Two hypomorphic alleles of mouse Ass1 as a new animal model of citrullinemia type I and other hyperammonemic syndromes

Citrullinemia type I (CTLN1, OMIM# 215700) is an inherited urea cycle disorder that is caused by an argininosuccinate synthetase (ASS) enzyme deficiency. In this report, we describe two spontaneous hypomorphic alleles of the mouse Ass1 gene that serve as an animal model of CTLN1. These two independent mouse mutant alleles, also described in patients affected with CTLN1, interact to produce a range of phenotypes. While some mutant mice died within the first week after birth, others survived but showed severe retardation during postnatal development as well as alopecia, lethargy, and ataxia. Notable pathological findings were similar to findings in human CTLN1 patients and included citrullinemia and hyperammonemia along with delayed cerebellar development, epidermal hyperkeratosis, and follicular dystrophy. Standard treatments for CTLN1 were effective in rescuing the phenotype of these mutant mice. Based on our studies, we propose that defective cerebellar granule cell migration secondary to disorganization of Bergmann glial cell fibers cause cerebellar developmental delay in the hyperammonemic and citrullinemic brain, pointing to a possible role for nitric oxide in these processes. These mouse mutations constitute a suitable model for both mechanistic and preclinical studies of CTLN1 and other hyperammonemic encephalopathies and, at the same time, underscore the importance of complementing knockout mutations with hypomorphic mutations for the generation of animal models of human genetic diseases.

Molecular mechanisms of protein kinase C-induced apoptosis in prostate cancer cells

Protein kinase C (PKC) isozymes, a family of serine-threonine kinases, are important regulators of cell proliferation and malignant transformation. Phorbol esters, the prototype PKC activators, cause PKC translocation to the plasma membrane in prostate cancer cells, and trigger an apoptotic response. Studies in recent years have determined that each member of the PKC family exerts different effects on apoptotic or survival pathways. PKCdelta, one of the novel PKCs, is a key player of the apoptotic response via the activation of the p38 MAPK pathway. Studies using RNAi revealed that depletion of PKCdelta totally abolishes the apoptotic effect of the phorbol ester PMA. Activation of the classical PKCalpha promotes the dephosphorylation and inactivation of the survival kinase Akt. Studies have assigned a pro-survival role to PKCepsilon, but the function of this PKC isozyme remains controversial. Recently, it has been determined that the PKC apoptotic effect in androgen-dependent prostate cancer cells is mediated by the autocrine secretion of death factors. PKCdelta stimulates the release of TNFalpha from the plasma membrane, and blockade of TNFalpha secretion or TNFalpha receptors abrogates the apoptotic response of PMA. Molecular analysis indicates the requirement of the extrinsic apoptotic cascade via the activation of death receptors and caspase-8. Dissecting the pathways downstream of PKC isozymes represents a major challenge to understanding the molecular basis of phorbol ester-induced apoptosis.

Carcinogenic effects of MGP-7 and B[a]P on the hamster cheek pouch

This study was performed to examine the carcinogenic effects of benzo[a]pyrene (B[a]P) and manufactured gas plant (MGP) residues on the hamster cheek pouch (HCP). Syrian hamsters were treated topically with a suspension of 2%, 10%, or 20% B[a]P or 50% or 100% MGP-7 (a mixture of residues from 7 MGP sites) in mineral oil for eight (short-term study) and sixteen, twenty, twenty-eight, and thirty-two weeks (long-term study). The short-term study showed that B[a]P induced p53 protein accumulation, indicative of genotoxic damage, as well as increased cell proliferation, hyperplasia, and inflammation, which is usually associated with promotional activity. In contrast, the MGP-7 presented only marginal p53 accumulation and induction of BrdU incorporation. In the long-term experiments, animals treated with 2% and 10% of B[a]P continued to show p53 protein accumulation as well as hyperplasia and increased cell proliferation and inflammation. By thirty weeks, all the animals treated with B[a]P had a 100% incidence of squamous cell carcinoma (SCC). Animals treated with 50% and 100% MGP-7 showed only weak hyperplasia and a low proliferation rate and accumulation of p53 protein through thirty-two weeks. Benzo[a]pyrene was highly carcinogenic when used at adequate doses. Manufactured gas plant residue, however, was not carcinogenic in this model.

