Glucose-mediated de novo lipogenesis in photoreceptors drives early diabetic retinopathy

Abstract

Diabetic retinopathy (DR) is an increasingly frequent cause of blindness across populations; however, the events that initiate pathophysiology of DR remain elusive. Strong preclinical and clinical evidence suggests that abnormalities in retinal lipid metabolism caused by diabetes may account for the origin of this disease. A major arm of lipid metabolism, de novo biosynthesis, is driven by elevation in available glucose, a common thread binding all forms of vision loss in diabetes. Therefore, we hypothesized that aberrant retinal lipid biogenesis is an important promoter of early DR. In murine models, we observed elevations of diabetes-associated retinal de novo lipogenesis ∼70% over control levels. This shift was primarily because of activation of fatty acid synthase (FAS), a rate-limiting enzyme in the biogenic pathway. Activation of FAS was driven by canonical glucose-mediated disinhibition of acetyl-CoA carboxylase, a major upstream regulatory enzyme. Mutant mice expressing gain-of-function FAS demonstrated increased vulnerability to DR, whereas those with FAS deletion in rod photoreceptors maintained preserved visual responses upon induction of diabetes. Excess retinal de novo lipogenesis—either because of diabetes or because of FAS gain of function—was associated with modestly increased levels of palmitate-containing phosphatidylcholine species in synaptic membranes, a finding with as yet uncertain significance. These findings implicate glucose-dependent increases in photoreceptor de novo lipogenesis in the early pathogenesis of DR, although the mechanism of deleterious action of this pathway remains unclear. Diabetic retinopathy (DR) is an increasingly frequent cause of blindness across populations; however, the events that initiate pathophysiology of DR remain elusive. Strong preclinical and clinical evidence suggests that abnormalities in retinal lipid metabolism caused by diabetes may account for the origin of this disease. A major arm of lipid metabolism, de novo biosynthesis, is driven by elevation in available glucose, a common thread binding all forms of vision loss in diabetes. Therefore, we hypothesized that aberrant retinal lipid biogenesis is an important promoter of early DR. In murine models, we observed elevations of diabetes-associated retinal de novo lipogenesis ∼70% over control levels. This shift was primarily because of activation of fatty acid synthase (FAS), a rate-limiting enzyme in the biogenic pathway. Activation of FAS was driven by canonical glucose-mediated disinhibition of acetyl-CoA carboxylase, a major upstream regulatory enzyme. Mutant mice expressing gain-of-function FAS demonstrated increased vulnerability to DR, whereas those with FAS deletion in rod photoreceptors maintained preserved visual responses upon induction of diabetes. Excess retinal de novo lipogenesis—either because of diabetes or because of FAS gain of function—was associated with modestly increased levels of palmitate-containing phosphatidylcholine species in synaptic membranes, a finding with as yet uncertain significance. These findings implicate glucose-dependent increases in photoreceptor de novo lipogenesis in the early pathogenesis of DR, although the mechanism of deleterious action of this pathway remains unclear. Therapy for diabetic retinopathy (DR), a common cause of visual disability, has evolved greatly over the past 2 decades but remains ineffective for up to one-third of patients (1Blinder K.J. Dugel P.U. Chen S. Jumper J.M. Walt J.G. Hollander D.A. Scott L.C. Anti-VEGF treatment of diabetic macular edema in clinical practice: Effectiveness and patterns of use (ECHO study report 1).Clin. Ophthalmol. 