Ayala King

Succinate dehydrogenase and fumarate hydratase: linking mitochondrial dysfunction and cancer

The phenomenon of enhanced glycolysis in tumours has been acknowledged for decades, but biochemical evidence to explain it is only just beginning to emerge. A significant hint as to the triggers and advantages of enhanced glycolysis in tumours was supplied by the recent discovery that succinate dehydrogenase (SDH) and fumarate hydratase (FH) are tumour suppressors and which associated, for the first time, mitochondrial enzymes and their dysfunction with tumorigenesis. Further steps forward showed that the substrates of SDH and FH, succinate and fumarate, respectively, can mediate a ‘metabolic signalling’ pathway. Succinate or fumarate, which accumulate in mitochondria owing to the inactivation of SDH or FH, leak out to the cytosol, where they inhibit a family of prolyl hydroxylase enzymes (PHDs). Depending on the PHD inhibited, two newly recognized pathways that support tumour maintenance may ensue: affected cells become resistant to certain apoptotic signals and/or activate a pseudohypoxic response that enhances glycolysis and is conveyed by hypoxia-inducible factor.

Transgenic mouse model for studying the transcriptional activity of the p53 protein: age‐ and tissue‐dependent changes in radiation‐induced activation during embryogenesis

The p53 tumor suppressor protein is a sequence‐specific transcriptional activator of target genes. Exposure of cells to DNA damage results in accumulation of biochemically active p53, with consequent activation of p53‐responsive promoters. In order to study how the transcriptional activity of the p53 protein is regulated in vivo, a transgenic mouse strain was generated. These mice harbor the p53‐dependent promoter of the mdm2 gene, fused to a lacZ reporter gene. Induction of lacZ activity by DNA damage (ionizing radiation) was monitored in embryos of different p53 genotypes. The transgenic promoter was substantially activated in vivo following irradiation; activation required functional p53. The activation pattern became more restricted with increasing embryo age, as well as with the state of differentiation of a given tissue. Generally, maximal p53 activation occurred in rapidly proliferating, relatively less differentiated cells. A striking extent of haploinsufficiency was revealed—induction of promoter activity was far less efficient in mice carrying only one wild‐type p53 allele. This suggests that normal levels of cellular p53 are limiting, and any further reduction already compromises the p53 response significantly. Thus, the activation potential of p53 is tightly controlled in vivo, both spatially and temporally, and an important element in this control is the presence of limiting basal levels of activatable p53.

Mitochondria‐derived Reactive Oxygen Species Mediate Blue Light‐induced Death of Retinal Pigment Epithelial Cells

Throughout the lifetime of an individual, light is focused onto the retina. The resulting photooxidative stress can cause acute or chronic retinal damage. The pathogenesis of age‐related macular degeneration (AMD), the leading cause of legal blindness in the developed world, involves oxidative stress and death of the retinal pigment epithelium (RPE) followed by death of the overlying photoreceptors. Evidence suggests that damage due to exposure to light plays a role in AMD and other age‐related eye diseases. In this work a system for lightinduced damage and death of the RPE, based on the human ARPE‐19 cell line, was used. Induction of mitochondriaderived reactive oxygen species (ROS) is shown to play a critical role in the death of cells exposed to short‐wavelength blue light (425 ± 20 nm). ROS and cell death are blocked either by inhibiting the mitochondrial electron transport chain or by mitochondria‐specific antioxidants. These results show that mitochondria are an important source of toxic oxygen radicals in blue light‐exposed RPE cells and may indicate new approaches for treating AMD using mitochondria‐targeted antioxidants.

Fumarate induces redox-dependent senescence by modifying glutathione metabolism

Mutations in the tricarboxylic acid (TCA) cycle enzyme fumarate hydratase (FH) are associated with a highly malignant form of renal cancer. We combined analytical chemistry and metabolic computational modelling to investigate the metabolic implications of FH loss in immortalized and primary mouse kidney cells. Here, we show that the accumulation of fumarate caused by the inactivation of FH leads to oxidative stress that is mediated by the formation of succinicGSH, a covalent adduct between fumarate and glutathione. Chronic succination of GSH, caused by the loss of FH, or by exogenous fumarate, leads to persistent oxidative stress and cellular senescence in vitro and in vivo. Importantly, the ablation of p21, a key mediator of senescence, in Fh1-deficient mice resulted in the transformation of benign renal cysts into a hyperplastic lesion, suggesting that fumarate-induced senescence needs to be bypassed for the initiation of renal cancers.

