Differential expression of cell survival and cell cycle regulatory proteins in cutaneous squamoproliferative lesions

Previous models of cutaneous carcinogenesis have primarily focused on the regulation of keratinocyte (KC) proliferation and differentiation. However, it has become clear in many neoplastic systems that altered rates of cell death and/or inabilty to undergo growth arrest can also contribute to the development of cancer. Apoptosis-regulatory proteins include those that block apoptosis such as Bcl-2 and Bcl-x, whilst a related protein Bax promotes apoptosis. Cell cycle regulatory proteins include those associated with growth arrest, i.e. p21^waf1, p53, and those associated with proliferation, i.e. Ki-67. Paraffin embedded samples from ten different lesions of squamous cell carcinoma (SCC), Bowen’s disease (BD), keratoacanthomas (KA), and nine normal adult skin samples were stained by immunohistochemistry to detect expression of Bcl-2, Bcl-x, Bax, Ki-67, p21^waf1, p53 and apoptosis (TUNEL assay). Compared to low levels of Bcl-x and Bcl-2 immunostaining in normal skin, all the squamoproliferative lesions had strong and diffuse KC expression of Bcl-x (>80%) but minimal to absent KC Bcl-2 expression (<15%). Bax immunopositivity was limited to the basal layer in normal skin and BD. In contrast, by examining serial sections both Bcl-x and Bax appeared to be coexpressed by the majority of malignant KCs in KA and SCC (>70%). These immunostaining profiles reveal that squamoproliferative lesions, including invasive transformed KCs, preferentially express Bcl-x over Bcl-2, in addition to upregulating their Bax levels. Even though there were numerous TUNEL positive cells in these squamoproliferative lesions, no other evidence of apoptosis was seen reinforcing the necessity to use caution when relying on TUNEL staining for identification of programmed cell death in skin biopsies. Normal sun-exposed skin had low but detectable p53 and rare p21^waf1 KC expression. Significantly higher numbers of p21^waf1 and p53 immunopositive KCs were noted throughout the lesions in BD and SCC in contrast to KA where p53 and rare p21^waf1 immunopositive KCs were primarily limited to the periphery of the tumor cell islands. In general, p53 KC expression was higher in all squamoproliferative lesions and sun-exposed normal skin compared to p21^waf1 expression. Summary of the expression of cell cycle regulatory proteins for both p21^waf1 and p53 KC expression was: SCC>BD>KA, in marked contrast to Ki-67 KC expression which was: BD>KA>SCC. The relatively few malignant cells in SCC that were actively participating in the cell cycle (i.e. Ki-67 positive) suggests that these neoplasms may arise primarily by increased cell survival and resistance to apoptosis rather than by hyperproliferation. These studies emphasize the importance of examining multiple members of protein families that regulate apoptosis, proliferation, growth arrest, and differentiation. It is the overall balance between these cellular phenomena that determine whether a cell remains viable or undergoes programmed cell death and contributes to the appearance of a neoplasm. The overexpression of Bcl-x may confer a survival advantage to malignant KCs unable to growth arrest to repair damaged DNA (mutant p53) and/or undergo terminal differentiation (increased p21^waf1). Thus, mutation or aberrant expression of such proteins may participate in the multistep process of carcinogenesis that gives rise to these squamoproliferative lesions.     

p53相关的细胞周期调控在UVB诱导的皮肤成纤维细胞早衰中的作用

目的探讨中波紫外线(UVB)诱导皮肤成纤维细胞(HSF)应激诱导的早衰(SIPS)后细胞周期调控的变化。探讨p53相关的生长停滞、细胞凋亡相关基因表达在UVB诱导的SIPS中的变化情况。方法对UVB诱导发生SIPS的HSF,流式细胞仪检测细胞周期;Western blot法检测衰老信号通路中p53,p21,p16的表达;实时荧光定量PCR芯片检测与p53相关的生长停滞(p53,p21,p16,p19)、凋亡(bax,bcl-2)基因的mRNA表达水平。结果流式细胞仪对UVB诱导的SIPS模型进行细胞周期检测发现,大部分细胞发生了G1期阻滞;衰老通路中p53,p21和p16的表达都明显增加;实时荧光定量PCR芯片检测发现,与生长停滞相关的抑癌基因p53,p21,p16,p19的表达均上调;p53下游的凋亡相关基因中的抑凋亡基因bcl-2表达稍上调,而凋亡基因bax表达下调。结论G1期阻滞和衰老信号通路蛋白表达增加进一步印证了UVB诱导HSF发生的SIPS。p53信号通路相关基因的表达变化提示UVB诱导的SIPS可能具有与p53相关的抗凋亡作用,但其中p53信号通路所起的作用还需更进一步的实验探索。     

