Susana Velasco-Miguel

Sirt1 protects against high-fat diet-induced metabolic damage

The identification of new pharmacological approaches to effectively prevent, treat, and cure the metabolic syndrome is of crucial importance. Excessive exposure to dietary lipids causes inflammatory responses, deranges the homeostasis of cellular metabolism, and is believed to constitute a key initiator of the metabolic syndrome. Mammalian Sirt1 is a protein deacetylase that has been involved in resveratrol-mediated protection from high-fat diet-induced metabolic damage, but direct proof for the implication of Sirt1 has remained elusive. Here, we report that mice with moderate overexpression of Sirt1 under the control of its natural promoter exhibit fat mass gain similar to wild-type controls when exposed to a high-fat diet. Higher energy expenditure appears to be compensated by a parallel increase in food intake. Interestingly, transgenic Sirt1 mice under a high-fat diet show lower lipid-induced inflammation along with better glucose tolerance, and are almost entirely protected from hepatic steatosis. We present data indicating that such beneficial effects of Sirt1 are due to at least two mechanisms: induction of antioxidant proteins MnSOD and Nrf1, possibly via stimulation of PGC1α, and lower activation of proinflammatory cytokines, such as TNFα and IL-6, via down-modulation of NFκB activity. Together, these results provide direct proof of the protective potential of Sirt1 against the metabolic consequences of chronic exposure to a high-fat diet.

PA26, a novel target of the p53 tumor suppressor and member of the GADD family of DNA damage and growth arrest inducible genes

Exposure of mammalian cells to hypoxia, radiation and certain chemotherapeutic agents promotes cell cycle arrest and/or apoptosis. Activation of p53 responsive genes is believed to play an important role in mediating such responses. In this study we identified a novel gene, PA26, which maps to chromosome 6q21 and encodes at least three transcript isoforms, of which two are differentially induced by genotoxic stress (UV, γ-irradiation and cytotoxic drugs) in a p53-dependent manner. A functional p53-responsive element was identified in the second intron of the PA26 gene, in consistence with a mechanism of transcriptional induction of the PA26 gene by p53. No clues to its functions were revealed by sequence analysis, although pronounced negative regulation by serum factors argues for a potential role of PA26 in growth regulation. Immunological analysis suggests that PA26 protein(s) is localized to the cell nucleus. Our results suggest that the PA26 gene is a novel p53 target gene with properties common to the GADD family of growth arrest and DNA damage-inducible stress-response genes, and, thus, a potential novel regulator of cellular growth.

Quantitative Analysis of Translesion DNA Synthesis across a Benzo[a]pyrene-Guanine Adduct in Mammalian Cells THE ROLE OF DNA POLYMERASE κ

Replication across unrepaired DNA lesions in mammalian cells is effected primarily by specialized, low fidelity DNA polymerases. We studied translesion DNA synthesis (TLS) across a benzo[a]pyrene-guanine (BP-G) adduct, a major mutagenic DNA lesion generated by tobacco smoke. This was done using a quantitative assay that measures TLS indirectly, by measuring the recovery of gapped plasmids transfected into cultured mammalian cells. Analysis of PolK+/+ mouse embryo fibroblasts (MEFs) showed that TLS across the BP-G adduct occurred with an efficiency of 48 ± 4%, which is an order of magnitude higher than in Escherichia coli. In PolK–/– MEFs, bypass was 16 ± 1%, suggesting that at least two-thirds of the BP-G adducts in MEFs were bypassed exclusively by polymerase κ (polκ). In contrast, polη was not required for bypass across BP-G in a human XP-V cell line. Analysis of misinsertion specificity across BP-G revealed that bypass was more error-prone in MEFs lacking polκ. Expression of polκ from a plasmid introduced into PolK–/– MEFs restored both the extent and fidelity of bypass across BP-G. Polκ was not required for bypass of a synthetic abasic site. In vitro analysis demonstrated efficient bypass across BP-G by both polκ and polη, suggesting that the biological role of polκ in TLS across BP-G is due to regulation of TLS and not due to an exclusive ability to bypass this lesion. These results indicate that BP-G is bypassed in mammalian cells with relatively high efficiency and that polκ bypasses BP-G in vivo with higher efficiency and higher accuracy than other DNA polymerases.

