RNAi screens reveal novel metabolic regulators: RIP140, MAP4k4 and the lipid droplet associated fat specific protein (FSP) 27.

Adipose tissue modulates whole body metabolism and insulin sensitivity by controlling circulating lipid levels and producing molecules that can regulate fatty acid metabolism in such tissues as muscle and liver. We have developed RNA interference (RNAi) screens to identify genes in cultured adipocytes that regulate insulin signalling and key metabolic pathways. These short interfering RNA (siRNA)-based screens identified the transcriptional corepressor receptor interacting protein 140 (RIP140) (J Clin Invest 116: 125, 2006) and the mitogen-activated protein kinase (MAP4k4) (Proc Natl Acad Sci USA 103: 2087, 2006) as negative regulators of insulin-responsive hexose uptake and oxidative metabolism. Gene expression profiling revealed that RIP140 depletion upregulates the expression of clusters of genes in the pathways of glucose uptake, glycolysis, tricarboxylic acid cycle, fatty acid oxidation, mitochondrial biogenesis and oxidative phosphorylation. RIP140-null mice resist weight gain on a high-fat diet and display enhanced glucose tolerance. MAP4k4 depletion in adipocytes increases many of the RIP140-sensitive genes, increases adipogenesis and mediates some actions of tumour necrosis factor-alpha (TNF-alpha). Remarkably, another hit in our RNAi screens was fat specific protein 27 (FSP27), a highly expressed isoform of Cidea. We discovered that FSP27 unexpectedly associates specifically with lipid droplets and regulates fat storage. We conclude that RIP140, MAP4k4 and the novel lipid droplet protein FSP27 are powerful regulators of adipose tissue metabolism and are potential therapeutic targets for controlling metabolic disease. The discovery of these novel proteins validates the power of RNAi screening for discovery of new therapeutic approaches to type 2 diabetes and obesity.

Analysis of insulin signalling by RNAi-based gene silencing.

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.

Activation of the Akt-related cytokine-independent survival kinase requires interaction of its phox domain with endosomal phosphatidylinositol 3-phosphate.

Protein kinases of the Akt and related serum- and glucocorticoid-regulated kinase (SGK) families are major downstream mediators of phosphatidylinositol (PI) 3-kinase signaling to many cellular processes including metabolic flux, membrane trafficking, and apoptosis. Activation of these kinases is thought to occur at the plasma membrane through their serine and threonine phosphorylation by the phosphoinositide-dependent kinase 1 (PDK1) protein kinase, which interacts with membrane 3'-polyphosphoinositides through its pleckstrin homology (PH) domain. Here, we demonstrate that the SGK family member cytokine-independent survival kinase (CISK) binds strongly and selectively to the monophosphoinositide PI(3)P through its phox homology (PX) domain. Comparing native green fluorescent protein-CISK (EGFP-CISK) to a mutant EGFP-CISK (Y51A) that displays attenuated binding to PI(3)P reveals that this interaction is both necessary and sufficient for its localization to early endosome antigen (EEA1)-positive endosomes. Furthermore, early endosome association of expressed epitope-tagged CISK in COS cells directed by binding of its PX domain to PI(3)P is required for activation of the CISK protein kinase by both insulin-like growth factor-1 and epidermal growth factor. Taken together, these results reveal a critical role of endosomal PI(3)P in the signal transmission mechanism whereby this survival kinase is activated in response to PI3-kinase stimulation by growth factors.

Phox homology domains specifically bind phosphatidylinositol phosphates.

The recruitment of specific cytosolic proteins to intracellular membranes through binding phosphorylated derivatives of phosphatidylinositol (PtdIns) controls such processes as endocytosis, regulated exocytosis, cytoskeletal organization, and cell signaling. Protein modules such as FVYE domains and PH domains that bind specifically to PtdIns 3-phosphate (PtdIns-3-P) and polyphosphoinositides, respectively, can direct such membrane targeting. Here we show that two representative Phox homology (PX) domains selectively bind to specific phosphatidylinositol phosphates. The PX domain of Vam7p selectively binds PtdIns-3-P, while the PX domain of the CPK PI-3 kinase selectively binds PtdIns-4,5-P(2). In contrast, the PX domain of Vps5p displays no binding to any PtdInsPs that were tested. In addition, the double mutant (Y42A/L48Q) of the PX domain of Vam7p, reported to cause vacuolar trafficking defects in yeast, has a dramatically decreased level of binding to PtdIns-3-P. These data reveal that the membrane targeting function of the Vam7p PX domain is based on its ability to associate with PtdIns-3-P, analogous to the function of FYVE domains.

Identification of an early endosomal protein regulated by phosphatidylinositol 3-kinase.