2017; 11: 393-401Crossref PubMed Scopus (73) Google Scholar). Development of new strategies for treatment could be facilitated by gaining better insight into the pathophysiology of early stage DR. Abnormalities in retinal lipid metabolism occur early in the course of diabetes and are therefore attractive candidates for mediation of mechanisms that eventually result in vision loss (2Busik J.V. Lipid metabolism dysregulation in diabetic retinopathy.J. Lipid Res. 2021; 62: 100017Abstract Full Text Full Text PDF PubMed Google Scholar). Retinal lipid abnormalities in diabetes involve both accumulation of deleterious lipids as well as depletion of beneficial ones. Strategies to correct such abnormalities have the potential to lower risk of vision loss from diabetes. For example, reversal of cholesterol accumulation in retinas of streptozotocin (STZ)-induced diabetic mice using liver X receptor agonists reduces severity of DR (3Zheng W. Mast N. Saadane A. Pikuleva I.A. Pathways of cholesterol homeostasis in mouse retina responsive to dietary and pharmacologic treatments.J. Lipid Res. 2015; 56: 81-97Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar). Similarly, lowering of retinal ceramide levels in diabetes by inhibiting acid sphingomyelinase activity (4Opreanu M. Tikhonenko M. Bozack S. Lydic T.A. Reid G.E. McSorley K.M. Sochacki A. Perez G.I. Esselman W.J. Kern T. Kolesnick R. Grant M.B. Busik J.V. The unconventional role of acid sphingomyelinase in regulation of retinal microangiopathy in diabetic human and animal models.Diabetes. 2011; 60: 2370-2378Crossref PubMed Scopus (66) Google Scholar) prevents vascular degeneration (5Chakravarthy H. Navitskaya S. O'Reilly S. Gallimore J. Mize H. Beli E. Wang Q. Kady N. Huang C. Blanchard G.J. Grant M.B. Busik J.V. Role of acid sphingomyelinase in shifting the balance between proinflammatory and reparative bone marrow cells in diabetic retinopathy.Stem Cells. 2016; 34: 972-983Crossref PubMed Scopus (32) Google Scholar). Complex and very long chain polyunsaturated fatty acids (VLC-PUFA), including docosahexaenoic acid (DHA) and eicosapentaenoic acid, are disproportionately abundant in the retina compared with other tissues (6Agbaga M.P. Mandal M.N. Anderson R.E. Retinal very long-chain PUFAs: New insights from studies on ELOVL4 protein.J. Lipid Res. 2010; 51: 1624-1642Abstract Full Text Full Text PDF PubMed Scopus (117) Google Scholar, 7Zemski Berry K.A. Gordon W.C. Murphy R.C. Bazan N.G. Spatial organization of lipids in the human retina and optic nerve by MALDI imaging mass spectrometry.J. Lipid Res. 2014; 55: 504-515Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar) and are needed to maintain retinal health but are reduced by diabetes (8Bennett L.D. Brush R.S. Chan M. Lydic T.A. Reese K. Reid G.E. Busik J.V. Elliott M.H. Anderson R.E. Effect of reduced retinal VLC-PUFA on rod and cone photoreceptors.Invest. Ophthalmol. Vis. Sci. 2014; 55: 3150-3157Crossref PubMed Scopus (28) Google Scholar, 9Tikhonenko M. Lydic T.A. Wang Y. Chen W. Opreanu M. Sochacki A. McSorley K.M. Renis R.L. Kern T. Jump D.B. Reid G.E. Busik J.V. Remodeling of retinal fatty acids in an animal model of diabetes: A decrease in long-chain polyunsaturated fatty acids is associated with a decrease in fatty acid elongases Elovl2 and Elovl4.Diabetes. 2010; 59: 219-227Crossref PubMed Scopus (98) Google Scholar). In experimental models, dietary PUFA replacement reduces DR severity (10Tikhonenko M. Lydic T.A. Opreanu M. Li Calzi S. Bozack S. McSorley K.M. Sochacki A.L. Faber M.S. Hazra S. Duclos S. Guberski D. Reid G.E. Grant M.B. Busik J.V. N-3 polyunsaturated Fatty acids prevent diabetic retinopathy by inhibition of retinal vascular damage and enhanced endothelial progenitor cell reparative function.PLoS One. 2013; 8e55177Crossref PubMed Scopus (66) Google Scholar), and in a randomized clinical trial, DHA supplementation was associated with lower DR severity scores (11Sala-Vila A. Diaz-Lopez A. Valls-Pedret C. Cofan M. Garcia-Layana A. Lamuela-Raventos R.M. Castaner O. Zanon-Moreno V. Martinez-Gonzalez M.A. Toledo E. Basora J. Salas-Salvado J. Corella D. Gomez-Gracia E. Fiol M. et al.Dietary marine omega-3 fatty acids and incident sight-threatening retinopathy in middle-aged and older individuals with type 2 diabetes: Prospective investigation from the PREDIMED trial.JAMA Ophthalmol. 