the thymic epithelium. Increased Foxn1 expression is required for the thymic expansion observed in Rb family mutant mice. Thus, the RB family promotes thymic involution and controls T cell production via a bone marrow–independent mechanism, identifying a novel pathway to target to increase thymic function in patients.

The mysterious human epidermal cell cycle, or an oncogene-induced differentiation checkpoint

Fifteen years ago, we reported that proto-oncogene MYC promoted differentiation of human epidermal stem cells, a finding that was surprising to the MYC and the skin research communities. MYC was one of the first human oncogenes identified, and it had been strongly associated with proliferation. However, it was later shown that MYC could induce apoptosis under low survival conditions. Currently, the notion that MYC promotes epidermal differentiation is widely accepted, but the cell cycle mechanisms that elicit this function remain unresolved. We have recently reported that keratinocytes respond to cell cycle deregulation and DNA damage by triggering terminal differentiation. This mechanism might constitute a homeostatic protection face to cell cycle insults. Here, I discuss recent and not-so-recent evidence suggesting the existence of a largely unexplored oncogene-induced differentiation response (OID) analogous to oncogene-induced apoptosis (OIA) or senescence (OIS). In addition, I propose a model for the role of the cell cycle in skin homeostasis maintenance and for the dual role of MYC in differentiation.

Immunohistological analysis of cell cycle and apoptosis regulators in thymus

The combined expression patterns of cell cycle and apoptosis regulators have not been analyzed in details in human thymus to the best of our knowledge. Our objective was to provide multiparametric and combined immunohistological information regarding the expression levels and the topographical distribution of major cell cycle and apoptosis regulators in postnatal human thymus. Ki67 and cyclins A, B1, D3 and E were frequently expressed by thymocytes with higher expression in cortical than medullary thymocytes. The expression of cyclin D2 was low in thymocytes. Thymic epithelial cells (TEC) exhibited low expression of Ki67 and cyclins. Bid was frequently expressed by thymocytes, Bcl-xL by cortical thymocytes and Bcl-2 by medullary thymocytes. The expression levels of Bim and survivin in thymocytes were low. The expression levels of Bax and Mcl-1 were higher in medullary than cortical thymocytes and TEC. Bak and Bad were mainly expressed in medullary TEC and Hassall Bodies (HB). c-FLIP and Fas were frequently expressed in TEC and FasL was mainly expressed by medullary TEC and HB. Cleaved caspase-3 was expressed by scattered thymocytes at the cortex and the corticomedullary junction and very rarely at the medulla. The different expression profiles and immunotopographical distribution of cell cycle and apoptosis regulators in thymocytes and TEC indicate that their expression is tightly regulated during thymic cell differentiation and that they are differentially involved in the cell survival/death regulation of thymocytes and TEC. Furthermore, this study indicates decrease of the proliferation and caspase-dependent apoptosis of thymocytes from the cortex to the medulla.

Translational repression of cyclin D3 by a stable G-quadruplex in its 5' UTR: implications for cell cycle regulation

cyclin D3 (CCND3) is one of the three D-type cyclins that regulate the G1/S phase transition of the cell cycle. Expression of CCND3 is observed in nearly all proliferating cells; however, the presence of high levels of CCND3 has been linked to a poor prognosis for several types of cancer. Therefore, further mechanistic studies on the regulation of CCND3 expression are urgently needed to provide therapeutic implications. In this study, we report that a conserved RNA G-quadruplex-forming sequence (hereafter CRQ), located in the 5' UTR of mammalian CCND3 mRNA, is able to fold into an extremely stable, intramolecular, parallel G-quadruplex in vitro. The CRQ G-quadruplex dramatically reduces the activity of a reporter gene in human cell lines, but it has little impact on its mRNA level, indicating a translational repression. Moreover, the CRQ sequence in its natural context inhibits translation of CCND3. Disruption of the G-quadruplex structure by G/U-mutation or deletion results in an elevated expression of CCND3 and an increased phosphorylation of Rb, a downstream target of CCND3, which promotes progression of cells through the G1 phase. Our results add to the growing understanding of the regulation of CCND3 expression and provide a potential therapeutic target for cancer treatment.

Combined effect of cyclin D3 expression and abrogation of cyclin D1 prevent mouse skin tumor development

We have previously demonstrated that ras-mediated skin tumorigenesis depends on signaling pathways that act preferentially through cyclin D1 and D2. Interestingly, the expression of cyclin D3 inhibits skin tumor development, an observation that conflicts with the oncogenic role of D-type cyclins in the mouse epidermis. Here, we show that simultaneous up and downregulation of particular members of the D-type cyclin family is a valuable approach to reduce skin tumorigenesis. We developed the K5D3/cyclin D1(-/-) compound mouse, which overexpresses cyclin D3 but lacks expression of cyclin D1 in the skin. Similar to K5D3 transgenic mice, keratinocytes from K5D3/cyclin D1(-/-) compound mice show a significant reduction of cyclin D2 levels. Therefore, this model allows us to determine the effect of cyclin D3 expression when combined with reduced or absent expression of the remaining two members of the D-type cyclin family in mouse epidermis. Our data show that induced expression of cyclin D3 compensates for the reduced level of cyclin D1 and D2, resulting in normal keratinocyte proliferation. However, simultaneous ablation of cyclin D1 and downregulation of cyclin D2 via cyclin D3 expression resulted in a robust reduction in ras-mediated skin tumorigenesis. We conclude that modulation of the levels of particular members of the D-type cyclin family could be useful to inhibit tumor development and, in particular, ras-mediated tumorigenesis.