DNA polymerase κ deficiency does not affect somatic hypermutation in mice

Somatic hypermutation (SH) in B cells undergoing T cell‐dependent immune responses generates high‐affinity antibodies that provide protective immunity. Most current models of SH postulate the introduction of a nick into the DNA and subsequent replication‐independent, error‐prone short‐patch synthesis by one or more DNA polymerases. The PolK (DinB1) gene encodes a specialized mammalian DNA polymerase called DNA polymerase κ (polκ), a member of the recently discovered Y family of DNA polymerases. The mouse PolK gene is expressed at high levels in the seminiferous tubules of the testis and in the adrenal cortex, and at lower levels in most other cells of the body including B lymphocytes. In vitro studies showed that polκ can act as an error‐prone polymerase, although they failed to ascribe a clear function to this enzyme. The ability of polκ to generate mutations when extending primers on undamaged DNA templates identifies this enzyme as a potential candidate for the introduction of nucleotide changes in the immunoglobulin (Ig) genes during the process of SH. Here we show that polκ‐deficient mice are viable, fertile and able to mount a normal immune response to the antigen (4‐hydroxy‐3‐nitrophenyl)acetyl‐chicken γ‐globulin (NP‐GC). They also mutate their Ig genes normally. However, polκ‐deficientembryonic fibroblasts are abnormally sensitive to killing following exposure to ultraviolet (UV) radiation, suggesting a role of polκ in translesion DNA synthesis.

Tumour biology: Policing of oncogene activity by p53.

The tumour-suppressor protein p53 provides the most important genetic defence against cancer and is activated in response to DNA damage and to oncogenic signalling, both of which occur almost universally in malignant tumours. But the relative contribution of these two pathways in inducing p53-dependent protection against cancer is unclear. Here we show that p53-dependent protection against cancer is lost in mice that have been genetically manipulated so that their p53 is activated in response to DNA damage but not to oncogenic signalling. We conclude that oncogenic signalling is the critical event that elicits p53-dependent protection and that the DNA-damage stimulus is less important.

Induction of p53-Dependent Senescence by the MDM2 Antagonist Nutlin-3a in Mouse Cells of Fibroblast Origin

Cellular senescence is emerging as an important in vivo anticancer response elicited by multiple stresses, including currently used chemotherapeutic drugs. Nutlin-3a is a recently discovered small-molecule antagonist of the p53-destabilizing protein murine double minute-2 (MDM2) that induces cell cycle arrest and apoptosis in cancer cells with functional p53. Here, we report that nutlin-3a induces cellular senescence in murine primary fibroblasts, oncogenically transformed fibroblasts, and fibrosarcoma cell lines. No evidence of drug-induced apoptosis was observed in any case. Nutlin-induced senescence was strictly dependent on the presence of functional p53 as revealed by the fact that cells lacking p53 were completely insensitive to the drug, whereas cells lacking the tumor suppressor alternative reading frame product of the CDKN2A locus underwent irreversible cell cycle arrest. Interestingly, irreversibility was achieved in neoplastic cells faster than in their corresponding parental primary cells, suggesting that nutlin-3a and oncogenic signaling cooperate in activating p53. Our current results suggest that senescence could be a major cellular outcome of cancer therapy by antagonists of the p53-MDM2 interaction, such as nutlin-3a.

UVB radiation-induced cancer predisposition in Cockayne syndrome group A (Csa) mutant mice

Cockayne syndrome (CS) is an inherited photosensitive neurodevelopmental disorder caused by a specific defect in the transcription-coupled repair (TCR) sub-pathway of NER. Remarkably, despite their DNA repair deficiency, CS patients do not develop skin cancer. Here, we present a mouse model for CS complementation group A. Like cells from CS-A patients, Csa-/- mouse embryonic fibroblasts (MEFs): (i) are ultraviolet (UV)-sensitive; (ii) show normal unscheduled DNA synthesis (indicating that the global genome repair sub-pathway is unaffected); (iii) fail to resume RNA synthesis after UV-exposure and (iv) are unable to remove cyclobutane pyrimidine dimers (CPD) photolesions from the transcribed strand of active genes. CS-A mice exhibit UV-sensitivity and pronounced age-dependent loss of retinal photoreceptor cells but otherwise fail to show the severe developmental and neurological abnormalities of the human syndrome. In contrast to human CS, Csa-/- animals develop skin tumors after chronic exposure to UV light, indicating that TCR in mice protects from UV-induced skin cancer development. Strikingly, inactivation of one Xpc allele (encoding a component of the damage recognition complex involved in the global genome repair sub-pathway) in Csa-/- mice resulted in a strongly enhanced UV-mediated skin cancer sensitivity, indicating that in a TC repair defective background, the Xpc gene product may be a rate-limiting factor in the removal of UV-induced DNA lesions.