2016; 134: 1142-1149Crossref PubMed Scopus (62) Google Scholar). Photoreceptors, which are increasingly recognized loci of early DR pathogenesis (12Majidi S.P. Rajagopal R. Photoreceptor responses to light in the pathogenesis of diabetic retinopathy.Vis. Neurosci. 2020; 37E007Crossref PubMed Scopus (3) Google Scholar), are particularly susceptible to perturbations in membrane lipid composition—specifically because of defects in biosynthetic machinery. For example, reduction of photoreceptor DHA because of targeted disruption of the biosynthetic enzyme lysophosphatidic acid acyltransferase 3 resulted in synaptic dysmorphology and vision loss (13Shindou H. Koso H. Sasaki J. Nakanishi H. Sagara H. Nakagawa K.M. Takahashi Y. Hishikawa D. Iizuka-Hishikawa Y. Tokumasu F. Noguchi H. Watanabe S. Sasaki T. Shimizu T. Docosahexaenoic acid preserves visual function by maintaining correct disc morphology in retinal photoreceptor cells.J. Biol. Chem. 2017; 292: 12054-12064Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). Diabetes also reduces DHA biosynthesis because of downregulation of elongation of VLC fatty acids protein 4 (ELOVL4), whereas ELOV4 overexpression can counteract some of the effects of diabetes on the retina (14Kady N.M. Liu X. Lydic T.A. Syed M.H. Navitskaya S. Wang Q. Hammer S.S. O'Reilly S. Huang C. Seregin S.S. Amalfitano A. Chiodo V.A. Boye S.L. Hauswirth W.W. Antonetti D.A. et al.ELOVL4-mediated production of very long-chain ceramides stabilizes tight junctions and prevents diabetes-induced retinal vascular permeability.Diabetes. 2018; 67: 769-781Crossref PubMed Scopus (31) Google Scholar). More broadly, lipid derangements because of disruption of rod photoreceptor fatty acid synthase (FAS)—the enzyme that catalyzes the committed step of de novo lipogenesis (15Semenkovich C.F. Regulation of fatty acid synthase (FAS).Prog. Lipid Res. 1997; 36: 43-53Crossref PubMed Scopus (195) Google Scholar)—causes loss of retinal DHA and VLC-PUFAs, resulting in a R. S. X. T. S. J. V. Chen S. J. C.F. Retinal de novo lipogenesis to maintain 2018; PubMed Scopus Google Scholar). FAS is a enzyme up of and from and acetyl-CoA using as a This pathway is for of lipid biosynthesis, including those for VLC fatty acid production R. S. X. T. S. J. V. Chen S. J. C.F. Retinal de novo lipogenesis to maintain 2018; PubMed Scopus Google Scholar). Diabetes is associated with in which may be of H. J. C. T. A. C.F. Fatty acid synthase responses to Biol. Chem. 2011; Full Text Full Text PDF PubMed Scopus (48) Google Scholar, Y. K. J.G. T. J. C.F. to maintain glucose, and cholesterol Full Text Full Text PDF PubMed Scopus Google Scholar). In FAS in diabetes in of membrane fatty or aberrant X. H. R. C.F. Fatty acid the membrane for in 2016; PubMed Scopus Google Scholar). In FAS activity and are increased in the of increased of membrane lipids K. H. X. J. T. C.F. lipogenesis and 2013; PubMed Scopus Google Scholar). the effects of diabetes on this important and lipid biogenesis pathway in the retina are we studies to FAS enzyme activity effects of FAS on DR and associated to the retinal lipid experimental diabetes. de novo lipogenesis is by we enzyme activity in retinal tissues for of mice and of type 2 diabetes and type are all associated with retinal and dysmorphology J. D. J. R. C. The A model for the study of diabetic retinal One. 2014; PubMed Scopus (117) Google Scholar, R. M. S. Kern Chen damage in a mouse model of diabetic to and Ophthalmol. Vis. Sci. PubMed Scopus Google Scholar, R. S. C.F. in mice with diabetic 2016; PubMed Scopus Google Scholar). of to diabetes in all models, de novo lipogenesis was by the of into by The for of the as for mice for the and mice in the all models, diabetes was associated with ∼70% in retinal FAS activity compared with is an important of biosynthetic we in using retinal from mice to the of on retinal FAS of retinal to for caused an in FAS activity compared with an control These increases to the effects of which is to FAS activity effects diabetes caused FAS activity in or protein we and in retinal FAS protein levels between retinas or those to of forms of experimental diabetes and