Terminal differentiation and senescence in the human melanocyte: repression of tyrosine-phosphorylation of the extracellular signal-regulated kinase 2 selectively defines the two phenotypes

Current Understanding of the Role of Senescent Melanocytes in Skin Ageing

Melanocytes reside within the basal epidermis of human skin, and function to protect the skin from ultraviolet light through the production of melanin. Prolonged exposure of the skin to UV light can induce irreparable DNA damage and drive cells into senescence, a sustained cell cycle arrest that prevents the propagation of this damage. Senescent cells can also be detrimental and contribute to skin ageing phenotypes through their senescence-associated secretory phenotype. Senescent cells can act in both an autocrine and paracrine manner to produce widespread tissue inflammation and skin ageing. Recently, melanocytes have been identified as the main senescent cell population within the epidermis and have been linked to a variety of skin ageing phenotypes, such as epidermal thinning and the presence of wrinkles. However, the literature surrounding melanocyte senescence is limited and tends to focus on the role of senescence in the prevention of melanoma. Therefore, this review aims to explore the current understanding of the contribution of senescent melanocytes to human skin ageing.

Protooncogene MYC drives human melanocyte melanogenesis and senescence

In spite of extensive research and advances on the molecular biology of melanoma, the process of melanocytic differentiation or its relationship with proliferation is poorly understood. The role of proto-oncogenes in normal melanocyte biology is also intriguing. Proto-oncogene MYC is overexpressed in 40% of melanomas. It has been suggested that MYC can mediate senescence bypass in malignant melanocytes, an important event in melanoma development, likely in cooperation with other oncogenic pathways. However, despite the apparent importance of MYC in melanoma, its functions in normal melanocytes are unknown. We have overexpressed MYC in freshly isolated human primary melanocytes and studied the effects on melanocytic proliferation and differentiation. MYC promoted a transient activation of melanocytes including cell cycle entry, DNA damage and cell migration. Subsequently, MYC induced melanogenesis, increased cellular size and complexity and senescence. Interestingly, we also found strong expression of MYC in regions of human nevi displaying high pigmentation and high expression of senescence marker p16. The results altogether show that MYC drives melanocytic differentiation and suggest that senescence is associated with differentiation. We discuss the implications into the mechanisms governing melanocytic differentiation and the development of melanoma.

Melanocyte abnormalities and senescence in the pathogenesis of idiopathic guttate hypomelanosis

BACKGROUND

Idiopathic guttate hypomelanosis (IGH) is a pigmentary disorder of unknown pathogenesis characterized by small discrete white macules. In the skin, epidermal melanin unit between melanocytes and keratinocytes is responsible for melanin synthesis and equal distribution of melanin pigment.

OBJECTIVE

Therefore, this study was designed to check the role of melanocytes in the pathogenesis of IGH.

METHODS

For this study, six IGH patients and six controls were enrolled. Melanin content was checked in the skin sections and in the cultured melanocytes. Senescence was checked in the lesional skin of IGH patients by comparing the mRNA and protein expression of senescence markers p16, hp1, and p21.

RESULTS

Cultured melanocytes from the IGH patients showed morphological changes in comparison to the control melanocytes. Melanocytes from IGH patients were bigger in size with very small and retracted dendrites as compared to the control melanocytes. Melanin accumulation was more in the IGH patients as compared to the controls. Our results showed that expression of p16, p21, and hp1 was significantly higher in lesional skin of IGH patient as compared to healthy controls.

CONCLUSION

This study revealed large-sized melanocytes with small and retracted dendrites in IGH patients. Accumulation of more melanin in the IGH melanocytes might be due to problem in the transfer of melanin from melanocytes to keratinocytes. Accumulation of melanin can lead to the senescence in the melanocytes of IGH patients.

Inhibition of melanogenesis by the pyridinyl imidazole class of compounds: possible involvement of the Wnt/β-catenin signaling pathway

While investigating the role of p38 MAPK in regulating melanogenesis, we found that pyridinyl imidazole inhibitors class compounds as well as the analog compound SB202474, which does not inhibit p38 MAPK, suppressed both α-MSH-induced melanogenesis and spontaneous melanin synthesis. In this study, we demonstrated that the inhibitory activity of the pyridinyl imidazoles correlates with inhibition of the canonical Wnt/β-catenin pathway activity. Imidazole-treated cells showed a reduction in the level of Tcf/Lef target genes involved in the β-catenin signaling network, including ubiquitous genes such as Axin2, Lef1, and Wisp1 as well as cell lineage-restricted genes such as microphthalmia-associated transcription factor and dopachrome tautomerase. Although over-expression of the Wnt signaling pathway effector β-catenin slightly restored the melanogenic program, the lack of complete reversion suggested that the imidazoles interfered with β-catenin-dependent transcriptional activity rather than with β-catenin expression. Accordingly, we did not observe any significant change in β-catenin protein expression. The independence of p38 MAPK activity from the repression of Wnt/β-catenin signaling pathway was confirmed by small interfering RNA knockdown of p38 MAPK expression, which by contrast, stimulated β-catenin-driven gene expression. Our data demonstrate that the small molecule pyridinyl imidazoles possess two distinct and opposite mechanisms that modulate β-catenin dependent transcription: a p38 inhibition-dependent effect that stimulates the Wnt pathway by increasing β-catenin protein expression and an off-target mechanism that inhibits the pathway by repressing β-catenin protein functionality. The p38-independent effect seems to be dominant and, at least in B16-F0 cells, results in a strong block of the Wnt/β-catenin signaling pathway.

Nevus senescence

Melanomas and nevi share many of the same growth-promoting mutations. However, melanomas grow relentlessly while benign nevi eventually undergo growth arrest and stabilize. The difference in their long-term growth potential may be attributed to activation of cellular senescence pathways. The primary mediator of senescence in nevi appears to be p16. Redundant, secondary senescence systems are also present and include the p14-p53-p21 pathway, the IGFBP7 pathway, the FBXO31 pathway, and the PI3K mediated stress induced endoplasmic reticulum unfolded protein response. It is evident that these senescence pathways result in an irreversible arrest in most instances; however, they can clearly be overcome in melanoma. Circumvention of these pathways is most frequently associated with gene deletion or transcriptional repression. Reactivation of senescence mechanisms could serve to inhibit melanoma tumor progression.

Molecular nevogenesis

Despite recent advances, the biology underlying nevogenesis remains unclear. Activating mutations in NRAS, HRAS, BRAF, and GNAQ have been identified in benign nevi. Their presence roughly correlates with congenital, Spitz, acquired, and blue nevi, respectively. These mutations are likely to play a critical role in driving nevogenesis. While each mutation is able to activate the MAP kinase pathway, they also interact with a host of different proteins in other pathways. The different melanocytic developmental pathways activated by each mutation cause the cells to migrate, proliferate, and differentiate to different extents within the skin. This causes each mutation to give rise to a characteristic growth pattern. The exact location and differentiation state of the cell of origin for benign moles remains to be discovered. Further research is necessary to fully understand nevus development given that most of the same developmental pathways are also present in melanoma.