HIRA orchestrates a dynamic chromatin landscape in senescence and is required for suppression of neoplasia

Cellular senescence is a stable proliferation arrest that suppresses tumorigenesis. Cellular senescence and associated tumor suppression depend on control of chromatin. Histone chaperone HIRA deposits variant histone H3.3 and histone H4 into chromatin in a DNA replication-independent manner. Appropriately for a DNA replication-independent chaperone, HIRA is involved in control of chromatin in nonproliferating senescent cells, although its role is poorly defined. Here, we show that nonproliferating senescent cells express and incorporate histone H3.3 and other canonical core histones into a dynamic chromatin landscape. Expression of canonical histones is linked to alternative mRNA splicing to eliminate signals that confer mRNA instability in nonproliferating cells. Deposition of newly synthesized histones H3.3 and H4 into chromatin of senescent cells depends on HIRA. HIRA and newly deposited H3.3 colocalize at promoters of expressed genes, partially redistributing between proliferating and senescent cells to parallel changes in expression. In senescent cells, but not proliferating cells, promoters of active genes are exceptionally enriched in H4K16ac, and HIRA is required for retention of H4K16ac. HIRA is also required for retention of H4K16ac in vivo and suppression of oncogene-induced neoplasia. These results show that HIRA controls a specialized, dynamic H4K16ac-decorated chromatin landscape in senescent cells and enforces tumor suppression.

Glucose metabolism and programmed cell death: an evolutionary and mechanistic perspective.

Over the last decade, cellular glucose metabolism has emerged as a central player in the mechanisms of programmed cell death (PCD). We examined the metabolic foundations of apoptosis from a Darwinian context and suggest that PCD has evolved from the cellular response to metabolic stress, most notably in relation to glucose metabolism. Whilst apoptosis and other forms of PCD are essential to the development, maintenance and survival of multicellular organisms, it is now evident that controlled and selective cell death confers fitness advantages in unicellular organisms. All species may thus harbour a fundamental relationship between the availability of basic nutrients and life/death decisions. This evolutionary perspective may inform our understanding of PCD in its many guises.

Generation of Cre Transgenic Mice with Postnatal RPE-Specific Ocular Expression

PURPOSE To generate and characterize a constitutively active, RPE-specific, cre-expressing transgenic mouse line. This line can be used to create RPE-specific knockouts by crossing with mice harboring loxP-flanked (floxed) genes. METHODS A transgene construct was assembled with the BEST1 promoter driving cre expression. Transgenic mice were generated on a C57BL/6 background. Cre expression was assessed by immunofluorescence and Western blot analysis. Cre enzymatic activity was tested by crossing to three lines with floxed DNA regions and detecting deletion of the intervening sequences or through histochemical detection of lacZ activity. Potential cre-mediated toxicity was assessed by retinal histology up to 24 months of age and by electroretinography. RESULTS The BEST1-cre line with expression in the highest percentage of RPE cells displayed a patchy mosaic expression pattern, with 50% to 90% of RPE cells expressing cre. In mice outcrossed to a mixed B6/129 background, expression was consistently found in 90% of RPE cells. Within the eye, only the RPE cells were immunoreactive with an anti-cre antibody. Maximum cre expression quantified by Western blot analysis occurred at P28. Crosses with three lines containing floxed sequences revealed RPE-specific cre activity in the eye and extraocular expression limited to the testes. Histology and electroretinography showed no cre-mediated RPE toxicity. CONCLUSIONS This BEST1-cre transgenic line enables generation of RPE-specific knockout mice. The mosaic expression pattern provides an internal control; the non-cre-expressing RPE cells continue to express the floxed genes. These mice should facilitate study of the multifunctional RPE and the generation of mouse models of human retinal disease.

