A transcriptomic analysis of the EK1.Br strain of human fibroblastoid keratocytes: the effects of growth, quiescence and senescence

Keratocyte biology

The study of corneal stromal keratocytes is motivated by its strong association with corneal health and visual function. They play a dominant role in the maintenance of corneal homeostasis and transparency through the production of collagens, proteoglycans and corneal crystallins. Trauma-induced apoptosis of keratocytes and replacement by fibroblasts and myofibroblasts disrupt the stromal matrix organization, resulting in corneal haze formation and vision loss. It is, therefore, important to understand the biology and behaviours of keratocytes and the associated stromal cell types (like fibroblasts, myofibroblasts, stromal stem cells) in wound healing, corneal pathologies (including keratoconus, keratitis, endothelial disorders) as well as different ophthalmic situations (such as collagen crosslinking/photodynamic treatment, keratoplasty and refractive surgery, and topical medications). The recent development of ex vivo propagation of keratocytes and stromal stem cells, and their translational applications, either via stromal injection or incorporated in bioscaffold, have been shown to restore the corneal transparency and regenerate native stromal tissue in animal models of corneal haze and other disorders.

Paracrine roles of cellular senescence in promoting tumourigenesis

Senescent cells activate genetic programmes that irreversibly inhibit cellular proliferation, but also endow these cells with distinctive metabolic and signalling phenotypes. Although senescence has historically been considered a protective mechanism against tumourigenesis, the activities of senescent cells are increasingly being associated with age-related diseases, including cancer. An important feature of senescent cells is the secretion of a vast array of pro-inflammatory cytokines, chemokines, and growth factors collectively known as the senescence-associated secretory phenotype (SASP). Recent research has shown that SASP paracrine signalling can mediate several pro-tumourigenic effects, such as enhancing malignant phenotypes and promoting tumour initiation. In this review, we summarise the paracrine activities of senescent cells and their role in tumourigenesis through direct effects on growth and proliferation of tumour cells, tumour angiogenesis, invasion and metastasis, cellular reprogramming and emergence of tumour-initiating cells, and tumour interactions with the local immune environment. The evidence described here suggests cellular senescence acts as a double-edged sword in cancer pathogenesis, which demands further attention in order to support the use of senolytic or SASP-modulating compounds for cancer treatment.

Stem cell senescence drives age-attenuated induction of pituitary tumours in mouse models of paediatric craniopharyngioma

Senescent cells may promote tumour progression through the activation of a senescence-associated secretory phenotype (SASP), whether these cells are capable of initiating tumourigenesis in vivo is not known. Expression of oncogenic β-catenin in Sox2+ young adult pituitary stem cells leads to formation of clusters of stem cells and induction of tumours resembling human adamantinomatous craniopharyngioma (ACP), derived from Sox2- cells in a paracrine manner. Here, we uncover the mechanisms underlying this paracrine tumourigenesis. We show that expression of oncogenic β-catenin in Hesx1+ embryonic precursors also results in stem cell clusters and paracrine tumours. We reveal that human and mouse clusters are analogous and share a common signature of senescence and SASP. Finally, we show that mice with reduced senescence and SASP responses exhibit decreased tumour-inducing potential. Together, we provide evidence that senescence and a stem cell-associated SASP drive cell transformation and tumour initiation in vivo in an age-dependent fashion.

Senescence in the aging process

The accumulation of 'senescent' cells has long been proposed to act as an ageing mechanism. These cells display a radically altered transcriptome and degenerative phenotype compared with their growing counterparts. Tremendous progress has been made in recent years both in understanding the molecular mechanisms controlling entry into the senescent state and in the direct demonstration that senescent cells act as causal agents of mammalian ageing. The challenges now are to gain a better understanding of how the senescent cell phenotype varies between different individuals and tissues, discover how senescence predisposes to organismal frailty, and develop mechanisms by which the deleterious effects of senescent cells can be ameliorated.

Changes in the Transcriptome of Human Astrocytes Accompanying Oxidative Stress-Induced Senescence

Aging is a major risk factor for many neurodegenerative disorders. A key feature of aging biology that may underlie these diseases is cellular senescence. Senescent cells accumulate in tissues with age, undergo widespread changes in gene expression, and typically demonstrate altered, pro-inflammatory profiles. Astrocyte senescence has been implicated in neurodegenerative disease, and to better understand senescence-associated changes in astrocytes, we investigated changes in their transcriptome using RNA sequencing. Senescence was induced in human fetal astrocytes by transient oxidative stress. Brain-expressed genes, including those involved in neuronal development and differentiation, were downregulated in senescent astrocytes. Remarkably, several genes indicative of astrocytic responses to injury were also downregulated, including glial fibrillary acidic protein and genes involved in the processing and presentation of antigens by major histocompatibility complex class II proteins, while pro-inflammatory genes were upregulated. Overall, our findings suggest that senescence-related changes in the function of astrocytes may impact the pathogenesis of age-related brain disorders.

