Lymphatic Wall Remodeling with Systemic and Tissue-Associated Inflammation in Obese Zucker Rats

In humans, lymphatic dysfunction and obesity appear to have a reciprocal relationship, such that lymphedema can cause adipose deposition, and obesity can impair lymphatic function. Recent studies in mice suggest that obesity induced by a high fat diet impairs lymphatic drainage in the tail and hind limbs. Lymphatics can also become excessively permeable, leaking out lymph in db/db mice that have both obesity and diabetes. These studies demonstrate lymphatic dysfunction is associated with obesity, however, the pathophysiological mechanisms remain unknown. The goal of this study was to investigate the extent to which obesity gives rise to inflammation and remodeling of the wall of rat mesenteric collecting lymphatic vessels. ZUCKER-Leprafa (obese) and ZUCKER- Leprafa/+ (lean) male and/or female rats, in conformance with the FASEB Statement of Principles for the use of Animals in Research and Education, were used for this study. Age groups include 8, 12 and 26-32 wks. Passive wall mechanics were measured using collecting lymphatics isolated and mounted onto resistance-matched glass micropipettes, in a 37°C albumin physiological salt solution (APSS without Ca2+) bath and luminal pressure was stepped from 1-5 cm H2O. Mesenteric arcades with adipose tissue and collecting lymphatics were dissected, fixed in 4% paraformaldehyde, processed and embedded in paraffin for sectioning and immunostaining with antibodies to rat-specific CD68 (macrophage marker) and caveolin-1 (adipocyte marker) for the quantification of crown-like structures (CLSs). Blood-serum and peri-lymphatic adipose tissue (PLAT) were also collected and analyzed for inflammatory biomarkers using the 67 Rat Biomarker Testing Service – RayBiotech. Average lymphatic wall thickness, wall:lumin ratio (X100) and medial cross sectional area were significantly greater in obese versus lean male rats at all ages. Many CLSs were observed surrounding mesenteric collecting lymphatics isolated from obese male 12 and 26-32 wk rats. Additionally, CLSs significantly increased with age. In an analyses of serum biomarkers obtained from 12 wk old lean and obese, male and female rats, we observed that many of the markers in PLAT (IL-6, IL-13, adiponectin) and in serum (GM-CSF, IL-4, Gas1) were differentially regulated by obesity and sex. The data support the ideas that obesity can permit changes to passive wall mechanics that can lead to a structural remodeling of lymphatic collecting vessels and that obesity is associated with both systemic and tissue-associated inflammation (PLAT) in both male and female Zucker rats.