Consequences of depleted SERCA2-gated calcium stores in the skin

Sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2 (SERCA2) pumps belong to the family of Ca2+-ATPases responsible for the maintenance of calcium in the endoplasmic reticulum. In epidermal keratinocytes, SERCA2-controlled calcium stores are involved in cell cycle exit and onset of terminal differentiation. Hence, their dysfunction was thought to provoke impaired keratinocyte cohesion and hampered terminal differentiation. Here, we assessed cultured keratinocytes and skin biopsies from a canine family with an inherited skin blistering disorder. Cells from lesional and phenotypically normal areas of one of these dogs revealed affected calcium homeostasis due to depleted SERCA2-gated stores. In phenotypically normal patient cells, this defect compromised upregulation of p21(WAF1) and delayed the exit from the cell cycle. Despite this abnormality it failed to impede the terminal differentiation process in the long term but instead coincided with enhanced apoptosis and appearance of chronic wounds, suggestive of secondary mutations. Collectively, these findings provide the first survey on phenotypic consequences of depleted SERCA-gated stores for epidermal homeostasis that explain how depleted SERCA2 calcium stores provoke focal lesions rather than generalized dermatoses, a phenotype highly reminiscent of the human genodermatosis Darier disease.

Pemphigus vulgaris identifies plakoglobin as key suppressor of c-Myc in the skin

The autoimmune disease pemphigus vulgaris (PV) manifests as loss of keratinocyte cohesion triggered by autoantibody binding to desmoglein (Dsg)3, an intercellular adhesion molecule of mucous membranes, epidermis, and epidermal stem cells. Here we describe a so far unknown signaling cascade activated by PV antibodies. It extends from a transient enhanced turn over of cell surface-exposed, nonkeratin-anchored Dsg3 and associated plakoglobin (PG), through to depletion of nuclear PG, and as one of the consequences, abrogation of PG-mediated c-Myc suppression. In PV patients (6/6), this results in pathogenic c-Myc overexpression in all targeted tissues, including the stem cell compartments. In summary, these results show that PV antibodies act via PG to abolish the c-Myc suppression required for both maintenance of epidermal stem cells in their niche and controlled differentiation along the epidermal lineage. Besides a completely novel insight into PV pathogenesis, these data identify PG as a potent modulator of epithelial homeostasis via its role as a key suppressor of c-Myc.

Epidermal caspase-3 cleavage associated with interferon-gamma-expressing lymphocytes in acute atopic dermatitis lesions

Keratinocyte apoptosis mediated by Fas/Fas ligand molecular interactions and subsequent caspase activation is believed to play an important role in the pathogenesis of atopic dermatitis (AD), in particular for the formation of spongiosis. To estimate epidermal caspase activation in normal and AD skin under in vivo conditions, we analysed caspase-3 cleavage by immunohistology. In normal skin as well as non-lesional AD skin, we detected caspase-3 cleavage in single cells of the basal layer. In contrast, in acute lesional AD skin, we not only obtained evidence for increased expression of cleaved caspase-3 in keratinocytes of the basal layer but also observed caspase-3 cleavage in one or more layers of the spinous cell layer, in particular in spongiotic areas. Short-term topical treatment of the skin lesions with tacrolimus or pimecrolimus abolished the expression of cleaved caspase-3 in the spinous layer. Moreover, epidermal caspase-3 cleavage correlated with the numbers of dermal interferon-gamma (IFN-gamma)-expressing CD4+ and CD8+ lymphocytes in skin lesions of AD patients, supporting the view that IFN-gamma is important for the activation of proapoptotic pathways in keratinocytes. This is also confirmed by the observation of increased Fas expression on keratinocytes in acute AD lesions that was markedly reduced following topical calcineurin inhibitor treatment. These data suggest that caspase-3 cleavage in the spinous layer of the epidermis is a pathologic event contributing to spongiosis formation in AD, whereas cleavage of caspase-3 in basal cells might represent a physiologic mechanism within the process of epidermal renewal.

Outside-in signaling through integrins and cadherins: a central mechanism to control epidermal growth and differentiation?