Hormetic effect of rotenone in primary human fibroblasts

BACKGROUND

Rotenone inhibits the electron transfer from complex I to ubiquinone, in this way interfering with the electron transport chain in mitochondria. This chain of events induces increased levels of intracellular reactive oxygen species, which in turn can contribute to acceleration of telomere shortening and induction of DNA damage, ultimately resulting in aging. In this study, we investigated the effect of rotenone treatment in human fibroblast strains.

RESULTS

For the first time we here describe that rotenone treatment induced a hormetic effect in human fibroblast strains. We identified a number of genes which were commonly differentially regulated due to low dose rotenone treatment in fibroblasts independent of their cell origin. However, these genes were not among the most strongly differentially regulated genes in the fibroblast strains on treatment with rotenone. Thus, if there is a common hormesis regulation, it is superimposed by cell strain specific individual responses. We found the rotenone induced differential regulation of pathways common between the two fibroblast strains, being weaker than the pathways individually regulated in the single fibroblast cell strains. Furthermore, within the common pathways different genes were responsible for this different regulation. Thus, rotenone induced hormesis was related to a weak pathway signal, superimposed by a stronger individual cellular response, a situation as found for the differentially expressed genes.

CONCLUSION

We found that the concept of hormesis also applies to in vitro aging of primary human fibroblasts. However, in depth analysis of the genes as well as the pathways differentially regulated due to rotenone treatment revealed cellular hormesis being related to weak signals which are superimposed by stronger individual cell-internal responses. This would explain that in general hormesis is a small effect. Our data indicate that the observed hormetic phenotype does not result from a specific strong well-defined gene or pathway regulation but from weak common cellular processes induced by low levels of reactive oxygen species. This conclusion also holds when comparing our results with those obtained for C. elegans in which the same low dose rotenone level induced a life span extending, thus hormetic effect.

Should we treat aging as a disease? The consequences and dangers of miscategorisation

The aging of the population represents one of the largest healthcare challenges facing the world today. The available scientific evidence shows that interventions are available now that can target fundamental "aging" processes or pathways. Sufficient economic evidence is available to argue convincingly that this approach will also save enormous sums of money which could then be deployed to solve other urgent global problems. However, as yet this scenario has barely entered the public consciousness and, far from being a point of vigorous debate, seems to be ignored by policy makers. Understanding why this lethargy exists is important given the urgent need to deal with the challenge represented by population aging. In this paper I hypothesize that one major cause of inaction is a widely held, but flawed, conceptual framework concerning the relationship between aging and disease that categorizes the former as "natural" and the latter as "abnormal." This perspective is sufficient in itself to act as a disincentive to intervention by rendering those who hold it prone to the "naturalistic fallacy" but can give rise to active hostility to biogerontology if coupled with loose and/or blurred understanding of the goals and potential of the field.

Cellular senescence: from growth arrest to immunogenic conversion

Cellular senescence was first reported in human fibroblasts as a state of stable in vitro growth arrest following extended culture. Since that initial observation, a variety of other phenotypic characteristics have been shown to co-associate with irreversible cell cycle exit in senescent fibroblasts. These include (1) a pro-inflammatory secretory response, (2) the up-regulation of immune ligands, (3) altered responses to apoptotic stimuli and (4) promiscuous gene expression (stochastic activation of genes possibly as a result of chromatin remodeling). Many features associated with senescent fibroblasts appear to promote conversion to an immunogenic phenotype that facilitates self-elimination by the immune system. Pro-inflammatory cytokines can attract and activate immune cells, the presentation of membrane bound immune ligands allows for specific recognition and promiscuous gene expression may function to generate an array of tissue restricted proteins that could subsequently be processed into peptides for presentation via MHC molecules. However, the phenotypes of senescent cells from different tissues and species are often assumed to be broadly similar to those seen in senescent human fibroblasts, but the data show a more complex picture in which the growth arrest mechanism, tissue of origin and species can all radically modulate this basic pattern. Furthermore, well-established triggers of cell senescence are often associated with a DNA damage response (DDR), but this may not be a universal feature of senescent cells. As such, we discuss the role of DNA damage in regulating an immunogenic response in senescent cells, in addition to discussing less established "atypical" senescent states that may occur independent of DNA damage.

