In dermatology, as in many specialties, there are a number of “tried and true” treatments that have been utilized, in some cases for millenia, to treat cutaneous diseases. These therapies continue to comprise an important part of our therapeutic armamentarium, despite the fact that the mechanisms by which these compounds improve clinical disease courses are often poorly understood. Modern efforts to understand pathologic disease mechanisms, particularly when undertaken or funded by pharmaceutical companies, tend to focus attention toward the development of novel classes of compounds whose marketing and prescription are likely to be financially lucrative. Rational drug design describes the concept of designing novel compounds that could structurally or functionally be predicted to modify a pathologic process or target through modification of gene or protein expression (Mandel et al., 2009). High-throughput computer-based algorithms can be constructed to screen a database of small molecules with unknown targets, or to screen a host of known targets for their potential response to a particular synthesized molecule, to rapidly and efficiently identify potential novel molecule-target combinations. Similarly, modeling programs can also be used to predict potential toxicities or untoward effects of these novel compounds (Mandel et al., 2009); these potential effects should, of course, then be investigated in experimental settings.
Given the potential for rational drug design methods to identify lucrative novel therapies, much less attention is often paid to gaining a better understanding of the mechanisms of compounds that have been used successfully for hundreds or thousands of years. Studies undertaken to elucidate the mechanisms by which these therapies exert their actions, however, represent valuable opportunities both to better understand their biological effects and to provide insight into physiologic processes that underlie the diseases these compounds affect. A recent study published in the Journal of Clinical Investigation provides a fine example of such work (van den Bogaard et al., 2013). The authors describe a mechanism by which coal tar, via activation of the aryl hydrocarbon receptor (AHR), restores epidermal barrier protein expression and attenuates Th2-cytokine-mediated effects in both in vitro models and in vivo in atopic dermatitis (van den Bogaard et al., 2013). A commentary in the same issue discusses that although coal tar is comprised of a wide variety of compounds that are likely to have effects on multiple targets in diseases like atopic dermatitis, a better understanding of known biological targets could allow for the extraction of specific active ingredients from coal tar which would hopefully allow for the creation of “cleaner” preparations that would allow patients to refrain from applying sticky, smelly, and stain-inducing coal tar to the skin (McLean and Irvine, 2013). Identification of new targets (like the AHR) in atopic dermatitis could also guide rational drug design methods to identify or construct novel compounds that could be added to the therapeutic armamentarium. These articles remind us that despite the current exciting climate of “smart” drug design, which has enabled the rapid introduction of new classes of therapeutic agents that revolutionize the way we treat various cutaneous diseases, we should not entirely neglect the study of “tried and true” therapies – there is likely a very good reason we use them, even if we don’t yet know what that reason is.
Before Walt Disney’s Fantasia featuring Mickey Mouse, Goethe gave the world the image of the sorcerer’s apprentice — cutting his broom progressively in half in an attempt to complete the task of cleaning up the sorcerer’s workshop. This is a perfect image for advances with ever finer examinations of the HLA locus with new genetic probes and methodologies. The sorcerers will be up all night at their computers, cleaning up the laboratory — flooded with data.
A recent advance appears in the current issue of Nature Genetics. Hughes et al extensively genotyped over six hundred Turkish and Italian patients with Behcet’s disease. The association of Behcet’s disease with HLA-B*51 tumbled, or was drowned, in the tsunami of data and will no longer be a correct answer on multiple choice exams. Strong associations with one of the loci in the HLA region associated with psoriasis (PSOR1C1) were present, as was the association with a locus between HLA-B and the MICA locus, in both the Turkish and the Italian cohorts. And the search and the analysis will go on. Clinicians and immunologists will ponder the possible links between Behcet’s disease and psoriasis. The publication ends with a prescient comment that many of the other ‘known’ HLA association may yield more and even useful information with the higher density forms of genetic analysis that are now possible. There will be many more buckets of data generated while the sorcerer’s workshop is being cleaned.
The questions and and answers below relate to the Research Techniques Made Simple entitled “Microarray Technique, Analysis and Applications in Dermatology” in the April 2013 issue of Journal of Investigative Dermatology.
1. The term “array” in microarray refers to the arrangement of which of the following on the chip:
a. the probe
b. the target
c. the fluorophore
d. the antigen
The correct answer is a: the probe
2. The most common types of probes used for microarrays are:
a. complementary DNA (cDNA)
b. single nucleotide polymorphisms (SNPs)
The correct answer is a: complementary DNA (cDNA)
3. Microarray analysis has been used to study
c. cutaneous T-cell lymphoma
e. all of the above
Correlation between local cortisol metabolism and epidermal hyperproliferative disorders
Recent findings have shown constitutive and regulated production of cortisol by human skin. Cutaneous cortisol can be generated through sequential metabolism of endogenously produced cholesterol or from progesterone that is delivered via the circulation. Thus, the recognized neuroendocrine functions of human skin are extended by a major glucocorticoidogenic capability.
Two key enzymes that regulate local cortisol availability for the glucocorticoid receptor are 11ß-HSD1 and 11ß-HSD2. 11β-HSD1 expresses ketoreductase activity (at a high
NADPH/NADP+ ratio) with the transformation of inactive cortisone into hormonally active cortisol. 11ß-HSD2, in turn, is an NADP+−dependent enzyme that acts exclusively as a dehydrogenase to inactivate cortisol to cortisone. Both of these enzymes are expressed in human skin.