The process of epidermal renewal persists throughout the entire life of an organism. It begins when a keratinocyte progenitor leaves the stem cell compartment, undergoes a limited number of mitotic divisions, exits the cell cycle, and commits to terminal differentiation. At the end of this phase, the postmitotic keratinocytes detach from the basement membrane to build up the overlaying stratified epithelium. Although highly coordinated, this sequence of events is endowed with a remarkable versatility, which enables the quiescent keratinocyte to reintegrate into the cell cycle and become migratory when necessary, for example after wounding. It is this versatility that represents the Achilles heel of epithelial cells allowing for the development of severe pathologies. Over the past decade, compelling evidence has been provided that epithelial cancer cells achieve uncontrolled proliferation following hijacking of a "survival program" with PI3K/Akt and a "proliferation program" with growth factor receptor signaling at its core. Recent insights into adhesion receptor signaling now propose that integrins, but also cadherins, can centrally control these programs. It is suggested that the two types of adhesion receptors act as sensors to transmit extracellular stimuli in an outside-in mode, to inversely modulate epidermal growth factor receptor signaling and ensure cell survival. Hence, cell-matrix and cell-cell adhesion receptors likely play a more powerful and wide-ranging role than initially anticipated. This Perspective article discusses the relevance of this emerging field for epidermal growth and differentiation, which can be of importance for severe pathologies such as tumorigenesis and invasive metastasis, as well as psoriasis and Pemphigus vulgaris.

Canine keratinocytes upregulate type I interferons and proinflammatory cytokines in response to poly(dA:dT) but not to canine papillomavirus.

Papillomaviruses (PV) are double stranded (ds) DNA viruses that infect epithelial cells within the skin or mucosa, most often causing benign neoplasms that spontaneously regress. The immune system plays a key role in the defense against PVs. Since these viruses infect keratinocytes, we wanted to investigate the role of the keratinocyte in initiating an immune response to canine papillomavirus-2 (CPV-2) in the dog. Keratinocytes express a variety of pattern recognition receptors (PRR) to distinguish different cutaneous pathogens and initiate an immune response. We examined the mRNA expression patterns for several recently described cytosolic nucleic acid sensing PRRs in canine monolayer keratinocyte cultures using quantitative reverse transcription-polymerase chain reaction. Unstimulated normal cells were found to express mRNA for melanoma differentiation associated gene 5 (MDA5), retinoic acid-inducible gene I (RIG-I), DNA-dependent activation of interferon regulatory factors, leucine rich repeat flightless interacting protein 1, and interferon inducible gene 16 (IFI16), as well as their adaptor molecules myeloid differentiation primary response gene 88, interferon-β promoter stimulator 1, and endoplasmic reticulum-resident transmembrane protein stimulator of interferon genes. When stimulated with synthetic dsDNA [poly(dA:dT)] or dsRNA [poly(I:C)], keratinocytes responded with increased mRNA expression levels for interleukin-6, tumor necrosis factor-α, interferon-β, RIG-I, IFI16, and MDA5. There was no detectable increase in mRNA expression, however, in keratinocytes infected with CPV-2. Furthermore, CPV-2-infected keratinocytes stimulated with poly(dA:dT) and poly(I:C) showed similar mRNA expression levels for these gene products when compared with expression levels in uninfected cells. These results suggest that although canine keratinocytes contain functional PRRs that can recognize and respond to dsDNA and dsRNA ligands, they do not appear to recognize or initiate a similar response to CPV-2.

