By Cheryl Guttman
San Diego — Understanding the pathogenesis of rosacea has been advanced by recent research and is expected to provide an important foundation for developing novel, rational approaches to therapy in the future, says Richard L. Gallo, M.D., Ph.D., professor and chief, division of dermatology, University of California, San Diego.
Dr. Gallo discusses findings from a series of research studies that show there is a dysfunction in antimicrobial peptide production and processing in rosacea and that it can arise via multiple pathways.
"This information indicates no one gene or stimulus can explain rosacea in all patients, and therefore, it is consistent with our longstanding frustration in trying to identify a solitary etiologic trigger,
" Dr. Gallo says.
Skin’s immune system
"Now, understanding of these antimicrobial peptides as a critical element of rosacea should offer us new targets of therapy," Dr. Gallo tells Dermatology Times.
Dr. Gallo and colleagues approached their investigations of rosacea pathogenesis from a biochemical and genetic standpoint, considering the key elements of the biology of the disease and their understanding of the functioning of the innate immune system of the skin.
Based on this knowledge, they hypothesized that rosacea reflects an abnormality in the reaction of the early response system to the variety of elements that have been identified as rosacea triggers.
More specifically, they hypothesized that elements of the antimicrobial peptide system and enzymes controlling that system may represent a "choke point" in the communication between the multiple different disease stimuli and the various clinical subtypes of rosacea.
Immunohistochemistry
A series of studies were designed to construct proof for this hypothesis. The first investigated levels of cathelicidin antimicrobial peptides in facial skin and showed significantly higher expression in patients with rosacea compared with unaffected controls as measured by immunohistochemistry, Enzyme-Linked ImmunoSorbent Assay (ELISA) and gene expression. Evaluation with mass spectroscopy showed the size of the cathelicidin peptides was also abnormal in the skin of all rosacea patients compared with controls, indicating a difference in proteolytic processing.
"The differences between rosacea patients and our control samples — uninvolved edges of basal cell carcinoma excisions — were dramatic, and the findings were somewhat surprising to us.
"However, it was the difference in peptide size that really led us to believe we had come upon something important," Dr. Gallo says.
Consistent with that belief, the researchers also found that all rosacea patients had abnormally increased activity of the serine protease enzymes responsible for cathelicidin peptide processing.
In previous research, Dr. Gallo and colleagues had already identified the genes for individual cathelicidin-processing enzymes. Based on that information, they evaluated the expression of the gene for stratum corneum tryptic enzyme (SCTE, kallikrein 5) in facial skin of rosacea patients and found it was also elevated and specifically in areas where the processed cathelicidin peptides were found.
Further experiments aimed to establish significance for the laboratory findings by demonstrating a cause and effect relationship. Applying Koch’s postulates, these experiments investigated the hypothesis that if the unique peptides found only in rosacea skin were important in disease pathogenesis, they could induce findings consistent with the clinical presentation of rosacea.
Cathelicidin peptides
A first in vitro experiment showed that production of pro-inflammatory cytokines by cultured keratinocytes was significantly greater when the skin cells were co-incubated with processed cathelicidin peptides found in rosacea skin compared with peptides from normal skin.
"Although supportive, this finding was not overly convincing. The results of a second experiment blew us away," Dr. Gallo says.
The latter research involved a murine model and evaluated the responses to dorsal skin injections of physiologically relevant concentrations of peptides from normal and rosacea skin.
After just two days of twice daily treatment, the animals injected with the rosacea-related peptides developed a phenotype that reproduced rosacea with the presence of inflammation, a vascular response, and ectasia.Adding strength to the cause and effect relationship was the observation of a dose-related response.
The research has now entered a new phase where studies are evaluating potential correlations between the proposed pathogenic pathway and both gene abnormalities and effective treatments.
These are our last questions, but really represent the beginning of our research project, not the end," Dr. Gallo says.Available evidence is still limited, but so far, it is entirely consistent with the existing hypothesis. One line of support is derived from an "experiment of nature," which is the finding that a polymorphism of the vitamin D receptor gene leading to excessive production of processed cathelicidin peptides is associated with rosacea fulminans.
Isotretinoin
In addition, it is also known that isotretinoin, which has been found to have beneficial effects in rosacea, influences genes involved in the cathelicidin and SCTE expression system.
Findings from a study examining serine protease levels in facial skin of rosacea patients after they start, stop, and restart minocycline therapy are preliminary but so far are providing a dramatic demonstration of how tetracyclines can affect this innate immune response pathway.Dr. Gallo says that marked decreases in SCTE levels have been observed after the initiation of minocycline therapy and occur in parallel with improvement in clinical disease activity.
Subsequent minocycline withdrawal and re-initiation corresponded to increases and decreases, respectively, in enzyme activity.
"The effects of minocycline withdrawal and rechallenge on SCTE show the initially observed change with minocycline initiation was not a coincidence and suggest that indeed, tetracycline affects the enzymes that we hypothesize are involved in the pathogenesis of rosacea," Dr. Gallo says. DT
Disclosure: The research has been supported in part by a seed grant from the National Rosacea Society.