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.

PTEN deficiency is fully penetrant for prostate adenocarcinoma in C57BL/6 mice via mTOR-dependent growth

The tumor suppressor phosphatase and tensin homolog (PTEN) is frequently involved in human prostate carcinoma. PTEN is therefore an attractive target for the development of preclinical animal models. Prostate intraepithelial neoplasia lesions develop in mice with Pten heterozygosity, but disease progression has been reported only in combination with either other tumor suppressor gene alterations or the conditional inactivation of both Pten alleles in prostate epithelial cells. We report that on a C57BL/6 background, in contrast to previous studies on mixed 129 genetic backgrounds, Pten locus heterozygosity is fully penetrant for the development of prostate adenocarcinoma. Grossly observable tumors were detected at 6 months of age, and, by 10 to 12 months, 100% of examined mice developed adenocarcinoma of the anterior prostate. Furthermore, double heterozygotes carrying both Pten and Tsc2-null alleles showed no increase relative to Pten(+/-) heterozygotes in either lesion development or progression. Lesions in both Pten(+/-); Tsc2(+/-), and Pten(+/-) mice exhibited loss of PTEN expression and activation of PI3K signaling. PI3K activation occurred early in prostate intraepithelial neoplasia lesion formation in these animals, consistent with loss of PTEN function, and contributed to the etiology of tumors that developed in Pten(+/-) mice. Furthermore, prostate lesion growth in Pten(+/-) mice was dependent on mTOR, as evidenced by a reduction in both phospho-S6 levels and proliferative index after rapamycin treatment.

Transcriptional regulation of estrogen receptor-alpha by p53 in human breast cancer cells

Estrogen receptor alpha (ER) and p53 are critical prognostic indicators in breast cancer. Loss of functional p53 is correlated with poor prognosis, ER negativity, and resistance to antiestrogen treatment. Previously, we found that p53 genotype was correlated with ER expression and response to tamoxifen in mammary tumors arising in mouse mammary tumor virus-Wnt-1 transgenic mice. These results lead us to hypothesize that p53 may regulate ER expression. To test this, MCF-7 cells were treated with doxorubicin or ionizing radiation, both of which stimulated a 5-fold increase in p53 expression. ER expression was also increased 4-fold over a 24-h time frame. In cells treated with small interfering RNA (siRNA) targeting p53, expression of both p53 and ER was significantly reduced (>60%) by 24 h. Induction of ER by DNA-damaging agents was p53 dependent as either ionizing radiation or doxorubicin failed to up-regulate ER after treatment with p53-targeting siRNA. To further investigate whether p53 directly regulates transcription of the ER gene promoter, MCF-7 cells were transiently transfected with a wild-type (WT) p53 expression vector along with a luciferase reporter containing the proximal promoter of ER. In cells transfected with WT p53, transcription from the ER promoter was increased 8-fold. Chromatin immunoprecipitation assays showed that p53 was recruited to the ER promoter along with CARM1, CBP, c-Jun, and Sp1 and that this multifactor complex was formed in a p53-dependent manner. These data show that p53 regulates ER expression through transcriptional control of the ER promoter, accounting for their concordant expression in human breast cancer.