Phosphatidylinositol 3-kinases (PI 3-kinases) have been implicated in membrane trafficking in the secretory and endocytic pathways of yeast and mammalian cells, but the molecular mechanisms by which these lipid kinases operate are not known. Here we identify a protein of 170 kDa that is rapidly released from cell membranes in response to wortmannin, a potent inhibitor of mammalian PI 3-kinases. The amino acid sequence of peptides from p170 reveal its identity to early endosomal antigen (EEA) 1, an endosomal antigen with homology to several yeast proteins genetically implicated in membrane trafficking. Immunofluorescence analysis of 3T3-L1 adipocytes with antisera against p170/EEA1 reveal a punctate peripheral pattern that becomes diffuse in response to wortmannin. In vitro, p170/EEA1 binds specifically to liposomes containing PIns(3)P, suggesting that the effect of wortmannin on cells is due to inhibition of PIns(3)P production. Thus, p170/EEA1 may define a family of proteins that mediate the regulatory effects of 3'-phosphoinositides on membrane trafficking in yeast and mammalian cells.

Signaling by phosphoinositide-3,4,5-trisphosphate through proteins containing pleckstrin and Sec7 homology domains.

Signal transmission by many cell surface receptors results in the activation of phosphoinositide (PI) 3-kinases that phosphorylate the 3' position of polyphosphoinositides. From a screen for mouse proteins that bind phosphoinositides, the protein GRP1was identified. GRP1 binds phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4, 5)P3] through a pleckstrin homology (PH) domain and displays a region of high sequence similarity to the yeast Sec7 protein. The PH domain of the closely related protein cytohesin-1, which, through its Sec7 homology domain, regulates integrin beta2 and catalyzes guanine nucleotide exchange of the small guanine nucleotide-binding protein ARF1, was also found to specifically bind PtdIns(3,4,5)P3. GRP1 and cytohesin-1 appear to connect receptor-activated PI 3-kinase signaling pathways with proteins that mediate biological responses such as cell adhesion and membrane trafficking.

Mouse p170 is a novel phosphatidylinositol 3-kinase containing a C2 domain.

Phosphatidylinositol (PI) 3-kinases catalyze the formation of 3'-phosphoinositides, which appear to promote cellular responses to growth factors and such membrane trafficking events as insulin-stimulated translocation of intracellular glucose transporters. We report here the cloning of a novel PI 3-kinase, p170, from cDNA of insulin-sensitive mouse 3T3-L1 adipocytes. Mouse p170 utilizes PI and to a limited extent PI 4-P as substrates, in contrast to the PI-specific yeast VPS34 homolog PtdIns 3-kinase and the p110 PI 3-kinases, which phosphorylate PI, PI 4-P, and PI 4,5-P2. Mouse p170 is also distinct from PtdIns 3-kinase or the p110 PI 3-kinases in exhibiting a 10-fold lower sensitivity to wortmannin. Unique structural elements of p170 include C-terminal sequences strikingly similar to the phosphoinositide-binding C2 domain of protein kinase C isoforms, synaptotagmins, and other proteins. These features of mouse p170 are shared with a recently cloned Drosophila PI 3-kinase, DmPI3K_68D. Together, these proteins define a new class of PI 3-kinase likely influenced by cellular regulators distinct from those acting upon p110- or VPS34-like PI 3-kinases.

Four structurally distinct, non-DNA-binding subunits of human nuclear respiratory factor 2 share a conserved transcriptional activation domain.

Nuclear respiratory factor 2 (NRF-2) was previously purified to near homogeneity from HeLa cells on the basis of its ability to bind tandem recognition sites in the rat cytochrome oxidase subunit IV (RCO4) promoter. It consisted of five subunits, alpha, beta 1, beta 2, gamma 1, and gamma 2. Sequencing of tryptic peptides from alpha and from mixtures of the two beta or two gamma subunits revealed sequence identities with subunits of the mouse GA-binding protein (GABP), a ubiquitously expressed ETS domain activator composed of three subunits, alpha, beta 1, and beta 2. To understand the precise relationship between NRF-2 and GABP, cDNAs for all five NRF-2 subunits have now been cloned and their products have been overexpressed. The results establish that the two additional NRF-2 subunits are molecular variants that differ from GABP beta 1 and beta 2 by having a 12-amino-acid insertion containing two serine doublets. PCR and RNase protection assays show that mRNAs for these variants are expressed in the human but not the rodent cells and tissues examined. The insertion did not alter the ability of the beta and gamma subunits to associate with alpha, the DNA-binding subunit, nor did it affect the ability of NRF-2 beta 1 or beta 2 to direct high-affinity binding of alpha to tandem sites in the RCO4 promoter. In addition, the four NRF-2 beta and gamma subunits were equally proficient in activating transcription in transfected cells when fused to a GAL4 DNA-binding domain. The domain responsible for this transcriptional activation was localized by deletion mapping to a region of approximately 70 amino acids that is conserved in all four NRF-2 beta and gamma subunits. The repeated glutamine-containing hydrophobic clusters within this region bear a strong resemblance to those recently implicated in protein-protein interactions within the transcriptional apparatus.

