The skin is the largest organ of the human body and serves as a barrier against the environment. One of the most common environmental insults to the skin is ultraviolet radiation (UVR), a ubiquitous carcinogen. Cutaneous UVB exposure initiates the pigmentation cascade, with the upregulation of pro- opiomelanocortin (POMC) in epidermal keratinocytes. POMC is post-translationally cleaved to yield melanocortin peptides essential for stimulating production of protective melanin. Interestingly, cleavage of POMC also yields the endogenous opioid, ?-endorphin (?-end), leading to a systemic increase in plasma levels of ?-end that is sufficient to cause `tanning addiction' in mice. ?-end is an endogenous opioid that binds to the -opioid receptor (-OPR) expressed in the central and peripheral nervous systems (CNS, PNS). Though the analgesic effects of opioids are well established, a role in suppressing inflammation has been demonstrated in animal models. Our observation that loss of ?-end or -OPR in mice results in a severe cutaneous inflammatory response to UVB suggests that opioids suppress inflammation in the skin. We hypothesize that UVR-induced ?-end may have acute beneficial effects. This proposal will elucidate the contribution of both the local, cutaneous effects, and the global, systemic, effects of opioid signaling on cutaneous inflammation.
Aim 1 will establish the experimental `sunburn' conditions to study the role of opioids in cutaneous inflammation using mice with and without functioning opioid signaling. In addition to studying the role of ?-end after UVR, we will determine if ?-end suppresses cutaneous inflammation in models of cutaneous inflammatory conditions, such as atopic dermatitis and psoriasis. Lastly, using histopathological analysis and flow cytometry techniques, we will characterize the local inflammatory response in ?-end KO, -OPR KO, and C57BL6 mice to identify contributing factors to the cutaneous phenotype.
In Aim 2 we will explore the mechanism by which ?-end exerts its anti-inflammatory effects. We hypothesize that keratinocyte-derived ?-endorphin acts on immune cells and peripheral nerve fibers to suppress inflammation. Using the murine model, we will immunophenotype opioid-deficient mice and control mice to determine differential numbers of immune cells in baseline and UV-exposed conditions. Next, using bone marrow transplantation, we will determine if expression of -OPR on immune cells is sufficient to affect the cutaneous inflammatory response to UVR. Lastly, to distinguish between potential contributions of the CNS and PNS to the anti-inflammatory effects of opioids, we will exploit pharmacological manipulation of the opioid signaling pathway. Combined, these studies may reveal a clinically relevant application and mechanism of topically-applied, peripherally-acting opioids as anti-inflammatory treatments for several cutaneous inflammatory conditions.

Public Health Relevance

Every year, over 35 million Americans will experience inflammatory skin conditions, including acute conditions such as sunburn and chronic conditions such as psoriasis. Unfortunately, current treatment options are limited due to negative side effects on the skin, such as thinning of the skin or increased susceptibility to infection. We will explore the anti-inflammatory effects of opioids in the skin and the potential use of topically applied opioids for inflammatory skin conditions.

National Institute of Health (NIH)
National Institute of Environmental Health Sciences (NIEHS)
Predoctoral Individual National Research Service Award (F31)
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Special Emphasis Panel (ZRG1)
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Humble, Michael C
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Harvard Medical School
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