Paracrine overexpression of insulin-like growth factor-1 enhances mammary tumorigenesis in vivo

Insulin-like growth factor-1 (IGF-1) stimulates proliferation, regulates tissue development, protects against apoptosis, and promotes the malignant phenotype in the breast and other organs. Some epidemiological studies have linked high circulating levels of IGF-1 with an increased risk of breast cancer. To study the role of IGF-1 in mammary tumorigenesis in vivo, we used transgenic mice in which overexpression of IGF-1 is under the control of the bovine keratin 5 (BK5) promoter and is directed to either the myoepithelial or basal cells in a variety of organs, including the mammary gland. This model closely recapitulates the paracrine exposure of breast epithelium to stromal IGF-1 seen in women. Histologically, mammary glands from transgenic mice were hyperplastic and highly vascularized. Mammary glands from prepubertal transgenic mice had significantly increased ductal proliferation compared with wild-type tissues, although this difference was not maintained after puberty. Transgenic mice also had increased susceptibility to mammary carcinogenesis, and 74% of the BK5.IGF-1 mice treated with 7,12-dimethylbenz[a]anthracene (20 microg/day) developed mammary tumors compared with 29% of the wild-type mice. Interestingly, 31% of the vehicle-treated BK5.IGF-1 animals, but none of the wild-type animals, spontaneously developed mammary cancer. The mammary tumors were moderately differentiated adenocarcinomas that expressed functional, nuclear estrogen receptor at both the protein and mRNA levels. These data support the hypothesis that tissue overexpression of IGF-1 stimulates mammary tumorigenesis.

CDK2 activation in mouse epidermis induces keratinocyte proliferation but does not affect skin tumor development

It has been widely assumed that elevated CDK2 kinase activity plays a contributory role in tumorigenesis. We have previously shown that mice overexpressing CDK4 under control of the keratin 5 promoter (K5CDK4 mice) develop epidermal hyperplasia and increased susceptibility to squamous cell carcinomas. In this model, CDK4 overexpression results in increased CDK2 activity associated with the noncatalytic function of CDK4, sequestration of p21(Cip1) and p27(Kip1). Furthermore, we have shown that ablation of Cdk2 reduces Ras-Cdk4 tumorigenesis, suggesting that increased CDK2 activity plays an important role in Ras-mediated tumorigenesis. To investigate this hypothesis, we generated two transgenic mouse models of elevated CDK2 kinase activity, K5Cdk2 and K5Cdk4(D158N) mice. The D158N mutation blocks CDK4 kinase activity without interfering with its binding capability. CDK2 activation via overexpression of CDK4(D158N), but not of CDK2, resulted in epidermal hyperplasia. We observed elevated levels of p21(Cip1) in K5Cdk2, but not in K5Cdk4(D158N), epidermis, suggesting that CDK2 overexpression elicits a p21(Cip1) response to maintain keratinocyte homeostasis. Surprisingly, we found that neither CDK2 overexpression nor the indirect activation of CDK2 enhanced skin tumor development. Thus, although the indirect activation of CDK2 is sufficient to induce keratinocyte hyperproliferation, activation of CDK2 alone does not induce malignant progression in Ras-mediated tumorigenesis.

Transgenic expression of E2F3a causes DNA damage leading to ATM-dependent apoptosis

Many early stage human tumors display markers of a DNA-damage response (DDR), including ataxia telangiectasia mutated (ATM) kinase activation. This suggests that DNA damage accumulates during the process of carcinogenesis and that the ATM-dependent response to this damage may function to suppress cancer progression. The E2F3a transcription factor plays an important role in regulating cell proliferation and is amplified in a subset of human cancers. Similar to human premalignant lesions, we find activated ATM and other markers of the DDR in the hyperplastic epidermis of transgenic mice expressing E2F3a through a keratin 5 (K5) promoter. Primary keratinocytes from K5 E2F3a transgenic mice contain increased levels of DNA breaks compared to wild-type cells. E2F3a overexpression also induced DNA damage in primary human fibroblasts that was inhibited by blocking DNA replication. The absence of ATM impaired apoptosis induced by E2F3a and treating K5 E2F3a transgenic mice with caffeine, an inhibitor of ATM and Rad3-related (ATR), promoted skin tumor development. These findings demonstrate that the deregulated expression of E2F3a causes DNA damage under physiological conditions and indicate that the ATM-dependent response to this damage is important for the induction of apoptosis and tumor suppression.