T-cadherin loss promotes experimental metastasis of squamous cell carcinoma

T-cadherin is gaining recognition as a determinant for the development of incipient invasive squamous cell carcinoma (SCC). However, effects of T-cadherin expression on the metastatic potential of SCC have not been studied. Here, using a murine model of experimental metastasis following tail vein injection of A431 SCC cells we report that loss of T-cadherin increased both the incidence and rate of appearance of lung metastases. T-cadherin-silenced SCC metastases were highly disordered with evidence of single cell dissemination away from main foci whereas SCC metastases overexpressing T-cadherin developed as compact, tightly organised sheets. SCC cell adhesion to vascular endothelial cells (EC) in culture was increased for T-cadherin-silenced SCC and decreased for T-cadherin-overexpressing SCC. Confocal microscopy showed that T-cadherin-silenced SCC adherent on EC display an elongated morphology with long thin extensions and a high degree of intercalation within the EC monolayer, whereas SCC overexpressing T-cadherin formed poorly-spread multicellular aggregates that remain on the outer surface of the EC monolayer. T-cadherin-deficient SCC or human keratinocyte cells exhibited increased transendothelial migration in vitro which could be attenuated in the presence of EGFR inhibitor gefitinib. Our data suggest that loss of T-cadherin can increase metastatic potential and aggressiveness of SCC, possibly due to facilitating arrest and extravasation through the vascular wall and/or more efficient establishment of metastases in the new microenvironment.

CD8+ T cells and NK cells from blister fluid of an EEM/SJS overlap highly express Fas ligand and Granulysin

INTRODUCTION Erythema exsudativum multiforme majus (EEMM) and Stevens-Johnson Syndrome (SJS) are severe cutaneous reaction patterns caused by infections or drug hypersensitivity. The mechanism by which widespread keratinocyte death is mediated by the immune system in EEMM/SJS are still to be elucidated. Here, we characterized the blister cells isolated from a patient with EEMM/SJS overlap and investigated its cause. METHODS Clinical classification of the cutaneous eruption was done according to the consensus definition of severe blistering skin reactions and histological analysis. Common infectious causes of EEMM were investigated using standard clinical techniques. T cell reactivity for potentially causative drugs was assessed by lymphocyte transformation tests (LTT). Lymphocytes isolated from blister fluid were analyzed for their expression of activation markers and cytotoxic molecules using flow cytometry. RESULTS The healthy 58 year-old woman suffered from mild respiratory tract infection and therefore started treatment with the secretolytic drug Ambroxol. One week later, she presented with large palmar and plantar blisters, painful mucosal erosions, and flat atypical target lesions and maculae on the trunc, thus showing the clinical picture of an EEMM/SJS overlap (Fig. 1). This diagnosis was supported by histology, where also eosinophils were found to infiltrate the upper dermis, thus pointing towards a cutaneous adverse drug reaction (cADR). Analysis of blister cells showed that they mainly consisted of CD8+ and CD4+ T cells and a smaller population of NK cells. Both the CD8+ T cells and the NK cells were highly activated and expressed Fas ligand and the cytotoxic molecule granulysin (Fig. 2). In addition, in comparison to NK cells from PBMC, NK cells in blister fluids strongly upregulated the expression of the skin-homing chemokine receptor CCR4 (Fig 4). Surprisingly, the LTT performed on PBMCs in the acute phase was positive for Ambroxol (SI=2.9) whereas a LTT from a healthy but exposed individual did not show unspecific proliferation. Laboratory tests for common infectious causes of EEMM were negative (HSV-1/-2, M. pneumoniae, Parvovirus B19). However, 6 weeks later, specific proliferation to Ambroxol could no longer be observed in the LTT (Fig 4.).