Regulation of the p21Sdi1/Cip1/Waf1DNA Synthesis Inhibitor in Senescent Human Diploid Fibroblasts

A large body of evidence has demonstrated that normal human fibroblasts have a limited division potential in culture and underwent senescence, a process whereby cells became arrested in the G1 phase of the cell cycle and overexpressed a DNA synthesis inhibitor(s). Cyclin-dependent kinase two (Cdk2) is required for the promotion of the Gi-to-S phase transition in human cells. Senescent fibroblasts contain intact cyclin-Cdk2 complexes but cannot induce Cdk2 protein kinase activity in response to mitogen stimulation. Recently, we cloned p21Sdi1, a potent inhibitor of DNA synthesis and Cdk2 kinase activity, from a senescent cell cDNA library and demonstrated that it was expressed at significantly higher levels in senescent cells than actively proliferating cells. In contrast to actively dividing cells, mitogen-stimulated senescent cells do not down-regulate the expression of p21Sdi1and do not express late G1 phase gene products that are required for entry into S phase. We suggest that the inability of mitogen-stimulated senescent cells to down-regulate p21Sdi1levels contributes to the resulting lack of late Gi gene expression and failure to traverse the G1/S phase boundary.

Genotoxic stress abrogates renewal of melanocyte stem cells by triggering their differentiation

Somatic stem cell depletion due to the accumulation of DNA damage has been implicated in the appearance of aging-related phenotypes. Hair graying, a typical sign of aging in mammals, is caused by the incomplete maintenance of melanocyte stem cells (MSCs) with age. Here, we report that irreparable DNA damage, as caused by ionizing radiation, abrogates renewal of MSCs in mice. Surprisingly, the DNA-damage response triggers MSC differentiation into mature melanocytes in the niche, rather than inducing their apoptosis or senescence. The resulting MSC depletion leads to irreversible hair graying. Furthermore, deficiency of Ataxia-telangiectasia mutated (ATM), a central transducer kinase of the DNA-damage response, sensitizes MSCs to ectopic differentiation, demonstrating that the kinase protects MSCs from their premature differentiation by functioning as a "stemness checkpoint" to maintain the stem cell quality and quantity.

Aging of the hair follicle pigmentation system

Skin and hair phenotypes are powerful cues in human communication. They impart much information, not least about our racial, ethnic, health, gender and age status. In the case of the latter parameter, we experience significant change in pigmentation in our journey from birth to puberty and through to young adulthood, middle age and beyond. The hair follicle pigmentary unit is perhaps one of our most visible, accessible and potent aging sensors, with marked dilution of pigment intensity occurring long before even subtle changes are seen in the epidermis. This dichotomy is of interest as both skin compartments contain melanocyte subpopulations of similar embryologic (i.e., neural crest) origin. Research groups are actively pursuing the study of the differential aging of melanocytes in the hair bulb versus the epidermis and in particular are examining whether this is in part linked to the stringent coupling of follicular melanocytes to the hair growth cycle. Whether some follicular melanocyte subpopulations are affected, like epidermal melanocytes, by UV irradiation is not yet clear. A particular target of research into hair graying or canities is the nature of the melanocyte stem compartment and whether this is depleted due to reactive oxygen species-associated damage, coupled with an impaired antioxidant status, and a failure of melanocyte stem cell renewal. Over the last few years, we and others have developed advanced in vitro models and assay systems for isolated hair follicle melanocytes and for intact anagen hair follicle organ culture which may provide research tools to elucidate the regulatory mechanisms of hair follicle pigmentation. Long term, it may be feasible to develop strategies to modulate some of these aging-associated changes in the hair follicle that impinge particularly on the melanocyte populations.

Low-energy helium-neon laser induces melanocyte proliferation via interaction with type IV collagen: visible light as a therapeutic option for vitiligo

BACKGROUND

The treatment of vitiligo remains a challenge for clinical dermatologists. We have previously shown that the helium-neon laser (He-Ne laser, 632.8 nm) is a therapeutic option for treatment of this depigmentary disorder.

OBJECTIVES

Addressing the intricate interactions between melanocytes, the most important cellular component in the repigmentation scheme of vitiligo, and their innate extracellular matrix collagen type IV, the current study aimed to elucidate the effects of the He-Ne laser on melanocytes.

METHODS

Cultured melanocytes were irradiated with the He-Ne laser. Relevant biological parameters including cell attachment, locomotion and growth were evaluated. In addition, the potentially involved molecular pathways were also determined.

RESULTS

Our results show that in addition to suppressing mobility but increasing attachment to type IV collagen, the He-Ne laser stimulates melanocyte proliferation through enhanced alpha2beta1 integrin expression. The expression of phosphorylated cyclic-AMP response element binding protein (CREB), an important regulator of melanocyte growth, was also upregulated by He-Ne laser treatment. Using a specific mitochondrial uncoupling agent [carbonyl cyanide m-chlorophenyl-hydrazone (CCCP)], the proliferative effect of the He-Ne laser on melanocytes was abolished and suppression of melanocyte growth was noted.

CONCLUSIONS

In summary, we have demonstrated that the He-Ne laser imparts a growth stimulatory effect on functional melanocytes via mitochondria-related pathways and proposed that other minor pathways including DNA damage may also be inflicted by laser treatment on irradiated cells. More importantly, we have completed the repigmentation scheme of vitiligo brought about by He-Ne laser light in vitro and provided a solid theoretical basis regarding how the He-Ne laser induces recovery of vitiligo in vivo.

Stem cell factor induces ERM proteins phosphorylation through PI3K activation to mediate melanocyte proliferation and migration

Stem cell factor (SCF) activates a variety of signals associated with stimulation of proliferation, differentiation, migration, and survival in melanocytes. However, the molecular mechanisms by which SCF and its receptor Kit activates these signaling pathways simultaneously and independently are still poorly defined. Here, we examined whether SCF induces ezrin/radixin/moesin (ERM) proteins phosphorylation as a downstream target of PI3K in melanocytes. ERM proteins are cross-linkers between the plasma membrane and the actin cytoskeleton and are activated by phosphorylation of a C-terminal threonine residue. Our results demonstrated that SCF-induced ERM proteins phosphorylation on threonine residue and Rac1 activation in cultured normal human melanocytes through the activation of PI3K. The functional role of phosphorylated-ERM proteins was examined using melanocytes infected with adenovirus carrying a dominant negative mutant (Ala-558, TA) or wild type of moesin. In the TA moesin-overexpressing melanocytes, SCF-induced cell proliferation and migration were inhibited. Thus, our results indicate that phosphorylation of ERM proteins plays an important role in the regulation of SCF-induced melanocyte proliferation and migration.