Transgenic overexpression of PKCε in the mouse prostate induces preneoplastic lesions

It is well established that protein kinase C (PKC) isozymes play distinctive roles in mitogenic and survival signaling as well as in cancer progression. PKCε, the product of the PRKCE gene, is up-regulated in various types of cancers including prostate, lung and breast cancer. To address a potential role for PKCs in prostate cancer progression we generated three mouse transgenic lines expressing PKCα, PKCδ, or PKCε in the prostate epithelium under the control of the rat probasin (PB) promoter. Whereas PB-PKCε and PB-PKCδ mice did not show any evident phenotype, PB-PKCε mice developed prostate hyperplasia as well as prostate intraepithelial neoplasia (PIN) that displayed enhanced phospho-Akt, phospho-S6, and phospho-Stat3 levels, as well as enhanced resistance to apoptotic stimuli. PKCε overexpression was insufficient to drive neoplastic changes in the mouse prostate. Notably, overexpression of PKCε by adenoviral means in normal immortalized RWPE-1 prostate cells confers a growth advantage and hyperactivation of Erk and Akt. Our results argue for a causal link between PKCε overexpression and prostate cancer development.

Unexpected reduction of skin tumorigenesis on expression of cyclin-dependent kinase 6 in mouse epidermis

Cyclin-dependent kinases (CDKs) 4 and 6 are important regulators of the G(1) phase of the cell cycle, share 71% amino acid identity, and are expressed ubiquitously. As a result, it was assumed that each of these kinases plays a redundant role regulating normal and neoplastic proliferation. In previous reports, we have described the effects of CDK4 expression in transgenic mice, including the development of epidermal hyperplasia and increased malignant progression to squamous cell carcinoma. To study the role of CDK6 in epithelial growth and tumorigenesis, we generated transgenic mice carrying the CDK6 gene under the keratin 5 promoter (K5CDK6). Similar to K5CDK4 mice, epidermal proliferation increased substantially in K5CDK6 mice; however, no hyperplasia was observed. CDK6 overexpression also triggered keratinocyte apoptosis in interfollicular and follicular epidermis as a compensatory mechanism to override aberrant proliferation. Unexpectedly, CDK6 overexpression results in decreased skin tumor development compared with wild-type siblings. The inhibition in skin tumorigenesis was similar to that previously reported in K5-cyclin D3 mice. Furthermore, biochemical analysis of the K5CDK6 epidermis showed preferential complex formation between CDK6 and cyclin D3, suggesting that this particular complex plays an important role in tumor restraint. These studies provide in vivo evidence that CDK4 and CDK6 play a similar role as a mediator of keratinocyte proliferation but differ in apoptosis activation and skin tumor development.

A mitosis block links active cell cycle with human epidermal differentiation and results in endoreplication

How human self-renewal tissues co-ordinate proliferation with differentiation is unclear. Human epidermis undergoes continuous cell growth and differentiation and is permanently exposed to mutagenic hazard. Keratinocytes are thought to arrest cell growth and cell cycle prior to terminal differentiation. However, a growing body of evidence does not satisfy this model. For instance, it does not explain how skin maintains tissue structure in hyperproliferative benign lesions. We have developed and applied novel cell cycle techniques to human skin in situ and determined the dynamics of key cell cycle regulators of DNA replication or mitosis, such as cyclins E, A and B, or members of the anaphase promoting complex pathway: cdc14A, Ndc80/Hec1 and Aurora kinase B. The results show that actively cycling keratinocytes initiate terminal differentiation, arrest in mitosis, continue DNA replication in a special G2/M state, and become polyploid by mitotic slippage. They unambiguously demonstrate that cell cycle progression coexists with terminal differentiation, thus explaining how differentiating cells increase in size. Epidermal differentiating cells arrest in mitosis and a genotoxic-induced mitosis block rapidly pushes epidermal basal cells into differentiation and polyploidy. These observations unravel a novel mitosis-differentiation link that provides new insight into skin homeostasis and cancer. It might constitute a self-defence mechanism against oncogenic alterations such as Myc deregulation.

Effects of MEK and DNMT inhibitors on arsenic-treated human uroepithelial cells in relation to Cyclin-D1 and p16

Arsenic compounds are well-known toxic and carcinogenic agents, and they are widely distributed throughout the earth's crust. These compounds are associated with various human malignancies. It has been reported that there is an elevated risk of bladder cancer in an area highly contaminated with arsenic on the southwest coast of Taiwan. However, the underlying mechanisms of arsenic-associated carcinogenesis are still unclear. The cell cycle regulatory proteins are important indicators in control of cell cycle progression. Moreover, the high expression of Cyclin-D1 and loss of p16 has been associated with a worse prognosis in a variety of human cancers. Therefore, we investigated the effect of arsenic on Cyclin-D1 and p16 expression and evaluated the role of the ERK signaling pathway and DNA methylation in arsenic carcinogenesis. Our study results showed that Cyclin-D1 high expression was found in 56.3% (9/16) of urothelial carcinomas (UC) from a blackfoot disease (BFD) area and 6.3% (1/16) of UC from a non-BFD area (p=0.002). The p16 low expression in 81.2% (13/16) of UC from BFD areas was significantly lower than in non-BFD areas (25.0%; 4/16) (p=0.001). In addition, the Cyclin-D1 increased expression but decreased p16 expression in arsenite-treated SV-HUC-1 cells. However, when cells were pretreated with inhibitors (5-aza-CdR or U0126), the effects of arsenite on Cyclin-D1 and p16 expression were suppressed. Finally, these results indicated that Cyclin-D1 and p16 both might play important roles in carcinogenesis as a result of arsenic.

