Possible role of oxidative stress in the pathogenesis of canine zinc-responsive dermatosis

Introduction: Zinc deficiency causes skin diseases in humans and animals but the underlyingpathogenic mechanisms still remain unclear. There is a growing body of evidence for therole of zinc in protecting skin against free radical-induced oxidative damage. Zinc ions mayinduce the synthesis of metallothioneins (MTs), sulfhydryl-rich proteins that store zinc and actas free radical scavengers. Cu/Zn superoxide dismutase (SOD) is also important for skin healthas a result of its antioxidant activity. Skin represents a major target for various environmentalstress agents able to induce synthesis of Heat Shock Proteins (HSPs) or Stress Proteins, whichare one of the most evolutionarily conserved classes of molecules playing a crucial role inthe maintenance of cellular homeostasis. A large number of studies have also demonstratedthe anti-apoptotic activity of several HSPs, that usually show elevated levels in proliferatingmammalian cells and cell cycle-dependent expression.Material and methods: The study was carried out on biopsy samples of canine zinc-responsivedermatosis from 8 Siberian Huskies and necropsy samples of normal skin from different sitesfrom 5 Siberian Huskies (excluding sites of frequent mechanical trauma but including thosewhere zinc-responsive dermatosis commonly occurs). Samples were fixed in 10% neutralbuffered formalin and embedded in paraffin wax. The immunohistochemical expression ofHsp27, Hsp72, Hsp73, Hsp90, Cu/Zn SOD, MT, Ki-67 and active caspase-3 were evaluatedusing a streptavidin-biotin-peroxidase technique. Ki-67 labelling index (KI) was calculated asthe percentage of positive nuclei divided by the total number of keratinocytes examined. Atleast 1000 keratinocytes per specimen were examined in ten randomly selected fields usinglight microscopy (x400).Results: All investigated HSPs showed intense cytoplasmic immunostaining in the affectedepidermis. Focal nuclear positivity of Hsp72 was also detected in isolated keratinocytes. AlthoughCu/Zn SOD expression was similar to that observed in normal skin, MT immunoreactivity wasrevealed in both the cytoplasm and nucleus of basal cells in normal skin but lacked in theaffected epidermis. Caspase 3 activation was also absent in the involved epidermis, whichrevealed a high Ki-67 index (a 3.5- to 9-fold increase compared to normal skin).Discussion: The results of this study support the hypothesis that cellular stress response, inparticular response to oxidative stress, is involved in the pathogenesis of skin lesions occurringin canine zinc-responsive dermatosis. The lack of MT immunoreactivity in the affectedepidermis may be indicative of low zinc levels, thus resulting in vulnerability to oxidativedamage. On the other hand, high levels of HSP expression in skin during zinc deficiency mayconfer protection against a variety of dangerous stimuli, contributing to inhibition of apoptosis,as well as to cell cycle regulation of proliferating keratinocytes. Detection of nuclear Hsp72expression in affected keratinocytes is consistent with its involvement in protection of thenucleus and repair of DNA damage, particularly induced by UV light exposure.[...]