WWOX hypomorphic mice display a higher incidence of B-cell lymphomas and develop testicular atrophy

WWOX is a putative tumor suppressor gene encoded within common chromosomal fragile site region FRA16D, in chromosome band 16q23. Multiple studies have demonstrated that WWOX expression is often reduced or lost in various tumor types. WWOX tumor suppressor activity was suggested by re-expressing WWOX in breast, ovarian, and lung tumor cell lines leading to tumor growth inhibition in vivo. To determine whether loss of Wwox gene expression has a role in tumorigenesis, we generated a mouse strain containing a Wwox gene mutated by a gene-trap vector. Homozygous Wwox gene-trap mice (Wwox(gt/gt)) had no detectable Wwox protein in most tissues examined, although, a low level could be detected in a minority of tissues. Because of these observations, we concluded that these mice are Wwox hypomorphs. Remarkably, Wwox hypomorphic mice are viable in contrast to the recently reported postnatal lethality of Wwox knockout mice. Testes from Wwox(gt/gt) males had high numbers of atrophic seminiferous tubules and reduced fertility when compared with wild-type counterparts. We observed that the Wwox(gt/gt) mice had a significantly shorter lifespan, and female hypomorphs had a higher incidence of spontaneous B-cell lymphomas. In conclusion, we describe a novel Wwox hypomorphic mouse model that overcomes postnatal lethality that was recently observed in Wwox knockout mice. Therefore, tumorigenesis studies using this model more closely recapitulates the loss of WWOX expression observed in human cancers. Importantly, our observation that Wwox hypomorphs had an increased incidence of B-cell lymphomas supports a role of Wwox as a tumor suppressor.

Inhibition of proprotein convertases: approaches to block squamous carcinoma development and progression

Most proprotein convertase (PC) inhibitors are compounds that act as competitive inhibitors. All of them contain the general cleavage motif RXK/RR that binds to the PC's active site impairing further interactions with their physiological substrates. The first inhibitors synthesized were the acyl-peptidyl-chloromethyl ketones that bind to the PC's active site through its peptidyl group and are able to transverse the plasma membrane due to the acyl moiety. For instance, one of the members of this family that exhibits reduced toxicity and has been widely used as an effective general PCs inhbitor is the derivative decanoyl-RVKR-chloromethylketone (CMK). Another approach to PC inhibition is based on proteins that contain either a natural or a bioengineered PC cleavage consensus site. In this context, the bioengineered serpin, alpha-1-antitrypsin Portland (alpha 1-PDX or PDX), proved to be a potent inhibitor of furin, the most studied of the cancer-related PCs. Both PDX and CMK were able to inhibit invasiveness of squamous cell carcinoma cell lines by blocking activation of cancer-associated PC substrates such as MT-MMPs, IGF-1R, and VEGF-C. A similar effect was produced by inhibiting PC-mediated processing using furin prosegment. PDX and CMK have also been assayed in vivo using skin carcinogenesis models. Newer promising small molecules and RNA interference approaches are also being developed to inhibit PCs.

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.

Differential susceptibility to chemically induced thymic lymphomas in SENCARB and SSIN inbred mice