Activation of the human mitochondrial transcription factor A gene by nuclear respiratory factors: a potential regulatory link between nuclear and mitochondrial gene expression in organelle biogenesis.

Mitochondrial transcription factor A (mtTFA), the product of a nuclear gene, stimulates transcription from the two divergent mitochondrial promoters and is likely the principal activator of mitochondrial gene expression in vertebrates. Here we establish that the proximal promoter of the human mtTFA gene is highly dependent upon recognition sites for the nuclear respiratory factors, NRF-1 and NRF-2, for activity. These factors have been previously implicated in the activation of numerous nuclear genes that contribute to mitochondrial respiratory function. The affinity-purified factors from HeLa cells specifically bind to the mtTFA NRF-1 and NRF-2 sites through guanine nucleotide contacts that are characteristic for each site. Mutations in these contacts eliminate NRF-1 and NRF-2 binding and also dramatically reduce promoter activity in transfected cells. Although both factors contribute, NRF-1 binding appears to be the major determinant of promoter function. This dependence on NRF-1 activation is confirmed by in vitro transcription using highly purified recombinant proteins that display the same binding specificities as the HeLa cell factors. The activation of the mtTFA promoter by both NRF-1 and NRF-2 therefore provides a link between the expression of nuclear and mitochondrial genes and suggests a mechanism for their coordinate regulation during organelle biogenesis.

NRF-1, an activator involved in nuclear-mitochondrial interactions, utilizes a new DNA-binding domain conserved in a family of developmental regulators.

Nuclear respiratory factor 1 (NRF-1) was first discovered as an activator of the cytochrome c gene and was subsequently found to play a broader role in nuclear-mitochondrial interactions. We have now cloned a HeLa cDNA encoding NRF-1 using degenerate oligomers derived from tryptic peptide sequences for PCR amplification. The cDNA-encoded protein was indistinguishable from the authentic HeLa cell factor on denaturing gels, displayed the expected NRF-1 DNA-binding specificity, and made the same guanine nucleotide contacts as HeLa NRF-1 on binding known NRF-1 recognition sites. Antiserum raised against the highly purified recombinant protein recognized the identical DNA-protein complex formed using either a crude nuclear fraction or nearly homogeneous HeLa NRF-1. Recombinant NRF-1 also activated transcription through specific sites from several NRF-1-responsive promoters, confirming both the transcriptional activity and specificity of the cDNA product. Portions of NRF-1 are closely related to sea urchin P3A2 and the erect wing (EWG) protein of Drosophila. Both are recently identified developmental regulatory factors. The region of highest sequence identity with P3A2 and EWG was in the amino-terminal half of the molecule, which was found by deletion mapping to contain the DNA-binding domain, whereas the carboxy-terminal half of NRF-1 was highly divergent from both proteins. The DNA-binding domain in these molecules is unrelated to motifs found commonly in DNA-binding proteins; thus, NRF-1, P3A2, and EWG represent the founding members of a new class of highly conserved sequence-specific regulatory factors.

Identity of GABP with NRF-2, a multisubunit activator of cytochrome oxidase expression, reveals a cellular role for an ETS domain activator of viral promoters.

The ETS domain proteins are a diverse family of transcriptional activators that have been implicated recently in the expression of a number of cell-specific and viral promoters. Nuclear respiratory factor 2 (NRF-2) is a nuclear transcription factor that activates the proximal promoter of the rat cytochrome c oxidase subunit IV (RCO4) gene through tandem sequence elements. These elements conform to the consensus for high-affinity ETS domain recognition sites. We have now purified NRF-2 to homogeneity from HeLa cells and find that it consists of five polypeptides, only one of which has intrinsic DNA-binding ability. The others participate in the formation of heteromeric complexes with distinct binding properties. NRF-2 also specifically recognizes multiple binding sites in the mouse cytochrome c oxidase subunit Vb (MCO5b) gene. As in the functionally related RCO4 gene, tandemly arranged NRF-2 sites are essential for the activity of the proximal MCO5b promoter, further substantiating a role for NRF-2 in respiratory chain expression. Determination of peptide sequences from the various subunits of HeLa NRF-2 reveals a high degree of sequence identity with mouse GA-binding protein (GABP), a multisubunit ETS domain activator of herpes simplex virus immediate early genes. A cellular role in the activation of nuclear genes specifying mitochondrial respiratory function is thus assigned to an ETS domain activator of viral promoters.