Enhanced skin carcinogenesis and lack of thymus hyperplasia in transgenic mice expressing human cyclin D1b (CCND1b)

Cyclin D1b is an alternative transcript of the cyclin D1 gene (CCND1) expressed in human tumors. Its abundance is regulated by a single base pair polymorphism at the exon 4/intron 4 boundary (nucleotide 870). Epidemiological studies have shown a correlation between the presence of the G870A allele (that favors the splicing for cyclin D1b) with increased risk and less favorable outcome in several forms of cancer. More recently, it has been shown that, unlike cyclin D1a, the alternative transcript D1b by itself has the capacity to transform fibroblasts in vitro. In order to study the oncogenic potential of cyclin D1b, we developed transgenic mice expressing human cyclin D1b under the control of the bovine K5 promoter (K5D1b mice). Seven founders were obtained and none of them presented any significant phenotype or developed spontaneous tumors. Interestingly, K5D1b mice do not develop the fatal thymic hyperplasia, which is characteristic of the cyclin D1a transgenic mice (K5D1a). Susceptibility to skin carcinogenesis was tested in K5D1b mice using two-stage carcinogenesis protocols. In two independent experiments, K5D1b mice developed higher papilloma multiplicity as compared with wild-type littermates. However, when K5D1b mice were crossed with cyclin D1KO mice, the expression of cyclin D1b was unable to rescue the carcinogenesis-resistant phenotype of the cyclin D1 KO mice. To further explore the role of cyclin D1b in mouse models of carcinogenesis we carried out in silico analysis and in vitro experiments to evaluate the existence of a mouse homologous of the human cyclin D1b transcript. We were unable to find any evidence of an alternatively spliced transcript in mouse Ccnd1. These results show that human cyclin D1b has different biological functions than cyclin D1a and confirm its oncogenic properties.

Expression of CDK4 or CDK2 in mouse oral cavity is retained in adult pituitary with distinct effects on tumorigenesis

The keratin 5 (K5) promoter drives transgenic expression to the basal cell layer of stratified epithelia. Surprisingly, analysis of K5CDK4 and K5CDK2 transgenic mouse embryos showed CDK4 and CDK2 expression not only in the expected tissues, but also in the adenohypophysis. This organ is derived from an upwards growth of the primitive oropharynx, a K5-expressing tissue. We show that transgenic expression of CDKs in the embryonic oral ectoderm is specifically retained in undifferentiated cells from the pars intermedia of the adenohypophysis. Interestingly, we found that K5CDK4 mice show a decreased number of pituitary stem cells, even though CDK4 is not expressed in the stem cells but in transit-amplifying (TA)-like cells. Interestingly, CDK4-expressing cells, but not CDK2-expressing cells, strongly synergize with lack of p27(Kip1) to generate pituitary carcinomas that appear with shortened latency and are drastically more aggressive than those arising in p27(-/-) mice. Thus, we show that deregulation of CDK expression in the primitive oral epithelium plays a unique function, providing a selective advantage that gives rise to transgene-positive TA-like pituitary cells. Furthermore, retention of CDK4 in these TA-like pituitary cells synergizes with loss of p27(Kip1) to induce pituitary adenocarcinomas. This model suggests that forced expression of CDK4 sensitizes cells and synergizes with a second change resulting in tumor development.

A chemical biology screen identifies glucocorticoids that regulate c-maf expression by increasing its proteasomal degradation through up-regulation of ubiquitin

The oncogene c-maf is frequently overexpressed in multiple myeloma cell lines and patient samples and contributes to increased cellular proliferation in part by inducing cyclin D2 expression. To identify regulators of c-maf, we developed a chemical screen in NIH3T3 cells stably overexpressing c-maf and the cyclin D2 promoter driving luciferase. From a screen of 2400 off-patent drugs and chemicals, we identified glucocorticoids as c-maf–dependent inhibitors of cyclin D2 transactivation. In multiple myeloma cell lines, glucocorticoids reduced levels of c-maf protein without influencing corresponding mRNA levels. Subsequent studies demonstrated that glucocorticoids increased ubiquitination-dependent degradation of c-maf and up-regulated ubiquitin C mRNA. Moreover, ectopic expression of ubiquitin C recapitulated the effects of glucocorticoids, demonstrating regulation of c-maf protein through the abundance of the ubiquitin substrate. Thus, using a chemical biology approach, we identified a novel mechanism of action of glucocorticoids and a novel mechanism by which levels of c-maf protein are regulated by the abundance of the ubiquitin substrate.