Human hair pigmentation--biological aspects

Skin and hair colour contribute significantly to our overall visual appearance and to social/sexual communication. Despite their shared origins in the embryologic neural crest, the hair follicle and epidermal pigmentary units occupy distinct, although open, cutaneous compartments. They can be distinguished principally on the basis of the former's stringent coupling to the hair growth cycle compared with the latter's continuous melanogenesis. The biosynthesis of melanin and its subsequent transfer from melanocyte to hair bulb keratinocytes depend on the availability of melanin precursors and on a raft of signal transduction pathways that are both highly complex and commonly redundant. These signalling pathways can be both dependent and independent of receptors, act through auto-, para- or intracrine mechanisms and can be modified by hormonal signals. Despite many shared features, follicular melanocytes appear to be more sensitive than epidermal melanocytes to ageing influences. This can be seen most dramatically in hair greying/canities and this is likely to reflect significant differences in the epidermal and follicular microenvironments. The hair follicle pigmentary unit may also serve as an important environmental sensor, whereby hair pigment contributes to the rapid excretion of heavy metals, chemicals and toxins from the body by their selective binding to melanin; rendering the hair fibre a useful barometer of exposures. The recent availability of advanced cell culture methodologies for isolated hair follicle melanocytes and for intact anagen hair follicle organ culture should provide the research tools necessary to elucidate the regulatory mechanisms of hair follicle pigmentation. In the longer term, it may be feasible to develop hair colour modifiers of a biological nature to accompany those based on chemicals.

Dynamic regulation of the human dopachrome tautomerase promoter by MITF, ER-alpha and chromatin remodelers during proliferation and senescence of human melanocytes

Senescent cells are known to display altered gene expression of differentiation-associated genes. We have previously demonstrated that the melanocyte transcriptional regulator microphthalmia-associated protein (MITF) is down-regulated in senescent melanocytes. Since virtually nothing is known regarding the differentiated function of senescent melanocytes, we analyzed the transcriptional regulation of Dopachrome tautomerase (DCT), a member of the tyrosinase gene family, in proliferating and in senescent human melanocytes. Computational analysis of the region containing the M-box that includes the MITF CATGTG binding motif demonstrated that this sequence overlaps with the estrogen receptor alpha (ER-alpha), USF-1, TFE-3, Isl-1 and AP-1 binding elements. Electrophoresis gel-shift analysis using an oligonucleotide containing MITF and ERE elements identified MITF and ER-alpha complexes in proliferating melanocytes, whereas only ER-alpha complexes were detected in senescent cells. Importantly, a promoter-reporter analysis demonstrated that the coactivator p300/CBP switched MITF from a repressor to an activator of DCT transcription. p300/CBP was also required by ER-alpha and MITF to induce high, synergistic activation of the DCT promoter. We have also found that transcription of the DCT gene is differentially regulated by major melanocyte mitogens. In contrast to the activating effect of cAMP inducers, 12-O-tetradecanoylphorbolacetate (TPA) was a potent repressor of DCT transcription, suggesting that this gene can be differentially regulated by multiple environmental signals and promoter context. In support of this conclusion, trichostatin A, a histone deacetylase inhibitor, counteracted the TPA-mediated repression, and restored high levels of DCT protein in cultured melanocytes. We conclude that senescent melanocytes display dramatic changes in the expression of differentiation-related proteins; such changes may in turn result in altered melanocyte function and survival to environmental stresses.

Mechanisms of hair graying: incomplete melanocyte stem cell maintenance in the niche

Hair graying is the most obvious sign of aging in humans, yet its mechanism is largely unknown. Here, we used melanocyte-tagged transgenic mice and aging human hair follicles to demonstrate that hair graying is caused by defective self-maintenance of melanocyte stem cells. This process is accelerated dramatically with Bcl2 deficiency, which causes selective apoptosis of melanocyte stem cells, but not of differentiated melanocytes, within the niche at their entry into the dormant state. Furthermore, physiologic aging of melanocyte stem cells was associated with ectopic pigmentation or differentiation within the niche, a process accelerated by mutation of the melanocyte master transcriptional regulator Mitf.

The role of B-RAF in melanoma

Melanoma is a form of skin cancer that has a poor prognosis and which is on the rise in Western populations. If detected early, it is easily treated by surgical excision. However, once melanoma metastasises it is notoriously resistant to existing therapies and for many patients the outlook is dismal. Thus a full description of melanoma etiology and a full understanding of the genetic lesions that underlie this disease is required to allow us to develop new and effective therapeutic strategies for its treatment. RAF proteins are a family of serine/threonine-specific protein kinases that form part of a signalling module that regulates cell proliferation, differentiation and survival. In mammals there are three isoforms, A-RAF, B-RAF and C-RAF, and recently it was shown that the B-RAF isoform is mutated in a high proportion of melanomas. In light of these exciting findings, we review what we have learned about B-RAF and its role in cutaneous melanoma.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

The role of telomeres in skin aging/photoaging

Recent work has substantially elucidated the mechanisms of skin aging and photoaging. In particular, a central role for telomere-based signaling can be inferred. Intrinsic aging is largely controlled by progressive telomere shortening, compounded by low grade oxidative damage to telomeres and other cellular constituents, the consequence of aerobic cellular metabolism. In sun exposed skin, UV irradiation also damages DNA and accelerates telomere shortening. Aging and photodamage appear to share a common final pathway that involves signaling through p53 following disruption of the telomere. These telomere-initiated responses, in combination with UV-induced damage to critical regulatory genes, lead to the familiar picture of "photoaging." These and other insights into the molecular basis for skin aging/photoaging may lead to enhanced management options.

Cutaneous photobiology. The melanocyte vs. the sun: who will win the final round?

Solar ultraviolet radiation (UV) is a major environmental factor that dramatically alters the homeostasis of the skin as an organ by affecting the survival, proliferation and differentiation of various cutaneous cell types. The effects of UV on the skin include direct damage to DNA, apoptosis, growth arrest, and stimulation of melanogenesis. Long-term effects of UV include photoaging and photocarcinogenesis. Epidermal melanocytes synthesize two main types of melanin: eumelanin and pheomelanin. Melanin, particularly eumelanin, represents the major photoprotective mechanism in the skin. Melanin limits the extent of UV penetration through the epidermal layers, and scavenges reactive oxygen radicals that may lead to oxidative DNA damage. The extent of UV-induced DNA damage and the incidence of skin cancer are inversely correlated with total melanin content of the skin. Given the importance of the melanocyte in guarding against the adverse effects of UV and the fact that the melanocyte has a low self-renewal capacity, it is critical to maintain its survival and genomic integrity in order to prevent malignant transformation to melanoma, the most fatal form of skin cancer. Melanocyte transformation to melanoma involves the activation of certain oncogenes and the inactivation of specific tumor suppressor genes. This review summarizes the current state of knowledge about the role of melanin and the melanocyte in photoprotection, the responses of melanocytes to UV, the signaling pathways that mediate the biological effects of UV on melanocytes, and the most common genetic alterations that lead to melanoma.

p16/cyclin-dependent kinase inhibitor 2A deficiency in human melanocyte senescence, apoptosis, and immortalization: possible implications for melanoma progression

BACKGROUND

The melanoma susceptibility locus cyclin-dependent kinase inhibitor 2A encodes two unrelated cell growth inhibitors, p16 and alternative reading frame (ARF). In fibroblasts, both proteins are implicated in cellular senescence, a key barrier to tumor development. The p16 coding sequence is more often mutated in melanoma families than is the ARF sequence. To investigate the role of p16 in melanocytes, we assessed aspects of growth, apoptosis, and immortalization in melanocytes cultured from two melanoma patients, both of whom had two inactive p16 alleles but functional ARF.