Similarly, a or treatment was associated with in FAS protein or levels in retinal retinas levels of such as synthase and protein levels as control These that FAS activity in the retina diabetes is caused by a mechanism and is responsive to therefore the of in the diabetic retina this enzyme is by and as an important of FAS activity by the production of of and in Fatty acid for Lipid Res. Full Text Full Text PDF PubMed Scopus Google Scholar). In all of experimental we that was to of increased retinal activity in diabetes and mice the and gain-of-function In we that deletion of from the retina was associated with a and early this for a DR R. S. X. T. S. J. V. Chen S. J. C.F. Retinal de novo lipogenesis to maintain 2018; PubMed Scopus Google Scholar). we as have These mice demonstrated a reduction in FAS protein in the retina compared with with and a reduction in FAS enzyme activity compared with rod photoreceptors up of the cell in the mouse we also with targeted loss of rod FAS using the S. Chen D. Y. J. Chen Chen mouse for PubMed Scopus Google Scholar). These mice in FAS protein and enzyme activity as retina mice that rod photoreceptors are the loci of retinal de novo induction of diabetes with for 3 retina and rod mice have FAS enzyme activity on and are on with that both the retinal FAS activity caused by also retinal of mice with of a FAS associated with human and H. H. M. T. S.S. Chen W. M. F. S. A. M. C. et for 2011; PubMed Scopus Google Scholar). This which in a acid of for in mice and in a in activity compared with the control in M. T. D. S. W.J. C. J. M. S. T. H. C.F. et lipid metabolism cell activity to and 2020; Full Text Full Text PDF PubMed Scopus Google Scholar). Similarly, we that mice this have elevation in retinal FAS activity compared with but with associated in protein the mice in enzyme activity induction of diabetes compared with of we or other in retina rod or compared with and DR are by FAS rod we to potential that are associated with early retinal damage in human diabetes and that has to the of vascular in this mouse model R. S. C.F. in mice with diabetic 2016; PubMed Scopus Google Scholar). for increased of across all major and loss of compared with in control such in mice with FAS rod to or and FAS rod this of the and These that in mice are by loss of FAS that the we for that are to to diabetes-induced in the mice levels of vascular endothelial and rod mice on such elevations compared with rod on or compared with control mice on mice for the FAS we diabetes by and retinal function by we that was associated with loss of effects on or and of to but an FAS on in this of and the severity of loss compared with FAS in of compared with we also observed such 3 of control yet the and FAS mice on in compared with on to that FAS activity is to the effects of diabetes on retinal function but that such with FAS on DR in mice that activity of this enzyme to the pathogenesis of vision loss in diabetes. a potential we lipid of retinal synaptic membranes, as are susceptible to in retinal FAS activity R. S. X. T. S. J. V. Chen S. J. C.F. Retinal de novo lipogenesis to maintain 2018; PubMed Scopus Google Scholar). for retinal synaptic membranes, we observed an in fatty acids those with diabetic mice compared with In diabetic lower of with fatty acids and long compared with with by using the in from FAS mice and to control we that FAS activity caused a elevation of retinal synaptic FAS mice have depletion of on the levels of compared with The we observed in synaptic membrane in diabetic mice observed In FAS we observed a in compared with observed in over in tissues from diabetic mice or FAS mice in retinal synaptic membrane associated with experimental by by mass to protein and to an or compared by in a new in retinal synaptic membrane associated with FAS by by mass to protein and to an or compared by in a new in retinal synaptic membrane associated with experimental to to to to to to by by mass to protein and to an or compared by in a new in retinal synaptic membrane associated with FAS by by mass to protein and to an or compared by in