CELLULAR AND DEVELOPMENTAL FUNCTIONS OF THE XENOPUS ARVCF-CATENIN:KAZRIN COMPLEX

Xenopus ARVCF (xARVCF), a member of p120-catenin subfamily, binds cadherin cytoplasmic domains to enhance cadherin metabolic stability, or when dissociated, modulates Rho-family GTPases. We previously found that xARVCF binds directly to Xenopus KazrinA (xKazrinA), a widely expressed, conserved protein that bears little homology to established protein families. xKazrinA is also known to influence keratinocyte proliferation-differentiation and cytoskeletal activity. In my study, I first evaluated the expression pattern of endogenous Kazrin RNA and protein in Xenopus embryogenesis as well as in adult tissues. We then collaboratively predicted the helical structure of Kazrin’s coiled-coil domain, and I obtained evidence of Kazrin’s dimerization/oligomerization. In considering the intracellular localization of the xARVCF-catenin:xKazrin complex, I did not resolve xKazrinA in a larger ternary complex with cadherin, nor did I detect its co-precipitation with core desmosomal components. Instead, screening revealed that xKazrinA binds spectrin. This suggested a potential means by which xKazrinA localizes to cell-cell junctions, and indeed, biochemical assays confirmed a ternary xARVCF:xKazrinA:xβ2-spectrin complex. Functionally, I demonstrated that xKazrin stabilizes cadherins by negatively modulating the RhoA small-GTPase. I further revealed that xKazrinA binds to p190B RhoGAP (an inhibitor of RhoA), and enhances p190B’s association with xARVCF. Supporting their functional interaction in vivo, Xenopus embryos depleted of xKazrin exhibited ectodermal shedding, a phenotype that could be rescued with exogenous xARVCF. Cell shedding appeared to be caused by RhoA activation, which consequently altered actin organization and cadherin function. Indeed, I was capable of rescuing Kazrin depletion with ectopic expression of p190B RhoGAP. In addition, I obtained evidence that xARVCF and xKazrin participate in craniofacial development, with effects observed upon the neural crest. Finally, I found that xKazrinA associates further with delta-catenin and p0071-catenin, but not with p120-catenin, suggesting that Kazrin interacts selectively with additional members of the p120-catenin sub-family. Taken together, my study supports Kazrin’s essential role in development, and reveals KazrinA’s biochemical and functional association with ARVCF-catenin, spectrin and p190B RhoGAP.

Mechanisms of UV induced systemic immunosuppression: An essential role for interleukin-10

The recognition of the skin as an immunocompetent organ has focused attention on the complex interaction between ultraviolet radiation and the immune system. How UV-radiation, which hardly penetrates past the epidermis, induces systemic immune suppression is not entirely clear. We propose that suppressive cytokines, released by UV-irradiated keratinocytes, play a role in the induction of immune suppression. Injecting supernatants from UV-exposed murine keratinocytes into mice impairs their ability to mount a delayed-type hypersensitivity response against allogeneic histocompatibility antigens. We tested the hypothesis that the down regulation of the immune response by UV is precipitated by the release of IL-10 after keratinocytes are UV-irradiated. After UV exposure IL-10 mRNA was upregulated. Western analysis indicated immunoreactive IL-10 was secreted by UV-exposed keratinocytes. The addition of supernatants from UV-irradiated keratinocytes to Th1 clones diminished their IFN production, whereas the addition of supernatants from normal keratinocytes had no suppressive effect on IFN production. Furthermore, treating supernatants from UV-irradiated keratinocytes with anti-IL-10 antibodies blocked the induction of immune suppression. To determine if IL-10 was responsible for the immunosuppression seen after total-body UV irradiation, UV-exposed mice were treated with anti-IL-10 antibodies. Treating UV-irradiated mice with anti-IL-10 reversed the induction of immune suppression. These findings suggest that keratinocyte-derived IL-10 was mediating UV-induced suppression in vivo. We also tested the hypothesis that UV-induced suppressor cells are Th2 cells. Mice were injected with spleen cells from either normal or UV-exposed donor mice immunized with alloantigen. At the time of spleen cell infusion, the recipient mice were then resensitized. Spleen cells from UV-exposed mice suppressed DTH. Mice treated identically and injected with anti-IL-10 antibodies were able to generate a DTH response. Taken together these data suggest that the suppressor cells that are induced by UV radiation are Th2 cells which mediate their suppressive effect by release of IL-10. ^

Bone marrow-derived mesenchymal stromal cells improve vascular regeneration and reduce leukocyte-endothelium activation in critical ischemic murine skin in a dose-dependent manner