Transcriptional activation through ETS domain binding sites in the cytochrome c oxidase subunit IV gene.

A mutational analysis of the rat cytochrome c oxidase subunit IV (RCO4) promoter region revealed the presence of a major control element consisting of a tandemly repeated pair of binding sites for a nuclear factor from HeLa cells. This factor was designated NRF-2 (nuclear respiratory factor 2) because a functional recognition site was also found in the human ATP synthase beta-subunit gene. Deletion or site-directed point mutations of the NRF-2 binding sites in the RCO4 promoter resulted in substantial loss of transcriptional activity, and synthetic oligomers of the NRF-2 binding sites from both genes stimulated a heterologous promoter when cloned in cis. NRF-2 binding and transcriptional activation required a purine-rich core sequence, GGAA. This motif is characteristic of the recognition site for a family of activators referred to as ETS domain proteins because of the similarity within their DNA-binding domains to the ets-1 proto-oncogene product. NRF-2 recognized an authentic Ets-1 site within the Moloney murine sarcoma virus long terminal repeat, and this site was able to compete for NRF-2 binding to the RCO4 promoter sequence. In addition, a single polypeptide of 55 kDa was detected following cross-linking of a partially purified NRF-2 fraction to RCO4, the human ATP synthase beta subunit, or Moloney murine sarcoma virus binding sites. However, in contrast to Ets-1, which appears to be exclusive to lymphoid tissues, NRF-2 has the broad tissue distribution expected of a regulator of respiratory chain expression.

The rat cytochrome c oxidase subunit IV gene family: tissue-specific and hormonal differences in subunit IV and cytochrome c mRNA expression.

We have isolated three members of the rat cytochrome c oxidase subunit IV gene family: one functional gene and two processed pseudogenes. The pseudogenes appear to represent the only other closely related sequences in this family. The functional gene encodes an isoform which is expressed in all tissues examined and has features characteristic of 'housekeeping' genes. These include multiple transcription start sites mapped to within an approximately 50 bp region and a GC-rich promoter lacking typical CCAAT or TATAA sequences. Although the subunit IV gene is expressed at its highest levels in cardiac and skeletal muscle, consistent with the high energy demand in those tissues, its expression differs from that of cytochrome c in several respects. 1) Subunit IV mRNA abundance in various tissues is relatively uniform when compared to the highly variable levels of cytochrome c mRNAs. 2) Unlike cytochrome c, subunit IV mRNA is expressed at a surprisingly high level in testis. 3) While cytochrome c mRNA levels in liver are increased markedly in response to thyroid hormone treatment, subunit IV mRNA is not significantly affected. Differences in the expression of these two nuclear-encoded respiratory genes are consistent with differences in regulatory elements within their promoters. Therefore, the regulation of nuclear-encoded respiratory genes in response to tissue demands for cellular energy may not be satisfactorily explained by a set of universal regulators common to all such genes.

Structure and expression of rodent genes encoding the testis-specific cytochrome c. Differences in gene structure and evolution between somatic and testicular variants.

Mammalian testis contains two forms of cytochrome c, one identical to the form found in somatic tissues and a second that is expressed in a stage-specific manner during spermatogenic differentiation. We have isolated both rat and mouse cDNA clones and the rat gene encoding the testis-specific cytochrome c and determined their DNA sequences. The testicular variant displays a number of notable differences with its somatic counterpart. 1) In contrast to the multipseudogene family derived from mammalian somatic cytochrome c genes, the testis gene is single-copy in genomic DNA with no detectable pseudogenes. 2) The rat testis gene is approximately 7 kilobases (kb) long with three introns totaling nearly 6.5 kb whereas the two introns dividing the 2.1-kb somatic gene occupy only 0.9 kb. Introns differ in position as well as size. 3) The testicular variant has a longer 5'-untranslated leader (230 versus 70 base pairs for the somatic gene) with an upstream open reading frame of 129 base pairs beginning with an AUG in a favorable translational context. 4) A single polyadenylation site in the testicular mRNA (approximately 900 nucleotides) contrasts with the three functionally equivalent sites observed in rat somatic messages. 5) Finally, rat and mouse testis cytochromes c differ at 4 amino acid residues as opposed to the complete sequence identity found in the somatic proteins suggesting a shorter unit evolutionary period for these molecules. These observations are consistent with a duplication of an ancestral cytochrome c gene leading to the emergence of novel structural features and regulatory properties likely associated with the striking tissue specificity of the testicular cytochrome c.