The retinoid, 6-[3-adamantyl-4-hydroxyphenyl]-2-napthalene carboxylic acid, controls proliferative, morphological, and inflammatory responses involved in microglial activation without cytotoxic effects.

Activation of microglia is regulated by controlling both its population size (through modulation of proliferation/death) and the production of inflammatory mediators. Retinoids control cellular proliferation, differentiation, and death. Natural retinoids have been shown to exhibit anti-inflammatory actions against activated microglia. However, the synthetic forms, which are regarded to be more stable in their actions, have not been explored for their capacity to modulate microglial activation, proliferation, and/or trigger cell death. The aim of the current study was to address these issues by using a model, lipopolysaccharide (LPS)-activated primary cultures of rat microglia, and the stable synthetic retinoid, 6-[3-adamantyl-4-hydroxyphenyl]-2-napthalene carboxylic acid (AHPN). Morphological observations of cluster of differentiation (CD) 11b (CD11b)-positive cells suggested that low concentration of AHPN (i.e. 5 μM) reduced LPS (1 μg/ml, 24 h)-activated morphology of microglia possibly toward a lower activated state, while attenuating nitrite production and the level of its synthesizing enzyme, inducible nitric oxide synthase (iNOS), as well as the chemokine, monocyte chemotactic protein-1 (MCP-1). The mechanisms behind these anti-inflammatory actions likely involved decreased activation of nuclear factor kappa B (NF-κB) as shown by the attenuated phosphorylation of its p65 subunit. In addition, fluorescence-activated cell sorting revealed that AHPN reduced the immunophenotypic marker of activation, CD68. LPS-mediated increase in cell number was reduced by low concentration AHPN, which resulted from inhibition of proliferation, based on decreased labeling for Ki-67 and reduced protein expression of cyclin D1, and not cell death. Higher concentrations of AHPN (50-100 μM) attenuated activation and cell number; however, the release of lactate dehydrogenase and appearance of annexin V and propidium iodide-positive cells suggested that cell death was its primary cause for reduced microglial activity. Overall, the current study shows that synthetic retinoids, such as AHPN, at low concentration attenuate microglial activation-associated responses, possibly via the inhibition of their cell proliferation without triggering cell death.