UV-induced squamous cell carcinoma – a role for antiapoptotic signalling pathways

The incidence of nonmelanoma skin cancer including squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) has dramatically increased in the last decades, and chronic sun exposure was identified as a main etiologic agent. UV radiation may produce DNA damage either directly or through reactive oxygen species (ROS). As mutations caused by UV may lead to skin cancer due to oncogene activation and tumor suppressor gene inactivation, efficient safeguard mechanisms have been developed during evolution. These enclose induction of apoptosis and formation sunburn cells aiming at the removal of premalignant cells. The keratinocyte apoptotic machinery in response to UV consists of both intrinsic/mitochondrial and extrinsic/death receptor-mediated cell-death pathways, which are particularly regulated by mitogen-activated protein kinases (MAPKs, JNK and p38) and the tumor-suppressor protein p53. For development of skin cancer, it appears that critical steps in apoptosis control are dysregulated leading to resistance both to death ligand-mediated and intrinsic proapoptotic pathways. These particularly include inactivation of p53, as well as activation of EGFR, COX-2 and MAPKs, which result in specific regulation of Bcl-2 proteins, death ligands and death receptors. The final unravelling of apoptosis regulation in epithelial skin cancer may allow the development of new targeted therapeutic strategies.     

Characterization of coordinated immediate responses by p16INK4A and p53 pathways in UVB-irradiated human skin cells

While the precise mechanisms of melanoma development are unknown, recent in vivo studies have revealed that the p16(Ink4a)/Rb pathway is disrupted in melanomagenesis. Here, we characterize the role of p16/Rb in coordinating the early events in UVB-irradiated skin. Foreskins and melanoma cell cultures were irradiated with low and high acute UVB doses and examined for cell-cycle- and apoptosis-associated genes. In melanoma cells, low UVB dose upregulated p16, p53, and p21 expression levels in Malme-3M, and high UVB dose accentuated the expression of p53 and p21(Cip1/Waf1), in particular; however, in SkMel-28 cells only p16 expression was upregulated in response to UV irradiation. In HaCaT cells, high UVB dose caused dramatic increase in p53 expression followed by upregulation of p21(Cip1/Waf1) and Bax, and downregulation of Bcl-2 leading to apoptosis. In HaCaT cells, reinstatement of p16 pathway restored cell-cycle arrest in response to low dose. Foreskin organ culture experiments confirmed our in vitro cell results. These data indicate that the p53 and p16 pathways respond independently to UVB insult. The p16 pathway is favored at low doses and results in cell-cycle arrest; the p53 pathway is more responsive to higher doses and induces apoptosis depending on p53 mutation status.     

Bexarotene activates the p53/p73 pathway in human cutaneous T-cell lymphoma

Background Bexarotene is the first synthetic retinoid X receptor‐selective retinoid (rexinoid) approved for the treatment of cutaneous T‐cell lymphoma (CTCL). However, little is known about the signalling pathways by which it exerts its anticarcinogenic effect. Objectives To characterize the effects of bexarotene in CTCL cell lines and elucidate the underlying molecular pathways of its antineoplastic effect. Methods The cell lines Hut‐78, HH and MJ were used. Cell viability was assessed with the XTT assay. The self‐renewal potential of cells after bexarotene treatment was studied with the methylcellulose clonogenic assay. Flow cytometry was used to analyse the effects on cell cycle, Ki‐67 expression and apoptosis induction. Cell cycle and apoptosis‐related protein expression were determined by Western blot and immunofluorescence. Results Bexarotene induced a loss of viability and more pronounced inhibition of clonogenic proliferation in HH and Hut‐78 cells, whereas the MJ line exhibited resistance. Bexarotene upregulated and activated Bax in sensitive lines, although not enough to signal significant apoptosis. Instead, all data point to the inhibition of proliferation, rather than apoptosis, as the main mechanistic action of the rexinoid. Bexarotene signals both G₁ and G₂/M arrest by the modulation of critical checkpoint proteins. We further found that bexarotene activates p53 by phosphorylation at Ser15, which influences the binding of p53 to promoters for cell cycle arrest, induces p73 upregulation, and, in concordance, also modulates some p53/p73 downstream target genes, such as p21, Bax, survivin and cdc2. Bexarotene‐mediated ataxia telangiectasia mutated protein (ATM) activation in all studied lines suggests that ATM is likely to be the p53/p73 upstream activator. Conclusions Our data indicate for the first time that bexarotene exerts its effect in CTCL mainly by triggering the p53/p73‐dependent cell cycle inhibition pathway, probably by upstream ATM activation. Therefore, bexarotene‐modulated genes represent potential biomarkers to assess the response to treatment of patients with CTCL.     

