Hypertensive Actions of Long Chain Fatty Acids are Paralleled by Toll‐Like Receptor 4 Upregulation and Nuclear Factor‐kB (NFkB) Activation in the Subfornical Organ

Abstract

Obesity‐induced hypertension is characterized by a low‐grade inflammatory state along with elevations in circulating long‐chain fatty acids (LCFA). LCFA have been associated with the development of hypertension, although the precise mechanisms remain incompletely defined. Importantly, toll‐like receptor 4 (TLR4) is a transmembrane receptor that is activated by LCFA, resulting in downstream inflammatory cellular processes. The central nervous system (CNS) is critical for the control of blood pressure, with the circumventricular subfornical (SFO) organ playing a critical role. However, the contribution of CNS TLR4 in blood pressure regulation, particularly within the SFO, remains unclear. As a first step, we hypothesized that an SFO LCFA‐TLR4‐inflammatory pathway is associated with elevations in blood pressure. Adult male C57B1/6 mice were instrumented with radiotelemeters for the measurement of blood pressure and implanted with an intracerebroventricular (ICV) cannula. Following recovery, a LCFA (oleic acid, 30 nmol), or vehicle control, was administered once a day over 2 days. Whereas mean arterial blood pressure was unchanged over the course of the study in control animals, 2‐day infusion of LCFA resulted in a pro‐hypertensive response (day 2: 95±2 vs. 110±2 mmHg, ICV vehicle vs. ICV LCFA, p<0.05, n=4). Interestingly, real‐time quantitative PCR analysis revealed SFO TLR4 mRNA upregulation following central LCFA administration when compared to vehicle (1.1±0.3 vs. 2.3±0.4 fold ICV vehicle, ICV vehicle vs. ICV LCFA, p<0.05, n=4). In contrast, TLR4 transcript in the paraventricular nucleus of hypothalamus, another cardioregulatory region, was not different between groups (1.1±0.2 vs. 1.4±0.3 fold ICV vehicle, ICV vehicle vs. ICV LCFA, p>0.05, n=4). Based on this, we further evaluated TLR4 protein expression in the SFO using immunohistochemical analyses. TLR4‐expressing cells were found predominately within the medial to caudal regions of the SFO. Interestingly, co‐immunolabeling indicated that SFO TLR4‐expressing cells were exclusively microglia (Iba1 colocalization: 75±4% TLR4 cells, n=3), whereas no co‐expression was found on astrocytes (GFAP) or neurons (NeuN). Given that TLR4 activation results in the downstream activation of the inflammatory transcription factor NFkB, we subsequently assessed LCFA‐TLR4 mediated activation of NFkB in the SFO using an in vivo bioluminescence imaging technique. In preliminary experiments (n=2), male C57B1/6 mice underwent SFO‐targeted injection of an adenovirus encoding firefly luciferase downstream of the NFκB consensus sequence; this approach allows for imaging of SFO NFκB activity in the same animal over time. When compared to basal levels, two day ICV LCFA administration resulted in an elevation in SFO NF‐κB activity that was evident within 24 hrs (1.3±0.2 fold baseline) and maintained at 48 hrs (1.3±0.4 fold baseline). Taken together, these findings indicate that LCFAs act within the CNS to elicit increases in arterial blood pressure. Moreover, the hypertensive actions of LCFA are paralleled by TLR4 upregulation and NFκB activation in the SFO, suggesting that a LCFA‐TLR4‐NFκB network in this nucleus may contribute to the development of hypertension, such as during obesity.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.