FRAP DNA-dependent protein kinase mediates a late signal transduced from ultraviolet-induced DNA damage

Irradiance-dependent UVB Photocarcinogenesis

Ultraviolet B (UVB) radiation from the sun may lead to photocarcinogenesis of the skin. Sunscreens were used to protect the skin by reducing UVB irradiance, but sunscreen use did not reduce sunburn episodes. It was shown that UVB-induced erythema depends on surface exposure but not irradiance of UVB. We previously showed that irradiance plays a critical role in UVB-induced cell differentiation. This study investigated the impact of irradiance on UVB-induced photocarcinogenesis. For hairless mice receiving equivalent exposure of UVB radiation, the low irradiance (LI) UVB treated mice showed more rapid tumor development, larger tumor burden, and more keratinocytes harboring mutant p53 in the epidermis as compared to their high irradiance (HI) UVB treated counterpart. Mechanistically, using cell models, we demonstrated that LI UVB radiation allowed more keratinocytes harboring DNA damages to enter cell cycle via ERK-related signaling as compared to its HI UVB counterpart. These results indicated that at equivalent exposure, UVB radiation at LI has higher photocarcinogenic potential as compared to its HI counterpart. Since erythema is the observed sunburn at moderate doses and use of sunscreen was not found to associate with reduced sunburn episodes, the biological significance of sunburn with or without sunscreen use warrants further investigation.

Rapamycin inhibits yeast nucleotide excision repair independently of tor kinases

The yeast target of rapamycin (Tor) kinases, Tor1 and Tor2, belong to the phosphatidylinositol 3-kinase-related family of proteins, which are involved in the cellular response to DNA damage and changes in nutrient conditions. In contrast to yeast, many eukaryotes possess a single Tor kinase. Regardless of the number of Tor kinases in an organism, two distinct complexes involving Tor proteins exist in eukaryotes, TORC1 and TORC2. The yeast TORC1, containing Tor1 or Tor2, is sensitive to the antibiotic rapamycin. The yeast TORC2 is insensitive to rapamycin. We examined the influence of rapamycin treatment upon yeast transcription-coupled nucleotide excision repair in a gene transcribed by RNA polymerase II. We also examined tor mutants for their ability to perform transcription-coupled repair in the absence or presence of rapamycin. Ostensibly lacking TORC1 and TORC2 function, a tor1tor2(ts) mutant grown at the nonpermissive temperature exhibited similar rates of repair as the wild-type strain. However, repair of both strands in genes decreases in the wild-type strain and the tor1tor2(ts) mutant exposed to rapamycin. Rapamycin may be inhibiting DNA repair independently of the Tor kinases. In yeast, FPR1 encodes the rapamycin-binding protein Fpr1 that inhibits the TORC1 kinase in the presence of rapamycin. Fap1 competes with rapamycin for Fpr1 binding. Deletion of the FPR1 or FAP1 gene abolishes the inhibitory effect of rapamycin on repair. Thus, the decreased repair observed following rapamycin treatment is independent of TORC1/2 function and likely due to a function of Fap1. We suggest that Fap1 and peptidyl-prolyl isomerases, particularly Fpr1, function in the cellular response to genotoxic stress. Our findings have clinical implications for genetic toxicities associated with genotoxic agents when coadministered with rapamycin.

The Receptor for Advanced Glycation End Products Is Highly Expressed in the Skin and Upregulated by Advanced Glycation End Products and Tumor Necrosis Factor-Alpha