Immunophenotypic analysis of the p53 gene in non-melanoma skin cancer and correlation with apoptosis and cell proliferation

BACKGROUND: Sunlight precipitates a series of genetic events that lead to the development of skin cancers such as basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). The p53 tumour suppressor gene, which plays a pivotal role in cell division and apoptosis, is frequently found mutated in sunlight-induced skin tumours. OBJECTIVE: To investigate the immunoreactivity of the p53 gene in non-melanoma skin cancers and to correlate its expression with apoptotic and cell proliferation markers. METHODS: We analysed 35 non-melanoma tumours including 19 BCCs and 16 SCCs from sun-exposed skin areas. p53 protein expression was studied immunohistochemically using the DO7 monoclonal antibody against wild-type and mutant p53 forms. The percentage of p53-immunopositive nuclei was measured by image analysis. Cell proliferation and apoptosis were also assessed by image analysis following Ki-67 immunostaining and application of the TUNEL method on paraffin sections, respectively. RESULTS: The percentage of p53-expressing cells varied from 3.5 to 90 in BCCs (median value 54.4%) and from 3.7 to 94 in SCCs (median value 40.3%). The mean value of Ki-67-positive cells was comparable in both groups of tumours with a mean value of 40.6% in BCCs and 34.6% in SCCs. Conversely, the TUNEL assay showed sporadic staining of apoptotic cells within the tumours with a mean value of 1.12% in BCCs and 1.8% in SCCs. p53 protein expression was correlated positively with cell proliferation (r = 0.75, P = 0.000001) and negatively with apoptosis (r = -0.23, P = 0.05). CONCLUSION: p53 immunoreactivity was high in the majority of the skin carcinomas examined and correlated positively with cell proliferation and negatively with apoptosis. The p53 protein overexpression appears to be related to an inactivated protein resulting from mutations of the p53 gene or other unclear molecular mechanisms.     

p63基因与表皮增殖、发育及肿瘤的研究现状

p63是最近发现的p53家族成员,它的生物学功能涉及到生长、凋亡、细胞周期停滞、肿瘤形成等多个方面,尤其突出的是对表皮增殖、发育的影响和在皮肤鳞状细胞癌中的高表达。     

A low UVB dose, with the potential to trigger a protective p53-dependent gene program, increases the resilience of keratinocytes against future UVB insults

One protein central in the response of human keratinocytes to ultraviolet B damage is p53. By transactivating genes involved in either cell cycle arrest or DNA repair, p53 has a leading role in the recovery from this damage. Considering this role, we wished to investigate whether the triggering of a p53-dependent gene program by repetitive ultraviolet B (UVB) exposure can induce an adaptive response in human skin cells. In particular, we examined two p53-target genes, p21/WAF1 and p53R2, with a crucial role in p53-induced cell cycle arrest and p53-induced DNA repair respectively. Exposure to a mild UVB dose was able to induce an adaptive response in human keratinocytes, leading to increased survival of cells that maintain their capacity to repair DNA damage upon exposure to apoptotic doses of UVB. Our study indicates that this adaptation response is only achieved if the interval between subsequent UVB insults allows sufficient time for the p53-induced protective gene program to be induced. Our results also demonstrate that small but quickly recurring UVB exposures are as harmful as one intense, continual exposure to UVB irradiation. Future research will be oriented toward investigating alternative ways to induce an adaptive response without pre-exposing the cells to UV.     