a new in retinal synaptic membrane associated with experimental by by mass to protein and to an or compared by in a new in retinal synaptic membrane associated with FAS to to to by by mass to protein and to an or compared by in a new by by mass to protein and to an or compared by by by mass to protein and to an or compared by by by mass to protein and to an or compared by by by mass to protein and to an or compared by by by mass to protein and to an or compared by by by mass to protein and to an or compared by loss from DR because of a that has A. E. M. Rajagopal R. insights and for diabetic Diabetes PubMed Scopus Google Scholar). damage to the retina in diabetes long the of such and cell of the In preclinical models, in of the rod visual and pharmacologic of or visual are all associated with reduction in DR severity or of DR K.A. D.A. T.A. is reduced experimental diabetes in a mouse model of retinal Ophthalmol. Vis. Sci. PubMed Scopus Google Scholar, H. J. Y. M. A. Antonetti D.A. J. J. K. Kern early diabetic retinopathy in Biol. Chem. 2015; Full Text Full Text PDF PubMed Scopus Google Scholar, H. J. Y. Saadane A. A. K. Kern and the of early diabetic Ophthalmol. Vis. Sci. 60: PubMed Scopus Google Scholar, C. S. C.F. Rajagopal R. reduces the severity of experimental diabetic 2020; PubMed Scopus Google Scholar). with the of photoreceptors H. by rod photoreceptors in and in Biol. Full Text Full Text PDF PubMed Scopus Google Scholar), implicate perturbations in photoreceptor visual metabolism as a in the pathogenesis of DR. The study insight into a role for an arm of photoreceptor of de novo lipogenesis by such pathway by diabetes. In type and type 2 of retinal FAS activity was associated with and disinhibition of a regulatory enzyme. all the de novo in the diabetic retina to rod whereas targeted loss of FAS in and we FAS activity to increased retinal synaptic membranes, to with diabetes are with those of lipid in the diabetic but some important to Busik et we that retinal from diabetic mice of In retinal DHA in diabetes to of and although the mechanism for of remains M. Lydic T.A. Wang Y. Chen W. Opreanu M. Sochacki A. McSorley K.M. Renis R.L. Kern T. Jump D.B. Reid G.E. Busik J.V. Remodeling of retinal fatty acids in an animal model of diabetes: A decrease in long-chain polyunsaturated fatty acids is associated with a decrease in fatty acid elongases Elovl2 and Elovl4.Diabetes. 2010; 59: 219-227Crossref PubMed Scopus (98) Google Scholar). we that those with such as in DR, studies in lipid because of diabetes in we retinal synaptic This in for the between compared with a study that in in DR M. Lydic T.A. Wang Y. Chen W. Opreanu M. Sochacki A. McSorley K.M. Renis R.L. Kern T. Jump D.B. Reid G.E. Busik J.V. Remodeling of retinal fatty acids in an animal model of diabetes: A decrease in long-chain polyunsaturated fatty acids is associated with a decrease in fatty acid elongases Elovl2 and Elovl4.Diabetes. 2010; 59: 219-227Crossref PubMed Scopus (98) Google Scholar). to study lipid in retinal synaptic for in studies of de novo lipogenesis in the we that retinal synaptic particularly by FAS which was associated with in synaptic and loss of R. S. X. T. S. J. V. Chen S. J. C.F. Retinal de novo lipogenesis to maintain 2018; PubMed Scopus Google Scholar). In mice with deletion of we a depletion of in the in for synaptic membranes, we in in retinal with we that FAS mice levels of retinal findings that FAS enzyme in diabetes function early in the course of to a potential on retinal synaptic This is lipid membrane including effects on and which can synaptic function M. S. S. S. T. R. H. Lipid cell membrane and by 2014; PubMed Scopus Google Scholar). because the lipid we observed in other to the retinal lipid as in retinal or to lipid in the study as mechanisms in FAS we a or in FAS mice in the of diabetes. This result suggests that of FAS the of is to to DR. we that in an in FAS mice on compared with mice on for findings is that retinal enzyme activity in FAS mice is to the of diabetes. is that FAS activity increases are of diabetes-associated in