α2 but Not α1 AMP-activated Protein Kinase Mediates Oxidative Stress-induced Inhibition of Retinal Pigment Epithelium Cell Phagocytosis of Photoreceptor Outer Segments

Abstract

Oxidative stress causes retinal pigment epithelium (RPE) cell dysfunction and is a major risk factor leading to the development of dry-type age-related macular degeneration. Taking pharmacological and genetic approaches, we address the mechanisms by which sublethal oxidative stress inhibits RPE cell phagocytosis. Sublethal oxidative stress dose-dependently inhibited RPE cell phagocytosis of photoreceptor outer segments (POS) and activated AMP-activated protein kinase (AMPK) as determined by increased Thr 172 and Ser 79 phosphorylation of AMPK␣ and its substrate acetyl-CoA carboxylase, respectively. Similar to oxidative stress, 5-aminoimidazole-4-carboxamide riboside (AICAR), a pharmacological activator of AMPK, inhibited RPE cell phagocytosis of POS in a dose-dependent manner. Inhibition of RPE cell phagocytosis by AICAR was fully reversed by blockade of AICAR translocation into cells by dipyridamole or inhibition of AICAR conversion to ZMP by adenosine kinase inhibitor 5-iodotubercidin. In agreement, AICAR-induced activation of AMPK was abolished by preincubation with dipyridamole or 5-iodotubercidin. Knock-out experiments further revealed that ␣2 but not ␣1 AMPK was involved in RPE cell phagocytosis and that activation of ␣2 AMPK contributed to the inhibition of RPE cell phagocytosis by oxidative stress. Inhibition of RPE cell phagocytosis by activation of ␣2 AMPK was associated with a dramatic increase in acetyl-CoA carboxylase phosphorylation. In comparison, AMPK had no role in oxidative stress-induced breakdown of RPE barrier function. Taken together, reduction in POS load under oxidative stress might direct RPE cells to a self-protected status. Thus, activating AMPK could have therapeutic potential in treating dry macular degeneration.The retinal pigment epithelium (RPE) 2 is a monolayer of pigmented cells forming a part of the blood-retina barrier (1). Basal membrane of the RPE is in contact with Bruch membrane, whereas the apical membrane faces the photoreceptor outer segments. The close structural interactions of RPE cells with the outer retina indicate that the major functions of the RPE layer are to maintain the survival and normal functioning of photoreceptors by controlling nutrients/waste products exchange (2), phagocytosing shed outer segments (3), shuttling retinoids to synthesize visual pigments (4), and producing trophic factors necessary for photoreceptor survival. Failure of any one of these functions can result in degeneration of the retina, loss of visual function, and blindness.Age-related macular degeneration (AMD) is an idiopathic retinal degenerative disease that predominates in the elderly in the Western world as a cause of irreversible, profound vision loss (5, 6). The pathogenic mechanisms whereby environmental factors contribute to the development of AMD remain elusive. However, growing evidence indicates that oxidative stress injury to the RPE plays an important role in the etiology of AMD. The RPE is at high risk for oxidative injury due to its location in a highly oxygenated envi...

Molecular mechanism of suppression of MDR1 by puerarin fromPueraria lobata viaNF-κB pathway and cAMP-responsive element transcriptional activity-dependent up-regulation of AMP-activated protein kinase in breast cancer MCF-7/adr cells

“…Activation of AMPK in experimental model systems is often achieved by administration of the AMPK activator aicar [35].…”

The Role of AMPK Pathway in Neuroprotection

“…In retinal RPE cells, autophagy regulating kinases have been proposed as potential therapeutic targets for age-related macular degeneration through activation of AMPK pathway (Kaarniranta et al 2012). In support of this hypothesis, another agonist of AMPK, AICAR, was found to protect RPE cells in response to oxidative stress (Qin and De Vries 2008). Moreover, AMPK-induced autophagy protected RPE cells from TRAIL-induced cell death (Herrero-Martin et al 2009).…”

“…These unique tissue and sub-cellular distributions suggest that complex types may have different substrates and therefore have unique functions. Consistent with this possibility, AMPK α2 but not α1 mediates oxidative stress-induced inhibition of RPE cell phagocytosis of photoreceptor outer segment (Qin and De Vries 2008). In addition, in vitro data have suggested AMPKα1 and α2 play distinct roles in regulating 4-HNE effects on RPE function and viability (Qin and Rodrigues 2010).…”

Vasopressin Regulates the Phosphorylation State of AMP-activated Protein Kinase (AMPK) in MDCK-C7 Cells

“…Furthermore, the biological effects of AICAR in Jurkat cells did not rely on the conversion of the activator into ZMP. In addition, inhibition of adenosine kinase prevented the activation of AMPK in human polarized retinal pigment epithelial cells [26]. Therefore, low adenosine kinase activity in MDCK-C7 cells may account for AICAR being unable to activate AMPK.…”