Cell cycle regulators in the keratinocyte (cyclin-cdk)

Abstract It has recently become clear that cyclin-dependent kinase (cdk) complex regulates the cell cycle by phosphorylating Rb protein, a tumor suppressor protein. It is likely that this complex is a target of various growth factors and anti-growth factors (UV TGF-β etc.) in keratinocyte (KC). It has also been suggested that abnormalities in the cell cycle regulating mechanism such as increased activity of cyclin-cdk due to mutation of p53, a tumor suppressor gene, and overexpression of cyclin D may be concerned with carcinogenesis of KC. Thus, recent studies indicate that the cyclin-cdk complex is a common target of proliferation and carcinogenesis in KC.     

p21 WAF1/cip1expression in non-melanoma skin tumors

Abnormal control of the cell cycle is closely linked to carcino-genesis. p21^WAF1/CIP1 protein is a universal inhibitor of Gl cyclin-dependent kinase and is induced by p53-dependent and -independent pathways. In order to elucidate the role of p21^WAF1/CIP1 in human skin carcinogenesis, protein expression in squamous cell carcinoma (SCC), basal cell carcinoma (BCC), Bowen's disease (BD), actinic keratosis (AK), keratoacanthoma (KA), seborrheic keratosis (SK), and normal skin was examined using an immunohistochemical method. In normal skin, a few positive cells were seen in some cases in the upper spinous layer of the epidermis; sebaceous glands also had positive cells. In cases of SK and KA, positive cells were found in the basal and suprabasal epidermal layers (proliferation pattern), and in cases of BD and AK, positive cells were seen mainly in the upper spinous layer (differentiation pattern). Cases of SCC had more positive cells and showed two staining patterns: proliferation, or mixed. Cases of BCC had no positive cells. p21^WAF1/CIP1 has some unidentified role in keratinocyte tumorigenesis, which may not be related directly to carcinogenesis.     

Endogenous survivin modulates survival and proliferation in UVB-treated human keratinocytes

Abstract:  Survivin is a bi-functional member of inhibitor of apoptosis protein family, as it is able to both inhibit apoptosis and to regulate cell cycle. We investigated the role of survivin in human keratinocytes under normal conditions and during UVB irradiation. Survivin siRNA decreases proliferation and induces apoptosis in human keratinocytes, in a mode consistent with the mitotic catastrophe. Low doses UVB increase survivin expression at earlier times, while high doses down-regulate survivin level. Low doses UVB induce cell cycle arrest in G2/M, while high doses UVB cause apoptosis. Moreover, overexpression of survivin protects keratinocytes from UVB-induced apoptosis, and silencing of survivin renders keratinocytes more susceptible to UVB-induced cell death. Finally, survivin siRNA increases UVB-induced reduction of cell proliferation. Taken together, these results indicate that survivin plays a critical role in epidermal homeostasis in normal conditions and during UVB exposure, with possible implication in skin carcinogenesis.     

Troglitazone inhibits cyclin D1 expression and cell cycling independently of PPARgamma in normal mouse skin keratinocytes

Troglitazone is one of the thiazolidinedione (TZD) class of anti-diabetic drugs and a ligand for peroxisome proliferator-activated receptor gamma (PPARgamma). Troglitazone and other PPARgamma ligands have been shown to inhibit cell proliferation and induce cell cycle arrest in a variety of cancer cells, and have been considered as potential tumor preventive and tumor therapeutic agents. Little is known, however, about how normal or initiated cells respond to these agents during mouse skin carcinogenesis. We report here that troglitazone and another TZD, ciglitazone, dramatically inhibited mitogen-induced cellular proliferation in normal mouse skin primary keratinocytes and in the C50 keratinocyte cell line. This was accompanied by induction of cell cycle G1 phase arrest and suppression of cyclin D1, cdk4, and cdk2 expression. Troglitazone suppressed cyclin D1 expression at multiple levels. In addition, we demonstrated that PPARgamma was not expressed at functional levels in cultured mouse skin keratinocytes, and that the inhibitory effects of troglitazone on cellular proliferation and cyclin D1 expression in these cells were PPARgamma-independent. Given the important role of keratinocyte proliferation in skin carcinogenesis, our data suggest that TZD may be useful tumor preventive agents in skin.     