In the past 20 yr, several inbred strains have been derived from SENCAR outbred mice. These strains display different susceptibility to the induction of papillomas and progression to squamous cell carcinomas (SCC) in the skin after chemical carcinogenesis. In the present study, we showed that one of these strains SENCARB/Pt was highly susceptible to the development of N-methyl-N-nitrosourea (MNU)- and 7,12-dimethylbenz[a]anthracene (DMBA)-induced lymphomas. In contrast, the SSIN/Sprd inbred strain is completely resistant to T-cell lymphomagenesis by both carcinogens. Within 175 d after a single injection of 75 mg/kilogram body weight (kbw) of MNU, SENCARB/Pt mice exhibited a 91.6% incidence of lymphoma. In addition, during an independent tumorigenesis study with repeated doses of intragastric DMBA, SENCARB/Pt mice showed an incidence of 75% lymphoma development 300 d after the last treatment. Histopathological and flow cytometric parameters indicated that the lymphomas were of the T-cell lineage. In order to study the genetics of MNU-induced tumorigenesis, we generated F1 hybrid mice between SSIN/Sprd and SENCARB/Pt mice. Tumor incidence in MNU-injected F1 mice suggested that the high tumor incidence is a dominant trait. Loss of heterozygosity (LOH) analysis in these tumor samples revealed allelic imbalances on chromosomes 15 and 19. Given that these inbred strains are closely related, it is likely that a relatively small number of loci are responsible for the observed differences in susceptibility. Therefore, these SENCAR inbred strains constitute important new tools to study the genetic basis of resistance and susceptibility to chemically induced thymic lymphoma formation.

Interaction between genetic susceptibility and early-life environmental exposure determines tumor-suppressor-gene penetrance

Gene-environment interactions are important determinants of cancer risk. Traditionally, gene-environment interactions are thought to contribute to tumor-suppressor-gene penetrance by facilitating or inhibiting the acquisition of additional somatic mutations required for tumorigenesis. Here, we demonstrate that a distinctive type of gene-environment interaction can occur during development to enhance the penetrance of a tumor-suppressor-gene defect in the adult. Using rats carrying a germ-line defect in the tuberous sclerosis complex 2 (Tsc-2) tumor-suppressor gene predisposed to uterine leiomyomas, we show that an early-life exposure to diethylstilbestrol during development of the uterus increased tumor-suppressor-gene penetrance from 65% to >90% and tumor multiplicity and size in genetically predisposed animals, but it failed to induce tumors in wild-type rats. This exposure was shown to impart a hormonal imprint on the developing uterine myometrium, causing an increase in expression of estrogen-responsive genes before the onset of tumors. Loss of function of the normal Tsc-2 allele remained the rate-limiting event for tumorigenesis; however, tumors that developed in exposed animals displayed an enhanced proliferative response to steroid hormones relative to tumors that developed in unexposed animals. These data suggest that exposure to environmental factors during development can permanently reprogram normal physiological tissue responses and thus lead to increased tumor-suppressor-gene penetrance in genetically susceptible individuals.

Persistent activation of the Akt pathway in head and neck squamous cell carcinoma: a potential target for UCN-01

Squamous carcinomas of the head and neck (HNSCC) represent the sixth most common cancer among men worldwide and a major cause of morbidity and mortality due to its relatively poor prognosis. As part of ongoing studies addressing the molecular events underlying tumor progression in HNSCC, we have explored the nature of the proliferative pathways in which dysregulation may promote aberrant cell growth in this tumor type. The serine/threonine protein kinase Akt is a downstream target of phosphatidylinositol 3-kinase and a key regulator of normal and cancerous growth and cell fate decisions. Therefore, in this study, we have examined the status of activation of Akt in different stages of squamous cell carcinoma development in mice and in clinical samples from HNSCC patients. By immunohistochemical analysis, using a recently developed phosphorylation state-specific antibody, we demonstrated that Akt activation correlates closely with the progression of mouse skin squamous cell carcinoma. We also observed that activation of Akt is a frequent event in human HNSCC because active Akt can be detected in these tumors with a pattern of expression and localization correlating with the progression of the lesions. In line with these observations, Akt was constitutively activated in a large fraction of HNSCC-derived cell lines. We also provide evidence that the Akt signaling pathway may represent a biologically relevant target for a novel antineoplastic agent, UCN-01, which recently has been shown to be active in cellular and xenograft models for HNSCC at concentrations safely achievable in clinically relevant situations.

