Background: Mechanisms by which RBP4 interacts with cells are not completely understood. Results: 1300002K09Rik (RBPR2) is identified as a Stra6-related protein expressed in liver, intestine, and obese fat that mediates RBP4 binding and retinol transport. Conclusion: RBPR2 is a novel RBP4 receptor that mediates retinol uptake. Significance: RBPR2 may be important for whole body retinol homeostasis or cellular actions of RBP4 in certain tissues.

Diacylglycerol kinases (DGKs) phosphorylate diacylglycerol (DAG) to terminate its signaling. To study DGK␦, we disrupted its gene in mice and found that DGK␦ deficiency reduced EGF receptor (EGFR) protein expression and activity. Similar to EGFR knockout mice, DGK␦-deficient pups were born with open eyelids and died shortly after birth. PKCs are activated by DAG and phosphorylate EGFR to reduce its expression and activity. We found DAG accumulation, increased threonine phosphorylation of EGFR, enhanced phosphorylation of other PKC substrates, and increased PKC autophosphorylation in DGK␦ knockout cells, indicating that DGK␦ regulates EGFR by modulating PKC signaling. D iacylglycerol kinases (DGKs) catalyze the phosphorylation of diacylglycerol (DAG) to produce phosphatidic acid (1, 2). DAG, the substrate of the DGK reaction, is a key intracellular signaling factor that activates PKCs, Ras guanyl nucleotidereleasing proteins, and some transient receptor potential channels (3, 4). DAG also recruits a number of proteins to membrane compartments, including the chimaerins, PKD, and the Munc13 proteins (3). Its effects on numerous and diverse targets underscores the importance of DAG signaling and indicates that DAG affects a broad array of signaling events. Because the consumption of DAG by DGKs is thought to attenuate these actions, the DGK reaction is biologically important and likely regulates numerous DAG signaling pathways.Mammalian DGKs differ in their structures, patterns of tissue expression, and catalytic properties. Ten of them have been identified and are classified into five subtypes based on their structural motifs (1, 2, 5, 6). Their structural diversity and distinct expression patterns indicate that each isoform may perform a different biological function. Supporting functional diversity, the DGK knockout mice that have been studied to date have distinct phenotypes, including resistance to seizures in DGK knockout mice (7), attenuated Ras signaling in DGK knockouts (8), and hyperactive T cell signaling in DGK -deficient mice (9).As a type II DGK, DGK␦ has a characteristic pleckstrin homology domain and a sterile ␣-motif domain (Fig. 1A). To determine its biological function, we generated mice with a targeted mutation of the DGK␦ gene. Our data indicate an important role for DGK␦ in modulating PKC and EGF receptor (EGFR) signaling. Results and DiscussionWe used mice to make a targeted deletion of the N-terminal portion of the DGK␦ catalytic domain (see Fig. 7, which is published as supporting information on the PNAS web site). Southern blot analysis of tail DNA confirmed proper insertion of the targeting vector (data not shown), and RT-PCR demonstrated absence of DGK␦ mRNA in homozygous mutant cells (Fig. 7C). Also confirming DGK␦ gene inactivation, DGK␦ protein was absent in knockout keratinocyte and dermal fibroblast cell lysates (Fig. 7D). Deleting DGK␦ did not significantly affect mRNA expression of other DGKs in brain tissue (Fig. 7E). Finally, to establish the expression pattern of DGK␦ in WT mice...

SUMMARY Autophagy is a homeostatic cellular process involved in the degradation of long-lived or damaged cellular components. The role of autophagy in adipogenesis is well recognized, but its role in mature adipocyte function is largely unknown. We show that the autophagy proteins Atg3 and Atg16L1 are required for proper mitochondrial function in mature adipocytes. In contrast to previous studies, we found that post-developmental ablation of autophagy causes peripheral insulin resistance independently of diet or adiposity. Finally, lack of adipocyte autophagy reveals cross talk between fat and liver, mediated by lipid peroxide-induced Nrf2 signaling. Our data reveal a role for autophagy in preventing lipid peroxide formation and its transfer in insulin-sensitive peripheral tissues.

Objective Impaired endothelial cell autophagy compromises shear-stress induced nitric oxide (NO) generation. We determined the responsible mechanism. Approach and Results Upon autophagy compromise in bovine aortic endothelial cells (ECs) exposed to shear-stress, a decrease in glucose uptake and EC glycolysis attenuated ATP production. We hypothesized that decreased glycolysis-dependent purinergic signaling via P2Y-1 receptors, secondary to impaired autophagy in ECs, prevents shear-induced p-eNOSS1177 and NO generation. Maneuvers that restore glucose transport and glycolysis (e.g., overexpression of GLUT1) or purinergic signaling (e.g., addition of exogenous ADP) rescue shear-induced p-eNOSS1177 and NO production in ECs with impaired autophagy. Conversely, inhibiting glucose-transport via GLUT1 siRNA, blocking purinergic signaling via ectonucleotidase-mediated ATP/ADP degradation (e.g., apyrase), or inhibiting P2Y1 receptors using pharmacological (e.g., MRS2179) or genetic (e.g., P2Y1-R siRNA) procedures, inhibits shear-induced p-eNOSS1177 and NO generation in ECs with intact autophagy. Supporting a central role for PKCδT505 in relaying the autophagy-dependent purinergic-mediated signal to eNOS, we find that: (i) shear-stress induced activating phosphorylation of PKCδT505 is negated by inhibiting autophagy; (ii) shear-induced p-eNOSS1177 and NO generation are restored in autophagy-impaired ECs via pharmacological (e.g., bryostatin) or genetic (e.g., CA-PKCδ) activation of PKCδT505 and (iii) pharmacological (e.g., rottlerin) and genetic (e.g., PKCδ siRNA) PKCδ inhibition prevents shear-induced p-eNOSS1177 and NO generation in ECs with intact autophagy. Key nodes of dysregulation in this pathway upon autophagy compromise were revealed in human arterial endothelial cells. Conclusion Targeted reactivation of purinergic signaling and/or PKCδ has strategic potential to restore compromised NO generation in pathologies associated with suppressed EC autophagy.