BACKGROUND AIMS Stem cells participate in vascular regeneration following critical ischemia. However, their angiogenic and remodeling properties, as well as their role in ischemia-related endothelial leukocyte activation, need to be further elucidated. Herein, we investigated the effect of bone marrow-derived mesenchymal stromal cells (BM-MSCs) in a critically ischemic murine skin flap model. METHODS Groups received either 1 × 10(5), 5 × 10(5), or 1 × 10(6) BM-MSCs or cell-free conditioned medium (CM). Controls received sodium chloride. Intravital fluorescence microscopy was performed for morphological and quantitative assessment of micro-hemodynamic parameters over 12 days. RESULTS Tortuosity and diameter of conduit-arterioles were pronounced in the MSC groups (P < 0.01), whereas vasodilation was shifted to the end arteriolar level in the CM group (P < 0.01). These effects were accompanied by angiopoietin-2 expression. Functional capillary density and red blood cell velocity were enhanced in all treatment groups (P < 0.01). Although a significant reduction of rolling and sticking leukocytes was observed in the MSC groups with a reduction of diameter in postcapillary venules (P < 0.01), animals receiving CM exhibited a leukocyte-endothelium interaction similar to controls. This correlated with leukocyte common antigen expression in tissue sections (P < 0.01) and p38 mitogen-activated protein kinase expression from tissue samples. Cytokine analysis from BM-MSC culture medium revealed a 50% reduction of pro-inflammatory cytokines (interleukin [IL]-1β, IL-6, IL-12, tumor necrosis factor-α, interferon-γ) and chemokines (keratinocyte chemoattractant, granulocyte colony-stimulating factor) under hypoxic conditions. DISCUSSION We demonstrated positive effects of BM-MSCs on vascular regeneration and modulation of endothelial leukocyte adhesion in critical ischemic skin. The improvements after MSC application were dose-dependent and superior to the use of CM alone.

Comparative assessment of a canine-specific medium to support colony formation from canine hair follicular keratinocytes.

BACKGROUND Follicular stem cells and their progeny are responsible for the cyclical renewal of hair follicles and maintenance of the hair coat. The understanding of pathways involved in this process is essential to elucidate the pathogenetic mechanisms of primary alopecia. Stem cells and their direct descendants are located in the bulge region of the isthmus of hair follicles. Although these cells have been studied extensively in mice and humans, data for canine isthmic keratinocyte activation and proliferation are not available. HYPOTHESIS/OBJECTIVES The aim was to establish an accurate and reliable in vitro system to study the growth potential of canine isthmic keratinocytes. We assessed the colony-promoting capability of a commercially available canine-specific medium, CELLnTEC (CnT-09), compared with a well-established home-made medium, complete FAD (cFAD). The CnT-09 medium is specific for the growth of canine keratinocytes, while the cFAD medium can support growth and colony formation of keratinocytes from several species. ANIMALS Skin biopsies were obtained from 15 recently euthanized dogs of various breeds with no skin abnormalities. METHODS The isthmic region of compound hair follicles was isolated by microdissection and cell growth monitored using several parameters with colony-forming assays. RESULTS The CnT-09 and cFAD media provided similar growth as measured by the total number and size of colonies, as well as rate of cell differentiation. CONCLUSIONS The commercial canine-specific CnT-09 medium was comparable to the home-made cFAD medium in supporting the growth and proliferation of canine follicular keratinocytes in vitro. The CnT-09 medium should be a viable alternative growth medium for molecular studies of alopecic disorders in dogs.

Preclinical studies identify non-apoptotic low-level caspase-3 as therapeutic target in pemphigus vulgaris

The majority of pemphigus vulgaris (PV) patients suffer from a live-threatening loss of intercellular adhesion between keratinocytes (acantholysis). The disease is caused by auto-antibodies that bind to desmosomal cadherins desmoglein (Dsg) 3 or Dsg3 and Dsg1 in mucous membranes and skin. A currently unresolved controversy in PV is whether apoptosis is involved in the pathogenic process. The objective of this study was to perform preclinical studies to investigate apoptotic pathway activation in PV pathogenesis with the goal to assess its potential for clinical therapy. For this purpose, we investigated mouse and human skin keratinocyte cultures treated with PV antibodies (the experimental Dsg3 monospecific antibody AK23 or PV patients IgG), PV mouse models (passive transfer of AK23 or PVIgG into adult and neonatal mice) as well as PV patients' biopsies (n=6). A combination of TUNEL assay, analyses of membrane integrity, early apoptotic markers such as cleaved poly-ADP-ribose polymerase (PARP) and the collapse of actin cytoskeleton failed to provide evidence for apoptosis in PV pathogenesis. However, the in vitro and in vivo PV models, allowing to monitor progression of lesion formation, revealed an early, transient and low-level caspase-3 activation. Pharmacological inhibition confirmed the functional implication of caspase-3 in major events in PV such as shedding of Dsg3, keratin retraction, proliferation including c-Myc induction, p38MAPK activation and acantholysis. Together, these data identify low-level caspase-3 activation downstream of disrupted Dsg3 trans- or cis-adhesion as a major event in PV pathogenesis that is non-synonymous with apoptosis and represents, unlike apoptotic components, a promising target for clinical therapy. At a broader level, these results posit that an impairment of adhesive functions in concert with low-level, non-lethal caspase-3 activation can evoke profound cellular changes which may be of relevance for other diseases including cancer.