Cyclin D2 and cyclin D3 play opposite roles in mouse skin carcinogenesis

D-type cyclins are components of the cell-cycle engine that link cell signaling pathways and passage throughout G1 phase. We previously described the effects of overexpression cyclin D1, D2 or D3 in mouse epidermis and tumor development. We now asked whether cyclin D2 and/or cyclin D3 play a relevant role in ras-dependent tumorigenesis. Here, we described the effect of cyclin D3 and cyclin D2 overexpression in mouse skin tumor development. Notably, overexpression of cyclin D3 results in reduced tumor development and malignant progression to squamous cell carcinomas (SCC). Biochemical analysis of keratinocytes shows that overexpression of cyclin D3 results in strong reduction of cyclin D2 and its associated kinase activity. Furthermore, we found that reinstatement of cyclin D2 level in the cyclin D3/cyclin D2 bigenic mice results in a complete reversion of the inhibitory action of cyclin D3. Supporting these results, ablation of cyclin D2 results in reduced tumorigenesis and malignant progression. On the other hand, overexpression of cyclin D2 results in an increased number of papillomas and malignant progression. We conclude that cyclin D3 and cyclin D2 play opposite roles in mouse skin tumor development and that the suppressive activity of cyclin D3 is associated with cyclin D2 downregulation.

Genetic mosaics reveal both cell-autonomous and cell-nonautonomous function of murine p27Kip1

Loss of the cyclin-dependent kinase inhibitor p27(Kip1) leads to an overall increase in animal growth, pituitary tumors, and hyperplasia of hematopoietic organs, yet it is unknown whether all cells function autonomously in response to p27(Kip1) activity or whether certain cells take cues from their neighbors. In addition, there is currently no genetic evidence that tumor suppression by p27(Kip1) is cell-autonomous because biallelic gene inactivation is absent from tumors arising in p27(Kip1) hemizygous mice. We have addressed these questions with tissue-specific targeted mouse mutants and radiation chimeras. Our results indicate that the suppression of pars intermedia pituitary tumors by p27(Kip1) is cell-autonomous and does not contribute to overgrowth or infertility phenotypes. In contrast, suppression of spleen growth and hematopoietic progenitor expansion is a consequence of p27(Kip1) function external to the hematopoietic compartment. Likewise, p27(Kip1) suppresses thymocyte hyperplasia through a cell-nonautonomous mechanism. The interaction of p27(Kip1) loss with epithelial cell-specific cyclin-dependent kinase 4 overexpression identifies the thymic epithelium as a relevant site of p27(Kip1) activity for the regulation of thymus growth.

Cyclin D1 and D3 Expression in Vestibular Schwannomas

OBJECTIVES

The G1 regulators of the cell cycle, cyclin D(1) and D(3), have been implicated in the regulation of Schwann cell proliferation and differentiation. The purpose of this study is to evaluate cyclin D(1) and D(3) protein expression and the corresponding clinical characteristics of vestibular schwannomas.

STUDY DESIGN AND METHODS

Tissue sections of 15 sporadic vestibular schwannomas were prepared. Immunohistochemical analysis of the vestibular schwannomas was performed with anticyclin D(1) and anticyclin D(3) antibodies. The immunoreactivity was evaluated in comparison with adjacent vestibular nerves. Tissue sections of breast carcinoma and prostate carcinoma were used as positive controls for cyclin D(1) and D(3) staining, respectively. Patient demographics, tumor characteristics, and cyclin D expression were reviewed, and statistical analysis was performed.

RESULTS

While the breast carcinoma control expressed abundant cyclin D(1) protein, none of the 15 vestibular schwannomas showed detectable cyclin D(1) staining. In contrast, seven of 15 vestibular schwannomas stained positive for the cyclin D(3) protein. Cyclin D(3) staining was taken up in the nucleus of schwannoma tumor cells in greater proportion than Schwann cells of adjacent vestibular nerve. Although sample size was small, no significant difference in the average age of presentation, tumor size, and male to female ratios for the cyclin D(3)(+) or cyclin D(3)(-) groups was found.

CONCLUSION

The Cyclin D(1) protein does not appear to play a prominent role in promoting cell cycle progression in vestibular schwannomas. In contrast, cyclin D(3) expression was seen in nearly half of the tumors examined, suggesting that it may have a growth-promoting role in some schwannomas. Further studies are needed to define its cellular mechanism.

A novel role for integrin-linked kinase in epithelial sheet morphogenesis

Integrin-linked kinase (ILK) is a multidomain protein involved in cell motility and cell-extracellular matrix interactions. ILK is found in integrin-containing focal adhesions in undifferentiated primary epidermal keratinocytes. Induction of keratinocyte differentiation by treatment with Ca(2+) triggers formation of cell-cell junctions, loss of focal adhesions, and ILK distribution to cell borders. We now show that Ca(2+) treatment of keratinocytes induces rapid (<or=1 h) translocation to the cell membrane of the adherens junction (AJ) proteins E-cadherin and beta-catenin. This is followed by slower (>6 h) localization of tight junction (TJ) proteins. The kinetics of ILK movement toward the cell periphery mimics that of AJ components, suggesting that ILK plays a role in the early formation of cell-cell contacts. Whereas the N terminus in ILK mediates localization to cell borders, expression of an ILK deletion mutant incapable of localizing to the cell membrane (ILK 191-452) interferes with translocation of E-cadherin/beta-catenin to cell borders, precluding Ca(2+)-induced AJ formation. Cells expressing ILK 191-452 also fail to form TJ and sealed cell-cell borders and do not form epithelial sheets. Thus, we have uncovered a novel role for ILK in epithelial cell-cell adhesion, independent of its well-established role in integrin-mediated adhesion and migration.

