In comparison to the internalization pathways of endocytosis, the recycling pathways are less understood. Even less defined is the process of regulated recycling, as few examples exist and their underlying mechanisms remain to be clarified. In this study, we examine the endocytic recycling of integrin b1, a process that has been suggested to play an important role during cell motility by mediating the redistribution of integrins to the migrating front. External stimulation regulates the endocytic itinerary of b1, mainly at an internal compartment that is likely to be a subset of the recycling endosomes. This stimulationdependent recycling is regulated by ARF6 and Rab11, and also requires the actin cytoskeleton in an ARF6-dependent manner. Consistent with these observations being relevant for cell motility, mutant forms of ARF6 that affect either actin rearrangement or recycling inhibit the motility of a breast cancer cell line.

Using an siRNA-based screen, we identified the transcriptional corepressor RIP140 as a negative regulator of insulin-responsive hexose uptake and oxidative metabolism in 3T3-L1 adipocytes. Affymetrix GeneChip profiling revealed that RIP140 depletion upregulates the expression of clusters of genes in the pathways of glucose uptake, glycolysis, TCA cycle, fatty acid oxidation, mitochondrial biogenesis, and oxidative phosphorylation in these cells. Conversely, we show that reexpression of RIP140 in mouse embryonic fibroblasts derived from RIP140-null mice downregulates expression of many of these same genes. Consistent with these microarray data, RIP140 gene silencing in cultured adipocytes increased both conversion of [ 14 C]glucose to CO 2 and mitochondrial oxygen consumption. RIP140-null mice, previously reported to resist weight gain on a high-fat diet, are shown here to display enhanced glucose tolerance and enhanced responsiveness to insulin compared with matched wild-type mice upon high-fat feeding. Mechanistically, RIP140 was found to require the nuclear receptor ERRα to regulate hexose uptake and mitochondrial proteins SDHB and CoxVb, although it likely acts through other nuclear receptors as well. We conclude that RIP140 is a major suppressor of adipocyte oxidative metabolism and mitochondrial biogenesis, as well as a negative regulator of whole-body glucose tolerance and energy expenditure in mice.

Components of intracellular signaling that mediate the stimulation-dependent recycling of integrins are being identified, but key transport effectors that are the ultimate downstream targets remain unknown. ACAP1 has been shown recently to function as a transport effector in the cargo sorting of transferrin receptor (TfR) that undergoes constitutive recycling. We now show that ACAP1 also participates in the regulated recycling of integrin beta1 to control cell migration. However, in contrast to TfR recycling, the role of ACAP1 in beta1 recycling requires its phosphorylation by Akt, which is, in turn, regulated by a canonical signaling pathway. Disrupting the activities of either ACAP1 or Akt, or their assembly with endosomal beta1, inhibits beta1 recycling and cell migration. These findings advance an understanding of how integrin recycling is achieved during cell migration, and also address a basic issue of how intracellular signaling can interface with transport to achieve regulated recycling.

The insulin-regulated glucose transporter GLUT4 is a key modulator of whole body glucose homeostasis, and its selective loss in adipose tissue or skeletal muscle causes insulin resistance and diabetes. Here we report an RNA interference-based screen of protein kinases expressed in adipocytes and identify four negative regulators of insulin-responsive glucose transport: the protein kinases PCTAIRE-1 (PCTK1), PFTAIRE-1 (PFTK1), I B kinase ␣, and MAP4K4͞NIK. Integrin-linked protein kinase was identified as a positive regulator of this process. We characterized one of these hits, MAP4K4͞NIK, and found that it is unique among mitogenactivated protein (MAP) kinases expressed in cultured adipocytes in attenuating hexose transport. Remarkably, MAP4K4͞NIK suppresses expression of the adipogenic transcription factors C͞EBP␣, C͞EBP␤, and PPAR␥ and of GLUT4 itself in these cells. RNA interference-mediated depletion of MAP4K4͞NIK early in differentiation enhances adipogenesis and triglyceride deposition, and even in fully differentiated adipocytes its loss up-regulates GLUT4. Conversely, conditions that inhibit adipogenesis such as TNF-␣ treatment or depletion of PPAR␥ markedly up-regulate MAP4K4͞ NIK expression in cultured adipocytes. Furthermore, TNF-␣ signaling to down-regulate GLUT4 is impaired in the absence of MAP4K4͞NIK, indicating that MAP4K4 expression is required for optimal TNF-␣ action. These results reveal a MAP4K4͞NIK-dependent signaling pathway that potently inhibits PPAR␥-responsive gene expression, adipogenesis, and insulin-stimulated glucose transport.GLUT4 function ͉ adipocyte differentiation ͉ protein kinase screening

