Disruption of the nucleotide excision repair (NER) pathway by mutations can cause xeroderma pigmentosum, a syndrome predisposing affected individuals to development of skin cancer. The xeroderma pigmentosum C (XPC) protein is essential for initiating global genome NER by recognizing the DNA lesion and recruiting downstream factors. Here we show that inhibition of the deacetylase and longevity factor SIRT1 impairs global genome NER through suppressing the transcription of XPC in a SIRT1 deacetylase-dependent manner. SIRT1 enhances XPC expression by reducing AKTdependent nuclear localization of the transcription repressor of XPC. Finally, we show that SIRT1 levels are significantly reduced in human skin tumors from Caucasian patients, a population at highest risk. These findings suggest that SIRT1 acts as a tumor suppressor through its role in DNA repair.ucleotide excision repair (NER) is a versatile DNA repair pathway that eliminates a wide variety of helix-distorting base lesions, including UV radiation-induced cyclobutane pyrimidine dimers (CPD) and (6-4) photoproducts (6-4PPs) (1, 2), as well as bulky adducts induced by numerous chemical compounds. Defects in NER by mutations cause the autosomal recessive xeroderma pigmentosum (XP) and Cockayne syndromes (3-5). XP patients are clinically characterized by cutaneous sensitivity to sunlight exposure and a predisposition to skin cancer. Seven NER-deficient genetic complementation groups of XP (XP-A to -G) have been identified, and all of the corresponding genes have now been cloned (3-5).Mammalian NER consists of two distinct subpathways: global genome NER (GG-NER), which operates throughout the genome, and transcription-coupled NER (TC-NER), which specifically removes lesions on the transcribed DNA strand of active genes. A major difference between these two pathways appears to lie in the strategies for detecting damaged bases. Accumulating evidence indicates that the XP group C (XPC) protein plays an essential role in GG-NER-specific damage recognition (6-8). Although biochemical and genetic analyses have characterized the function of XPC in considerable detail, much remains to be elucidated with regard to its regulation and interaction with other critical cellular pathways.Sirtuin 1 (SIRT1), a mammalian counterpart of the yeast silent information regulator 2 (Sir2) and a proto member of the sirtuin family, is an NAD-dependent longevity-promoting deacetylase. SIRT1 is crucial for cell survival, metabolism, senescence, and stress response in several cell types and tissues (9-13). Both histone and nonhistone targets of SIRT1 have been identified, including FOXO, p53, and PPARγ (10,12,14). SIRT1 has been implicated as an important player in cancer. However, it remains unclear whether SIRT1 serves as a tumor suppressor or a tumor promoter (13-16). SIRT1 expression is relatively higher in many malignancies, including colon, breast, prostate, and skin cancers and leukemia, as compared with their corresponding normal tissues (17)(18)(19)(20)(21)(22). But the specific c...
Skin and dental findings comprise 4 of 11 major features and 3 of 6 minor features in the diagnostic criteria. A definite diagnosis of TSC is defined as the presence of at least 2 major features or 1 major and 2 or more minor features; in addition, a pathological mutation in TSC1 or TSC2 is diagnostic. Skin and oral examinations should be performed annually and every 3 to 6 months, respectively. Intervention may be indicated for TSC skin or oral lesions that are bleeding, symptomatic, disfiguring, or negatively affecting function. Options presented include surgical excision, laser(s), or use of a mammalian target of rapamycin inhibitor.
Non-melanoma skin cancer is the most common cancer in the U.S., where DNA-damaging UVB radiation from the sun remains the major environmental risk factor. However, the critical genetic targets of UVB radiation are undefined. Here we show that attenuating PTEN in epidermal keratinocytes is a predisposing factor for UVB-induced skin carcinogenesis in mice. In skin papilloma and squamous cell carcinoma (SCC), levels of PTEN were reduced compared to skin lacking these lesions. Likewise, there was a reduction in PTEN levels in human premalignant actinic keratosis and malignant SCC, supporting a key role for PTEN in human skin cancer formation and progression. PTEN downregulation impaired the capacity of global genomic nucleotide excision repair (GG-NER), a critical mechanism for removing UVB-induced mutagenic DNA lesions. In contrast to the response to ionizing radiation, PTEN downregulation prolonged UVB-induced growth arrest and increased the activation of the Chk1 DNA damage pathway in an AKT-independent manner, likely due to reduced DNA repair. PTEN loss also suppressed expression of the key GG-NER protein xeroderma pigmentosum C (XPC) through the AKT/p38 signaling axis. Reconstitution of XPC levels in PTEN-inhibited cells restored GG-NER capacity. Taken together, our findings define PTEN as an essential genomic gatekeeper in the skin, through its ability to positively regulate XPC-dependent GG-NER following DNA damage.
Ultraviolet (UV) radiation in sunlight is the major environmental cause of skin cancer. PTEN (phosphatase and tensin homolog deleted on chromosome 10) is a proven critical tumor suppressor. We report here that UVB down-regulates PTEN in primary human keratinocytes, human HaCaT keratinocytes, and mouse skin. As compared to normal skin, the PTEN levels are reduced in human actinic keratosis, a precancerous skin lesion caused by solar UV. PTEN down-regulation is mediated by two mechanisms: (1) PTEN is cleaved by active caspase in apoptotic cells in which AKT activation is reduced; and (2) PTEN transcription is suppressed in surviving cells, and this suppression is independent of caspase activation and occurs in parallel with increased ERK and AKT activation. We report here that the combination of ERK and AKT activation is crucial for PTEN suppression in surviving cells following UVB irradiation. PTEN remains suppressed in these cells. AKT activation is higher in UVB-irradiated surviving cells as compared to UVB protected control cells. ERK and AKT pathways are involved in sustaining PTEN suppression in UVB-exposed cells. Increasing PTEN expression enhances apoptosis of keratinocytes in response to UVB radiation. Our findings indicate that (1) UVB radiation suppresses PTEN expression in keratinocytes, and (2) the ERK/AKT/PTEN axis may form a positive feedback loop following UVB irradiation. Identification of PTEN as a critical molecular target of UVB will add to our understanding of the pathogenesis of skin cancer.
Macroautophagy (hereafter autophagy) is a cellular "self-eating" process that is implicated in many human cancers, where it can act to either promote or suppress tumorigenesis. However, the role of autophagy in regulation of inflammation during tumorigenesis remains unclear. Here we show that autophagy is induced in the epidermis by ultraviolet (UV) irradiation and autophagy gene Atg7 promoted UV-induced inflammation and skin tumorigenesis. Atg7 regulated UV-induced cytokine expression and secretion, and promoted Ptgs2/Cox-2 expression through both a CREB1/CREB-dependent cell autonomous mechanism and an IL1B/IL1β-dependent non-cell autonomous mechanism. Adding PGE increased UV-induced skin inflammation and tumorigenesis, reversing the epidermal phenotype in mice with Atg7 deletion in keratinocytes. Similar to ATG7 knockdown in human keratinocytes, ATG5 knockdown inhibited UVB-induced expression of PTGS2 and cytokines. Furthermore, ATG7 loss increased the activation of the AMPK pathway and the phosphorylation of CRTC1, and led to endoplasmic reticulum (ER) accumulation and reduction of ER stress. Inducing ER stress and inhibiting calcium influx into the ER by thapsigargin reverses the inflammation and tumorigenesis phenotype in mice with epidermal Atg7 deletion. Taken together, these findings demonstrate that deleting autophagy gene Atg7 leads to a suppression of carcinogen-induced protumorigenic inflammatory microenvironment and tumorigenesis of the epithelium.
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