Biological evaluation of potential retinoid X receptor-selective agonists: the search for a more effective treatment for Alzheimer’s disease

Abstract

Alzheimer’s disease (AD) is the most common form of dementia affecting millions of people; however, treatment options are currently limited. Previous studies have shown potential to slow the progression of AD by 4-[1-(5,6,7,8-Tetrahydro-3,5,5,8,8-pentamethyl-2-naphthalenyl)- ethenyl]benzoic acid (bexarotene), an antineoplastic agent modeled after a vitamin A derivative, in animal models. Bexarotene binds to the retinoid X receptors (RXRs) and stimulates RXR homodimerization and activation of RXR target genes. It also impacts RXR heterodimerization with other nuclear receptors, including the liver-X-receptor (LXR) and retinoic acid receptor (RAR). Bexarotene’s reversal of beta amyloid (Aβ) plaques in mouse models likely occurs via RXR-LXR activation and induction of apolipoprotein E (ApoE) expression. However, bexarotene has many adverse effects, including hyperlipidemia, skin toxicity, hypothyroidism, etc. The current study developed and tested novel bexarotene analogues for their ability to upregulate ApoE expression, crucial in AD neuroprotection, without producing the significant adverse effects of hyperlipidemia and skin toxicity through upregulation of sterol regulatory element-binding protein (SREBP) and activation of RAR, respectively. In order to test bexarotene analogues, luciferase assays were performed in both human colon cancer and human embryonic kidney cell lines. Two screening assays were completed to assess their ability (1) to induce RXR homodimerization through mammalian two-hybrid assays (M2H) and (2) to induce RXR responsive element DNA sequence (RXRE) based transcription. We then assessed the ability of the analogues (1) to induce ApoE expression through LXRE-based luciferase assays, while evaluating their potential for adverse effects through (2) SREBP- and (3) RARE-based reporter systems. Results from the M2H and RXRE assays revealed that our novel analogues produce a wide range of transcriptional activity. LXRE, SREBP, and RARE assays revealed similar results. Specifically, analogue A44 had significantly higher activity with LXRE (p < 0.05) and significantly lower activation via SREBP (p < 0.05) and RARE (p < 0.05) as compared to bexarotene. These assays revealed that our novel bexarotene analogues can potentially be more effective and potent RXR ligands than bexarotene with the capability to circumvent RAR cross-over and elevated SREBP expression, and thus the adverse effects of bexarotene. These analogues may have the potential to slow the progression of AD through increased ApoE expression without the current limitations associated with bexarotene use.