Transgenic Mice Overexpressing Serum Retinol-Binding Protein Develop Progressive Retinal Degeneration through a Retinoid-Independent Mechanism

Abstract

Serum retinol-binding protein 4 (RBP4) is the sole specific transport protein for retinol in the blood, but it is also an adipokine with retinol-independent, proinflammatory activity associated with obesity, insulin resistance, type 2 diabetes, and cardiovascular disease. Moreover, two separate studies reported that patients with proliferative diabetic retinopathy have increased serum RBP4 levels compared to patients with mild or no retinopathy, yet the effect of increased levels of RBP4 on the retina has not been studied. Here we show that transgenic mice overexpressing RBP4 (RBP4-Tg mice) develop progressive retinal degeneration, characterized by photoreceptor ribbon synapse deficiency and subsequent bipolar cell loss. Ocular retinoid and bisretinoid levels are normal in RBP4-Tg mice, demonstrating that a retinoid-independent mechanism underlies retinal degeneration. Increased expression of pro-interleukin-18 (pro-IL-18) mRNA and activated IL-18 protein and early-onset microglia activation in the retina suggest that retinal degeneration is driven by a proinflammatory mechanism. Neither chronic systemic metabolic disease nor other retinal insults are required for RBP4 elevation to promote retinal neurodegeneration, since RBP4-Tg mice do not have coincident retinal vascular pathology, obesity, dyslipidemia, or hyperglycemia. These findings suggest that elevation of serum RBP4 levels could be a risk factor for retinal damage and vision loss in nondiabetic as well as diabetic patients. S erum retinol-binding protein 4 (RBP4) is the sole specific transport protein for vitamin A (retinol) in the blood (1-4).The eye is the organ most dependent on the RBP4-mediated delivery of retinol to maintain optimal function, as RBP4 loss of function in either mice or humans impairs retinal function, leading to visual impairment, while other organ systems remain intact (5-7). However, in the past decade numerous clinical studies have linked increased serum levels of RBP4 to disease, including obesity (8, 9), insulin resistance (8-13), type 2 diabetes (9, 13), and cardiovascular disease (hypertension, atherosclerosis, stroke) (14-18). This appears to be more than a spurious correlation, since recent studies have demonstrated that RBP4 is an adipokine (adipose-derived cytokine) with retinoid-independent, proinflammatory activity that contributes to the development of insulin resistance (13,(19)(20)(21). Moreover, a human single nucleotide polymorphism that increases RBP4 promoter activity confers a 2-fold increase for the risk of type 2 diabetes (22, 23).Several studies have provided mechanistic insights into RBP4-induced insulin resistance. Mice with genetic or pharmacologic elevation of RBP4 levels develop insulin resistance (13), whereas lowering of RBP4 levels improves insulin sensitivity in mice (13,24). RBP4 inhibits insulin signaling in adipocytes indirectly by activating proinflammatory cytokine production in macrophages through retinol-independent and Toll-like receptor 4 (TLR4)-and c-Jun N-terminal kinase (JNK)-dependent ...

Adipocyte‐specific overexpression of retinol‐binding protein 4 causes hepatic steatosis in mice

“…With the reports from Yang et al (8) and Graham et al (9) a decade ago, there has been considerable research interest focused on understanding how RBP4 synthesized by adipocytes contributes to the development of metabolic disease, including liver disease. Although much of this literature reports associations between elevated serum/plasma and/or tissue RBP4 levels and metabolic disease, (1)(2)(3)(4)(5)(6)(8)(9)(10)(11)(12)(13)(14)(15)(16) there are a number of studies reporting evidence to the contrary, (29)(30)(31)(32) including ones that failed to establish associations between RBP4 and NAFLD. (33,34) Undoubtedly, some of this disagreement in the literature arises from differences in methodologies employed by different research groups, including methodologies needed for grouping and studying human subjects as well as laboratory techniques used to evaluate RBP4 levels and actions.…”

“…(7) Retinol bound to RBP4 accounts for approximately 95% of the retinoid that is present in the fasting circulation. (7) Many investigators have reported findings that associate elevated RBP4 levels with the development of type 2 diabetes, (8)(9)(10)(11)(12) cardiovascular disease, (13)(14)(15)(16) as well as NAFLD. (1)(2)(3)(4)(5)(6) Kahn and colleagues proposed that the development of obesity leads to increased expression of RBP4 by adipocytes, influencing metabolic disease development.…”

Local and Systemic Inflammatory Biomarkers of Diabetic Retinopathy: An Integrative Approach

“…39 Moreover, in an animal model overexpressing RBP4, an earlyonset microglia activation followed by a progressive retinal degeneration mediated by an increased expression of pro-IL-18 was observed. 40 Therefore, RBP4 could be involved in the inflammatory process of DR and could be considered a biomarker of early stages of DR. Takebayashi et al 41 found elevated RBP4 levels in T2DM patients in comparison with nondiabetic subjects, and significant increased levels in patients with PDR versus no DR or nonproliferative DR. However, this relation was reduced after adjusting for urinary albumin excretion (UAE).…”

Liver-secreted RBP4 does not impair glucose homeostasis in mice

“…In humans, serum RBP4 in obese and obese/diabetic patients was found ϳ1.5-3-fold higher when compared with lean controls (4 -7), and this increase was mirrored by elevated expression of RBP4 in adipose tissue (8). Mice deficient in RBP4 were more insulin sensitive than control littermates, whereas mice with transgenic expression of RBP4 were prone to high-fat diet (HFD)-induced insulin resistance (4), although not all studies could reproduce these phenotypes (9,10). Interestingly, the sole injection of Escherichia coli-derived recombinant RBP4 into mice led to impaired glucose tolerance (4,11).…”