A comparison of oncogene-induced senescence and replicative senescence: implications for tumor suppression and aging

Cellular senescence is a stable proliferation arrest associated with an altered secretory pathway, the senescence-associated secretory phenotype. However, cellular senescence is initiated by diverse molecular triggers, such as activated oncogenes and shortened telomeres, and is associated with varied and complex physiological endpoints, such as tumor suppression and tissue aging. The extent to which distinct triggers activate divergent modes of senescence that might be associated with different physiological endpoints is largely unknown. To begin to address this, we performed gene expression profiling to compare the senescence programs associated with two different modes of senescence, oncogene-induced senescence (OIS) and replicative senescence (RS [in part caused by shortened telomeres]). While both OIS and RS are associated with many common changes in gene expression compared to control proliferating cells, they also exhibit substantial differences. These results are discussed in light of potential physiological consequences, tumor suppression and aging.

Better immunity in later life: a position paper

Ageing is the greatest challenge that health-care systems will have to deal with this century. This is because a wide spectrum of pathological impairments emerge in the later part of the human life course which sharply increase mortality and reduce quality of life. Dysfunction of the immune system with advancing age is of crucial importance to the development of disability in later life and finally death. Understanding immune ageing, immunosenescence, has long been recognised as an essential prerequisite for the delivery of effective interventions which will improve late life health. Ten years ago, the ImAginE consortium undertook a broad ranging series of projects which added significantly to our understanding of how fundamental ageing mechanisms drove immune decline. In the decade which followed, abundant evidence has accumulated from nonhuman model systems that ageing results from the progressive operation of a relatively few common processes which act across the major organ systems. These advances in fundamental understanding both allow better clarification of the potential cross-system dysregulation that occurs in ageing and open new avenues for intervention. Over the course of a 2-day workshop, the original ImAginE participants have considered these issues and present some suggestions for current priority areas in immunosenescence.

Cellular senescence and the senescent secretory phenotype: therapeutic opportunities

Aging is the largest risk factor for most chronic diseases, which account for the majority of morbidity and health care expenditures in developed nations. New findings suggest that aging is a modifiable risk factor, and it may be feasible to delay age-related diseases as a group by modulating fundamental aging mechanisms. One such mechanism is cellular senescence, which can cause chronic inflammation through the senescence-associated secretory phenotype (SASP). We review the mechanisms that induce senescence and the SASP, their associations with chronic disease and frailty, therapeutic opportunities based on targeting senescent cells and the SASP, and potential paths to developing clinical interventions.

Insights into CNS ageing from animal models of senescence

In recent years, novel model systems have made significant contributions to our understanding of the processes that control the ageing of whole organisms. However, there are limited data to show that the mechanisms that gerontologists have identified as having a role in organismal ageing contribute significantly to the ageing of the central nervous system. Two recent discoveries illustrate this particularly well. The first is the consistent failure of researchers to demonstrate a simple relationship between organismal ageing and oxidative stress--a mechanism often assumed to have a primary role in brain ageing. The second is the demonstration that senescent cells play a causal part in organismal ageing but remain essentially unstudied in a CNS context. We argue that the animal models now available (including rodents, flies, molluscs and worms), if properly applied, will allow a paradigm shift in our current understanding of the normal processes of brain ageing.

Characterization of cellular senescence mechanisms in human corneal endothelial cells

The human cornea is a tri-laminar structure composed of several cell types with substantial mitotic potential. Age-related changes in the cornea are associated with declining visual acuity and the onset of overt age-related corneal diseases. Corneal transplantation is commonly used to restore vision in patients with damaged or diseased corneas. However, the supply of donor tissue is limited, and thus there is considerable interest in the development of tissue-engineered alternatives. A major obstacle to these approaches is the short replicative lifespan of primary human corneal endothelial cells (HCEC). Accordingly, a comprehensive investigation of the signalling pathways and mechanisms underpinning proliferative lifespan and senescence in HCEC was undertaken. The effects of exogenous human telomerase reverse transcriptase expression, p53 knockdown, disruption of the pRb pathway by over-expression of CDK4 and reduced oxygen concentration on the lifespan of primary HCEC were evaluated. We provide proof-of-principle that forced expression of telomerase, when combined with either p53 knockdown or CDK4 over-expression, is sufficient to produce immortalized HCEC lines. The resultant cell lines express an HCEC-specific transcriptional fingerprint, and retain expression of the corneal endothelial temperature-sensitive potassium channel, suggesting that significant dedifferentiation does not occur as a result of these modes of immortalization. Exploiting these insights into proliferative lifespan barriers in HCEC will underpin the development of novel strategies for cell-based therapies in the human cornea.

