Amyloid-beta Interactions with ABC Transporters and Resistance Modifiers

Results and Discussion

Membrane transporters in various barriers e.g. BBB, blood placenta barrier, blood retinal barrier, etc. have important roles in maintaining the normal function of cells in the organism. The effectiveness of cancer chemotherapy as in the case for AD therapy, lies in its ability to reach its target(s). This ability is challenged by membrane transporters called pgp or ABC transporters. These transporters extrude anticancer agents and when over-expressed by suboptimal therapy, render the cancer cell resistant not only to the specific agent, initially employed, but to other anticancer agents as well. Amyloid-beta peptides are implicated in the development of AD inasmuch as it is the main component of non-soluble amyloid plaques that accumulate intracerebrally (10). Amyloid-beta peptide is chemically “sticky” and gradually builds up into plaques and interferes with nerve transmission signals. As mentioned before, pgp, also known as ABCB1 protein, plays an important and substantial role in the elimination of amyloid from the brain via its activity at the BBB. Relevant to this fact, is the proven ability of this ABC transporter to extrude amyloid from MDR1 cancer cells (9).

The reversal of the MDR phenotype of cancer cells takes place by inhibition of the ABC transporter that is over-expressed in our model experiments. The effects of MDR modifiers such as disiloxanes sila-409 and sila-421 alone and memantine complexed with amyloid-beta peptides reduced rhodamine accumulation in the tumor cells. These findings may indicate some new directions for possible drug design to modify amyloid efflux (Table I). Amyloid-beta peptides 1-42 have toxic effects on cells. The complexes formed between amyloid-beta and resistance modifier disiloxanes were not able to increase Rhodamine123 accumulation in the treated cells despite the fact that the disiloxanes and the positive verapamil control alone significantly inhibited Rhodamine123 efflux from the treated tumor cells resulting in increased intracellular accumulation of rhodamine.

These results demonstrate that amyloid-beta peptides and memantine alone have opposite effects on the ABC transporter of human MDR1 transfected into mouse cells. In the case of the amyloid-beta peptides 1-42 complexed with the two disiloxanes, the sila-409-amyloid complex reduced rhodamine accumulation, possibly due to direct complex formation with amyloid. In the case of sila-421-amyloid complex, the amyloid was less effective.

The effect of the resistance modifiers alone and their amyloid complexes on apoptosis induction was examined using MDR mouse lymphoma cells (Table II). Small differences were found in the frequency of early apoptosis induction which indicates some non-specific membrane effects. The rate of viable cells was found above 90-95% percent in the presence of the tested compounds and their complexes with amyloid-beta 1-42. The MDR modifiers verapamil, sila-421, sila-409, and amyloid alone resulted in the reversal of resistance to doxorubicin. Whereas amyloid-beta 1-42 prevented the MDR reversal effect of the resistance modifiers including memantine, sila-409 and sila-421. The compounds studied showed an early apoptosis induction. The memantine and sila-421-complexes of amyloid increased apoptosis. Late apoptosis and viability of cells were not changed significantly by memantine and disiloxane-complexes.

The authors suggest that resistance modifiers form complexes with amyloid. The ABC transporters may have theoretical interest since they are responsible for the functions of barriers in different tissues e.g. BBB, gut, kidney, placenta, testis, liver, adrenal cortex and breast by regulating efflux of endogenous and exogenous substrates including amyloids, hormones and chemotherapeutics. One strategy for reversal of the resistance in tumor cells expressing ABC transporters is the combination of antitumor chemotherapeutics with resistance modifiers. The clearance of amyloid-beta from the brain represents a therapeutic target for a human brain endothelial cell line hCEMC/D (3). Tai et al. suggested that pgp and BCRP might act to prevent blood borne amyloid-beta from entering the brain (8). Brenn and coworkers had shown that amyloid beta 1-42 itself down regulates the expression of pgp and amyloid beta transporters could enhance the intracerebral accumulation of amyloid-beta. As a consequence, accelerate neurodegeneration in AD and cerebral beta-amyloid angiopathy in vivo (10).

The two main problems related to chemotherapy of cancer and AD are theoretically similar. Chemotherapy inefficiency due to drug resistance can be overcome by inhibition of ABC transporter. In case of cancer, the reversal of drug resistance of tumor cells expressing ABC transporters by the combination of anticancer drugs and chemosensitizers holds a great promise for improved chemotherapy. However, in AD the reduced clearance across BBB and accumulation of beta-amyloid –as a pgp substrate– are responsible for the neurologic dysfunction, that has a key role in the development of beta-amyloid mediated angiopathy. In this case beta-amyloid is considered as endogenous substrate, however in cancer the anthracyclines are exogenous substrates. Consequently, simultaneous administration of resistance modifiers to block the membrane transporter can improve the effectiveness chemotherapy both in cancer and in AD. The reversal of MDR and induction of apoptosis might offer a potential benefit in cancer. We suppose that a synergistic combination between some resistance modifiers e.g. memantine or sila compounds with amyloid-beta may offer further opportunities for chemotherapy of cancer and AD.

Behrens (3) had shown an inverse association between AD and cancer. The question arises: what is the possible common factor in AD and cancer. Nevertheless, a considerable attention has been given to the mechanisms by which amyloid-beta might be transported between the brain and blood and evidence suggests that MDR pgp plays a role in amyloid-beta transport (11), The conclusion of this publication emphasizes that treatments currently under investigation to prevent or to treat AD might lead to a greater risk of cancer development, and inversely, treatments to halt cancer, could predispose to development of AD (6, 7). In cancer, cell regulation mechanisms are disrupted with augmentation of cell survival and proliferation, whereas conversely AD is associated with increased neuronal death, either caused by concomitant beta-amyloid complexes and tau deposition. Association between cancer and AD is made more imperative considering that treatments currently under investigation to prevent, and treat AD, might lead to a greater risk of cancer development (7, 8).

Significantly lower expression of ABC transporters was found following expression of amyloid-beta 1-42, but not its scrambled or complexed equivalent (12). In our experiments the two studied disiloxanes had shown some chemopreventive effects in vivo (13). Similar effect was found also in vivo in human pancreatic cancer xenograft-inoculated mice (14). In addition to the beneficial effects of the sila-409 and sila-421 on chemoprevention, the identified vascular activity of the two compounds can be promising for further in vivo studies (model experiments) in rational drug design (15).