Concurrence of replicative senescence and elevated expression of p16(INK4A) with subculture-induced but not calcium-induced differentiation in normal human oral keratinocytes

Epithelial cell senescence: an adaptive response to pre-carcinogenic stresses?

Senescence is a cell state occurring in vitro and in vivo after successive replication cycles and/or upon exposition to various stressors. It is characterized by a strong cell cycle arrest associated with several molecular, metabolic and morphologic changes. The accumulation of senescent cells in tissues and organs with time plays a role in organismal aging and in several age-associated disorders and pathologies. Moreover, several therapeutic interventions are able to prematurely induce senescence. It is, therefore, tremendously important to characterize in-depth, the mechanisms by which senescence is induced, as well as the precise properties of senescent cells. For historical reasons, senescence is often studied with fibroblast models. Other cell types, however, much more relevant regarding the structure and function of vital organs and/or regarding pathologies, are regrettably often neglected. In this article, we will clarify what is known on senescence of epithelial cells and highlight what distinguishes it from, and what makes it like, replicative senescence of fibroblasts taken as a standard.

Decrease in nicotinamide adenine dinucleotide dehydrogenase is related to skin pigmentation

Skin pigmentation is caused by various physical and chemical factors. It might also be influenced by changes in the physiological function of skin with aging. Nicotinamide adenine dinucleotide (NADH) dehydrogenase is an enzyme related to the mitochondrial electron transport system and plays a key role in cellular energy production. It has been reported that the functional decrease in this system causes Parkinson's disease. Another study reports that the amount of NADH dehydrogenase in heart and skeletal muscle decreases with aging. A similar decrease in the skin would probably affect its physiological function. However, no reports have examined the age-related change in levels of NADH dehydrogenase in human skin. In this study, we investigated this change and its effect on skin pigmentation using cultured human epidermal keratinocytes. The mRNA expression of NDUFA1, NDUFB7, and NDUFS2, subunits of NADH dehydrogenase, and its activity were significantly decreased in late passage keratinocytes compared to early passage cells. Conversely, the mRNA expression of melanocyte-stimulating cytokines, interleukin-1 alpha and endothelin 1, was increased in late passage cells. On the other hand, the inhibition of NADH dehydrogenase upregulated the mRNA expression of melanocyte-stimulating cytokines. Moreover, the level of NDUFB7 mRNA was lower in pigmented than in nonpigmented regions of skin in vivo. These results suggest the decrease in NADH dehydrogenase with aging to be involved in skin pigmentation.

Khat induces G1-phase arrest and increased expression of stress-sensitive p53 and p16 proteins in normal human oral keratinocytes and fibroblasts

Khat is a psychostimulant plant used by over 10 million people daily, mainly in eastern Africa and the Middle East. Previous studies have suggested an association between khat use and oral lesions such as hyperkeratosis and oral cancer. This study investigated the effects of an extract of khat on primary normal human oral keratinocytes (NOK) and normal human oral fibroblasts (NOF). Low (sublethal) concentrations of khat inhibited the proliferation of both cell types in a dose-dependent and time-dependent manner. Both NOK and NOF treated with khat accumulated in the G1-phase of the cell cycle and showed increased expression of the stress-sensitive p53 protein after 24 h. Normal human oral keratinocytes showed a profound increase in p16(INK4A) (p16) after 24 h and showed morphological changes suggesting cell differentiation. Normal human oral fibroblasts showed growth inhibition and increased expression of p21(WAF1/CIP1) (p21) within 24 h. The concentrations of khat tested in this study were within the range of those found in the oral cavity of khat chewers. The results show that stress induced by khat modulates the cell cycle in oral keratinocytes and fibroblasts. It is further speculated whether khat could have similar effects in vivo, especially in keratinocytes.