Glucose homeostasis is controlled by insulin in part through the stimulation of glucose transport in muscle and fat cells. This insulin signaling pathway requires phosphatidylinositol (PI) 3-kinase-mediated 3-polyphosphoinositide generation and activation of Akt/protein kinase B. Previous experiments using dominant negative constructs and gene ablation in mice suggested that two phosphoinositide phosphatases, SH2 domain-containing inositol 5-phosphatase 2 (SHIP2) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulate this insulin signaling pathway. Here we directly tested this hypothesis by selectively inhibiting the expression of SHIP2 or PTEN in intact cultured 3T3-L1 adipocytes through the use of short interfering RNA (siRNA). Attenuation of PTEN expression by RNAi markedly enhanced insulin-stimulated Akt and glycogen synthase kinase 3␣ (GSK-3␣) phosphorylation, as well as deoxyglucose transport in 3T3-L1 adipocytes. In contrast, depletion of SHIP2 protein by about 90% surprisingly failed to modulate these insulin-regulated events under identical assay conditions. In control studies, no diminution of insulin signaling to the mitogen-activated protein kinases Erk1 and Erk2 was observed when either PTEN or SHIP2 were depleted. Taken together, these results demonstrate that endogenous PTEN functions as a suppressor of insulin signaling to glucose transport through the PI 3-kinase pathway in cultured 3T3-L1 adipocytes.Insulin is the primary hormone that regulates glucose homeostasis, and impairment of insulin action and secretion plays a critical role in the pathogenesis of diabetes mellitus (1, 2). One of the primary metabolic responses mediated by insulin is the stimulation of glucose transport and glycogen synthesis in muscle and adipose tissue (3, 4). It is now established that translocation of glucose transporter 4 (GLUT4) 1 from intracellular compartments to the plasma membrane is mainly responsible for the insulin-stimulated increase in glucose uptake in these tissues (5-8). This process is initiated when insulin binds and activates its receptor tyrosine kinase at the cell surface. The activated insulin receptor phosphorylates the insulin receptor substrate (IRS) family of proteins on tyrosine residues. IRS proteins propagate insulin signaling to the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase, which activates the p110 catalytic subunit (3, 4). A growing number of experiments indicate that insulin-induced PI 3-kinase activation is critical for insulin-induced metabolic actions, including glucose uptake and glycogen synthesis (9 -11). Thus, phosphorylation of phosphatidylinositol 4,5-bisphosphate by activated PI 3-kinase increases levels of PI(3,4,5)P 3 at cellular membranes. This phosphorylated lipid is a potent modulator of the protein kinases PDK1, atypical protein kinase C /, and Akt/Protein kinase B (12, 13). Blockade of the PI 3-kinase signaling pathway through the use of specific inhibitors or by the expression of a dominant negative p85 regulatory sub...

Using siRNA-mediated gene silencing in cultured adipocytes, we have dissected the insulin-signalling pathway leading to translocation of GLUT4 glucose transporters to the plasma membrane. RNAi (RNA interference)-based depletion of components in the putative TC10 pathway (CAP, CrkII and c-Cbl plus Cbl-b) or the phospholipase Cgamma pathway failed to diminish insulin signalling to GLUT4. Within the phosphoinositide 3-kinase pathway, loss of the 5'-phosphatidylinositol 3,4,5-trisphosphate phosphatase SHIP2 was also without effect, whereas depletion of the 3'-phosphatase PTEN significantly enhanced insulin action. Downstream of phosphatidylinositol 3,4,5-trisphosphate and PDK1, silencing the genes encoding the protein kinases Akt1/PKBalpha, or CISK(SGK3) or protein kinases Clambda/zeta had little or no effect, but loss of Akt2/PKBbeta significantly attenuated GLUT4 regulation by insulin. These results show that Akt2/PKBbeta is the key downstream intermediate within the phosphoinositide 3-kinase pathway linked to insulin action on GLUT4 in cultured adipocytes, whereas PTEN is a potent negative regulator of this pathway.

Cultured adipocyte cell lines are a model system widely used to study adipose function, but they exhibit significant physiological differences compared with primary cells from adipose tissue. Here we report short interfering RNA-based methodology to selectively attenuate gene expression in mouse and human primary adipose tissues as a means of rapidly validating findings made in cultured adipocyte cell lines. The method is exemplified by depletion of the PTEN phosphatase in white adipose tissue (WAT) from mouse and humans, which increases Akt phosphorylation as expected. This technology is also shown to silence genes in mouse brown adipose tissue. Previous work revealed upregulation of the mitochondrial protein UCP1 in adipose cells from mice lacking the gene for the transcriptional corepressor RIP140, whereas in cultured adipocytes, loss of RIP140 has a little effect on UCP1 expression. Application of our method to deplete RIP140 in primary mouse WAT elicited markedly increased oxygen consumption and expression of UCP1 that exactly mimics the phenotype observed in RIP140-null mice. This ex-vivo method of gene silencing should be useful in rapid validation studies as well as in addressing the depot-and species-specific functions of genes in adipose biology.-Puri, V., A. Chakladar, J. V. Virbasius, S. Konda, A. M. Powelka, M. Chouinard, G. N. Hagan, R. Perugini, and M. P. Czech. RNAi-based gene silencing in primary mouse and human adipose tissues. J. Lipid Res. 2007. 48: 465-471.

ARF6 regulates membrane trafficking between the plasma membrane and endosomes. We investigated the role of ARF6 in synaptic vesicle biogenesis as this process occurs both at the plasma membrane and at endosomes. We used a synaptic vesicle marker protein, p-selectin-horseradish peroxidase (HRP), to follow the effects of ARF6 expression on synaptic vesicle biogenesis in PC12 neuroendocrine cells. Expression of a constitutively active ARF6 mutant increased, while expression of a nucleotide-free ARF6 mutant decreased, p-selectin-HRP levels in the synaptic vesicle peak. These results provide the first direct evidence for a role for ARF6 in synaptic vesicle biogenesis.