The role of endogenous IFN-beta in cutaneous melanoma: Consequences of epidermal hyperplasia

The purpose of this study was to characterize epidermal hyperplasia overlying malignant melanoma, to determine the mitogenic factor responsible for the induction of this hyperplasia and to investigate its biological consequence. Whether increased keratinocyte proliferation overlying melanoma is due to production of growth factors by the tumor cells or to other mechanisms is unknown. Epidermal hyperplasia overlying human melanoma was found overlying thick (>4.0mm), but not thin (<1.0mm) tumors. Immunostaining of the sections for growth factors related to angiogenesis revealed that epidermal hyperplasia was associated with loss of IFN-β production by the keratinocytes directly overlying the tumors. Since previous studies from our laboratory have demonstrated that exogenous administration of IFN-β negatively regulates angiogenesis, we hypothesize that tumors are able to produce growth factors which stimulate the proliferation of cells in the surrounding tissues. This hyperplasia leads to a decrease in the endogenous negative regulator of angiogenesis, IFN-β. ^ The human melanoma cell line, DM-4 and several of its clones were studied to identify the mitogenic factor for keratinocytes. The expression of TGF-α directly correlated with epidermal hyperplasia in the DM-4 clones. A375SM, a human melanoma cell line that produces high levels of TGF-α, was transfected with a plasmid encoding full-length antisense TGF-α. The parental and transfected cells were implanted intradermally into nude mice. The extent of epidermal hyperplasia directly correlated with expression of TGF-α and decreased production of IFN-β, hence, increased angiogenesis. ^ In the next set of experiments, we determined the role of IFN-β on angiogenesis, tumor growth and metastasis of skin tumors. Transgenic mice containing a functional mutation in the receptor for IFN α/β were obtained. A375SM melanoma cells were implanted both s.c. and i.v. into IFN α/βR −/− mice. Tumors in the IFN α/β R −/− mice exhibited increased angiogenesis and metastasis. IFN α/βR −/− mice were exposed to chronic UV irradiation. Autochthonous tumors developed earlier in the transgenic mice than the wild-type mice. ^ Collectively, the data show that TGF-α produced by tumor cells induces proliferation of keratinocytes, leading to epidermal hyperplasia overlying malignant melanoma associated with loss of IFN-β and enhanced angiogenesis, tumorigenicity and metastasis. ^