Does p16ink4a expression increase with the number of cell doublings in normal and malignant lymphocytes?

p16(ink4a) is known to be a major inhibitor of cyclin-dependent kinases of G1-phase. Its accumulation is associated with replicative senescence. We analyzed to what extent the number of cell doublings may participate to p16(ink4a) expression in normal and malignant lymphocytes. p16(ink4a) expression, not found in normal quiescent B or T-lymphocytes, was observed after stimulation of B-lymphocytes (72 h) and T-lymphocytes (2 weeks) before the occurrence of replicative senescence markers such as senescence-associated-beta-galactosidase activity. Afterwards, in lymphocyte long-term cultures, the increase in p16(ink4a) followed the expression of features of cell ageing. In acute lymphoblastic leukemia, the analysis of the individual differences between peripheral blood and blood compartments (34 cases) showed a decrease in cell proliferation (p<0.005), in telomerase activity (p<0.0005), and in hTERT expression (p<0.04), associated with an increase of p16(ink4a) (p<0.035) in blood leukemic cells. These results support the hypothesis that (i) an increase in p16(ink4a) expression in normal lymphocytes is linked, in part, to the number of cell doublings before the occurrence of replicative senescence and (ii) this process is maintained in leukemic cell populations of numerous patients.

Co-expression of p16INK4A and laminin 5 by keratinocytes: a wound-healing response coupling hypermotility with growth arrest that goes awry during epithelial neoplastic progression

The replicative lifespan of human keratinocytes in culture is restricted by a telomere-unrelated induction of p16INK4A (p16) and p14ARF. We have found that, in vivo, p16 is expressed by epidermal and oral keratinocytes at the migrating fronts of healing wounds and at the stromal interface of severely dysplastic and early invasive lesions and that such cells also invariably display increased expression of Laminin 5 (Lam5). In culture, p16 and Lam5 are coexpressed in keratinocytes at senescence, at the edges of wounds made in confluent cultures, and when cells are plated on dishes coated with the gamma2 precursor form of Lam5 (Lam5gamma2pre). Lam5/p16 coexpression in all three in vitro settings is associated with directional hypermotility and growth arrest. Hypermotility and growth arrest are uncoupled in p16- and p14ARF/p53-deficient keratinocytes and squamous cell carcinoma (SCC) cells; such cells become hypermotile is response to Lam5gamma2pre but do not growth arrest. Thus, the Lam5/p16 response is activated in normal wound healing, causing growth arrest of migratory keratinocytes that lead wound reepithelialization. This response also becomes activated at a critical stage of neoplastic progression, acting as a tumor suppressor mechanism. Rare premalignant cells that lose p16 remain motile and proliferative, thereby resulting in invasive growth as SCC.

Beta ig-h3 induces keratinocyte differentiation via modulation of involucrin and transglutaminase expression through the integrin alpha3beta1 and the phosphatidylinositol 3-kinase/Akt signaling pathway

Beta ig-h3 is an extracellular matrix protein whose expression is highly induced by transforming growth factor (TGF)-beta1. Whereas beta ig-h3 is known to mediate keratinocyte adhesion and migration, its effects on keratinocyte differentiation remain unclear. In the present study, it was demonstrated that expression of both beta ig-h3 and TGF-beta1 was enhanced during keratinocyte differentiation and that expression of the former was strongly induced by that of the latter. This study also asked whether changes in beta-h3 expression would affect keratinocyte differentiation. Indeed, down-regulation of beta ig-h3 by transfection with antisense beta ig-h3 cDNA constructs effectively inhibited keratinocyte differentiation by decreasing the promoter activities and thus expression of involucrin and transglutaminase. The result was an approximately 2-fold increase in mitotic capacity of the cells. Conversely, overexpression of beta ig-h3, either by transfection with beta ig-h3 expression plasmids or by exposure to recombinant beta ig-h3, enhanced keratinocyte differentiation by inhibiting cell proliferation and concomitantly increasing involucrin and transglutaminase expression. Recombinant beta ig-h3 also promoted keratinocyte adhesion through interaction with integrin alpha3beta1. Changes in beta ig-h3 expression did not affect intracellular calcium levels. Subsequent analysis revealed not only induction of Akt phosphorylation by recombinant beta ig-h3 but also blockage of Akt phosphorylation by LY294002, an inhibitor of phosphatidylinositol 3-kinase. Taken together, these findings indicate that enhanced beta ig-h3, induced by enhanced TGF-beta during keratinocyte differentiation, provoked cell differentiation by enhancing involucrin and transglutaminase expression through the integrin alpha3beta1 and phosphatidylinositol 3-kinase/Akt signaling pathway. Lastly, it was observed that beta ig-h3-mediated keratinocyte differentiation was caused by promotion of cell adhesion and not by calcium regulation.