Constitutive and regulated expression of the mouse Dinb (Polκ) gene encoding DNA polymerase kappa

A recently discovered group of novel polymerases are characterized by significantly reduced fidelity of DNA synthesis in vitro. This feature is consistent with the relaxed fidelity required for the replicative bypass of various types of base damage that frequently block high fidelity replicative polymerases. The present studies demonstrate that the specialized DNA polymerase kappa (polkappa) is uniquely and preferentially expressed in the adrenal cortex and testis of the mouse, as well as in a variety of other tissues. The adrenal cortex is the sole site of detectable expression of the Polkappa gene in mouse embryos. This adrenal expression pattern is consistent with a requirement for polkappa for the replicative bypass of DNA base damage generated during steroid biosynthesis. The expression pattern of polkappa in the testis is specific for particular stages of spermatogenesis and is distinct from the expression pattern of several other low fidelity DNA polymerases that are also expressed during spermatogenesis. The mouse (but not the human) Polkappa gene is primarily regulated by the p53 gene and is upregulated in response to exposure to various DNA-damaging agents in a p53-dependent manner.

Genetic dissection of the role of p21Cip1/Waf1 in p53-mediated tumour suppression.

Protein p21Cip1/Waf1 is transcriptionally activated by the tumour suppressor p53 and previous studies have shown that p21 plays a role in tumour suppression. However, the involvement of p21 in p53-mediated tumour suppression remains to be directly demonstrated in vivo. Tumour suppression mediated by p53 can be measured by comparing tumour susceptibility in animals carrying two (wild-type mice) or three (super-p53 mice) copies of the p53 gene. We have taken advantage of this genetically defined system to measure p53-mediated cell-cycle arrest, apoptosis and tumorigenesis, in a p21 wild-type and in a p21-null context. The absence of p21 significantly impaired the enhanced p53-mediated cell-cycle arrest characteristic of super-p53 cells, but did not affect the enhanced apoptosis. Importantly, in an experimental model of fibrosarcoma induction, the absence of p21 significantly decreased the tumour suppression benefit of super-p53 mice. We conclude that cell-cycle arrest through p21 plays a significant role in mediating p53-dependent cancer protection.

A novel p53 mutational hotspot in skin tumors from UV-irradiated Xpc mutant mice alters transactivation functions

A mutation in codon 122 of the mouse p53 gene resulting in a T to L amino acid substitution (T122→L) is frequently associated with skin cancer in UV-irradiated mice that are both homozygous mutant for the nucleotide excision repair (NER) gene Xpc (Xpc−/−) and hemizygous mutant for the p53 gene. We investigated the functional consequences of the mouse T122→L mutation when expressed either in mammalian cells or in the yeast Saccharomyces cerevisiae. Similar to a non-functional allele, high expression of the T122→L allele in p53−/− mouse embryo fibroblasts and human Saos-2 cells failed to suppress growth. However, the T122→L mutant p53 showed wild-type transactivation levels with Bax and MDM2 promoters when expressed in either cell type and retained transactivation of the p21 and the c-Fos promoters in one cell line. Using a recently developed rheostatable p53 induction system in yeast we assessed the T122→L transactivation capacity at low levels of protein expression using 12 different p53 response elements (REs). Compared to wild-type p53 the T122→L protein manifested an unusual transactivation pattern comprising reduced and enhanced activity with specific REs. The high incidence of the T122→L mutant allele in the Xpc−/− background suggests that both genetic and epigenetic conditions may facilitate the emergence of particular functional p53 mutations. Furthermore, the approach that we have taken also provides for the dissection of functions that may be retained in many p53 tumor alleles.