Signalling in the epidermis: the E2F cell cycle regulatory pathway in epidermal morphogenesis, regeneration and transformation

The epidermis is the outermost layer in the skin, and it is the first line of defence against the environment. The epidermis also provides a barrier against loss of fluids and electrolytes, which is crucial for life. Essential in the maintenance of this tissue is its ability to continually self-renew and regenerate after injury. These two characteristics are critically dependent on the ability of the principal epidermal cell type, the keratinocyte, to proliferate and to respond to differentiation cues. Indeed, the epidermis is a multilayered tissue composed of keratinocyte stem cells and their differentiated progeny. Central for the control of cell proliferation is the E2F transcription factor regulatory network. This signaling network also includes cyclins, cdk, cdk inhibitors and the retinoblastoma (pRb) family of proteins. The biological importance of the E2F/pRb pathway is emphasized by the fact that a majority of human tumours exhibit alterations that disrupt the ability of pRb proteins to inhibit E2F, leading to permanent activation of the latter. Further, E2F is essential for normal epidermal regeneration after injury. Other member of the E2F signaling pathway are also involved in epidermal development and pathophysiology. Thus, whereas the pRb family of proteins is essential for epidermal morphogenesis, abnormal regulation of cyclins and E2F proteins results in tumorgenesis in this tissue. In this review, we discuss the role of each member of this important growth regulatory network in epidermal formation, homeostasis and carcinogenesis.

Targeted expression of cyclin D2 results in cardiomyocyte DNA synthesis and infarct regression in transgenic mice

Restriction point transit and commitment to a new round of cell division is regulated by the activity of cyclin-dependent kinase 4 and its obligate activating partners, the D-type cyclins. In this study, we examined the ability of D-type cyclins to promote cardiomyocyte cell cycle activity. Adult transgenic mice expressing cyclin D1, D2, or D3 under the regulation of the alpha cardiac myosin heavy chain promoter exhibited high rates of cardiomyocyte DNA synthesis under baseline conditions. Cardiac injury in mice expressing cyclin D1 or D3 resulted in cytoplasmic cyclin D accumulation, with a concomitant reduction in the level of cardiomyocyte DNA synthesis. In contrast, cardiac injury in mice expressing cyclin D2 did not alter subcellular cyclin localization. Consequently, cardiomyocyte cell cycle activity persisted in injured hearts expressing cyclin D2, ultimately resulting in infarct regression. These data suggested that modulation of D-type cyclins could be exploited to promote regenerative growth in injured hearts.

G1/S phase cyclin-dependent kinase overexpression perturbs early development and delays tissue-specific differentiation in Xenopus

Cell division and differentiation are largely incompatible but the molecular links between the two processes are poorly understood. Here, we overexpress G1/S phase cyclins and cyclin-dependent kinases in Xenopus embryos to determine their effect on early development and differentiation. Overexpression of cyclin E prior to the midblastula transition (MBT), with or without cdk2, results in a loss of nuclear DNA and subsequent apoptosis at early gastrula stages. By contrast, overexpressed cyclin A2 protein does not affect early development and, when stabilised by binding to cdk2, persists to tailbud stages. Overexpression of cyclin A2/cdk2 in post-MBT embryos results in increased proliferation specifically in the epidermis with concomitant disruption of skin architecture and delay in differentiation. Moreover, ectopic cyclin A2/cdk2 also inhibits differentiation of primary neurons but does not affect muscle. Thus, overexpression of a single G1/S phase cyclin/cdk pair disrupts the balance between division and differentiation in the early vertebrate embryo in a tissue-specific manner.

Driving the cell cycle to cancer

Cell cycle progression requires the co-ordinated activation of several kinases, some of which are activated upon the binding of a cyclin subunit. At least four of these so-called cyclin-dependent kinases, namely Cdk4, Cdk6, Cdk2 and Cdk1, have specific roles at particular stages of the cell cycle, including passage through the various cell cycle transitions and the response to specific checkpoints. Not surprisingly, most human tumors carry mutations that deregulate at least one of these kinases. To analyze their specific role in vivo, we are generating strains of gene-targeted mice carrying either activated or defective alleles of these Cdks. As an example, Cdk4 expression appears to be expendable in most cell types since mice lacking Cdk4 are viable. Yet, Cdk4 mutant mice are smaller in size and infertile (only partial infertility in males). In addition, Cdk4 defective mice develop insulin dependent diabetes early in life. However, the importance of these Cdks in tumor cell cycles is underscored by the phenotype of knock in mice where the normal Cdk4 gene has been replaced by a Cdk4 R24C (insensitive to INK inhibitors) mutant. These animals develop a wide spectrum of spontaneous tumors and are highly susceptible to specific carcinogenic treatments. These models are being used now to understand how deregulation of these Cdks leads to cancer development and will be a valuable tool to design and validate new therapeutic strategies against tumour development.