Autolysosomes accumulate during in vitro CD8+ T-lymphocyte aging and may participate in induced death sensitization of senescent cells

As autophagic inclusions accumulate in senescent fibroblasts, we wondered whether an increase in cellular fragility during in vitro lymphocyte aging may be related to an autolysosome accumulation. We established that, during long-term cultures, repeatedly stimulated T-lymphocytes acquired characteristics of replicative senescence and became progressively intolerant to activation. Cell death following stimulations: (i) corresponded to apoptosis, associated with necrosis at the end of the culture; (ii) was not, for its main part, mediated through CD95/CD178 or TNFRII/TNF alpha interactions; and (iii) occurred in spite of bcl-2 increased expression. After 14 weeks of culture, the percentage of lymphocytes containing at least one autophagic inclusion (p<0.0001) and the lipofuscin autofluorescence in lymphocytes (p<0.0001) were significantly increased. The expression of several genes regulating autophagy did not significantly vary with the age of the culture. Forty-eight hours after each stimulation, the percentage of induced cell death rose while, in the remaining living cells, the percentage of lymphocytes with autophagic vacuoles (p<0.05), with beta-galactosidase activity and the lipofuscin autofluorescence (p<0.001) significantly decreased, suggesting the preferential death of cells with autophagy. Our data support the view that the accumulation of autolysosomes in senescent lymphocytes might aggravate cellular fragility, leading to apoptosis and necrosis mainly induced by lymphocyte activation.

Co-expression of p16(INK4A) and laminin 5 gamma2 by microinvasive and superficial squamous cell carcinomas in vivo and by migrating wound and senescent keratinocytes in culture

The high frequency of mutation, deletion, and promoter silencing of the gene encoding p16(INK4A) (p16) in premalignant dysplasias and squamous cell carcinomas (SCC) of epidermis and oral epithelium classifies p16 as a tumor suppressor. However, the point during neoplastic progression at which this protein is expressed and presumably impedes formation of an SCC is unknown. Induction of p16 has been found to be responsible for the senescence arrest of normal human keratinocytes in culture, suggesting the possibility that excessive or spatially abnormal cell growth in vivo triggers p16 expression. We examined 73 skin and oral mucosal biopsy specimens immunohistochemically to test this hypothesis. p16 was not detectable in benign hyperplastic lesions, but instead was expressed heterogeneously in some dysplastic and carcinoma in situ lesions and consistently at areas of microinvasion and at superficial margins of advanced SCCs. p16-positive cells in these regions coexpressed the gamma2 chain of laminin 5, identified previously as a marker of invasion in some carcinomas. Normal keratinocytes undergoing senescence arrest in culture proved to coordinately express p16 and gamma2 and this was frequently associated with increased directional motility. Keratinocytes at the edges of wounds made in confluent early passage cultures also coexpressed p16 and gamma2, accompanying migration to fill the wound. These results have identified the point during neoplastic progression in stratified squamous epithelial at which the tumor suppressor p16 is expressed and suggest that normal epithelia may use the same mechanism to generate non-dividing, motile cells for wound repair.