Decreased growth inhibitory responses of squamous carcinoma cells to interferon-gamma involve failure to recruit cki proteins into cdk2 complexes.

Interferon-gamma induces an irreversible growth arrest and squamous differentiation in normal human epidermal keratinocytes. We present for the first time a careful biochemical analysis of the cell-cycle-related events that occur during interferon-gamma treatment of normal human epidermal keratinocytes. The interferon-gamma-induced irreversible growth arrest state is characterized by inhibition of cyclin-dependent kinases, prevention of Rb and p130 (Rb2) phosphorylation, and increases in p27(Kip1), p16(Ink4a), and p130 proteins, together with a transient increase in p21(Waf1/Cip1). Cells derived from squamous cell carcinomas are less responsive to interferon-gamma and do not terminally differentiate. We exploited these differences in response to interferon-gamma in order to identify the particular molecular defects in cell cycle control that promote carcinogenesis in squamous epithelia. In several squamous cell carcinoma cell lines as well as in interferon-gamma-insensitive HaCaT cells, interferon gamma was unable to significantly induce levels of p130 and/or p16 protein. In addition, p21 association with cdk2 complexes was undetectable in either the absence or the presence of interferon-gamma and, unlike normal human epidermal keratinocytes, p27 association with cdk2 did not increase with interferon-gamma treatment. These multiple defects appear to be intrinsic to the mechanisms of cell cycle regulation rather than due to defects in the interferon-gamma signaling pathway, as induction of several interferon-gamma-responsive genes including Stat 1, IRF-1, and p21 itself was normal. Interestingly, exogenous expression of p21 protein in the squamous cell carcinoma cell lines by adenovirus carrying wildtype p53 or p21 cDNA cooperated with interferon-gamma to produce a greater inhibition of growth than either agent alone, even though p21 protein could barely be detected in cdk2 complexes. We conclude that squamous cell carcinoma cells have intrinsic defects in their ability to regulate cdk-cki complexes in response to differentiation signals.     

皮肤鳞状细胞癌和基底细胞癌中p53和C-myc突变基因及蛋白的检测

目的 研究和探讨ｐ５３和Ｃ－ｍｙｃ基因突变在皮肤鳞状细胞癌（ＳＣＣ）和基底细胞癌（ＢＣＣ）的发生,发展中的作用。方法 采用多聚酶链反应－单链构象多态性分析（ＰＣＲ－ＳＳＣＰ）技术检测石蜡标本中ｐ５３与Ｃ－ｍｙｃ基因突变及免疫组化方法检测Ｃ－ｍｙｃ蛋白及突变型ｐ５３基因产物的表达。结果 ３０例ＳＣＣ中１２例ｐ５３基因突变（４０％）和１例Ｃ－ｍｙｃ基因突变,Ｐ５３蛋白表达占５０％（１５／３０）,Ｃ－ｍ     

Molecular mechanism of carcinogenesis by human papillomavirus-16

Human papillomaviruses (HPVs) are common DNA viruses in humans. Recently, epithelial cancers associated with HPV infection have been used as models of virus-induced carcinogenesis. HPVs can be divided into two groups, mucosal and cutaneous. HPV-16 is the most frequent mucosal type associated with cervical cancer. Although the molecular mechanisms of carcinogenesis by HPV-16 have not been completely elucidated, it is apparent that HPV infection is the major risk factor in cervical carcinogenesis. Two viral early genes, E6 and E7, and an upstream regulatory region (URR) are preserved in cervical carcinoma cell lines as well as in clinical samples of cervical cancer, indicating that these regions are important in cancer development. E6 and E7 function as transforming genes. E6 protein binds to and promotes degradation of the tumor suppressor protein, p53, while E7 protein complexes and inactivates the Rb protein; together, they disrupt cell cycle regulation. E6 and E7 are transcribed from a promoter, P97. P97 is regulated by complex interactions between multiple, positive and negative, cellular factors and the viral E2 product. E2 disruption caused by the integration into the cellular genome may induce overexpression of E6 and E7. The E6 and E7 proteins are thought to act as critical factors in cervical carcinogenesis by inactivating the two tumor suppressor proteins, p53 and Rb, which are commonly mutated in other human cancers.