Increased xanthine oxidase in the skin of preeclamptic women

Abstract

Xanthine oxioreductase is the holoenzyme responsible for terminal purine catabolism. Under conditions of metabolic stress or heightened proinflammatory cytokine production, this enzyme is preferentially in its oxidized form, xanthine oxidase, with catalytic action that generates uric acid and the free radical superoxide. As preeclampsia is characterized by heightened inflammation, oxidative stress, and hyperuricemia, it has been proposed that xanthine oxidase plays a pivotal role in this hypertensive disorder of pregnancy. We sought to determine whether xanthine oxidase protein content was higher in maternal tissue of preeclamptic mothers, compared to healthy pregnant controls, using immunohistochemical analysis of skin biopsies. We further compared xanthine oxidase immunoreactivity in skin biopsies from preeclamptic women and patients with several inflammatory conditions. In preeclamptic women, intense xanthine oxidase immunoreactivity was present within the epidermis. By contrast, only very faint xanthine oxidase staining was observed in skin biopsies from healthy pregnant controls. Further, a role for inflammation in the increase of xanthine oxidase was suggested by similar findings of heightened xanthine oxidase immunoreactivity in the skin biopsies from nonpregnant individuals diagnosed with conditions of systemic inflammation. The finding of increased xanthine oxidase in maternal tissue, most likely as the result of heightened maternal inflammation, suggests maternal xanthine oxidase as a source of free radical and uric acid generation in preeclampsia.

Figure 1

Xanthine oxidase immunoreactivty (green staining) is present in stratum granulosum layer of epidermis in preeclamptic women (panel B) compared to faint XO immunoreactivity in the same epidermal layer of healthy pregnant controls (panel A). Healthy non-pregnant controls demonstrate minimal epidermal XO immunoreactivity (panel C). These patterns of immunohistochemical staining were observed in all 5 preeclamptic skin biopsies (panel E), all 5 healthy pregnant controls (panel F) and all 5 healthy non-pregnant controls (panel G). Omission of the primary antibody directed against XO results in no observable immunoreactivity (panel D). Scalebar = 50μm.

Figure 2

Quantitative analysis of XO immunohistochemical fluorescent intensity within the squamous epithelial layers of skin biopsies collected from healthy non-pregnant (n=5), healthy pregnant (n=5) and preeclamptic (n=5) study subjects. Data is presented as median ± IQR. P<0.05, Kruskal-Wallis one-way analysis of variance; (a) P<0.05, Dunn’s post hoc analysis compared to healthy non-pregnant subjects; (b) P<0.05, Dunn’s post hoc analysis compared to healthy pregnant subjects.

Figure 3

Xanthine oxidase immunoreactivity (green staining) is present in the stratum granulosum layer of the epidermis from patients with conditions of chronic inflammation including systemic lupus erythematosis (panel A), dermatitis (panel B), lichen simplex (panel C), mixed connective tissue disease (panel D) and bullous phemphigod (panel E). Patients with bullous phemphigod in which the disease is in remission demonstrate little epidermal XO immunoreactivity (panel F). Similarly, minimal XO immunoreactivity is observed in epidermis of healthy non-pregnant controls (panel G). Omission of the primary antibody directed against XO results in no observable immunoreactivity (panel H). Representative images are shown, along with quantitative analysis of XO immunofluorescent intensity within the squamous epithelial layers (panel I) Scalebar = 50μm.