Elevated lactate dehydrogenase A (LDHA) expression is associated with poor outcome in tumor patients. Here we show that LDHA-associated lactic acid accumulation in melanomas inhibits tumor surveillance by T and NK cells. In immunocompetent C57BL/6 mice, tumors with reduced lactic acid production (Ldha) developed significantly slower than control tumors and showed increased infiltration with IFN-γ-producing T and NK cells. However, in Rag2γc mice, lacking lymphocytes and NK cells, and in Ifng mice, Ldha and control cells formed tumors at similar rates. Pathophysiological concentrations of lactic acid prevented upregulation of nuclear factor of activated T cells (NFAT) in T and NK cells, resulting in diminished IFN-γ production. Database analyses revealed negative correlations between LDHA expression and T cell activation markers in human melanoma patients. Our results demonstrate that lactic acid is a potent inhibitor of function and survival of T and NK cells leading to tumor immune escape.

Chronic sun exposure causes photoaging of human skin, a process that is characterized by clinical, histological and biochemical changes which differ from alterations in chronologically aged but sun-protected skin. Within recent years, substantial progress has been made in unraveling the underlying mechanisms of photoaging. Induction of matrix metalloproteinases as a consequence of activator protein (AP)-1 and nuclear factor (NF)-kB activation as well as mutations of mitochondrial DNA have been identified recently.T he term photoaging describes distinct clinical, histological and functional features of chronically sunexposed skin. It has evolved from a variety of terms such as heliodermatosis, actinic dermatosis, and accelerated skin aging. Photoaged, chronically sun-exposed skin has characteristics in common with sun-protected, chronologically aged skin. However, there are features which are found exclusively in photoaged skin, making it an independent entity with its own pathophysiology.Extended life-span, more spare time and excessive exposure to ultraviolet (UV) radiation from natural sunlight or tanning devices, especially in the western population, has resulted in an ever increasing demand to protect human skin against the detrimental effects of UV-exposure of the skin to ultraviolet light. Therefore, photoaging will be of increasing concern in the future.The clinical and histological characteristics of photoaged skin have been known for some time (1); however, not until recently have the underlying molecular mechanisms responsible for the specific macro-and microAbbreviations: EGF, epidermal growth factor; ERK, extracellular signal-regulated kinase; GAG, glycosaminoglycans; JNK, c-Jun amino terminal kinase; mt, mitochondrial; MAP, mitogen-activated protein; MMP, matrix metalloproteinase; MED, minimal erythema dose; NF-kB, Nuclear factor kB; nm, nanometer; OXPHOS, oxidative phosphorylation; RA, retinoic acid; ROS, reactive oxygen species; TIMP, tissue specific inhibitor of matrix metalloproteinases. 239This has increased our understanding of photoaging significantly and has led to new prophylactic and therapeutic strategies aimed at the prevention and repair of the detrimental effects of chronic sun-exposure on the skin.Key words: antioxidants; mitochondrial DNA; photoaging; reactive oxygen species; repetitive sun exposure; retinoic acid; sunscreens; ultraviolet light.scopic alterations been discovered. The role of selected transcription factors (AP-1, NF-kB) in photoaging has been demonstrated and it has been found that mutations of mitochondrial DNA may also be involved. The elucidation of these pathophysiological mechanisms provides the basis for evaluating the efficacy of photo(aging)protective substances and might help in the development of new strategies which will provide protection and repair of photoaged human skin. Previous reviews on this topic have described the different aspects of photoaging (2 -4). Hence, this review will only briefly summarize the clinical and histological features of photoa...

Mutations of mitochondrial (mt) DNA accumulate during normal aging. The most frequent mutation is a 4,977-base pair deletion also called the common deletion, which is increased in photoaged skin. Oxidative stress may play a major role in the generation of large scale mtDNA deletions, but direct proof for this has been elusive. We therefore assessed whether the common deletion can be generated in vitro through UV irradiation and whether reactive oxygen species are involved in this process. Normal human fibroblasts were repetitively exposed to sublethal doses of UVA radiation and assayed for the common deletion employing a semiquantitative polymerase chain reaction technique. There was a time/dose-dependent generation of the common deletion, attributable to the generation of singlet oxygen, since the common deletion was diminished when irradiating in the presence of singlet oxygen quenchers, but increased when enhancing singlet oxygen half-life by deuterium oxide. The induction of the common deletion by UVA irradiation was mimicked by treatment of unirradiated cells with singlet oxygen produced by the thermodecomposition of an endoperoxide. These studies provide evidence for the involvement of reactive oxygen species in the generation of aging-associated mtDNA lesions in human cells and indicate a previously unrecognized role of singlet oxygen in photoaging of human skin.Oxidative phosphorylation in mitochondria is carried out by five protein complexes encoded by both the nuclear DNA and the mitochondrion's own genome, the mitochondrial (mt) 1 DNA. Mutations of mtDNA have been shown previously to play a role in a variety of degenerative diseases mainly affecting muscle and nerve tissues (1-3) as well as diseases such as familial diabetes mellitus (4). Their relevance is not restricted to degenerative diseases, however; e.g. mtDNA mutations are also critically involved in the normal aging process (5-8).The most frequent and best characterized mutation in mtDNA is a deletion of 4,977 base pairs in length, also called the common deletion. This common deletion is considered to be a marker for mutations in the mitochondrial genome, and substantial efforts have been made to elucidate the mechanism by which it is generated. A modified slip-replication mechanism has been proposed (9 -12) involving the misannealing of direct repeats (Scheme 1). Hotspots for the common deletion exhibit structural abnormalities facilitating the misannealing of direct repeats from the light to the heavy strand of the mtDNA (13). This then leads to loop formation of both the heavy and the light strand of mtDNA. The initiation of loop exclusion is thought to be mediated by reactive oxygen species (13,14).Reactive oxygen species can damage mtDNA (15-17), and damage by hydrogen peroxide is more extensive in mtDNA than in nuclear DNA (18). Furthermore it has been shown recently that increased oxidative stress is correlated to an altered mitochondrial function in vivo (19). In addition, oxidative stress induced by solar radiation may also be respon...

Induction of apoptosis of keratinocytes by ultraviolet (UV) radiation is a protective phenomenon relevant in limiting the survival of cells with irreparable DNA damage. Changes in UV-induced apoptosis may therefore have significant impact on photocarcinogenesis. We have found that the immunomodulatory cytokine IL-12 suppresses UV-mediated apoptosis of keratinocytes both in vitro and in vivo. IL-12 caused a remarkable reduction in UV-specific DNA lesions which was due to induction of DNA repair. In accordance with this, IL-12 induced the expression of particular components of the nucleotide-excision repair complex. Our results show that cytokines can protect cells from apoptosis induced by DNA-damaging UV radiation by inducing DNA repair, and that nucleotide-excision repair can be manipulated by cytokines.

The results suggest that, whilst accepted causes do affect onset of melasma, a combination of these factors often triggers this disorder. These factors may provide further insights into how physicians can manage individual melasma cases, support recommendation of preventative measures and even anticipate treatment results and recurrence.