Expression profiles of p63, p53, survivin, and hTERT in skin tumors

Effects of hTERT on metal ion-induced genomic instability

There is currently a great interest in delayed chromosomal and other damaging effects of low-dose exposure to a variety of pollutants which appear collectively to act through induction of stress-response pathways related to oxidative stress and ageing. These have been studied mostly in the radiation field but evidence is accumulating that the mechanisms can also be triggered by chemicals, especially heavy metals. Humans are exposed to metals, including chromium (Cr) (VI) and vanadium (V) (V), from the environment, industry and surgical implants. Thus, the impact of low-dose stress responses may be larger than expected from individual toxicity projections. In this study, a short (24 h) exposure of human fibroblasts to low doses of Cr (VI) and V (V) caused both acute chromosome damage and genomic instability in the progeny of exposed cells for at least 30 days after exposure. Acutely, Cr (VI) caused chromatid breaks without aneuploidy while V (V) caused aneuploidy without chromatid breaks. The longer-term genomic instability was similar but depended on hTERT positivity. In telomerase-negative hTERT- cells, Cr (VI) and V (V) caused a long lasting and transmissible induction of dicentric chromosomes, nucleoplasmic bridges, micronuclei and aneuploidy. There was also a long term and transmissible reduction of clonogenic survival, with an increased beta-galactosidase staining and apoptosis. This instability was not present in telomerase-positive hTERT+ cells. In contrast, in hTERT+ cells the metals caused a persistent induction of tetraploidy, which was not noted in hTERT- cells. The growth and survival of both metal-exposed hTERT+ and hTERT- cells differed if they were cultured at subconfluent levels or plated out as colonies. Genomic instability is considered to be a driving force towards cancer. This study suggests that the type of genomic instability in human cells may depend critically on whether they are telomerase-positive or -negative and that their sensitivities to metals could depend on whether they are clustered or diffuse.

Apoptosis induced by ultraviolet B in HPV-immortalized human keratinocytes requires caspase-9 and is death receptor independent

Ultraviolet B (UVB) induces both apoptosis and skin cancer. We found that human keratinocytes (KC) immortalized by Human Papillomavirus (HPV)16 E6/E7 were sensitized to UVB-induced apoptosis, possibly representing a transient regression-prone precancerous stage equivalent to actinic keratosis. To further examine which caspases are apical and essential, we utilized retroviral constructs expressing dominant-negative caspase-9 (caspase-9-DN) or Fas-associated protein with death domain (FADD)-DN as well as caspase inhibitor peptides. Caspase-9-DN and zLEHD-fmk both suppressed caspase-9, -3, and -8 activity after UVB exposure, as well as proteolytic processing of procaspase-3 into its active form, DNA fragmentation factor 45 cleavage, and internucleosomal DNA fragmentation. By contrast, stable expression of FADD-DN in HPV-immortalized KC did not inhibit UVB-induced activation of caspases-9, -3, and -8 nor downstream apoptotic events, although inhibition of caspase-8 with zIETD-fmk attenuated apoptosis. This study indicates that caspase-9 activation is upstream of caspases-3 and -8 and that UVB-induced apoptosis in HPV-immortalized human KC is death receptor (DR) independent and requires both caspase-9 upstream and caspase-8 downstream for maximal apoptosis. These studies further indicate that cell type as well as transformation state determine the sensitivity and mode of cell death (DR vs. mitochondrial apoptotic pathways) in response to UVB and explain the high regression rates of premalignant lesions.

Pathways involved in sunburn cell formation: deregulation in skin cancer

The incidence of squamous cell carcinoma of the skin is rising worldwide for decades. Chronic exposure to sunlight is the most important environmental risk factor for this type of skin cancer. This is predominantly due to the DNA damaging effect of ultraviolet-B (UVB) in sunlight. UVB induces also sunburn cells, i.e. apoptotic keratinocytes, which is a crucial protective mechanism against the carcinogenic effects of UVB irradiation. This process is regulated by a wide range of molecular determinants involved in the balance between pro- and anti-apoptotic pathways. Growing evidence suggests that the deregulation of this balance by chronic UVB irradiation, contributes to the development of skin cancer. This review gives a brief summary of major known pathways involved in the regulation of keratinocyte survival and cell death upon UVB damage and discusses the contribution of the deregulation of these cascades to photocarcinogenesis.

Unique domain functions of p63 isotypes that differentially regulate distinct aspects of epidermal homeostasis

p63 is critical for squamous development and exists as multiple isotypes of two subclasses, TA and DeltaN. DeltaNp63 isotypes can antagonize transcription by TAp63 and p53, and are highly expressed in squamous cell cancers. Using mouse keratinocytes as a biological model of squamous epithelium, we show that multiple p63 isotypes, DeltaN- and TA-containing, are expressed and differentially modulated during in vitro murine keratinocyte differentiation. DeltaNp63alpha declines with Ca2+-induced differentiation, while a smaller DeltaN-form, DeltaNp63s, persists, suggesting unique functions of the two DeltaN-forms. To investigate the impact of dysregulated p63 expression that is observed in cancers and to define the biological contribution of the different domains of the p63 isotypes, DeltaNp63alpha, DeltaNp63p40, TAp63alpha, TAp63gamma or beta-galactosidase were overexpressed in primary murine keratinocytes. Microarray, RT-PCR and western blot analyses revealed that overexpression of DeltaNp63p40, which lacks the entire alpha-tail present in DeltaNp63alpha, permits expression of a full panel of differentiation markers. This is in contrast to overexpression of the full-length DeltaNp63alpha, which blocks induction of keratin 10, loricrin and filaggrin. These findings support a role for the alpha-tail of DeltaNp63alpha in blocking differentiation-specific gene expression. Overexpression of either TAp63 isotype permits keratin 10 and loricrin expression, thus the alpha-terminus requires the cooperation of the DeltaN domain in blocking early differentiation. However, both TA isotypes block filaggrin induction. The DeltaN-terminus is sufficient to maintain keratinocytes in a proliferative state, as both DeltaN forms block Ca2+-mediated p21WAF1 induction and S-phase arrest, while sustaining elevated PCNA levels. No alteration in cell cycle regulation was observed in keratinocytes overexpressing TAp63alpha or TAp63gamma. Clarifying the functional distinctions between p63 isotypes and domains will help to elucidate how their dysregulation impacts tumor biology and may suggest novel therapeutic strategies for modulating behavior of tumor cells with altered expression of p53 family members.