photoreceptors that initiate early DR. a model account for finding that retinas from FAS mice levels of VLC-PUFA and 2 and compared with in with tissues from diabetic which loss of VLC-PUFA compared with that effects of accumulation because of FAS with other retinal membrane in such as loss of VLC-PUFA because of ELOVL4 or could cause the lipid needed to initiate DR the evidence for a photoreceptor of early in DR is some evidence from patients with retinal degeneration from between such and the of vascular in DR are Chen Chen M.S. Chen Wang reduces the risk of diabetic A One. PubMed Scopus Google Scholar, The of diabetic retinopathy in patients with for pathophysiology and J. Ophthalmol. PubMed Scopus Google Scholar, M.B. M. The of and diabetic J. Ophthalmol. Full Text PDF PubMed Scopus Google Scholar). from Busik et that the loss of VLC-PUFA in diabetes may be the of photoreceptor are to have the of that endothelial PUFA in of in the of could and to (14Kady N.M. Liu X. Lydic T.A. Syed M.H. Navitskaya S. Wang Q. Hammer S.S. O'Reilly S. Huang C. Seregin S.S. Amalfitano A. Chiodo V.A. Boye S.L. Hauswirth W.W. Antonetti D.A. et al.ELOVL4-mediated production of very long-chain ceramides stabilizes tight junctions and prevents diabetes-induced retinal vascular permeability.Diabetes. 2018; 67: 769-781Crossref PubMed Scopus (31) Google Scholar). is that diabetes-induced lipid perturbations photoreceptor in the 2018; PubMed Scopus Google Scholar). These effects could be by in synaptic membrane and effects on M. S. S. S. T. R. H. Lipid cell membrane and by 2014; PubMed Scopus Google Scholar). is that retinal could have deleterious on photoreceptor metabolism, which could in vascular For example, in because of in to damage J.M. M.A. The of dietary and fatty acids on nerve function in murine of Neurosci. PubMed Scopus Google Scholar). membrane lipid could for example, in acids or which are to in the retina cells and A mechanism of Neurosci. PubMed Scopus Google Scholar). to vascular that experimental and clinical diabetes reduces in the by diabetes may account for such by the and of this could a for early in DR. mice from and of or receptor also from with the which are which are and which from from and from or from from and from In some was to diabetes in mice the of mice to this mice for 4 and of in or for The was for a of was by a using a of from with was also and with reduction in compared with in by a to prevent For we FAS and mouse the was from from of was on the resulting using the endothelial to the of and retinas animal in and as of was to a and This was in a for and using of acid with The resulting was with of was and the was and to an long-chain fatty such with and the was to a to the from and mice in 4 for into and with and A was for with and of with was maintained with a and on both with and The was a for with for the and for the using a The of the was from the to the of the and the was from that to the The of the was on using a and to for was retinas from animal into of and in a with a of was and in to for protein using the an of and was the was and in with A was into a mass using an with which a of of with of species in the using of with a of to the species as the species as the in the using of with a of of and species was compared with and to the protein of the light membrane for synaptic retinas from in of and on for and by for with a to of and of and in an 4 for from to by and The as the light membrane and the the membrane to lipid and by mass as In are as In are as with the the of the and and For with in the using For with in the with was was and with was or using In all and the for in and for the of and by in this including synaptic membrane fatty acid mass be upon all to The that have of with the of this This was by the and to New Development and an to the of and of the and the R. and C. F. S. X. M. S. and C. F. S. R. S. S. X. and C. F. S. R. X. and C. F. S. M. and S. J. R. S. S. and H. R. and C. F. S. R. S. and C. F. S. and R. R. and C. F. S. The is the of the and the of the of

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