Lipoic Acid Stimulates cAMP Production in Healthy Control and Secondary Progressive MS Subjects

Abstract

Lipoic acid (LA) exhibits antioxidant and anti-inflammatory properties; supplementation reduces disease severity and T lymphocyte migration into the central nervous system in a murine model of multiple sclerosis (MS), and administration in secondary progressive MS (SPMS) subjects reduces brain atrophy compared to placebo. The mechanism of action (MOA) of LA's efficacy in suppression of MS pathology is incompletely understood. LA stimulates production of the immunomodulator cyclic AMP (cAMP) in vitro. To determine whether cAMP could be involved in the MOA of LA in vivo, we performed a clinical trial to examine whether LA stimulates cAMP production in healthy control and MS subjects, and whether there are differences in the bioavailability of LA between groups. We administered 1200 mg of oral LA to healthy control, relapsing remitting MS (RRMS) and SPMS subjects, and measured plasma LA and cAMP levels in peripheral blood mononuclear cells (PBMCs). There were no significant differences between the groups in pharmacokinetic (PK) parameters. Healthy and SPMS subjects had increased cAMP at 2 and 4 h post-LA treatment compared to baseline, while RRMS subjects showed decreases in cAMP. Additionally, plasma concentrations of prostaglandin E2 (PGE2, a known cAMP stimulator) were significantly lower in female RRMS subjects compared to female HC and SPMS subjects 4 h after LA ingestion. These data indicate that cAMP could be part of the MOA of LA in SPMS, and that there is a divergent response to LA in RRMS subjects that may have implications in the efficacy of immunomodulatory drugs. This clinical trial, "Defining the Anti-inflammatory Role of Lipoic Acid in Multiple Sclerosis," NCT00997438, is registered at https://ichgcp.net/clinical-trials-registry/NCT00997438 .

Keywords: Cyclic AMP; Lipoic acid; Multiple sclerosis; Pharmacokinetics; Prostaglandin E2; Thioctic acid.

Conflict of interest statement

Conflict of Interest: The authors declare that they have no conflict of interest.

Figures

Fig. 1. Study flow diagram

N=number of subjects in each group

Fig. 2. Plasma lipoic acid concentration-time profiles

Data points are mean plasma LA concentrations for healthy control (HC, N=19), relapsing remitting (RRMS, N=21), and secondary progressive (SPMS, N=15) subjects. Standard error is indicated by bars

Fig. 3. Cyclic adenosine monophosphate (cAMP) response of peripheral blood mononuclear cells (PBMCs) at three time points relative to lipoic acid treatment

Bold symbols are the estimated mean cAMP level at each time point and the lines represent the estimated change in cAMP between time points for each subject group: healthy control (HC, N=19), relapsing remitting (RRMS, N=19), and secondary progressive (SPMS, N=16). Mean cAMP levels and estimated changes in cAMP were estimated from general estimating equations. The range of the y-axis has been truncated in order to display changes in cAMP over time. Full graph is available as Supplemental Figure 1

Fig. 4. Plasma prostaglandin E2 (PGE2) concentrations

PGE2 enzyme-linked immunosorbent assays (ELISAs, Enzo Life Sciences) were performed on plasma from the same healthy control (HC, N=19), relapsing remitting (RRMS, N=21), and secondary progressive (SPMS, N=15) subjects as in Fig. 2 according to manufacturer's instructions. Results were analyzed in aggregate (top panel), as well as by sex (middle panel females, bottom panel males). Center lines show the medians; box limits indicate the 25th and 75th percentiles as determined by R software; whiskers extend 1.5 times the interquartile range from the 25th and 75th percentiles, outliers are represented by dots; crosses represent sample means. These graphs were generated by BoxPlotR: a web-tool for generation of box plots. * indicates significant differences (p<0.05) between 4 h samples from HC and RRMS donors and between SPMS and RRMS donors. # indicates an outlier value not pictured on the chart due to the chosen graph scale: 4738.9 pg/ml PGE2 for a baseline value of one SPMS subject

Fig. 5. Model of in vitro LA, PGE2, and cAMP interactions

In this figure, arrow heads indicate stimulation, and circles indicate inhibition. A. LA and PGE2 can each bind to and activate EP2/EP4 receptors, which in turn stimulates production of cAMP. Dual stimulation has a synergistic effect on cAMP production (dashed arrows). Conversely, in certain cell types, LA has been shown to suppress PGE2 production, which has a negative effect on cAMP production via EP2/EP4 receptors. B. PGE2 binds to and activates EP3 receptors, which inhibit cAMP production. In this case, LA suppression of PGE2 would result in higher cAMP levels, since less activation of EP3 receptors by PGE2 would lead to a reduction in inhibition of cAMP production by EP3. The nature of the LA/EP3 receptor relationship is unknown