Advanced glycation end products (AGEs) form non-enzymatically from reactions of proteins with reducing sugars. In the skin, AGEs were reported to accumulate in dermal elastin and collagens and to interact nonspecifically with the cell membrane of dermal fibroblasts. Therefore, AGEs may influence the process of skin aging. We investigated the presence of the AGE receptor RAGE in skin and the influence of AGEs on receptor expression and the formation of extracellular matrix (ECM). Sections of sun-protected and sun-exposed skin were analyzed with monoclonal antibodies against (RAGE), heat-shock protein 47, factor XIIIa, CD31, and CD45. RAGE was mainly expressed in fibroblasts, dendrocytes, and keratinocytes and to a minor extent in endothelial and mononuclear cells. Human foreskin fibroblasts (HFFs) highly expressed RAGE on the protein and mRNA level when analyzed by quantitative Western blotting and real-time PCR. Incubation of HFFs with the specific RAGE ligand Nepsilon-(carboxymethyl)lysine-modified BSA (CML-BSA) and tumor necrosis factor-alpha resulted in significant upregulation of RAGE expression. CML-BSA induced a mildly profibrogenic pattern, increasing connective tissue growth factor, transforming growth factor-beta (TGF-beta) 1, and procollagen-alpha1(I) mRNA, whereas expression of matrix metalloproteinase (MMP)-1, -2, -3, and -12 was unaffected. We conclude that in HFFs, AGE-RAGE interactions may influence the process of skin aging through mild stimulation of ECM gene expression.

Cilomilast, tacrolimus and rapamycin modulate dendritic cell function in the elicitation phase of allergic contact dermatitis

BACKGROUND

Cilomilast and tacrolimus as well as rapamycin are potential drugs for the treatment of allergic skin diseases like atopic dermatitis and allergic contact dermatitis.

OBJECTIVES

To compare the in vitro and in vivo immunomodulatory effects of the phosphodiesterase 4 inhibitor cilomilast with those of tacrolimus and rapamycin.

METHODS

The in vitro action of cilomilast, tacrolimus and rapamycin were tested in a mixed leucocyte reaction (MLR). In vivo, the inhibitory action of the immunomodulatory drugs was compared in the toluene-2,4-diisocyanate (TDI)-induced allergic inflammatory response with particular focus on dendritic cell (DC) function.

RESULTS

Cilomilast, tacrolimus and rapamycin were all able to inhibit DC-mediated T-cell activation in a MLR. But it was demonstrated for cilomilast that the target cells are T cells rather than DC. In vivo, a combination of systemic and topical administration of each of these three substances significantly inhibited swelling in the murine ear 16 h after TDI challenge. There was also a reduction in the weight of the draining auricular lymph node, in lymphocyte cell count, and in the number of emigrated DC. The density of Langerhans cells in the epidermis was correspondingly higher in mice treated with cilomilast, tacrolimus and rapamycin than in those treated with vehicle. All three substances were found to inhibit DC migration ex vivo in a skin DC migration assay performed on ear tissue after TDI challenge.

CONCLUSIONS

DC migration into the draining lymph node also takes place in the elicitation phase of allergic contact dermatitis and this migration can be influenced by tacrolimus and rapamycin, and, to a lesser extent, by cilomilast.

Murine coronavirus nonstructural protein p28 arrests cell cycle in G0/G1 phase

Murine coronavirus mouse hepatitis virus (MHV) gene 1 encodes several nonstructural proteins. The functions are unknown for most of these nonstructural proteins, including p28, which is encoded at the 5' end of the MHV genome. Transient expression of cloned p28 in several different cultured cells inhibited cell growth, indicating that p28 expression suppressed cell proliferation. Expressed p28 was exclusively localized in the cytoplasm. Cell cycle analysis by flow cytometry demonstrated that p28 expression induced G(0)/G(1) cell cycle arrest. Characterization of various cellular proteins that are involved in regulating cell cycle progression demonstrated that p28 expression resulted in an accumulation of hypophosphorylated retinoblastoma protein (pRb), tumor suppressor p53, and cyclin-dependent kinase (Cdk) inhibitor p21(Cip1). Expression of p28 did not alter the amount of p53 transcripts yet increased the amount of p21(Cip1) transcripts, suggesting that p28 expression increased p53 stability and that p21(Cip1) was transcriptionally activated in a p53-dependent manner. Our present data suggest the following model of p28-induced G(0)/G(1) cell cycle arrest. Expressed cytoplasmic p28 induces the stabilization of p53, and accumulated p53 causes transcriptional upregulation of p21(Cip1). The increased amount of p21(Cip1) suppresses cyclin E/Cdk2 activity, resulting in the inhibition of pRb hyperphosphorylation. Accumulation of hypophosphorylated pRb thus prevents cell cycle progression from G(0)/G(1) to S phase.

