Supplementation with oral lipoic acid (LA) significantly reduces brain atrophy in progressive multiple sclerosis (MS) subjects. However, its mechanisms of action are unclear. We hypothesize that LA provides protection by acting on peripheral immune cells; alterations in the peripheral immune system in MS are well established. Peripheral monocytes (Mo) migrate into the CNS and differentiate into macrophages, which are the predominant cell type in acute inflammatory lesions in MS. LA treatment results in generally less inflammatory phenotypes of human Mo and monocyte-derived macrophages (MDM), including alterations in inflammatory cytokines, inhibition of phagocytosis, and stimulation of the immunomodulator cyclic AMP (cAMP). LA is also an antioxidant that activates Nuclear factor erythroid-2-related factor 2 (Nrf2) in other cell types.
Identify the mechanisms by which LA attenuates inflammatory responses of Mo and MDM. For cAMP-dependent immunomodulatory functions, determine the involvement of direct effectors (protein kinase A, PKA and exchange-protein activated by cAMP, EPAC). For Nrf2-dependent antioxidant functions, determine whether target gene NQO1 [NAD(P)H dehydrogenase quinone 1] expression is induced. Finally, examine expression of heme-oxygenase-1 (HO-1), a downstream effector of both cAMP and Nrf2 signaling, and determine the relative contribution of each signaling pathway to its activation.
Treat human Mo and MDM with LA in the presence of inhibitors of cAMP and Nrf2 signaling and perform qPCR, ELISAs, and phagocytosis assays to examine expression of NQO1 and HO-1, inflammatory cytokine secretion, and phagocytic activity.
LA treatment increases mRNA expression of NQO1; this increase is blocked by Nrf2 inhibition. LA stimulates mRNA and protein expression of HO-1; this increase is blocked by inhibition of either Nrf2 or PKA, but not EPAC. Additionally, PKA appears to (partially) mediate the effects of LA on cytokine secretion in Mo, while Nrf2 and EPAC mediate these effects in MDM. Finally, inhibition of Nrf2 in Mo and inhibition of either Nrf2 or EPAC in MDM (partially) blocks LA inhibition of phagocytosis.
Both cAMP and Nrf2 signaling mediate LA’s modulation of inflammation, with differential contributions in Mo vs. MDMs. This information will facilitate the use of LA in MS therapy to achieve optimal efficacy, either on its own or as a targeted co-therapy with current disease-modifying drugs.