Regulation of the signaling pathways involved in autophagy is certainly a potential therapeutic target for AMD treatment. However, targeting autophagy may be more complex than that, since a) autophagy is a fundamental housekeeping process in all cells, and too little or too much autophagy can result in cellular dysfunction, b) autophagy pathways will differ dependending on the stimulus signal(s), and c) AMD has both degenerative characteristics including protein deposits, and in certain cases proliferative characteristics as occurs in the wet AMD form. To date, there is no consensus as to whether autophagy inhibitors or activators would be beneficial in AMD therapy, and how they should be used in different phenotypes of AMD in prevention or in therapy. The induction of autophagy can be attained through inhibition of the mitogen-activated protein kinase/extracellular signal-regulated kinase, class I phosphoinositide 3-kinase-AKT and MTOR signaling pathways. Autophagy inhibitors can be broadly classified into early or late stage inhibitors depending on the autophagy pathway they act upon ().
Figure 7. Pharmacological targets of autophagy. (A) Early stage inhibitors acting upon PIK3CA and MTOR, the most important checkpoints of autophagy induction. (B) Late stage inhibitors of autophagy such as chloroquine and its modified derivatives, (more ...)
Early stage inhibitors of autophagy, such as wortmannin, 3-methyladenine (3-MA) and LY294002, can act upon an upstream regulator of autophagy, the class III PtdIns3K. In retinal ganglion cells following optic nerve transection, the inhibition of autophagy by wortmannin and 3-MA, as well as the late stage inhibitor bafilomycin A1
, can decrease cell viability, suggesting a cell-protective role of autophagy in neurodegenerative diseases.130
Conversely, blocking autophagy with 3-MA in developing retinal neuroepithelium or after optic nerve axotomy results in abnormal retinal tissue formation and function or attenuation of axonal swelling and degeneration, respectively.131,132
Late stage inhibitors of autophagy including antimalarial drugs and broad-spectrum antibiotics such as fluoroquinolones, impose their action on the lysosomal part of the autophago-lysosomal pathway and prevent fusion of autophagosomes with lysosomes. Similarly to early stage inhibitors, the late ones have also been tested in cancer cell lines and cancer animal models. The antimalarial drug chloroquine (CQ) has been used in cancer therapy due to its inhibitory effect on autophagy, besides its additional, autophagy-independent toxic effect on cancer cells.133-136
CQ and other quinines have been associated with cases of retinal toxicity, particularly when provided at higher doses for longer times.137,138
In vitro, CQ can induce lipid accumulation and block phagocytosis in ARPE-19 cells in a dose- and time-dependent manner.44
Neuronal transmitter blockers such as HTR1A (5-hydroxytryptamine (serotonin) receptor 1A, G protein-coupled) agonists offer a therapeutic option for retinal degenerations such as AMD, diabetic retinopathy or retinitis pigmentosa. In both, an in vitro system and an in vivo mice model of atrophic AMD, the HTR1A receptor agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH DPAT) can protect the retina from degeneration by reducing oxidative damage. This study showed that autophagy- and photo-oxidative stress-derived lipofuscin accumulation can be reduced by 8-OH DPAT in cultured ARPE-19 cells, possibly through stimulation of MTOR phosphorylation, which can lead to decreased autophagy induction ().139
Recent findings have revealed that rapamycin-induced inhibition of MTOR complex 1 (MTORC1), and, therefore, activation of autophagy, can slow the aging and neurodegenerative processes in mice.130,140,141
Interestingly, RPE degeneration is associated with increased sensitivity and enhanced activity of MTORC1 in experimental AMD studies.142,143
Rapamycin prevents the development of harmful AMD-related aging signs in RPE cells. MTOR regulates the detrimental dedifferentiation and hypertrophy of RPE cells exposed to oxidative stress, whereas rapamycin treatment can prevent these effects and preserve photoreceptor functions.142
In addition, rapamycin inhibits choroidal neovascularisation (CNV) by interfering with the function of VEGFA (vascular endothelial growth factor A).143,144
Co-culture assays of RPE and endothelial cells revealed that rapamycin is an effective VEGF inhibitor and it can reduce sprouting of endothelial cells.145
However, even though rapamycin prevents retinal degeneration in animal models, it has a number of off-target effects, which have limited its usefulness in age-related neurological disorders such as Parkinson and Huntington diseases.
AMP-activated protein kinase (AMPK) is classically activated by energy depletion and hypoxia. Moreover, a variety of chemicals including the adenosine analog AICAR (5-aminoimidazole-4-carboxamide ribonucleoside) have been used to investigate the role of AMPK in the regulation of the MTORC1 pathway (). AICAR protects RPE cells from oxidative stress.146
Moreover, AMPK-induced autophagy protects the RPE cells from TNFSF10/TRAIL (tumor necrosis factor (ligand) superfamily, member 10)-induced cell death.147
Our unpublished data reveal that AICAR accelerates cleansing of protesome inhibitor-induced protein aggregation via autophagy and improves cell survival in ARPE-19 cells. Taken together, autophagy is a plausible therapy target in AMD, but may be complex due to a variation in the AMD phenotypes. Dry AMD involves degenerative changes without cellular proliferation, whereas CNV development is based on the choroidal endothelial cell proliferation in wet AMD. The inhibition of autophagy potentiates anti-angiogenic effects and might be used together with anti-VEGF therapy in wet AMD.148,149
However, the autophagy inducer rapamycin functions as a VEGF inhibitor and reduces CNV activity, as discussed above.143,144
Due to these opposite effects, further results and experimental models are required to determine whether autophagy activation or inhibition is a goal in AMD therapy.150
Recent clinical experiences provide the possibility to apply drugs through intravitreal injections. This could elegantly circumvent a number of side effects of putative autophagy-related therapies when applied systemically. Autophagy-targeted gene therapy for treatment of AMD is also an interesting future option.151,152
However, accumulating lysosomal lipofuscin may be a limiting factor in the regulation of autophagy flux. Overall, considering AMD as a degenerative age-related disease it may be tempting to state that functional autophagy may prevent RPE cell degeneration and AMD development, although documentation varies in the different models studied.