Chronic stress accelerates ultraviolet-induced cutaneous carcinogenesis

BACKGROUND

Physical and emotional stressors have been found to mediate a wide variety of biological changes including the facilitation of tumor progression; however most of these paradigms utilized artificial sources of neoplasms and stress.

METHODS

Skh mice were exposed to carcinogenic doses of ultraviolet light (UV). The stressed group was subjected to the close proximity of fox urine as a source of stress from the presence of the odor of their natural predator, while the control group remained stress free.

RESULTS

A significant acceleration in the development of cutaneous neoplasms was observed in mice that had been exposed to the stressor. The first tumor appeared in the group after 8 weeks, whereas nonstressed mice began to develop these by week 21.

CONCLUSION

These results suggest that stress plays a role in potentiating cutaneous carcinogenesis.

Endothelial cell development, vasculogenesis, angiogenesis, and tumor neovascularization: an update

In the embryo, blood vessel formation de novo (vasculogenesis) and from existing vessels (angiogenesis) results in blood vessels lined by endothelial cells (ECs). The relationship between ECs and blood cells suggested by their physical closeness was recently confirmed with the demonstration of progenitors that give rise to both cell types. In tumors, new blood vessel formation has been thought to occur primarily via angiogenesis. Recent evidence, however, suggests that postnatal vasculogenesis also contributes to tumor neovascularization. In this article, we provide an update on EC development, including early lineage specification, morphogenesis or differentiation to form functional blood vessels, and regulation of EC survival and senescence. Furthermore, we review the latest findings on tumor neovascularization and therapeutic potentials of molecules critical to this process.

Simple duplex fecal PCR assay that allows identification of false-negative results in Helicobacter sp.-infected mice

We designed a simple and sensitive duplex polymerase chain reaction (PCR) assay for detection of false-negative results during routine Helicobacter sp. feces analysis. We took advantage of the various Lactobacillus species that form part of the normal intestinal flora of laboratory rodents to improve our PCR diagnostic assays. Using this one-step PCR assay, we were able to rule out false-negative results without the need of adding internal standard molecules. This is an important quality control for PCR diagnostic tests, since the presence of inhibitors in feces can prevent detection of Helicobacter infections using PCR analysis. Use of this Lactobacillus group-specific PCR assay can be extended to other feces tests used in mouse quality-control programs.

Lack of cyclin-dependent kinase 4 inhibits c-myc tumorigenic activities in epithelial tissues

The proto-oncogene c-myc encodes a transcription factor that is implicated in the regulation of cellular proliferation, differentiation, and apoptosis and that has also been found to be deregulated in several forms of human and experimental tumors. We have shown that forced expression of c-myc in epithelial tissues of transgenic mice (K5-Myc) resulted in keratinocyte hyperproliferation and the development of spontaneous tumors in the skin and oral cavity. Although a number of genes involved in cancer development are regulated by c-myc, the actual mechanisms leading to Myc-induced neoplasia are not known. Among the genes regulated by Myc is the cyclin-dependent kinase 4 (CDK4) gene. Interestingly, previous studies from our laboratory showed that the overexpression of CDK4 led to keratinocyte hyperproliferation, although no spontaneous tumor development was observed. Thus, we tested the hypothesis that CDK4 may be one of the critical downstream genes involved in Myc carcinogenesis. Our results showed that CDK4 inhibition in K5-Myc transgenic mice resulted in the complete inhibition of tumor development, suggesting that CDK4 is a critical mediator of tumor formation induced by deregulated Myc. Furthermore, a lack of CDK4 expression resulted in marked decreases in epidermal thickness and keratinocyte proliferation compared to the results obtained for K5-Myc littermates. Biochemical analysis of the K5-Myc epidermis showed that CDK4 mediates the proliferative activities of Myc by sequestering p21Cip1 and p27Kip1 and thereby indirectly activating CDK2 kinase activity. These results show that CDK4 mediates the proliferative and oncogenic activities of Myc in vivo through a mechanism that involves the sequestration of specific CDK inhibitors.