High incidence of thymic epithelial tumors in E2F2 transgenic mice

In virtually all human tumors, genetic and epigenetic alterations have been found which affect the INK4/-CYCLIN D/RB pathway, which regulates cell cycle entry and exit in normal cells. E2F transcription factors are important downstream components of this pathway, which act by controlling the expression of genes involved in DNA replication and cell cycle progression. To determine whether E2F2 deregulation promotes proliferation and tumorigenesis in vivo, we generated E2F2 transgenic mice, in which the Emu and murine pim1 promoter (pp) direct high expression of E2F2 in thymic epithelial cells. Emu-pp-E2F2 mice start to develop cytokeratin- and ER-TR4-positive cortical thymomas from the age of 20 weeks, and within 1 year, nearly all mice succumb to gross thymic epithelial tumors. General thymic morphology is largely maintained, but T cell development is perturbed in thymomas, with proportionately less CD4(+)CD8(+) double-positive thymocytes. In the first 3 months, E2F2 transgenic thymi exhibit only mild epithelial hyperplasia, and thereafter thymomas arise stochastically, probably following additional mutations. Interestingly, Emu-pp-E2F1 mice do not display cortical thymomas. These data argue that E2F2 promotes unscheduled cell division and oncogenic transformation of thymic epithelial cells.

Differential expression of cyclin D1 in the human hair follicle

The proliferation of keratinocytes in the hair follicle varies from slowly cycling, intermittently proliferating stem cells in the bulge to rapidly proliferating, transient cells in the bulb. To better understand the biological differences between these two compartments, we sought to identify differentially expressed genes using cDNA macroarray analysis. Cyclin D1 was one of 13 genes increased in the bulge compared to the bulb, and its differential expression was corroborated by quantitative real-time polymerase chain reaction (PCR) on the original samples. Using immunohistochemical staining, laser-capture microdissection (LCM) and quantitative real-time PCR, we localized cyclin D1 to the suprabasal cells of the telogen bulge and anagen outer root sheath (ORS). Surprisingly, cyclin D1, D2, and D3 were not detectable by immunohistochemistry in the rapidly proliferating hair-producing cells of the anagen bulb (matrix cells), while these cells were strongly positive for Ki-67 and retinoblastoma protein. In contrast, pilomatricoma, a tumor thought to be derived from matrix cells, was positive for cyclin D1, D2, and D3. Our results suggest that cyclin D1 may mediate the proliferation of stem cells in the bulge to more differentiated transient amplifying cells in the suprabasal ORS. In contrast, non-cyclin D1-proteins appear to control cell division of the highly proliferative bulb matrix cells. This non-cyclin D1-mediated proliferation may provide a protective mechanism against tumorigenesis, which is overridden in pilomatricomas. Our data also demonstrate that the combination of DNA macroarray, LCM and quantitative real-time PCR is a powerful approach for the study of gene expression in defined cell populations with limited starting material.

Laser capture microdissection-based in vivo genomic profiling of wound keratinocytes identifies similarities and differences to squamous cell carcinoma

Keratinocytes undergo a dramatic phenotypic conversion during reepithelialization of skin wounds to become hyperproliferative, migratory, and invasive. This transient healing response phenotypically resembles malignant transformation of keratinocytes during squamous cell carcinoma progression. Here we present the first analysis of global changes in keratinocyte gene expression during skin wound healing in vivo, and compare these changes to changes in gene expression during malignant conversion of keratinized epithelium. Laser capture microdissection was used to isolate RNA from wound keratinocytes from incisional mouse skin wounds and adjacent normal skin keratinocytes. Changes in gene expression were determined by comparative cDNA array analyses, and the approach was validated by in situ hybridization. The analyses identified 48 candidate genes not previously associated with wound reepithelialization. Furthermore, the analyses revealed that the phenotypic resemblance of wound keratinocytes to squamous cell carcinoma is mimicked at the level of gene expression, but notable differences between the two tissue-remodeling processes were also observed. The combination of laser capture microdissection and cDNA array analysis provides a powerful new tool to unravel the complex changes in gene expression that underlie physiological and pathological remodeling of keratinized epithelium.

Impaired hair follicle morphogenesis and cycling with abnormal epidermal differentiation in nackt mice, a cathepsin L-deficient mutation

We previously described an autosomal-recessive mutation named nackt (nkt) exhibiting partial alopecia associated with CD4(+) T-cell deficiency. Also, we recently reported that nkt (now Ctsl(nkt)) comprises a deletion in the cathepsin L (Ctsl) gene. Another recent study reported that Ctsl knockout mice have CD4(+) T-cell deficiency and periodic shedding of hair, which recapitulate the nkt mutation and the old furless (fs) mutation. The current study focuses on the dermatological aspects of the nkt mutation. Careful histological analysis of skin development of homozygous nkt mice revealed a delayed hair follicle morphogenesis and late onset of the first catagen stage. The skin of Ctsl(nkt)/Ctsl(nkt) mice showed mild epidermal hyperplasia and hyperkeratosis, severe hyperplasia of the sebaceous glands, and structural alterations of hair follicles. Epidermal differentiation seems to be affected in nkt skin, with overexpression of involucrin and profilaggrin/filaggrin along with focal areas of keratin 6 expression in the interfollicular epidermis. Severe epidermal hyperplasia, acanthosis, orthokeratosis, and hyperkeratosis were only observed in mice maintained in nonpathogen-free environments. The analysis of Rag2-/- Ctsl(nkt)/Ctsl(nkt) double-mutant mice indicates that the skin defect remains under the absence of T and B cells. This animal model provides in vivo evidence that cysteine protease cathepsin L plays a critical role in hair follicle morphogenesis and cycling, as well as epidermal differentiation.