Immunosuppressants and skin cancer in transplant patients: focus on rapamycin

BACKGROUND

The responsibility of immunosuppressants for the increased risk of skin cancers in organ transplant recipients is widely recognized. Discerning the role of each drug is complicated owing to the fact that most patients generally have combinations of several medications.

OBJECTIVE

This article will discuss the role of the main immunosuppressants in the pathogenesis of skin cancers.

METHODS

This work consists of a review of the most significant publications.

RESULTS

Experimental and clinical studies suggest that corticosteroids, azathioprine, cyclosporine (CsA), and tacrolimus increase the incidence of skin cancer. Each drug may act through two different mechanisms including the impairment of the systemic immunosurveillance and a direct oncogenic effect. CsA was shown to be oncogenic independently of its immunosuppressive effect. By contrast, several works on mice have found that rapamycin inhibits tumor growth while being immunosuppressive. Furthermore, rapamycin was shown to inhibit several UV-induced mechanisms involved in skin carcinogenesis. Preliminary clinical studies have reported a lower incidence of skin malignancy in patients treated with rapamycin compared to CsA from the time of transplantation.

CONCLUSION

New immunosuppressive strategies for transplant patients with skin cancer are not only based on minimizing immunosuppression. Data suggest that rapamycin could have a protective effect against skin cancer. Further studies are required to assess accurately the efficacy and tolerance of rapamycin in these patients.

Association of transcription-coupled repair but not global genome repair with ultraviolet-B-induced Langerhans cell depletion and local immunosuppression

Exposure to ultraviolet-B radiation impairs cellular immune responses. This immunosuppression seems to be associated with Langerhans cell migration. DNA damage appears to play a key role because enhanced nucleotide excision repair, a pathway essential for elimination of ultraviolet-B-induced DNA lesions, strongly counteracts immunosuppression. To determine the effect of DNA repair on ultraviolet-B-induced local immunosuppression and Langerhans cell disappearance, three mouse strains carrying different defects in nucleotide excision repair were compared. XPC mice, which were defective in global genome repair, were as sensitive to ultraviolet-B-induced local suppression of contact hypersensitivity to picryl chloride as their wild-type littermates. CSB mice, defective in transcription-coupled repair, were far more sensitive for immunosuppression as were XPA mice, defective in both transcription-coupled repair and global genome repair. Only a moderate depletion of Langerhans cells was observed in XPC mice and wild-type littermates. Ultraviolet-B-induced Langerhans cell depletion was enhanced in CSB and XPA mice. Hence, the major conclusion is that local immunosuppression is only affected when transcription-coupled DNA repair is impaired. Furthermore, a defect in transcription-coupled repair was linked to enhanced ultraviolet-B-induced Langerhans cell depletion. In combination with earlier experiments, it can be concluded that Langerhans cell disappearance is related to ultraviolet-B-induced local but not to systemic immunosuppression.

Frap, FKBP12 rapamycin-associated protein, is a candidate gene for the plasmacytoma resistance locus Pctr2 and can act as a tumor suppressor gene