The role of the sonic hedgehog signaling pathway in epidermal homeostasis

Non-melanoma skin cancer is the most frequently diagnosed malignancy in the United States of which basal cell carcinoma (BCC) accounts for 65%. It has recently been determined that deregulation of the sonic hedgehog (shh) pathway leads to the development of BCC. Shh, gli-1, gli-2 gli-3, ptc and smo are overexpressed in BCC and overexpression of these genes in the epidermis results in formation of BCC-like tumors. Despite these observations, the mechanisms by which the pathway controls epidermal homeostasis and the development of the malignant phentotype are unknown. This study assessed the role of the shh pathway in epidermal homeostasis through regulation of apoptosis and differentiation. ^ The anti-apoptotic protein, bcl-2 is overexpressed in BCC, however transcriptional regulators of bcl-2 in the epidermis are unknown. Transient transfection of primary keratinocytes with gli-1 resulted in an increase of bcl-2 expression. Database analysis revealed seven candidate gli binding sites on the bcl-2 promoter. Cotransfection of increasing amounts of gli-1 in keratinoycytes resulted in a corresponding dose-dependent increase in bcl-2 promoter luciferase activity. An N-terminal mutant of gli-3 inhibited gli-1 transactivation of the bcl-2 promoter. The region −428 to −420 was found to be important for gli-1 regulation through gel shift, luciferase assays and site-directed mutagenesis. ^ In order to assess the ability of the shh pathway to regulate keratinocyte differentiation, HaCaT keratinocytes overexpressing sonic hedgehog, were grown in organotypic raft culture. Overexpression of shh induced a basal cell phenotype compared to vector control, as evidenced by transmural staining of cytokeratin 14 and altered Ki67 staining. Shh also induced keratinocyte invasion into the underlying collagen. This was associated with increased phosphorylation of EGFR, jnk and raf and increased expression of c-jun, mmp-9 and Ki67. Interestingly, shh overexpression in HaCaTs did not induce the typical downstream effects of shh signaling, suggesting a gli-independent mechanism. Sonic hedgehog's ability to induce an invasive phenotype was found to be dependent on activation of the EGF pathway as inhibition of EGFR activity with AG1478 and c-225 was able to reduce the invasiveness of HaCaT shh keratinocytes, whereas treatment with EGF augmented the invasiveness of the HaCaT shh clones. ^ These studies reveal the importance of the sonic hedgehog pathway in epidermal homeostasis by regulation of apoptosis through bcl-2, and control of keratinocyte differentiation and invasion through activation of the EGF pathway. They further suggest potential mechanisms by which deregulation of the shh pathway may lead to the development of the malignant phenotype. ^

Role of the cyclin dependent kinase-4 in normal and neoplastic proliferation

In the last few years, our laboratory has studied the regulatory mechanisms of proliferation and differentiation in epidermal tissues. Our results showed differences in the roles of cyclin dependent-kinases 4 and 6, and the three D-type cyclins, during normal epidermal proliferation and neoplastic development. Thus, to elucidate the role of the different cell cycle regulators, we developed transgenic mice that overexpress CDK4 (K5-CDK4), or their cognate D-type cyclins, in epithelial tissues. The most severe phenotype was observed in K5-CDK4 animals that developed dermal fibrosis, epidermal hyperplasia and hypertrophy. Forced expression of CDK4 in the epidermal basal cell layer increased the malignant conversion of skin papillomas to squamous cell carcinomas (SCC). Contrastingly, lack of CDK4 completely inhibited tumor development, suggesting that CDK4 is required in this process. Biochemical studies demonstrated that p21 Cip1 and p27Kip1 inhibitors are sequestered by CDK4 resulting in indirect activation of Cyclin E/CDK2, implicating the non-catalytic activity of CDK4 in deregulation of the cell cycle progression. ^ It has been proposed that the proliferative and oncogenic role of Myc is linked to its ability to induce the transcription of CDK4, cyclin D1, and cyclin D2 in vitro. Deregulation of Myc oncogene has been found in several human cancers. Also it has been demonstrated that CDK4 has the ability to functionally inactivate the product of the tumor suppressor gene Rb, providing a link between Myc and the CDK4/cyclin D1/pRb/p16 pathway in some malignant tumors. Here, we sought to determine the role of CDK4 as a mediator of Myc activities by developing a Myc overexpressing mouse nullizygous for CDK4. We demonstrated that lack of CDK4 results in reduced keratinocyte proliferation and epidermal thickness in K5-Myc/CDK4-null mice. In addition, complete reversion of tumor development was observed. All together, this work demonstrates that CDK4 acts as an oncogene independent of the D-type cyclin levels and it is an important mediator of the tumorigenesis induced by Myc. In addition, we showed that the sequestering activity of CDK4 is critical for the development of epidermal hyperplasia during normal proliferation, malignant progression from papillomas to squamous cell carcinomas, and tumorigenesis induced by Myc. ^