Differential regulation of cyclins D1 and D3 in hepatocyte proliferation

Substantial evidence suggests that cyclin D1 plays a pivotal role in the control of the hepatocyte cell cycle in response to mitogenic stimuli, whereas the closely related protein cyclin D3 has not been extensively evaluated. In the current study, we examined the regulation of cyclins D1 and D3 during hepatocyte proliferation in vivo after 70% partial hepatectomy (PH) and in culture. In contrast to cyclin D1, which was nearly undetectable in quiescent liver and substantially up-regulated after PH, cyclin D3 was constitutively expressed and induced only modestly. In the regenerating liver, the concentration of cyclin D3 was only about 10% of that of cyclin D1. Cyclin D1 formed complexes primarily with cyclin-dependent kinase 4 (cdk4), which were markedly activated in the regenerating liver and readily sequestered the cell cycle inhibitory proteins, p21 and p27. Cyclin D3 bound to both cdk4 and cdk6. Cyclin D3/cdk6 activity was readily detectable in quiescent liver and changed little after PH, and this complex appeared to play a minor role in sequestering p21 and p27. In cultured hepatocytes, epidermal growth factor or insulin had little effect, but the combination of these agents substantially induced cyclin D1 and cell cycle progression. Inhibition of Mek1 or phosphoinositide 3-kinase markedly inhibited cyclin D1 expression and replication. In contrast, cyclin D3 was expressed in the absence of mitogens and was only modestly affected by these manipulations. In addition, growth-inhibitory extracellular matrix conditions inhibited cyclin D1 but not cyclin D3 expression. In conclusion, these results support the concept that cyclin D1 is critically regulated by extracellular stimuli that control proliferation, whereas cyclin D3 is regulated through different pathways and plays a distinct role in the liver.

Hyperproliferation, induction of c-Myc and 14-3-3σ, but no cell fragility in keratin-10-null mice

In the past, keratins have been established as structural proteins. Indeed,mutations in keratin 10 (K10) and other epidermal keratins lead to severe skin fragility syndromes. Here, we present adult K10-/- mice, which reveal a novel connection between the regulation of cell proliferation and K10. Unlike most keratin mutant mice, the epidermis of adult K10-/-mice showed no cytolysis but displayed hyperproliferation of basal keratinocytes and an increased cell size. BrdU labelling revealed a shortened transition time for keratinocytes migrating outwards and DAPI staining of epidermal sheets uncovered an impaired organization of epidermal proliferation units. These remarkable changes were accompanied by the induction of c-Myc,cyclin D1, 14-3-3σ and of wound healing keratins K6 and K16. The phosphorylation of Rb remained unaltered. In line with the downregulation of K10 in squamous cell carcinomas and its absence in proliferating cells in vivo, our data suggest that the tissue-restricted expression of some members of the keratin gene family not only serves structural functions. Our results imply that the altered composition of the suprabasal cytoskeleton is able to alter the proliferation state of basal cells through the induction of c-Myc. A previous model based on transfection of K10 in immortalized human keratinocytes suggested a direct involvement of K10 in cell cycle control. While those experiments were performed in human cultured keratinocytes, our data establish, that in vivo, K10 acts by an indirect control mechanism in trans.

Activation of cyclin D1 and D2 promoters by human T-cell leukemia virus type I tax protein is associated with IL-2-independent growth of T cells

Our aim was to examine the involvement of G(1) cell-cycle regulators in cell growth dysregulation induced by HTLV-I. Compared to uninfected cells, higher expression levels of cyclin D1 and D2 mRNA were detected in HTLV-I-infected T-cell lines, which were at least in part mediated by the viral transforming protein Tax since Tax activated both cyclin D1 and D2 promoters in the human T-cell line Jurkat. A Tax mutant that did not activate NF-kappaB failed to activate cyclin D1 and D2 promoters. Inhibitors of NF-kappaB (dominant negative IkappaBs mutants) suppressed Tax-dependent activation of cyclin D1 and D2 promoters, indicating that Tax-induced activation was mediated by NF-kappaB. Wild-type and mutant Tax capable of activating NF-kappaB, but not Tax mutant incapable of activating NF-kappaB, converted cell growth of a T-cell line from being IL-2-dependent to being IL-2-independent; and this conversion was associated with IL-2-independent induction of cyclins D1 and D2. Our data suggest that induction of cyclins D1 and D2 by Tax is involved in IL-2-independent cell-cycle progression as well as IL-2-independent transformation of primary human T cells by HTLV-I. High expression levels of cyclin D1 and D2 mRNAs were also detected in some patients with ATL. Our findings link HTLV-I infection to changes in cellular D-type cyclin gene expression, transformation of T cells and subsequent development of T-cell leukemia.

