Maria Garmyn

Activation of p38 MAPK is required for Bax translocation to mitochondria, cytochrome c release and apoptosis induced by UVB irradiation in human keratinocytes

This study establishes that activation of p38 MAPK by UVB represents a crucial signal required for the conformational change and translocation of Bax to the mitochondria in human keratinocytes. UVB‐induced Bax translocation and mitochondrial cytochrome c release, which precede caspase activation and other endpoints of the apoptotic program such as chromatin fragmentation and loss of mitochondrial transmembrane potential, are blocked by genetic or pharmacological inhibition of the p38α MAPK. Inhibition of p38 MAPK strongly reduces the UVB‐induced formation of sunburn cells and blocks Bax conformational change both in cultured human keratinocytes and in human skin, providing clear evidence for the physiological role of the p38 MAPK‐Bax pathway in the removal of precancerous, UVB‐damaged keratinocytes. Furthermore, we show that Bcl‐2 overexpression, but not the pan‐caspase inhibitor zVAD‐fmk, blocks Bax conformational change and its subsequent translocation downstream of p38 MAPK. These data indicate that the activation of p38 MAPK by UVB engages a caspase‐independent death signal leading to mitochondrial membrane permeabilization and apoptosis in human keratinocytes and suggest that p38 MAPK might have a preventive role in the process of photocarcinogenesis.

A p38MAPK/HIF-1 Pathway Initiated by UVB Irradiation Is Required to Induce Noxa and Apoptosis of Human Keratinocytes

The signal transduction pathways leading to apoptosis of human keratinocytes responding to UVB irradiation are complex and not completely understood. Previously, we reported that in UVB-irradiated keratinocytes, p38(MAPK) instigates Bcl-2-associated X protein (Bax) activation and mitochondrial apoptosis. However, the molecular mechanism underlying the pro-apoptotic function of p38(MAPK) remained unclear. Here, we show that in UVB-treated human primary keratinocytes the activation of p38(MAPK) is necessary to upregulate Noxa, a BH3-only pro-apoptotic dominantly induced by UVB and required for apoptosis. Whereas p53-silencing was marginally cytoprotective and poorly affected Noxa expression, p38(MAPK) inhibition in p53-silenced keratinocytes or in p53(-/-) cells could still efficiently prevent Noxa induction and intrinsic apoptosis after UVB, indicating that p38(MAPK) signals mainly through p53-independent mechanisms. Furthermore, p38(MAPK) was required for the induction and activation of hypoxia-inducible factor 1 (HIF-1) in response to UVB, and HIF-1 knockdown reduced Noxa expression and apoptosis. In UVB-irradiated keratinocytes, Noxa targeted the anti-apoptotic myeloid cell leukemia sequence 1 (Mcl-1) for degradation, and small-interfering RNA (siRNA)-mediated knockdown of Noxa or p38(MAPK) inhibition restored levels of Mcl-1 and abolished apoptosis. Thus, the pro-apoptotic mechanisms orchestrated by p38(MAPK) in human keratinocytes in response to UVB involve an HIF-1/Noxa axis, which prompts the downregulation of anti-apoptotic Mcl-1, thereby favoring Bax-mediated mitochondrial apoptosis of UVB-damaged keratinocytes.

Case–control study to identify melanoma risk factors in the Belgian population: the significance of clinical examination

Background  Although numerous studies have evaluated risk factors associated with cutaneous malignant melanoma (CMM), no such study has been carried out in Belgium.

The Effect of Acute and Chronic Photodamage on Gene Expression in Human Keratinocytes

We identified genes involved in the normal response to acute UV damage, as they were modulated in cultured newborn keratinocytes by a single sublethal UV dose, appropriately filtered to mimic solar radiation. Their gene products encode proteins involved in the regulation of cell growth (proto-oncogenes c-myc and c-fos), a gene inducible by growth arrest and DNA damage (GADD153), the cytokine interleukin (IL) 1 alpha and beta and finally a differentiation-associated small proline-rich gene (SPR2). Because chronically sun-exposed skin is known to have altered immune responsiveness and a statistical predisposition to skin cancer, we then examined UV induction of these genes in cultured keratinocytes derived from habitually sun-exposed adult skin, and for the older donors in paired cultures derived from sun-protected site of the same donors. Aging alone increased the baseline expression of two differentiation-associated genes (SPR2 and IL-1 receptor antagonist) in cultures from sun-protected skin. In contrast, photoaging increased the UV inducibility of c-fos but decreased the baseline expression of the differentiation-associated genes IL-1 receptor antagonist and SPR2.

Skin mild hypoxia enhances killing of UVB-damaged keratinocytes through reactive oxygen species-mediated apoptosis requiring Noxa and Bim

The naturally occurring skin hypoxia has emerged as a crucial host factor of the epidermal microenvironment. We wanted to systematically investigate how reduced oxygen availability of the epidermis modulates the response of keratinocytes and melanocytes to noxious ultraviolet B radiation (UVB). We report that the exposure of normal human keratinocytes (NHKs) or melanocytes (NHEMs) to mild hypoxia drastically impacts cell death responses following UVB irradiation. The hypoxic microenvironment favors survival and reduces apoptosis of UVB-irradiated NHEMs and their malignant counterparts (melanoma cells). In contrast, NHKs, but not the transformed keratinocytes, under hypoxic conditions display increased levels of reactive oxygen species (ROS) and are significantly sensitized to UVB-mediated apoptosis as compared to NHKs treated under normoxic conditions. Prolonged exposure of UVB-treated NHKs to hypoxia triggers a sustained and reactive oxygen species-dependent activation of the stress kinases p38(MAPK) and JNKs, which in turn, engage the activation of Noxa and Bim proapoptotic proteins. Combined silencing of Noxa and Bim significantly inhibits UVB-mediated apoptosis under hypoxic conditions, demonstrating that hypoxia results in an amplification of the intrinsic apoptotic pathway. Physiologically occurring skin hypoxia, by facilitating the specific removal of UVB-damaged keratinocytes, may represent a decisive host factor impeding important steps of the photocarcinogenesis process.

Influence of aging and malignant transformation on keratinocyte gene expression

Skin cancer is the most common malignancy in humans (Yuspa 1987). Most skin cancers are strongly linked to UV exposure (Aubry and MacGibbon 1985; Stoll and Schwartz 1987; Streilein 1991), but other agents such as chemical carcinogens and immunosuppressants (Yuspa et al. 1980) may also contribute. In addition, age-specific skin cancer incidence rises exponentially during adulthood and available data suggest that advanced age itself, independent of exposure history, may be a risk factor (Lin et al. 1989).

Deregulation of cell-death pathways as the cornerstone of skin diseases

Deregulation of cell‐death pathways plays a key role in the pathogenesis of various skin diseases. The different types of cell death are mainly defined by morphological criteria, and include apoptosis, autophagic cell death, and necrosis. The process of apoptosis is well characterized at the molecular level and involves the activation of two main pathways, the intrinsic and extrinsic pathways, converging into the execution of apoptosis by intracellular cysteine proteases, called caspases. The relevance and implication of these apoptotic pathways in the pathophysiology of skin diseases, such as toxic epidermal necrolysis, graft‐versus‐host disease and skin cancer, has been extensively studied. The role of autophagic cell death in progression of skin tumours and response to cytotoxic drugs is only beginning to be elucidated.