Susceptibility to mouse plasmacytomagenesis is a complex genetic trait controlled by several Pctr loci (Pctr1, Pctr2, etc). Congenic strain analysis narrowed the genetic interval surrounding the Pctr2 locus, and genes identified in the interval were sequenced from susceptible BALB/c and resistant DBA/2 mice. Frap (FKBP12 rapamycin-associated protein, mTOR, RAFT) was the only gene differing in amino acid sequence between alleles that correlated with strain sensitivity to tumor development. The in vitro kinase activity of the BALB/c FRAP allele was lower than the DBA/2 allele; phosphorylation of p53 and PHAS1/4EBP1 (properties of heat and acid stability/eukaryotic initiation factor 4E-binding protein) and autophosphorylation of FRAP were less efficient with the BALB/c allele. FRAP also suppressed transformation of NIH 3T3 cells by ras, with DBA/2 FRAP being more efficient than BALB/c FRAP. Rapamycin, a specific inhibitor of FRAP, did not inhibit growth of plasmacytoma cell lines. These studies identify Frap as a candidate tumor suppressor gene, in contrast to many reports that have focused on its prooncogenic properties. Frap may be similar to Tgfb and E2f in exerting both positive and negative growth-regulatory signals, depending on the timing, pathway, or tumor system involved. The failure of rapamycin to inhibit plasma cell tumor growth suggests that FRAP antagonists may not be appropriate for the treatment of plasma cell tumors. Pctr2 joins Pctr1 in possessing alleles that modify susceptibility to plasmacytomagenesis by encoding differences in efficiency of function (efficiency alleles), rather than all-or-none, gain-of-function, or loss-of-function alleles. By analogy, human cancer may also result from the combined effects of several inefficient alleles.

Enhanced DNA repair of cyclobutane pyrimidine dimers changes the biological response to UV-B radiation

The goal of DNA repair enzyme therapy is the same as that for gene therapy: to rescue a defective proteome/genome by introducing a substitute protein/DNA. The danger of inadequate DNA repair is highlighted in the genetic disease xeroderma pigmentosum. These patients are hypersensitive to sunlight and develop multiple cutaneous neoplasms very early in life. The bacterial DNA repair enzyme T4 endonuclease V was shown over 25 years ago to be capable of reversing the defective repair in xeroderma pigmentosum cells. This enzyme, packaged in an engineered delivery vehicle, has been shown to traverse the stratum corneum, reach the nuclei of living cells of the skin, and enhance the repair of UV-induced cyclobutane pyrimidine dimers (CPD). In such a system, changes in DNA repair, mutagenesis, and cell signaling can be studied without manipulation of the genome.

Measurement of UVB-Induced DNA damage and its consequences in models of immunosuppression

Exposure to UVB results in formation of cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts in DNA. These can be quantified by a variety of techniques including alkaline gel electrophoresis, ELISAs, Southwestern blotting, and immunohistochemistry. Damage to DNA results in activation of damage response pathways, as indicated by Western blotting using antibodies specific for p53 and breast cancer-associated gene 1 (BRCA1) phosphorylation. The signal from DNA damage to activation of these response pathways appears to be mediated by FKBP12-rapamycin-associated protein (FRAP), since these phosphorylation events are blocked by rapamycin. UVB-induced DNA damage also leads to induction of immunosuppressive cytokines including tumor necrosis factor alpha (TNF-alpha) and interleukin (IL)-10 in skin. Induction of TNF-alpha by UVB is readily detectable in cultured normal human epidermal keratinocytes (NHEKs) using ELISA, while induction of IL-10 is readily detectable in cultured mouse keratinocytes but not in NHEKs. Induction of DNA damage by liposome-encapsulated HindIII results in induction of immunosuppressive responses similar to UVB. Clinical testing shows that liposome-encapsulated T4 endonuclease V or photolyase stimulates repair of CPDs in the skin of human subjects, and prevents UVB-induced immunosuppression. Stimulation of repair and prevention of immunosuppression have been linked to prevention of skin cancer by liposome-encapsulated T4 endonuclease V in repair-deficient xeroderma pigmentosum patients.