METHODS

Growth and senescence were evaluated by cumulative population-doubling curves, and apoptosis by terminal deoxytransferase labeling. Expression of p53 and p21, which are associated with fibroblast senescence, was assessed by immunoblotting. Amphotropic retroviruses were used to transfer exogenous gene sequences into the melanocytes.

RESULTS

Both melanocyte cultures showed high rates of apoptosis, which were reduced when the cells were grown in the presence of keratinocyte feeder cells or human stem cell factor plus endothelin 1. With these growth factors, both cultures proliferated for 45-55 net population doublings, markedly longer than the maximum of 10 net population doublings of normal adult human melanocytes in similar media, indicating impaired senescence. One of the cultures developed chromosomal aberrations, with numerous dicentric chromosomes at senescence, consistent with telomere dysfunction. p53 and p21 levels were not elevated in senescent normal melanocytes but were elevated in senescent p16-deficient melanocytes. Interference with p53 function by transfer of human papillomavirus 16-E6 further extended the lifespan of p16-deficient melanocytes. Human telomerase reverse transcriptase was sufficient to immortalize both these cell strains but not normal melanocytes.

CONCLUSION

Normal senescence in human melanocytes requires p16 activity. p53 contributes to a delayed form of senescence that requires telomere shortening, in p16-deficient melanocytes. These findings provide some basis for the role of p16 in melanoma susceptibility.

Human melanocyte senescence and melanoma susceptibility genes

The molecular mechanisms and biology of cellular senescence in human melanocytes are discussed, including similarities to and differences from senescence in fibroblasts and other cell lineages. Special reference is made to the fact that the known melanoma susceptibility genes in the human, Inhibitor A of [cyclin-dependent] kinase 4-alternative reading frame (INK4A-ARF) and cyclin-dependent kinase 4, are involved in the regulation of cellular senescence, and possible reasons why this should be so. Based on the evidence including growth and survival kinetics of human and mouse melanocytes carrying germline deficiencies in the INK4A sequence, it is suggested that an 'M0' or p16/RB-dependent form of senescence may be particularly important in melanocytes. A speculative model is proposed, relating current concepts of early melanoma progression to the processes of cellular senescence and immortalization. This includes the suggestion that moles or nevi are senescent clones of melanocytes.

Role of p44/p42 MAP kinase in the age-dependent increase in vascular smooth muscle cell proliferation and neointimal formation

OBJECTIVE

Age-dependent increase in vascular smooth muscle cell (VSMC) proliferation is thought to contribute to the pathology of atherosclerotic diseases. In this study, we investigated the role of mitogen-activated protein kinases (MAPKs) on VSMC proliferation and neointimal formation in the context of aging.

METHODS AND RESULTS

VSMCs were isolated from the aorta of young and old rabbits. The proliferative index after serum stimulation was significantly increased in old versus young VSMCs. This was associated with a significant and specific age-dependent increase in p44/p42 MAPK activation. Treatment with MEK inhibitor PD98059 successfully inhibited p44/p42 MAPK activities and VSMC proliferation. These results were confirmed in vivo using a model of balloon injury in rabbit iliac arteries. p44/p42 MAPK activities were rapidly induced by angioplasty in young and old animals. However, the levels of p44/p42 MAPK activities achieved in arteries of old rabbits were significantly higher than those of young rabbits. This was associated with a higher cellular proliferative index and a significant increase in neointimal formation in old animals. Local delivery of PD98059 in old rabbits successfully inhibited p44/p42 MAPK activities after angioplasty, which led to a significant reduction in cellular proliferation and neointimal formation in treated animals.

CONCLUSIONS

Our study suggests for the first time that increased p44/p42 MAPK activation contributes to augmented VSMC proliferation and neointimal formation with aging. p44/p42 MAPK inhibition could represent a novel therapeutic avenue against atherosclerotic diseases.

Identification and cloning of human polynucleotide phosphorylase, hPNPase old-35, in the context of terminal differentiation and cellular senescence

Terminal differentiation and cellular senescence display common properties including irreversible growth arrest. To define the molecular and ultimately the biochemical basis of the complex physiological changes associated with terminal differentiation and senescence, an overlapping-pathway screen was used to identify genes displaying coordinated expression as a consequence of both processes. This approach involved screening of a subtracted cDNA library prepared from human melanoma cells induced to terminally differentiate by treatment with fibroblast IFN and mezerein with mRNA derived from senescent human progeria cells. This strategy identified old-35, which encodes an evolutionary conserved gene, human polynucleotide phosphorylase (hPNPase(old-35)), that is regulated predominantly by type I IFNs. The hPNPase(OLD-35) protein localizes in the cytoplasm of human cells and induces RNA degradation in vitro, as does its purified bacterial protein homologue. Ectopic expression of hPNPase(old-35) in human melanoma cells reduces colony formation, confirming inhibitory activity of this RNA-degradation enzyme. Identification of hPNPase(old-35), an IFN-inducible 3'-5' RNA exonuclease, provides additional support for a relationship between IFN action and RNA processing and suggests an important role for this gene in growth control associated with terminal differentiation and cellular senescence.

Fifty years of skin aging

In developed countries, interest in cutaneous aging is in large part the result of a progressive, dramatic rise over the past century in the absolute number and the proportion of the population who are elderly (Smith et al, 2001). The psychosocial as well as physiologic effects of skin aging on older individuals have created a demand for better understanding of the process and particularly for effective interventions. Skin aging is a complex process determined by the genetic endowment of the individual as well as by environmental factors. The appearance of old skin and the clinical consequences of skin aging have been well known for centuries, but only in the past 50 y have mechanisms and mediators been systematically pursued. Still, within this relatively short time there has been tremendous progress, a progress greatly enhanced by basic gerontologic research employing immunologic, biochemical, and particularly molecular biologic approaches (Figs 1, 2).

Molecular regulation of melanocyte senescence

Cell senescence is the loss of ability to divide after a finite number of divisions, seen in normal mammalian somatic cells and often disrupted in cancer cells. The three genes so far associated with familial melanoma susceptibility--INK4A, CDK4 and ARF, are all implicated in the molecular pathways controlling cell senescence. Here we review those pathways, both as generally studied in fibroblasts and epithelial cells, and as specifically analysed in melanocytes. Key molecular effectors in melanocyte senescence appear to include some in common with other cell types - telomere attrition and the p16/RB pathway, and one that is not commonly mentioned in this connection, the cAMP signalling pathway that also regulates melanocyte differentiation. These findings are discussed in relation to the role of cell senescence in the development and molecular genetics of melanoma and its precursor lesions.

Molecular mechanisms of skin ageing

Cutaneous ageing is a complex biological phenomenon consisting of two components; intrinsic ageing, which is largely genetically determined and extrinsic ageing caused by environmental exposure, primarily UV light. In sun-exposed areas, these two processes are superimposed. The process of intrinsic skin ageing resembles that seen in most internal organs and is thought to involve decreased proliferative capacity leading to cellular senescence, and altered biosynthetic activity of skin derived cells. Extrinsic ageing, more commonly termed photoageing, also involves changes in cellular biosynthetic activity but leads to gross disorganisation of the dermal matrix. The molecular mechanisms underlying some of these changes are now beginning to be unravelled and are discussed. As these mechanisms are identified, further insights into the underlying processes of skin ageing should emerge and better strategies to prevent the undesirable effects of age on skin appearance should follow.