Bradykinin receptor modulation in cellular models of aging and Alzheimer's disease

Human fibroblast cell culture systems have been used to model both molecular events associated with the aging process and the biochemical anomalies found in the aging-associated neurodegenerative disorder Alzheimer's disease (AD). We demonstrate modulation of bradykinin (BK) B2 receptors that results in Intermediate (I, Kd 2.5-5 nM) and Low (L, Kd 44 nM) receptor affinity states in two cellular model systems that target aging and aging-associated disorders: the human lung fibroblast cell line WI-38 model for cellular aging and a skin fibroblast cell line from a patient with early onset familial Alzheimer's disease. In both cellular models the generation of I and L BK B2 receptors is extremely rapid, occurring within 1 min of activation of protein kinase C (PKC) by phorbol ester. Blocking phosphoprotein phosphatase activity further augments the cellular content of I and L receptors in the Alzheimer's skin fibroblast cell line. These two lines of evidence suggest that a phosphorylation cascade modifying the receptors is responsible for the I and L states. The I and L receptors remain biologically active and enhance cellular responsiveness to elevated levels of BK that are found in tissue injury, one of the major risk factors for development of Alzheimer's disease. The Alzheimer's disease skin fibroblast cell line presents a cellular environment highly enriched in the amyloid Abeta1-42 peptide that is the hallmark of Alzheimer's plaque lesions in the brain. This Abeta-rich environment may serve to foster the signal transduction mechanism that generates I and L BK B2 receptors.

PML a target of translocations in APL is a regulator of cellular senescence

PML is the most frequent fusion partner of the RARalpha in the specific translocations associated with acute promyelocytic leukemia (APL). Models to explain the origin of this leukemia propose a block in cell differentiation due to aberrant repression of retinoic acid responsive genes and/or disruption of the function of the PML-containing nuclear bodies. Recently, PML has been identified as a regulator of replicative senescence and the premature senescence that occurs in response to oncogenic ras. This review discusses the idea that senescence is a general tumor suppressor mechanism related to terminal differentiation and disrupted during the establishment of APL and other cancers. According to this idea the PML-RARalpha fusion protein promotes leukemogenesis not only through repression of retinoic acid responsive genes, but also by way of interfering with several tumor suppressor proteins that cooperate to establish senescence. Retinoids and other drugs effective against APL do so by re-establishment of the senescence program, which also includes features of cell differentiation.

p16(MTS-1/CDKN2/INK4a) in cancer progression

Since its discovery as an inhibitor of cyclin-dependent kinases 4 and 6, the tumor suppressor p16 has continued to gain widespread importance in cancer. The high frequency of deletions of p16 in tumor cell lines first suggested an important role for p16 in carcinogenesis. This initial genetic evidence was subsequently strengthened by numerous studies documenting p16 inactivation in kindreds with familial melanoma. Moreover, a high frequency of p16 gene alterations was found in primary tumors, while recent studies have identified p16 promoter methylation as a major mechanism of tumor-suppressor-gene silencing. Additional insight into p16's role in cancer has come from the genetic analysis of precancerous lesions and various tissue culture models. It is now believed that loss of p16 is an early and often critical event in tumor progression. Consequently, p16 is a major tumor-suppressor gene whose frequent loss occurs early in many human cancers.

Mechanisms involved in ultraviolet light-induced immunosuppression

Ultraviolet light (UV) represents one of the most relevant environmental factors influencing humans, especially with regard to its hazardous health effects, which include premature skin aging, skin cancer, and exacerbation of infectious diseases. Several of these effects are mediated by the immunosuppressive properties of UV. UV can compromise the immune system in several ways, e.g., by affecting the function of antigen-presenting cells, inducing the release of cytokines, and modulating the expression of surface molecules. Recently a link between UV-induced immunosuppression and apoptosis was recognized. In the following, the basic mechanisms underlying UV-induced immunosuppression will be discussed.