Stage-specific alterations of cyclin expression during UVB-induced murine skin tumor development

We have evaluated the in vivo correlation between the expression of cell cycle markers and skin tumor development in SKH-1 hairless mice in a complete photocarcinogenesis protocol. Irradiated mice developed an average of 16 tumors per animal by week 23 with the average number of carcinomas per mouse being 2.1. The expression of p53 and cyclins A and D1 was confined initially to sporadic single cells and gradually developed into foci of patchy intense staining in the basal and granular layers of UVB-exposed epidermis. p53 was expressed in all the papilloma sections examined, whereas cyclins D1 and A were expressed in 68 and 71% of these lesions, respectively. In UVB-induced squamous cell carcinomas (SCC), p53 was expressed in >90% of the tumors, whereas cyclin D1 was detected in 55% of the lesions, and cyclin A staining was limited to 27%. These immunohistochemical observations were confirmed by Western blotting and protein kinase assays. We observed an early wave of cyclin A overexpression and cyclin A protein kinase activity preceding the appearance of detectable tumors. Cyclin D1 and p53 overexpression were coupled with the development of tumors, and these changes are likely to be relevant to the pathogenesis of these lesions.

Thymic hyperplasia and lung carcinomas in a line of mice transgenic for keratin 5-driven HPV16 E6/E7 oncogenes

Human Papillomavirus type 16 (HPV-16) is the cause of both benign lesions and ano-genital cancers. In HPV-associated cancers the transforming properties of the expressed viral E6 and E7 proteins have been revealed by a number of different assays. We have generated transgenic mice expressing HPV-16 E6/E7 genes under the control of the murine keratin 5 gene promoter, which should confer cell-type specific expression in the basal cells of squamous stratified epithelia. Transgenic mice developed thymic hyperplasia and lung neoplasia with 100% frequency, the thymus showing a size increase at 2 months and reaching the maximum dimension at 6 months, when lung carcinomas appeared. After this time the size of hyperplastic thymi decreased, while malignant formations invaded the mediastinal area. Hepatic metastasis could be also observed in some of the animals at the autopsy and death invariably occurred around 10-11 months of age.

Basal cell proliferation in female SKH-1 mice treated with alpha- and beta-hydroxy acids

Alpha- and beta-hydroxy acids are compounds that have been used extensively in cosmetic and dermatological formulations. Clinical and qualitative effects of alpha- and beta-hydroxy acids have been well characterized, but little is known about their mechanism of action or acute and chronic biochemical effects. In the present study, we examined the acute proliferative effects of glycolic and salicylic acids on cell proliferation in the epidermis of SKH-1 female mice, using BrdU incorporation as a marker of epidermal proliferation. In preliminary experiments, we observed an increase in the rate of proliferation after 3 days of treatment with 10% glycolic acid-containing cream and this was sustained throughout a 6.5-week (treatment 5 days/week) time course compared with untreated control animals. After each treatment with cream containing glycolic acid there was a wave of proliferation that was maximal 12 to 16 h (significant at p < 0.05) after treatment, followed by a subsequent increase in epidermal thickness at 18 to 20 h (significant at p < 0.05). The effects of the concentration and pH level of glycolic acid- and salicylic acid-containing creams on the rate of proliferation and increases in skin thickness in SKH-1 epidermis were also investigated. We observed a dose-dependent increase in epidermal proliferation of animals treated with either glycolic or salicylic acid. A similar time-dependent response was observed in the epidermal thickness in animals treated with salicylic acid, but not with glycolic acid. Differences in pH (3.5 or 4.0) had no significant effect on either epidermal proliferation or skin thickness. The data that we present here should be useful in characterizing not only the beneficial but also the adverse effects that occur following acute or chronic usage of alpha-hydroxy acids.

Transgenic expression of cyclin-dependent kinase 4 results in epidermal hyperplasia, hypertrophy, and severe dermal fibrosis

In a previous report we have described the effects of expression of D-type cyclins in epithelial tissues of transgenic mice. To study the involvement of the D-type cyclin partner cyclin-dependent kinase 4 (CDK4) in epithelial growth and differentiation, transgenic mice were generated carrying the CDK4 gene under the control of a keratin 5 promoter. As expected, transgenic mice showed expression of CDK4 in the epidermal basal-cell layer. Epidermal proliferation increased dramatically and basal cell hyperplasia and hypertrophy were observed. The hyperproliferative phenotype of these transgenic mice was independent of D-type cyclin expression because no overexpression of these proteins was detected. CDK4 and CDK2 kinase activities increased in transgenic animals and were associated with elevated binding of p27(Kip1) to CDK4. Expression of CDK4 in the epidermis results in an increased spinous layer compared with normal epidermis, and a mild hyperkeratosis in the cornified layer. In addition to epidermal changes, severe dermal fibrosis was observed and part of the subcutaneous adipose tissue was replaced by connective tissue. Also, abnormal expression of keratin 6 associated with the hyperproliferative phenotype was observed in transgenic epidermis. This model provides in vivo evidence for the role of CDK4 as a mediator of proliferation in epithelial cells independent of D-type cyclin expression.