Terao et al. (2013) now report that 11ß-HSD1 decreases in hyperpoliferative disorders like seborrhoeic keratosis and squamous and basal cell carcinoma, while 11ß-HSD2 increases in seborrhoeic keratosis and basal cell carcinoma. Furthermore, treatment of mouse skin with TPA reduces expression of 11ß-HSD1, while forced 11ß-HSD2 overexpression stimulates keratinocyte proliferation.
These important findings offer new insights not only into the potential role of 11ß-HSD1 and 11ß-HSD2 in the development of cutaneous hyperproliferative disorders and perhaps skin carcinogenesis, but also raise the possibility that these enzymes are notable regulators of epidermal homeostasis under physiological conditions. Targeting these enzymes, for example by topically applied small molecules, may therefore represent an exciting novel strategy for the therapeutic manipulation of hyperproliferative human skin disorders.
Are NK cells really important in the pathogenesis of psoriasis?
There is considerable evidence suggesting that NK cells play a role in the pathogenesis of psoriasis. In their recent study, Batista et al (2013) specifically looked at the expression of CD57, which is known to be associated with the senescence of NK cells. The investigators found that the frequency of CD7-CD56+CD16+ (NK cell markers) was much higher in involved psoriatic skin. This implies that IFN-g production is higher in the involved area, due to the decreased frequency of CD57+CD56+CD16+ NK senescence cells. Furthermore, their study showed increased expression of NKG2A, a key NK cell activating receptor whose expression correlates well with the level of IFN-g production by NK cells, in involved versus uninvolved psoriatic skin. Therefore, this study further strengthens the concept that NK cells play an important role in the pathogenesis of psoriasis and should, thus, be specifically targeted by future anti-psoriatic therapy.
Environmental factors predominately contribute to phenotype variations in XP-C patients
Xeroderma pigmentosum (XP) patients have defects in the nucleotide excision DNA repair pathway. XP complementation group C (XP-C) constitutes one-third of all cases and is therefore the most frequent form of XP. This recessive disorder is characterized by increased sun sensitivity, freckling, pigmentary changes, skin atrophy and UV-induced skin cancer.
To date, there are only four major reports on the genetic background of XP-C. Schäfer et al (2013) now complement these studies by identifying 16 additional German XP-C patients from different ethnic backgrounds. All patients carried homozygous mutations, indicating parental consanguinity. Five mutations are novel, and all of them, except for a single amino acid deletion, lead to premature stop codons and nonsense-mediated mRNA decay. This genetic uniformity may be reflected in the homogeneous phenotypes of the patients.
The authors demonstrated diminished post-UV cell survival and nucleotide excision DNA repair capability of fibroblasts for all patients. Interestingly, they note that one-third of their patients reported sun sensitivity. This particular symptom could not be correlated to a particular mutation or functional outcome. The authors observe that skin cancer occurs mostly in the patients who do not experience sun sensitivity and consequently are less likely to avoid UV exposure. Thus, photosensitivity is a protective factor. Why it affects some patients and not others, even though they have the same mutation, is one of the many mysteries of XP that remains to be solved.
Selected by P.M. Steijlen, Maastricht, the Netherlands
The hair follicle continuously undergoes cycles of regeneration coupled with a high proliferation and protein synthesis activity (anagen), followed by an apoptosis-driven organ involution (catagen) and a relative resting phase (telogen). Hair follicle cycling is governed by signaling interactions between specialized, inductive fibroblasts (dermal papilla cells) and hair matrix keratinocytes. Numerous soluble factors, transcription factors, and adhesion molecules play indispensable roles in these signaling interactions.
Kellenberger AJ and Tauchi M (2013) now show that HF cycling is regulated by yet another biologically important molecule, which increasingly attracts interest in several areas of investigative dermatology and has become an important frontier in skin research: mammalian target of rapamycin complex 1 (mTORC1).
The investigators reveal a phase-specific mTORC1 kinase activity: it is high during anagen and low during telogen. Immunohistochemical investigation shows co-expression of an important stem cell marker (keratin 15) in the majority of phophorylated mTOR-positive cells. Moreover, a specific mTORC1 inhibitor, rapamycin, delays spontaneous anagen induction, suggesting that mTORC1 may be involved in the onset of anagen.
These intriguing findings in mice may pave the way for a new treatment of human hair growth disorders, i.e. hirsutism and alopecia, by selectively up- or down-regulating mTORC1 kinase activity.
PPRE-luciferase mice: A powerful new tool for translational PPAR biology research
Peroxisome proliferator-activated receptors (PPARs) are fatty acid-activated transcription factors that belong to the nuclear hormone receptor family. PPARs were primarily shown to play important roles in lipid and glucose metabolism. There are three PPAR isotypes, PPARα, PPARβ/δ and PPARγ, with distinct tissue expression. All three are expressed in skin, where they regulate various aspects of skin homeostasis. PPARs control keratinocyte proliferation and differentiation, regulate wound healing, and modulate skin inflammation. PPAR activation exerts anti-inflammatory effects in various skin conditions such as irritant and allergic contact dermatitis, atopic dermatitis, and UV-induced erythema.
New experimental tools will accelerate the discovery of novel drugs targeting PPARs. El-Jamal et al. (2013) recently generated new PPAR responsive element-luciferase (PPRE-Luc) mice. These were topically treated with PPARα and PPARγ agonists to determine optimal ligand doses, bioluminescence kinetics, and isoform specific effects. Using these new PPRE-luciferase mice will be useful for screening and characterizing novel PPAR ligands.
Novel compounds optimized in this system will improve our ability to treat many inflammatory skin disorders. It is hoped that these mice will become widely available in the near future to facilitate further in vivo-research into the complex roles of PPARs in skin and to ultimately allow the development of new therapeutic strategies that target these fatty acid-activated transcription factors.