Retinol-binding protein-4 attenuates insulin-induced phosphorylation of IRS1 and ERK1/2 in primary human adipocytes.

Reduced sensitivity to insulin in adipose, muscle, and liver tissues is a hallmark of type 2 diabetes. Animal models and patients with type 2 diabetes exhibit elevated levels of circulating retinol-binding protein (RBP4), and RBP4 can induce insulin resistance in mice. However, little is known about how RBP4 affects insulin signaling. We examined the mechanisms of action of RBP4 in primary human adipocytes. RBP4-treated adipocytes exhibited the same molecular defects in insulin signaling, via IRS1 to MAP kinase, as in adipocytes from patients with type 2 diabetes. Without affecting autophosphorylation of the insulin receptor, RBP4 blocked the insulin-stimulated phosphorylation of IRS1 at serine (307) [corresponding to serine (302) in the murine sequence] and concomitantly increased the EC50 (from 0.5 to 2 nM) for insulin stimulation of IRS1 phosphorylation at tyrosine. The phosphorylation of IRS1 at serine (312) [corresponding to serine (307) in the murine sequence] was not affected in cells from diabetic patients and was also not affected by RBP4. The EC50 for insulin stimulation of downstream phosphorylation of MAP kinase ERK1/2 was increased (from 0.2 to 0.8 nM) by RBP4. We show that ERK1/2 phosphorylation is similarly impaired in adipocytes from patients with type 2 diabetes. However, the sensitivity to insulin for downstream signaling to control of protein kinase B and glucose uptake was not affected by RBP4. When insulin-resistant adipocytes from patients with type 2 diabetes were incubated with antibodies against RBP4, insulin-induced phosphorylation of IRS1 at serine (307) was normalized and the EC50 for insulin stimulation of ERK1/2 phosphorylation was reduced. Endogenous levels of RBP4 were markedly reduced in adipocytes from obese or type 2 diabetic subjects, whereas expression levels of RBP4 mRNA were unaffected. These findings indicate that RBP4 may be released from diabetic adipocytes and act locally to inhibit phosphorylation of IRS1 at serine (307), a phosphorylation site that may integrate nutrient sensing with insulin signaling.

The C-terminal region of cis-retinol/androgen dehydrogenase 1 (CRAD1) confers ER localization and in vivo enzymatic function.

Retinoic acid is generated from retinol (vitamin A) by the sequential actions of two different classes of enzymes, retinol dehydrogenases and retinal dehydrogenases. Several enzymes implicated in this process have been identified and characterized in vitro. However, our understanding of the cell biological function and regulation of this process is limited. To get further knowledge regarding the regulation of RA biosynthesis, we have determined possible regulatory mechanisms at the transcriptional and post-transcriptional levels for the prototypic microsomal retinol dehydrogenase cis-retinol/androgen dehydrogenase 1 (CRAD1). We note that the expression and stability of the enzyme are only moderately controlled by the retinoid status. Instead, we find that the cytosolic tail dramatically affects the activity of the enzyme, and we have mapped the structural elements required for ER retention and in vivo functional activity, respectively. Although inactive tail-deletion mutants display an abnormal subcellular localization, restoration of ER localization per se is not sufficient for enzymatic activity suggesting that additional trans-acting components interacting with, or modifying, the cytosolic tail are required for controlling the activity of the enzyme in vivo.

Activation of innate immunity, inflammation, and potentiation of DNA vaccination through mammalian expression of the TLR5 agonist flagellin.

Improving DNA vaccination remains a fundamental goal in vaccine research. Theoretically, this could be achieved by molecules encoded by DNA capable of activating TLRs to mimic inflammatory responses generated by infection. Therefore, we constructed an expression vector that allows mammalian cells to express the TLR5 agonist flagellin (FliC) at the cell surface. In vitro, cell lines expressing FliC stimulated production of proinflammatory cytokines and the up-regulation of costimulatory molecules on monocytes. Mice given the FliC expression vector intradermally exhibited site-specific inflammation and, in combination with vectors expressing Ags, developed dramatic increases in Ag-specific IgG as well as IgA. Surprisingly, mice also developed strong Ag-specific MHC class I-restricted cellular immunity. To determine whether vaccination using FliC vectors could elicit protective immunity to an infectious agent, mice were given dermal injections of FliC expression vector together with a vector encoding the influenza A virus nucleoprotein. This vaccination strategy elicited protective immunity to lethal influenza A virus infection. These results demonstrate that expression of DNA-encoded TLR agonists by mammalian cells greatly enhance and broaden immune responses, imposing new possibilities on DNA vaccination to infectious agents and cancer.

Development of a versatile reporter assay for studies of retinol uptake and metabolism in vivo.

The two isomers of retinoic acid (RA), all-trans RA and 9-cis RA, are produced in several tissues in order to allow specific control of target gene transcription. Given the high potency of these receptor ligands, it seems likely that the cellular uptake and metabolic activation of the precursor, retinol (vitamin A), should be a highly regulated process. Several retinol dehydrogenases and components involved in the downstream events have been identified and partially characterized. However, less is known about the cellular uptake of retinol, and the isomerase activity giving rise to the 9-cis and 11-cis branches of the pathway. In this work, we show that the 9-cis RA biosynthesis pathway can be fully reconstituted in cultured HEK293A cells expressing a reporter system, including an endogenous isomerase activity converting all-trans retinol into 9-cis retinol. This assay allows for functional studies of known components, as well as screening for yet unidentified genes involved in the pathway. In addition to free all-trans retinol, we find that these cells can take up retinol from plasma retinol binding protein (RBP) by a mechanism that can be efficiently inhibited by blocking antibodies, suggesting that the uptake may involve a cellular receptor. We also demonstrate that overexpression of CRBPI can drive the accumulation of intracellular retinol from unbound retinol added to the medium. Thus, this versatile cellular assay can be used to study several aspects of retinol uptake and metabolism in vivo.

Structure and function of retinol dehydrogenases of the short chain dehydrogenase/reductase family.

All-trans-retinol is the common precursor of the active retinoids 11-cis-retinal, all-trans-retinoic acid (atRA) and 9-cis-retinoic acid (9cRA). Genetic and biochemical data supports an important role of the microsomal members of the short chain dehydrogenases/reductases (SDRs) in the first oxidative conversion of retinol into retinal. Several retinol dehydrogenases of this family have been reported in recent years. However, the structural and functional data on these enzymes is limited. The prototypic enzyme RDH5 and the related enzyme CRAD1 have been shown to face the lumen of the endoplasmic reticulum (ER), suggesting a compartmentalized synthesis of retinal. This is a matter of debate as a related enzyme has been proposed to have the opposite membrane topology. Recent data indicates that RDH5, and presumably other members of the SDRs, occur as functional homodimers, and need to interact with other proteins for proper intracellular localization and catalytic activity. Further analyses on the compartmentalization, membrane topology, and functional properties of microsomal retinol dehydrogenases, will give important clues about how retinoids are processed.