Mitogen- and ultraviolet-B-induced signaling pathways in normal human melanocytes

In normal human melanocytes various mitogens activate the mitogen-activated protein kinases ERK1/2 and the downstream transcription factor CREB (Ca2+/cAMP response element binding protein). Endothelin-1, basic fibroblast growth factor, and alpha-melanotropin interact synergistically to stimulate human melanocyte proliferation. The former two mitogens phosphorylated ERK1/2, its substrate p90rsk, and CREB. Alpha-melanotropin, forskolin, or dibutyryl cAMP failed to phosphorylate any of those targets, however. The concomitant presence of endothelin-1, basic fibroblast growth factor, and alpha-melanotropin significantly potentiated CREB phosphorylation. The mitogen-induced phosphorylation of p90rsk and CREB was dependent on ERK1/2 activation, and was mediated by intracellular calcium mobilization and by protein kinase C and tyrosine kinase activation, but not by activation of the cAMP-dependent protein kinase A. Exposure of melanocytes to ultraviolet radiation B resulted in the phosphorylation of the stress-induced mitogen- activated protein kinases p38 and JNK/SAPK, but not ERK1/2. Ultraviolet radiation B induced the phosphorylation of CREB via a pathway that was partially dependent on p38, but had no effect on p90rsk or ERK1/2. Therefore, in human melanocytes, CREB is a common downstream target for distinct effectors that are involved in either mitogenic signaling or stress signaling initiated by ultraviolet radiation B.

Calorie restriction influences cell cycle protein expression and DNA synthesis during liver regeneration

Calorie restriction without essential nutrient deficiency (calorie restriction, CR) abrogates experimental carcinogenesis and extends healthful life span. To test whether CR influences cell-cycle protein expression during the hepatocellular proliferation induced by 70% partial hepatectomy (PH), BALB/c mice were separated into two groups, fed comparable semi-purified diets for 10 weeks that differed 40% in caloric offering, and were then subjected to PH. When PH was performed, CR mice weighed 36% less than ad libitum (AL)-fed mice (P < 0.01), but liver-to-body weight ratios were similar. During the regenerative hyperplasia, hepatocytes of CR mice demonstrated evidence of accelerated entrance and passage through G1 and S phases, and an earlier exit from the cell cycle. The first peak of DNA synthesis occurred 6 hr earlier, and the second peak was significantly greater among CR mice with 38% +/- 13% bromodeoxyuridine (BrdU)-positive hepatocytes, compared with 14% +/- 4% in AL mice (P < 0.01). More E2F-1 expression was induced at the hepatic G1/S boundary just prior to each peak of DNA synthesis in regenerating livers of CR mice (P < 0.01), and 8 hr earlier among CR mice. More hyperphosphorylated retinoblastoma p110 was detected during hepatic G1 and the G1-S transition among CR mice, coincident with the early hepatocellular proliferative wave. Cyclin A was induced during the first peak of DNA synthesis 4 hr earlier among CR mice, and it continued 4 hr longer in AL mice, indicating an earlier post-replicative exit by hepatocytes in CR mice. p21 was induced during the G1 phase at 4 hr post-PH, and was maximally expressed during and after peak DNA synthesis in both dietary groups. These results indicate that CR influences cell cycle protein expression levels, causing hepatocytes to enter into S phase earlier and exit abruptly from the cell cycle, and they support the premise that CR enhances induced cell responsiveness by influencing cell cycle regulatory controls.

Lack of Elk-1 phosphorylation and dysregulation of the extracellular regulated kinase signaling pathway in senescent human fibroblast

Replicative senescence is characterized by numerous phenotypic alterations including the loss of proliferative capacity in response to mitogens and numerous changes in gene expression including impaired serum inducibility of the immediate-early genes c-fos and erg-1. Transcription of c-fos in response to mitogens depends on the activation of a multiprotein complex formed on the c-fos serum response element (SRE), which includes the transcription factors SRF (serum response factor) and TCF (ternary complex factor). Our data indicate that at least two defects are responsible for the decreased c-fos transcription in senescent cells, one caused by diminished DNA binding activity of the SRF and another resulting from impaired activation of the TCF, Elk-1. In nuclei isolated from serum stimulated senescent cells the activating phosphorylation of p62(TCF)/Elk-1, which is catalyzed by the members of the extracellular-regulated kinase (ERK) family was strikingly diminished and correlated with a decrease in the abundance of activated ERK proteins. In contrast, in total cell lysates ERK phosphorylation and ERK activity (normalized to total protein) reached similar levels following stimulation of early- and late-passage cells. Interestingly, senescent cells consistently exhibited higher ERK protein abundance. Thus, the proportion of phosphorylated (active) ERK molecules in stimulated senescent cells was lower than in early passage cells. The accumulation of unphosphorylated ERK molecules in senescent cells correlated with the diminished abundance of phosphorylated (active) MEK. These data indicate that in senescent cells there is a general dysregulation in the ERK signaling pathway, which results in the accumulation of inactive ERK molecules, decreased abundance of active ERK in the nucleus of senescent cells, and subsequent lack of activation of the transcription factor TCF(Elk-1). These impairments, together with the impaired DNA binding activity of SRF, could potentially account for the lack of c-fos expression in senescent cells and for multiple other molecular changes dependent upon this pathway.

The human melanocyte: a model system to study the complexity of cellular aging and transformation in non-fibroblastic cells

The melanocyte is a neural crest-derived cell that localizes in humans to several organs including the epidermis, eye, inner ear and leptomeninges. In the skin, melanocytes synthesize and transfer melanin pigments to surrounding keratinocytes, leading to skin pigmentation and protection against solar exposure. We have investigated the process of replicative senescence and accompanying irreversible cell cycle arrest, in melanocytes in culture. As was found in other cell types, progressive telomere shortening appears to trigger replicative senescence in normal melanocytes. In addition, senescence is associated with increased binding of the cyclin-dependent kinase inhibitor (CDK-I) p16(INK4a) to CDK4, down-regulation of cyclin E protein levels (and consequent loss of cyclin E/CDK2 activity), underphosphorylation of the retinoblastoma protein RB and subsequent increased levels of E2F4-RB repressive complexes. In contrast to fibroblasts, however, the CDK-Is p21(Waf-1) and p27(Kip-1) are also down-regulated. These changes appear to be important for replicative senescence because they do not occur in melanocytes that overexpress the catalytic subunit of the enzyme telomerase (hTERT), or in melanomas, which are tumors that originate from melanocytes or melanoblasts. In contrast to unmodified melanocytes, hTERT overexpressing (telomerized) melanocytes displayed telomerase activity, stable telomere lengths and an extended replicative life span. However, telomerized melanocytes show changes in cell cycle regulatory proteins, including increased levels of cyclin E, p21(Waf-1) and p27(Kip-1). Cyclin E, p21(Waf-1) and p27(Kip-1) are also elevated in many primary melanomas, whereas p16(INK4a) is mutated or deleted in many invasive and metastatic melanomas. Thus, the molecular mechanisms leading to melanocyte senescence and transformation differ significantly from fibroblasts. This suggests that different cell types may use different strategies to halt the cell cycle in response to telomere attrition and thus prevent replicative immortality.

