The Potential Effect of Insulin on AChE and Its Interactions with Rivastigmine In Vitro

Abstract

There is no definite cure for Alzheimer's disease (AD) due to its multifactorial origin. Drugs that inhibit acetylcholinesterase (AChE), such as rivastigmine, are promising symptomatic treatments for AD. Emerging evidence suggests that insulin therapy can hinder several aspects of AD pathology. Insulin has been shown to modify the activity of AChE, but it is still unknown how insulin and AChE interact. Combination therapy, which targets several features of the disease based on existing medications, can provide a worthy therapy option for AD management. However, to date, no studies have examined the potential interaction of insulin with AChE and/or rivastigmine in vitro. In the present study, we employed the Response Surface Methodology (RSM) as an in vitro assessment to investigate the effect of insulin on both AChE activity and rivastigmine inhibitory action using a common spectrophotometric assay for cholinesterase activity, Ellman's method. Our results showed that insulin, even at high concentrations, has an insignificant effect on both the activity of AChE and rivastigmine's inhibitory action. The variance of our data is near zero, which means that the dispersion is negligible. However, to improve our understanding of the possible interaction of insulin and rivastigmine, or its target AChE, more in silico modelling and in vivo studies are needed.

Keywords: Alzheimer disease; acetylcholinesterase inhibition; combinational therapy; insulin; rivastigmine.

Figure 1

A summary of the roles of rivastigmine and insulin in AD treatment based on cholinergic hypothesis and distorted cerebral insulin metabolism. The onset and progression of AD symptoms are in close relation to some cellular and molecular hallmarks: the accumulation of extracellular insoluble Aβ, intracellular hyper-phosphorylated NFTs, inflammation, insulin resistance, and metabolic malfunction in brain cells, and the progressive degeneration of the cholinergic neurons. AChE can also accelerate Aβ deposition and the accumulation of senile plaques. The loss of function in neurons and synapses is responsible for the incidence of AD symptoms. Combination therapy with rivastigmine and insulin can target various features of AD. Rivastigmine blocks ACh hydrolysis, elevates synaptic ACh levels, suppresses Aβ deposition, and promotes cholinergic function. Insulin enhances brain bioenergy, synaptogenesis, and synaptic remodelling, improves dendritic spine formation, boosts turnover of neurotransmitters, influences the clearance of Aβ and the phosphorylation of tau.

Figure 2

RSM plots of the integrated effects of rivastigmine and ATCh (a) at concentrations of 0 and (b) 50 µM of insulin (in both diagrams A and B, the red and black curves represent ATCh concentrations of 1000 and 0 µM, respectively). (c) A 3D response surface plot illustrating the integrated effect of rivastigmine and ATCh at an insulin concentration of 25 µM, all based on the standard Ellman’s method (p-value > 0.0001).

Figure 3

RSM plots for the integrated effect of insulin and ATCh at concentrations of (a) 0 and (b) 4000 µM of rivastigmine (in both diagrams A and B, the red and black curves represent ATCh concentrations of 1000 and 0 µM, respectively). (c) A 3D response surface plot illustrates the integrated effect of insulin and ATCh at a rivastigmine concentration of 0 µM, whereby insulin has an insignificant effect on AChE activity. All the tests were conducted based on the standard Ellman’s method (p-value = 0.039).

Figure 4

RSM plots of the integrated effect of insulin and rivastigmine at concentrations of (a) 500 and (b) 1000 µM of ATCh (in both diagrams, the red and black curves represent rivastigmine concentrations of 8000 and 0 µM, respectively). (c) A 3D response surface plot illustrates the integrated effect of insulin and rivastigmine at 1000 µM of ATCh concentration, whereby the increase in insulin concentration has an insignificant effect on the enzyme activity. All the tests were conducted based on the standard Ellman’s method (p-value = 0.8346).