Chronic exposure to aluminum (Al) is associated with some pathological changes related to neurotoxicity and Alzheimer's disease (AD), including amyloid beta (Aβ) plaques in senile plaque, hyperphosphorylated tau (p-tau) in neurofibrillary tangles (NFTs), and neuronal loss [1], [2], [3]. Epidemiological studies have shown that a higher daily intake of Al from drinking water is accompanied by the increased prevalence of AD in the elderly [4], and the aggregation of senile plaques in the hippocampus and temporal lobe [5]. Aβ is produced by the sequential cleavage of the amyloid precursor protein (APP) through β-secretase (BACE1) and γ-secretase, and BACE1 is the rate-limiting enzyme [6]. Numerous studies have shown that Al exposure can increase BACE1mRNA, BACE1, and Aβ levels in the rat brain [7] and primary cultured neurons [8]. This pathological and clinical correlation prompted our interest in understanding the mechanism(s) through which aluminum exposure contributes to the increase in BACE1.

Studies have shown the regulatory role of microRNAs (miRNAs) in neurodegenerative disease by the degradation or translation inhibition of the target RNA [9]. Previous preliminary studies have demonstrated that the expression of BACE1 is associated with miR29 regulation. Our studies [10] using aluminum-maltolate (Al(mal)3)-treated neuron cells and HEK293 cells transferred by miR29a/b1 mimics through liposome transfection technology have suggested that Al(mal)3 increases the Al levels in the serum/brain of rats and induces the deposition of Aβ1-42 in rats, which are regulated by miR29a, miR29b1 through BACE1mRNA, and BACE1. However, the “trigger” involved in the Al-induced elevation of BACE1 through decreasing miR29 is still unknown.

Studies have confirmed that the abnormal activation of nuclear factor kappa B (NF-κB), which is a fast-acting nuclear transcription factor, present in eukaryotic cells is closely related to a series of neurologic injuries as well as neurodegenerative diseases. The NF-κB signaling system is evolutionarily conserved and plays the role of common transcriptional activator in various complex immune signaling pathways. In general, in addition to mature B cells and plasma cells, the DNA binding capacity of NF-κB in cells is masked by the formation of a trimer with an inhibitor (IκB). The IκB kinase induces the activation and phosphorylates IκB, leading to its degradation in the cytoplasm after being stimulated by the signal, and NF-κB is then released and translocates to the nucleus where it binds to DNA and activates gene expression. This ability to respond to a signal makes NF-κB an inducible factor that transmits information directly to its nuclear target [11], [12]. NF-κB/Rel comprises a family of five proteins, namely, NF-κB1(p50), NF-κB2(p52), p65(RelA), Rel B, and c-Rel, that may form different transcriptionally active homodimeric and heterodimeric complexes [12]. In some cases, the dimer repertoire may shift with different cell types during cell differentiation and development. However, RelA, of all the possible DNA-binding dimers, is the most complex with a DNA target and potent activator. Additionally, most monocyte lineages rely on RelA:p50 [13]. In the central nervous system (CNS), p65:p50 dimers are predominantly neurotoxic compared with the neuroprotective c-Rel-containing dimers [14].

NF-κB can regulate the expression of numerous cytokines and adhesion molecules, and it also acts as an anti-apoptotic protein in certain situations. Our study also confirmed that NF-κB is involved in the cell division cycle and the maintenance of the cell differentiation phenotype. Notably, NF-κB is involved in the brain programming of systemic aging in the nervous system as well as in the pathogenesis of several neurodegenerative diseases. The nuclear content of RelA abnormally increases in nigral dopamine (DA) neurons and glial cells in patients of Parkinson's disease (PD) [15]. Postmortem anatomy in patients with AD showed an increased proportion of neurons with nuclear p65 in the hippocampus and cortex around Aβ plaques [16]. Furthermore, an increase in the p65:p50 dimer mediates an increase in BACE1 transcription, which enhances Aβ production [17]. Recently, NF-κB was found to regulate the expression of a range of miRNAs, such as miR183, miR155, miR9, miR125b, and miR146a [9], [18], [19]. Additionally, myoblast and rhabdomyosarcoma studies showed that miR29 expression is negatively regulated by NF-κB [20]. Studies on liver fibrosis [21] and leukemia [22] have shown similar results. Thus, a ‘‘feed-forward’’ effect may be a possible mechanism through which NF-κB enhances the transcriptional inhibition of miR29 expression induced by Al.

To determine whether Al-induced NF-κB can negatively regulate miR29 in animal and cell model systems, we established an animal model of Al exposure and added an NF-κB inhibitor into the cell media of adrenal phaeochromocytoma (PC12) cells. We hypothesized that negative Al regulation of miR29a/b1 may be associated with NF- κB, and the addition of Al(mal)3 with the NF-κB inhibitor into the culture during PC12 growth would significantly reduce the expression of miR29 in Aβ-plaque signaling pathways.