Resveratrol, but not dihydroresveratrol, induces premature senescence in primary human fibroblasts

Resveratrol, trans-3,5,4'-trihydroxystilbene, is a polyphenolic compound which has been reported to mimic the gene expression patterns seen in whole animals undergoing dietary restriction. The mechanism of action of resveratrol remains poorly understood, but modulation of both cellular proliferation and apoptosis has been proposed as important routes by which the molecule may exert its effects. This study reports the effects of both resveratrol and dihydroresveratrol (a primary in vivo metabolite) on the proliferative capacity of human primary fibroblasts. No generalised reduction in the growth fraction was observed when fibroblasts derived from three different tissues were treated with resveratrol at concentrations of 10 μm or less. However, concentrations above 25 μm produced a dose-dependent reduction in proliferation. This loss of the growth fraction was paralleled by an increase in the senescent fraction as determined by staining for senescence associated beta galactosidase and dose recovery studies conducted over a 7-day period. Entry into senescence in response to treatment with resveratrol could be blocked by a 30-min preincubation with the p38 MAP kinase inhibitor SB203580. No effects on proliferation were observed when cells were treated with dihydroresveratrol at concentrations of up to 100 μm.

Mechanisms of corneal tissue cross-linking in response to treatment with topical riboflavin and long-wavelength ultraviolet radiation (UVA)

PURPOSE

Treatment of de-epithelialized human corneas with riboflavin (RF) + long-wavelength ultraviolet light (UVA; RFUVA) increases corneal stroma tensile strength significantly. RFUVA treatment retards the progression of keratoconus, perhaps by cross-linking of collagen molecules, but exact molecular mechanisms remain unknown. Research described here tested possible chemical mechanisms of cross-linking.

METHODS

Corneas of rabbits and spiny dogfish sharks were de-epithelialized mechanically, subjected to various chemical pretreatments, exposed to RFUVA, and then subjected to destructive tensile stress measurements. Tensile strength was quantified with a digital force gauge to measure degree of tissue cross-linking.

RESULTS

For both rabbit and shark corneas, RFUVA treatment causes significant cross-linking by mechanism(s) that can be blocked by the presence of sodium azide. Conversely, such cross-linking is greatly enhanced in the presence of deuterium oxide (D(2)O), even when RF is present at only one tenth the currently used clinical concentrations. Blocking carbonyl groups preexisting in the stroma with 2,4-dinitrophenylhydrazide or hydroxylamine blocks essentially all corneal cross-linking. In contrast, blocking free amine groups preexisting in the stroma with acetic anhydride or ethyl acetimidate does not affect RFUVA corneal cross-linking. When both carbonyl groups are blocked and singlet oxygen is quenched, no RFUVA cross-linking occurs, indicating the absence of other cross-linking mechanisms.

CONCLUSIONS

RFUVA catalyzes cross-linking reactions that require production of singlet oxygen ((1)O(2)), whose half-life is extended by D(2)O. Carbonyl-based cross-linking reactions dominate in the corneal stroma, but other possible reaction schemes are proposed. The use of D(2)O as solution media for RF would enable concentration decreases or significant strength enhancement in treated corneas.

Can we intervene in human ageing?

Ageing is a progressive failure of defence and repair processes that produces physiological frailty (the loss of organ reserve with age), loss of homeostasis and eventual death. Over the past ten years exceptional progress has been made in understanding both why the ageing process happens and the mechanisms that are responsible for it. The study of natural mutants that accelerate some, but not all, of the features of the human ageing process has now progressed to a degree that drug trials are either taking place or can be envisaged. Simultaneously, a series of mutations have been identified in different species that confer extended healthy life, indicating that the ageing process is much more malleable than might have been expected and that single interventions have the potential to delay the onset of multiple age-associated conditions. Data generated using these organisms have led to the formulation of a powerful new hypothesis, the ‘green theory’ of ageing. This proposes that a finite capacity to carry out broad-spectrum detoxification and recycling is the primary mechanistic limit on organismal lifespan. This is turn suggests important new experimental approaches and potential interventions designed to increase healthy lifespan.