Differential response of mature TrkA/p75(NTR) expressing human and pig oligodendrocytes: aging, does it matter?

A differential morphological response of mature oligodendrocytes (OL) isolated from human and pig brains to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and to the nerve growth factor (NGF) was observed. In both cases, OL regenerate their processes; however, the rate and the extension of the process formation of human OL were behind that of pig OL. Presumably, the advanced age of the human tissue in these experiments might have contributed to this decrease in process formation, an effect that was already observed for rat OL [Yong et al. (1991) J Neurosci Res 29:87-99]. The less effectivity of NGF via TrkA, which was immunocytochemically shown in human OL, and of TPA via the protein kinase C (PKC) pathway, may have its common focus on the mitogen-activated protein kinase (MAPK) cascade. In this context, it was noted that only a few studies on aging of mature OL are available. It is conceivable that age-related changes in the properties of OL could be an important factor for their cellular responsiveness during longer lasting demyelinating diseases such as multiple sclerosis. Hence, this review would like to provide a basis for future investigations on the aging of mature OL. The data presently available suggest a preliminary classification of mature OL into three categories.

Significance of nuclear relocalization of ERK1/2 in reactivation of c-fos transcription and DNA synthesis in senescent fibroblasts

Two of mitogen-activated protein kinases (MAPK), p44(mapk)/p42(mapk) extracellular signal-regulated kinases (ERK1/2), translocate into nuclei following activation and play critical roles in connecting the signal to gene expression and allowing cell-cycle entry. Here we found that the nuclear translocation of ERK1/2 in response to growth stimuli was significantly inhibited in senescent cells that were irreversibly growth arrested, compared with presenescent cells. The activation step of these enzymes was not impaired, since ERK1/2 were phosphorylated and activated in senescent cells as efficiently as in presenescent cells. By elaborately localizing ERK2 in the nuclei of senescent cells, we could restore c-fos transcriptional activity upon growth stimuli, which was repressed in senescent cells. Furthermore, the nuclear localization of ERK1/2 has been suggested to potentiate the proliferative activity of the senescent cells in collaboration with adenovirus E1A protein. More importantly, SV40 large T antigen, the strong inducer of DNA synthesis, had the inherent ability to restore nuclear relocalization of active ERK1/2 in senescent cells, which was essentially required for the reinitiation of DNA synthesis. Thus, manipulating the relocalization of ERK1/2 into nuclei was expected to open the way to overcome some of the senescent phenotypes.

Melanin accumulation accelerates melanocyte senescence by a mechanism involving p16INK4a/CDK4/pRB and E2F1

Cellular and molecular evidence suggests that senescence is a powerful tumor-suppressor mechanism that prevents most higher eukaryotic cells from dividing indefinitely in vivo. Recent work has demonstrated that alpha-melanocyte stimulating hormone (alpha-MSH) or cholera toxin (CT) can activate a cAMP pathway that elicits proliferative arrest and senescence in normal human pigmented melanocytes. In these cells, senescence is associated with increased binding of p16INK4a to CDK4 and loss of E2F-binding activity. Because senescence may provide defense against malignant transformation of melanocytes, and because pigmentation is a strong defense against melanoma, we examined the ability of melanocytes derived from light and dark skin to respond to CT. Here we demonstrate that in melanocytes derived from dark-skinned individuals, CT-induced melanogenesis is associated with accumulation of the tumor suppressor p16INK4a, underphosphorylated retinoblastoma protein (pRb), downregulation of cyclin E, decreased expression of E2F1, and loss of E2F-regulated S-phase gene expression. In contrast to other senescent cell types, melanocytes have reduced or absent levels of the cyclin-dependent kinase inhibitors p27Kip1 and p21Waf-1. Importantly, melanocytes derived from light-skinned individuals accumulated smaller amounts of melanin than did those from dark-skinned individuals under the same conditions, and they continued to proliferate for several more division cycles. This delayed senescence may result from reduced association of p16 with CDK4, reduced levels of underphosphorylated pRb, and steady levels of cyclin E and E2F1. Because cyclin E-CDK2 inhibition is required for p16-mediated growth suppression, upregulation of p16 and downregulation of cyclin E appear essential for maintenance of terminal growth and senescence. Given the rising incidence of melanoma, identification of major growth regulatory proteins involved in senescence should shed light on the biology of this genetically mysterious tumor.

Deregulated E2F transcriptional activity in autonomously growing melanoma cells

Inactivation of the retinoblastoma tumor suppressor protein (pRb) has been implicated in melanoma cells, but the molecular basis for this phenotype has not yet been elucidated, and the status of additional family members (p107 and p130, together termed pocket proteins) or the consequences on downstream targets such as E2F transcription factors are not known. Because cell cycle progression is dependent on the transcriptional activity of E2F family members (E2F1-E2F6), most of them regulated by suppressive association with pocket proteins, we characterized E2F-pocket protein DNA binding activity in normal versus malignant human melanocytes. By gel shift analysis, we show that in mitogen-dependent normal melanocytes, external growth factors tightly controlled the levels of growth-promoting free E2F DNA binding activity, composed largely of E2F2 and E2F4, and the growth-suppressive E2F4-p130 complexes. In contrast, in melanoma cells, free E2F DNA binding activity (E2F2 and E2F4, to a lesser extent E2F1, E2F3, and occasionally E2F5), was constitutively maintained at high levels independently of external melanocyte mitogens. E2F1 was the only family member more abundant in the melanoma cells compared with normal melanocytes, and the approximately fivefold increase in DNA binding activity could be accounted for mostly by a similar increase in the levels of the dimerization partner DP1. The continuous high expression of cyclin D1, A2, and E, the persistent cyclin-dependent kinase 4 (CDK4) and CDK2 activities, and the presence of hyperphosphorylated forms of pRb, p107, and p130, suggest that melanoma cells acquired the capacity for autonomous growth through inactivation of all three pocket proteins and release of E2F activity, otherwise tightly regulated in normal melanocytes by external growth factors.