Transcriptional regulation of the mdm2 oncogene by p53 requires TRRAP acetyltransferase complexes

The p53 tumor suppressor regulates the cellular response to genetic damage through its function as a sequence-specific transcription factor. Among the most well-characterized transcriptional targets of p53 is the mdm2 oncogene. Activation of mdm2 is critical in the p53 pathway because the mdm2 protein marks p53 for proteosome-mediated degradation, thereby providing a negative-feedback loop. Here we show that the ATM-related TRRAP protein functionally cooperates with p53 to activate mdm2 transcription. TRRAP is a component of several multiprotein acetyltransferase complexes implicated in both transcriptional regulation and DNA repair. In support of a role for these complexes in mdm2 expression, we show that transactivation of the mdm2 gene is augmented by pharmacological inhibition of cellular deacetylases. In vitro analysis demonstrates that p53 directly binds to a TRRAP domain previously shown to be an activator docking site. Furthermore, transfection of cells with antisense TRRAP blocks p53-dependent transcription of mdm2. Finally, using chromatin immunoprecipitation, we demonstrate direct p53-dependent recruitment of TRRAP to the mdm2 promoter, followed by increased histone acetylation. These findings suggest a model in which p53 directly recruits a TRRAP/acetyltransferase complex to the mdm2 gene to activate transcription. In addition, this study defines a novel biochemical mechanism utilized by the p53 tumor suppressor to regulate gene expression.

Activation of protein kinase CK2 is an early step in the ultraviolet B-mediated increase in interstitial collagenase (matrix metalloproteinase-1; MMP-1) and stromelysin-1 (MMP-3) protein levels in human dermal fibroblasts

Enhanced expression of matrix metalloproteinase (MMP)-1/interstitial collagenase and MMP-3/stromelysin-1 in skin fibroblasts and subsequent damage of dermal connective tissue in the context of sun-induced premature aging and skin tumour progression is causally linked to UVB irradiation. Here, we were interested in identifying components of the complex signal-transduction pathway underlying UVB-mediated up-regulation of these delayed UV-responsive genes and focused on components maximally activated early after irradiation. A 2.3-fold increase in protein kinase CK2 activity was measured at 20-40 min after low-dose UVB irradiation (at 10 mJ/cm2) of dermal fibroblasts. This UVB-mediated increase in CK2 activity was abrogated by pharmacological approaches using non-toxic concentrations of the CK2 inhibitor 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole (DRB). Preincubation of fibroblasts with DRB prior to UVB irradiation lowered MMP-1 by 49-69% and MMP-3 protein levels by 55-63% compared with UVB-irradiated controls. By contrast, the CK2 inhibitor did not affect the UVB-triggered transcription of MMPs. Furthermore, UVB irradiation of fibroblasts overexpressing a kinase-inactive mutant of CK2 (CK2alpha-K68A-HA) resulted in lowering of the protein levels of MMP-1 by 25% and MMP-3 by 22% compared with irradiated fibroblasts transfected with the vector control. This reduction in MMP protein levels correlated with the transfection efficiency. Taken together, we describe a novel aspect of protein kinase CK2, namely its inducible activity by UVB irradiation, and provide evidence that CK2 is an early mediator of the UVB-dependent up-regulation of MMP-1 and MMP-3 translation, whereas their major tissue inhibitor of matrix metalloproteinase-1 is not affected by CK2.

Human immunodeficiency virus 1 envelope glycoprotein complex-induced apoptosis involves mammalian target of rapamycin/FKBP12-rapamycin-associated protein-mediated p53 phosphorylation