Wnt signaling potentiates nevogenesis

Significance Human benign nevi (moles) are clonal neoplasms that rarely progress to melanoma because their cells (melanocytes) are arrested in a viable but nonproliferating state (senescence). However, at low frequency, nevus melanocytes do progress to melanoma. Consequently, it is important to understand the factors that determine nevus formation and progression to melanoma. We present evidence that repression of a proliferation-promoting cell signaling pathway (Wnt signaling pathway) contributes to senescence of melanocytes in vitro. However, Wnt signaling remains active in some senescent human melanocytes in nevi, and activation of Wnt signaling leads to a delay in melanocyte senescence in a mouse model. We suggest that activated Wnt signaling in human nevi delays senescence to promote nevus formation, and thereafter, persistent Wnt signaling might undermine senescence-mediated tumor suppression. Cellular senescence is a stable proliferation arrest associated with an altered secretory pathway (senescence-associated secretory phenotype). Cellular senescence is also a tumor suppressor mechanism, to which both proliferation arrest and senescence-associated secretory phenotype are thought to contribute. The melanocytes within benign human nevi are a paradigm for tumor-suppressive senescent cells in a premalignant neoplasm. Here a comparison of proliferating and senescent melanocytes and melanoma cell lines by RNA sequencing emphasizes the importance of senescence-associated proliferation arrest in suppression of transformation. Previous studies showed that activation of the Wnt signaling pathway can delay or bypass senescence. Consistent with this, we present evidence that repression of Wnt signaling contributes to melanocyte senescence in vitro. Surprisingly, Wnt signaling is active in many senescent human melanocytes in nevi, and this is linked to histological indicators of higher proliferative and malignant potential. In a mouse, activated Wnt signaling delays senescence-associated proliferation arrest to expand the population of senescent oncogene-expressing melanocytes. These results suggest that Wnt signaling can potentiate nevogenesis in vivo by delaying senescence. Further, we suggest that activated Wnt signaling in human nevi undermines senescence-mediated tumor suppression and enhances the probability of malignancy.

Research into cancer metabolomics: Towards a clinical metamorphosis

The acknowledgement that metabolic reprogramming is a central feature of cancer has generated high expectations for major advances in both diagnosis and treatment of malignancies through addressing metabolism. These have so far only been partially fulfilled, with only a few clinical applications. However, numerous diagnostic and therapeutic compounds are currently being evaluated in either clinical trials or pre-clinical models and new discoveries of alterations in metabolic genes indicate future prognostic or other applicable relevance. Altogether, these metabolic approaches now stand alongside other available measures providing hopes for the prospects of metabolomics in the clinic. Here we present a comprehensive overview of both ongoing and emerging clinical, pre-clinical and technical strategies for exploiting unique tumour metabolic traits, highlighting the current promises and anticipations of research in the field.

Protein Phosphatase 1 binds strongly to the retinoblastoma protein but not to p107 or p130 in vitro and in vivo

Purpose. To identify and characterize retinoblastoma protein (pRb) binding proteins that may influence retinoblast proliferation and retinal pigment epithelial cell survival. Methods. The yeast two-hybrid system was used to screen a bovine retinal cDNA library and to characterize positive clones. DNA sequencing and site-directed mutagenesis were used for further analysis. Co-immunoprecipitation experiments were used to confirm the results of the two-hybrid system in vivo. Results. In the two-hybrid system, Protein Phosphatase 1a1 (PP1a1) binds the retinoblastoma protein. Unlike several other pRb binding proteins, PP1a1 binds only weakly to the Rb family member p107, and does not demonstrate detectable binding to p130. Confirming the two-hybrid results, endogenous PP1 in a human retinal pigment epithelial (RPE) cell line co-immunoprecipitates with endogenous pRb but not p107 or p130. Site directed mutagenesis of two pRb binding motifs in PP1a1 from LXSXE to LXCXE leads to slight increases in its two-hybrid interaction with pRb but does not alter its binding preference for pRb over the other family members. The complete sequence of bovine PP1a1 is reported. Conclusion. The strong two-hybrid interaction between PP1a1 and pRb, but not p107 or p130, suggests that the phosphorylation status of members of the pRb family may be regulated by different phosphatases, contributing to fine control of cell cycle progression. Conversely, PP1 activity may be specifically regulated by pRb and not p107 or p130. Mutagenesis studies suggest that PP1a1s LXSXE motif is not responsible for its binding preference for pRb over p107 and p130. Disruption of the PP1-pRb interaction may influence retinoblastoma tumorigenesis as well as RPE cell proliferation and survival.