Impaired signal transduction in mitogen activated rat splenic lymphocytes during aging

Mitogen activated protein kinases (MAPK) are activated by a wide variety of signals leading to cell proliferation and differentiation in different cell types. With aging, there is a marked decrease in proliferation of T-lymphocytes in response to a variety of mitogens. Several age-related changes in the activation of MAPK pathways in T-lymphocytes activated via the T-cell receptor (TCR) have been described in different species. This way, some TCR proximal defects in tyrosine kinase activity have been delineated. In this study, we have used rat splenic lymphocytes to measure the effect of aging on the activation of two MAP kinase families: ERK and JNK. In order to bypass the receptor-proximal age-dependent defects previously described, we used phorbol ester (PMA) and Ca2+ ionophore (A23187) as co-mitogens. Our results demonstrate that splenic lymphocytes from old rats have a disturbance in the activation of the ERK and JNK MAPK signal transduction pathways, that are located downstream of the receptor-proximal events. At least part of the age-related defect leading to decreased ERK activity appears to be located upstream of ERK itself, since activation of MEK is also impaired. On the other hand, the observed defects in MAPK activation do result in decreased activation of downstream events, such as c-Jun phosphorylation. Thus, we conclude that aging of splenic lymphocytes results in a functional decline in signal transduction, and at least some of these defects are located downstream of the receptor-proximal events previously described by others. The impaired activity of these two MAP kinase pathways is likely to play a role in the diminished lymphoproliferation observed in old individuals.

Activation of a cAMP pathway and induction of melanogenesis correlate with association of p16(INK4) and p27(KIP1) to CDKs, loss of E2F-binding activity, and premature senescence of human melanocytes

There is strong evidence that the senescent phenotype, whether induced by telomere shortening, oxidative damage, or oncogenic stimuli, is an important tumor suppressive mechanism. The melanocyte is a cell of neural crest origin that produces the pigment melanin and can develop into malignant melanomas. To understand how malignant cells escape senescence, it is first crucial to define what genes control senescence in the normal cell. Prolonged exposure to high levels of cAMP results in accumulation of melanin and terminal differentiation of human melanocytes. Here we present evidence that activation of a cAMP pathway correlates with multiple cellular changes in these cells: (1) increased expression of the transcription factor microphthalmia; (2) increased melanogenesis; (3) increased association of the cyclin-dependent kinase inhibitors (CDK-Is) p27(KIP1) and p16(INK4) with CDK2 and CDK4, respectively; (4) failure to phosphorylate the retinoblastoma protein (pRB); (5) decreased expression of E2F1, E2F2, and E2F4 proteins; (6) loss of E2F DNA-binding activity; and (7) phenotypic changes characteristic of senescent cells. Senescent melanocytes have potent E2F inhibitory activity, because extracts from these cells completely abolished E2F DNA-binding activity that was present in extracts from the early proliferative phase. We propose that increased activity of the CDK-Is p27 and p16 and loss of E2F activity in human melanocytes characterize a senescence program activated by the cAMP pathway. Disruption of cAMP-mediated and melanogenesis-induced senescence may cause immortalization of human melanocytes, an early step in the development of melanomas.

High levels of expression of p27KIP1 and cyclin E in invasive primary malignant melanomas

Cancer cells have abnormal cell cycle regulation which favors accelerated proliferation, chromosomal instability, and resistance to the senescence response. Although the p16INK4a locus is the most prominent susceptibility locus for familial melanomas, the low frequency of p16 mutations in sporadic melanomas suggests additional alterations in other cell cycle regulatory genes. Here we used primary melanoma tumors to reveal early cell cycle alterations that could be masked in advanced metastatic lesions due to their inherently high genetic instability. Unexpectedly, the cyclin-dependent kinase inhibitors p27KIP1 and/or p21Waf-1/SDI-1 were found to be expressed in 13 of 18 (72%) of the primary melanomas with a Breslow thickness greater than 0.076 mm. In general, p27 and/or p21 staining in the primary tumors correlated with low Ki-67 index. Importantly, most of the p21- and p27-positive tumors expressed high levels of cyclin D1 and cyclin E. In proliferating cells p27 is predominantly associated with cyclin D-CDK4 complexes, but does not inhibit the kinase activity, whereas in quiescent cells p27 is found associated with inactive CDK2 complexes. p27 was also expressed at high levels in proliferating primary melanomas in culture, and found to be associated with active cyclin E-CDK2 complexes containing high levels of cyclin E. It is thus likely that accumulation of cyclin E overcomes the potent inhibitory activity of p27 and p21 in CDK2 complexes. Of the primary melanomas with no indication of invasiveness, only three of 15 (20%) were positive for p27 and/or p21. We propose that high levels of p27 and p21 may confer upon melanoma tumors their characteristic resistance to conventional therapies. In turn, high levels of cyclins E and D1 may contribute to unlimited proliferation in primary melanomas that express the tumor suppressor p16INK4. J Invest Dermatol 113:1039-1046 1999

Effects of commonly used mitogens on the cytotoxicity of 4-tertiary butylphenol to human melanocytes

In the search for environmental compounds responsible for contact or occupational vitiligo, it was found that the most potent was 4-tertiary butylphenol (4-TBP). Exposure to 4-TBP is widespread both in industry and in consumer items including synthetic leather, plastic, glues, and germicidal phenolic detergents. How 4-TBP causes depigmentation and the death of melanocytes is currently unclear. Growth mitogens for human melanocytes include alpha-melanocyte stimulating hormone (alpha-MSH), basic fibroblast growth factor (bFGF) and 12-o-tetradecanoylphorbol-13-acetate (TPA). The former two mitogens are physiological growth factors for melanocytes. We have studied the effects of these mitogens on the cytotoxicity of 4-TBP in human melanocytes. Our results demonstrated that deprivation of alpha-MSH or bFGF from melanocyte cultures resulted in reduced cytotoxicity to 4-TBP. Similar results were obtained upon treatment of melanocytes with an inhibitor of cAMP-dependent protein kinase A (PKA), that is known to be activated by alpha-MSH, or with an inhibitor of the tyrosine kinase bFGF receptor. In contrast, removal of fetal bovine serum or TPA from the culture medium did not influence the susceptibility of melanocytes to 4-TBP. These results suggest that activation of the cAMP and tyrosine kinase signaling pathways, both of which are involved in the mitogenic response of melanocytes, increase the susceptibility of these cells to the cytotoxic effects of 4-TBP.

Age-related loss of proliferative activity of human vascular smooth muscle cells in culture

This work studied the proliferation activity in cultures of vascular smooth muscle cells (SMC) from individuals of different ages. The cells derived from arteries of 12 donors of both sexes from 45 to 91 years of age. The main parameter considered was the 'proliferation rate' (cells grown per day in the different culture passages) taking into account the age of the donor. No significant relationship between age of the donor and the cell life in proliferation was found. On the contrary, the mean time of passage duration for reaching the maximum of proliferation as well as its 'efficiency' (maximum of proliferation rate registered/mean time of passage duration) show a statistically significant dependence on the age of the donor. Furthermore, the proliferation rate measured in each passage is statistically significant related to donor age. The regressions obtained show a similar negative slope (VC 4%). Considering the first five culture passages, the regression crosses the x-axis at the age of 105.6+/-11.7 years. This age in which no proliferative activity of human SMC would be expected lies near the limit of maximum life potential for human beings. Our results suggest that with advancing donor age there is an increasing number of senescent SMC either primarily transferred or appeared in the culture. Vascular SMC of individuals whose life is near the end would almost be all senescent and therefore show extremely low proliferation rates in the culture. If the proliferative activity of arterial SMC is a condition for atherogenesis, the proportion of senescent cells would be inversely related to the propensity of developing the atheroma because of the inability of these cells to divide.