Syncytia arising from the fusion of cells expressing a lymphotropic human immunodeficiency virus (HIV)-1-encoded envelope glycoprotein complex (Env) gene with cells expressing the CD4/CXCR4 complex undergo apoptosis through a mitochondrion-controlled pathway initiated by the upregulation of Bax. In syncytial apoptosis, phosphorylation of p53 on serine 15 (p53S15) precedes Bax upregulation, the apoptosis-linked conformational change of Bax, the insertion of Bax in mitochondrial membranes, subsequent release of cytochrome c, caspase activation, and apoptosis. p53S15 phosphorylation also occurs in vivo, in HIV-1(+) donors, where it can be detected in preapoptotic and apoptotic syncytia in lymph nodes, as well as in peripheral blood mononuclear cells, correlating with viral load. Syncytium-induced p53S15 phosphorylation is mediated by the upregulation/activation of mammalian target of rapamycin (mTOR), also called FKBP12-rapamycin-associated protein (FRAP), which coimmunoprecipitates with p53. Inhibition of mTOR/FRAP by rapamycin reduces apoptosis in several paradigms of syncytium-dependent death, including in primary CD4(+) lymphoblasts infected by HIV-1. Concomitantly, rapamycin inhibits p53S15 phosphorylation, mitochondrial translocation of Bax, loss of the mitochondrial transmembrane potential, mitochondrial release of cytochrome c, and nuclear chromatin condensation. Transfection with dominant negative p53 has a similar antiapoptotic action as rapamycin, upstream of the Bax upregulation/translocation. In summary, we demonstrate that phosphorylation of p53S15 by mTOR/FRAP plays a critical role in syncytial apoptosis driven by HIV-1 Env.

Effects of solar radiation on the human immune system

On UV irradiation of the skin, a complex cascade of immunological changes results, initiated by cutaneous chromophores and ending in suppression of some local and systemic immune responses. In this review, the stages in this process are outlined first, concentrating on the roles of DNA and urocanic acid as photoreceptors. Evidence indicating UV-induced immunomodulation of delayed hypersensitivity and resistance to infectious diseases in human subjects follows. Aspects of genetic susceptibility to the immunosuppressive effects of UV exposure and extrapolation of the data obtained in animal models to the human situation are included. Finally uncertain and unknown factors relating to the impact of UV on the human immune system are discussed.

Carcinogen-induced inflammation and immunosuppression are enhanced in xeroderma pigmentosum group A model mice associated with hyperproduction of prostaglandin E2

Xeroderma pigmentosum group A (XPA) gene-deficient mice easily developed skin cancers by the application of topical chemical carcinogens as well as by UV irradiation. As certain chemical carcinogens have been shown to be immunosuppressive, we examined the inflammatory and immunosuppressive effects of dimethylbenz(a)anthracene (DMBA) on XPA mice. Compared with wild-type mice, XPA mice showed greater ear swelling and reduction of epidermal Langerhans cells after DMBA application. Topical application of DMBA impaired the induction of contact hypersensitivity, initiated either locally or at distant sites. These DMBA-induced local and systemic immunosuppressions were more greatly enhanced in XPA mice than in wild-type mice. DMBA application induced pronounced production of PGE(2), IL-10, and TNF-alpha in the skin of XPA mice. Treatment with indomethacin, a potent inhibitor of PG biosynthesis, inhibited DMBA-induced inflammation and local immunosuppression. In XPA mice, increased serum IL-10 was detected after DMBA treatment. Excess production of PGE(2), TNF-alpha, and IL-10 after DMBA application may be involved in the enhanced local and systemic immunosuppression in DMBA-treated XPA mice. Susceptibility to DMBA-induced skin tumors in XPA mice may be due to easy impairment of the immune system by DMBA in addition to a defect in the repair of DMBA-DNA adduct. Enhanced immunosuppression by chemical carcinogens as well as the mutagenicity of these mutagens might be associated with the high incidence of internal malignancies seen in XP patients. Moreover, these results supported the hypothesis that persistent DNA damage is a trigger for the production of immunoregulatory cytokines.

Why is DNA damage signaling so complicated? Chaos and molecular signaling

Molecular signaling in eukaryotic cells is accomplished by complex and redundant pathways converging on key molecules that are allosterically controlled by a limited number of signaling proteins. The p53-signaling pathway is an example of a complicated sequence of signals produced in response to DNA damage. This pattern of signaling may arise from chance occurrences at the origin of life and the necessities imposed on a nanomolar system. From this viewpoint, chaos theory may explain the origin, complexity, and convergence of these pathways.