Epidermal abnormalities and increased malignancy of skin tumors in human epidermal keratin 8-expressing transgenic mice

Keratins K8 and K18 are the major components of the intermediate filament cytoskeleton of simple epithelia. Increased levels of these keratins have been associated with invasive growth and progression to malignancy in different types of human and murine epithelial tumors (including skin tumors), and even in tumors from nonepithelial origin. However, it has not yet clarified whether K8/K18 expression in tumors is cause or consequence of malignancy. Given the increasing incidence of epidermal cancer in humans (40% of all tumors diagnosed), we generated a mouse model to examine the role of simple epithelium keratins in the establishment and progression of human skin cancer. Transgenic mice expressing human K8 in the epidermis showed severe epidermal and hair follicle dysplasia with concomitant alteration in epidermal differentiation markers. The severity of the skin phenotype of these transgenic mice increases with age, leading to areas of preneoplastic transformation. Skin carcinogenesis assays showed a dramatic increase in the progression of papillomas toward malignancy in transgenic animals. These results support the idea that K8 alters the epidermal cell differentiation, favors the neoplastic transformation of cells, and is ultimately responsible of the invasive behavior of transformed epidermal cells leading of conversion of benign to malignant tumors.

Protein kinase C and prostate carcinogenesis: targeting the cell cycle and apoptotic mechanisms

A series of both genetic and epigenetic factors have been implicated in the genesis and progression of prostate cancer. Recent evidence revealed that protein kinase C (PKC) isozymes play a crucial role in the control of cell proliferation and apoptosis in prostate cancer models, as well as in the transition from an androgen-dependent to an androgen-independent status. Indeed, PKCalpha and PKCdelta promote apoptosis in androgen-dependent prostate cancer cells. Due to the relevance of PKC isozymes in the control of cell cycle, both in G1/S and G2/M, the elucidation of such complex intracellular networks using cellular and animal models has become of outmost importance. In this review, we present the current knowledge on the regulation of apoptosis and tumorigenicity by PKC isozymes and the functional roles of cell cycle regulators in prostate carcinogenesis. The development of animal models where overexpression of discrete PKCs or cell cycle regulators is targeted to the prostate will greatly contribute to the understanding of the molecular basis of the disease, and more importantly, it will have profound implications for the development of novel strategies for prostate cancer therapy.

Mutation frequencies in murine keratinocytes as a function of carcinogenic status

A link between genetic abnormalities and carcinogenesis is well established. It follows that a correlation exists between mutation frequency and malignant progression. We have determined the spontaneous and DNA damage-induced mutation frequencies for a series of cell lines derived from SENCAR mouse keratinocytes at various stages of malignant progression. Nontumorigenic mouse keratinocytes (3PC), papillomas (MT1/2), squamous-cell carcinomas (CH72), and spindle-cell carcinomas (CH72T4) were transfected with damaged or undamaged shuttle vectors containing a supF mutation reporter gene. The plasmid mutation frequencies were determined by blue/white screening. The spontaneous plasmid mutation frequency of the squamous-cell carcinoma line was slightly higher than the mutation frequencies of the other cell lines tested. The DNA damage induced by triplex-directed psoralen crosslinks increased the mutation frequencies sixfold to eighteenfold in all cell lines tested, with no significant differences among the cell lines. Sequence analyses revealed that the spindle-cell carcinoma line had a different spontaneous mutation spectrum from the other cell lines. DNA damage-induced mutations were predominantly point mutations at the triplex-duplex junction in all of the cell lines tested, as expected. These data suggested that a strong mutator phenotype was not required for progression to an advanced malignant phenotype in our model system.