The melanocyte differentiation pathway in spitz nevi

The nature of Spitz nevi is poorly understood, and their distinction from malignant melanoma can be difficult. Although there is general agreement on the diagnostic criteria, experts continue to have some differences, and controversial cases are not rare. A major obstacle to progress in this area is the lack of basic knowledge about melanocyte differentiation in Spitz nevi, as compared with ordinary nevi and malignant melanomas. Based on the hypothesis that normal melanocytes may have a differentiation pathway with discrete stages, it is suggested that the features of Spitz nevi may reflect homeostatic mechanisms governing maturation in the melanocyte differentiation pathway, whereas those of malignant melanomas may reflect carcinogen-induced aberrations. This perspective may be helpful in the continuing effort to develop optimal criteria for the differential diagnosis of Spitz nevi from malignant melanomas.

The expression of heat shock protein 70 decreases with cellular senescence in vitro and in cells derived from young and old human subjects

Because heat shock proteins have been shown to play a critical role in protecting cells from hyperthermia and other types of stresses, it was of interest to determine what effect cellular senescence in vitro and cells cultured in vitro from young and old human donors have on the ability of cells to regulate the expression of heat shock protein 70 (hsp70), the most prominent and most evolutionary conserved of the heat shock proteins. The ability of early and late passage IMR-90 lung fibroblasts and epidermal melanocytes and skin fibroblasts obtained from young and old human donors to express hsp70 was determined after a brief heat shock. We found that the levels of hsp70 protein and mRNA were lower in late passage cells and cells from old donors than in early passage cells and cells from young donors. The binding activity of the heat shock transcription factor HSF1, as measured by a gel shift assay, was significantly higher in early passage cells and cells from young donors in comparison to late passage cells and cells from old donors. In addition, the levels of HSF1 decreased significantly in late passage cells and cells from old donors in comparison to early passage cells and cells from young donors. Thus, our study demonstrates that the induction of hsp70 by hyperthermia in fibroblasts is significantly lower in late passage fibroblasts and in fibroblasts from old donors. In addition, our study shows that the decline in hsp70 expression during cellular senescence in vitro and in cells derived from old human subjects is paralleled by a decrease in the levels of HSF1.

Cell aging in vivo and in vitro

It has become a staple assumption of biology that there is an intrinsic fixed limit to the number of divisions that normal vertebrate cells can undergo before they senesce, and this limit is in some way related to aging of the organism. The notion of such a limited replicative lifespan arose from the often repeated observation that diploid fibroblasts cannot proliferate indefinitely in monolayer culture, and that the number of divisions before senescence is directly related to the in vivo lifespan of different species. The in vitro evidence is countered by estimates that the number of cell divisions in some organs of rodents and man are one or more orders of magnitude higher than the in vitro limit, with no indication of the degenerative changes seen in culture. Serial transplantation experiments in animals also exhibit many more cell divisions than the in vitro studies, with some indicating an indefinite replicative lifespan. I present evidence that vertebrate cells are severely stressed by enzymatic dispersion and sustain cumulative damage during serial subcultivations. The evidence includes large increases in cell size and its heterogeneity, reductions in replicative efficiency at low seeding densities, appearance of abnormal structures in the cytoplasm, changes in metabolism to a common cell culture type, continuous loss of methyl groups and reiterated sequences from DNA, and a constant rate of decline of growth rate with passage. This evidence is complemented by the reduction induced in the replicative life span of diploid cells by a large array of treatments which have different primary targets in the cells. The most consistent and general observation of cell behavior in aging animals, with only a few exceptions, is a reduction in the rate of cell proliferation. This reduction is perpetuated when the cells are grown in culture, indicating it is an enduring and intrinsic property of the cells rather than a systemic effect of the aging organism. A similar heritable reduction in growth rate can be induced in established cell lines by prolonged incubation at quiescence. The reduction can be exaggerated by subculturing the quiescent cells under suboptimal conditions, just as the effects of age are exaggerated under stress. The constant decline of growth rate that occurs during serial passage of diploid cells may represent a similar decay of cell function. I propose that the limit on replicative lifespan is an artifact that reflects the failure of diploid cells to adapt to the trauma of dissociation and the radically foreign environment of cell culture. It is, however, a useful artifact that has given us much information about cell behavior under stressful conditions. The overall evidence indicates cell in vivo accumulate damage over a lifetime that results in gradual loss of differentiated function and growth rate accompanied by an increased probability for the development of cancer. Such changes are normally held to a minimum by the organized state of the tissues and homeostatic regulation of the organism. The rejection of an intrinsic limit on the number of cell divisions eliminates the need for a cellular clock, such as telomere length, that counts mitoses. I offer a heuristic explanation for the gradual reduction of cell function and growth capacity with age based on a cumulative discoordination of interacting pathways within and between cells and tissues. I also make a case for the use of established cell lines as model systems for studying heritable damage to cell populations that simulates the effects of aging in vivo, and represents a relatively unexplored area of cell biology.

Replicative senescence: implications for in vivo aging and tumor suppression

Normal cells have limited proliferative potential in culture, a fact that has been the basis of their use as a model for replicative senescence for many years. Recent molecular analyses have identified numerous changes in gene expression that occur as cells become senescent, and the results indicate that multiple levels of control contribute to the irreversible growth arrest. These include repression of growth stimulatory genes, overexpression of growth inhibitory genes, and interference with downstream pathways. Studies with cell types other than fibroblasts will better define the role of cell senescence in the aging process and in tumorigenesis.

Studies demonstrating the complexity of regulation and action of the growth inhibitory gene SDI1

The identification of the DNA synthesis inhibitory gene SDI1 by investigators studying cell senescence, tumor suppression, cell cycle control and differentiation suggest a key regulatory role for this gene. To better understand the growth regulatory activity of this gene we proceeded with the experiments described here. The data demonstrate that SDI1 is an important downstream effector of p53, but here we report that it can also cause inhibition of DNA synthesis in several immortal human cell lines, independent of p53 or Rb status. Levels of SDI1 mRNA expression in immortal cells are consistently much lower than that of normal cells, indicating that immortalization is not compatible with high expression of SDI1. These results highlight the complex nature of regulation of this gene and its mode of action.

Origins of G1 arrest in senescent human fibroblasts

Human diploid fibroblasts have a finite proliferative lifespan in culture, at the end of which they are arrested with G1 phase DNA contents. Upon serum stimulation, senescent cells are deficient in carrying out a subset of early signal transduction events such as activation of protein kinase C and induction of c-fos. Later in G1, they uniformly fail to express late G1 genes whose products are required for DNA synthesis, implying that they are unable to pass the R point. Failure to pass the R point may occur because senescent cells are unable to phosphorylate the retinoblastoma protein, owing to the accumulation of inactive complexes of cyclin E/Cdk2 and possibly cyclin D/Cdk4. Senescent cells contain high amounts of p21, a potent cyclin-dependent kinase inhibitor whose levels are also elevated in cells arrested in G1 following DNA damage, suggesting that both arrests might share a common mechanism. Cell aging is accompanied by a progressive shortening of chromosomal telomeres, which could be perceived by the cells as a form of DNA damage that gives rise to the signals that inactive the cell cycle machinery.