Sterols and sphingolipids are limited to eukaryotic cells, and their interaction has been proposed to favor formation of lipid microdomains. Although there is abundant biophysical evidence demonstrating their interaction in simple systems, convincing evidence is lacking to show that they function together in cells. Using lipid analysis by mass spectrometry and a genetic approach on mutants in sterol metabolism, we show that cells adjust their membrane composition in response to mutant sterol structures preferentially by changing their sphingolipid composition. Systematic combination of mutations in sterol biosynthesis with mutants in sphingolipid hydroxylation and head group turnover give a large number of synthetic and suppression phenotypes. Our unbiased approach provides compelling evidence that sterols and sphingolipids function together in cells. We were not able to correlate any cellular phenotype we measured with plasma membrane fluidity as measured using fluorescence anisotropy. This questions whether the increase in liquid order phases that can be induced by sterol-sphingolipid interactions plays an important role in cells. Our data revealing that cells have a mechanism to sense the quality of their membrane sterol composition has led us to suggest that proteins might recognize sterol-sphingolipid complexes and to hypothesize the coevolution of sterols and sphingolipids.

Natamycin is a polyene antibiotic that is commonly used as an antifungal agent because of its broad spectrum of activity and the lack of development of resistance. Other polyene antibiotics, like nystatin and filipin are known to interact with sterols, with some specificity for ergosterol thereby causing leakage of essential components and cell death. The mode of action of natamycin is unknown and is investigated in this study using different in vitro and in vivo approaches. Isothermal titration calorimetry and direct binding studies revealed that natamycin binds specifically to ergosterol present in model membranes. Yeast sterol biosynthetic mutants revealed the importance of the double bonds in the B-ring of ergosterol for the natamycin-ergosterol interaction and the consecutive block of fungal growth. Surprisingly, in strong contrast to nystatin and filipin, natamycin did not change the permeability of the yeast plasma membrane under conditions that growth was blocked. Also, in ergosterol containing model membranes, natamycin did not cause a change in bilayer permeability. This demonstrates that natamycin acts via a novel mode of action and blocks fungal growth by binding specifically to ergosterol.Fungal infections have recently become a growing threat to human health, especially in persons whose immune systems are compromised (for example, by human immunodeficiency virus and cancer chemotherapy). Only a few effective antifungal agents are currently in use; these include the polyenes, the fluorocytes, and the azole derivatives. One important problem is the increase of drug resistance, particularly against azole antimyotics and fluorocytosine (1). Resistance against polyene antibiotics is still a rare event, which makes these antibiotics particularly interesting as antifungal agents. The polyene antibiotics have a ring structure in which a conjugated double bond system is located opposite to a number of hydroxyl functions. Often a mycosamine group is present in combination with a carboxyl moiety, rendering the molecule amphoteric (Fig. 1). In the past convincing evidence has been presented that several members of this class of antibiotics target sterols and in particular ergosterol, the abundant and main sterol of fungal membranes (2, 3). Different types of polyene antibiotics were shown to have different modes of action despite that they share a common target. The larger polyenes like amphotericin B and nystatin form pores together with ergosterol in the plasma membrane that collapse vital ion gradients, thereby killing the cells. The smaller uncharged filipin also destroys the membrane barrier, but by a completely different mechanism. Filipin forms large complexes with sterols between the leaflets of the lipid bilayer, resulting in loss of the barrier function (2). Natamycin (also called pimaricin) is a very effective member of the polyene antibiotic family with a large standing record of applications. It is produced by Streptomyces natalensis and used against fungal infections, but it is also widely utiliz...

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The proline-, glutamic acid-, serine- and threonine-rich (PEST) family of protein tyrosine phosphatases (PTPs) includes proline-enriched phosphatase (PEP)/lymphoid tyrosine phosphatase (LYP), PTP-PEST, and PTP-hematopoietic stem cell fraction (HSCF). PEP/LYP is a potent inhibitor of T-cell activation, principally by suppressing the activity of Src family protein tyrosine kinases (PTKs). This function seems to be dependent, at least in part, on the ability of PEP to bind C-terminal Src kinase (Csk), a PTK also involved in inactivating Src kinases. Interestingly, a polymorphism of LYP in humans (R620W) is a significant risk factor for autoimmune diseases including type 1 diabetes, rheumatoid arthritis, and lupus. The R620W mutation may be a 'gain-of-function' mutation. In non-hematopoietic cells, PTP-PEST is a critical regulator of adhesion and migration. This effect correlates with the aptitude of PTP-PEST to dephosphorylate cytoskeletal proteins such as Cas, focal adhesion associated-kinase (FAK), Pyk2, and PSTPIP. While not established, a similar function may also exist in immune cells. Additionally, overexpression studies provided an indication that PTP-PEST may be a negative regulator of lymphocyte activation. Interestingly, mutations in a PTP-PEST- and PTP-HSCF-interacting protein, PSTPIP1, were identified in humans with pyogenic sterile arthritis, pyoderma gangrenosum, and acne (PAPA) syndrome and familial recurrent arthritis, two autoinflammatory diseases. These mutations abrogate the ability of PSTPIP1 to bind PTP-PEST and PTP-HSCF, suggesting that these two PTPs may be negative regulators of inflammation.

Background: PTP-PEST is a phosphatase essential for embryonic viability. Results: PTP-PEST is critical for adhesion and migration of endothelial cells. Its absence in endothelial cells results in mouse embryonic lethality. Conclusion:The embryonic viability seen in constitutive PTP-PEST-deficient mice is due to a defect in endothelial cell functions. Significance: PTP-PEST is a key regulator of endothelial cell functions in vitro and in vivo.

This article is available online at http://www.jlr.org role in the formation of membrane microdomains ( 1 ). Until now, only very limited and expensive sources of labeled cholesterol were available. Cholesterol enriched at carbons 3 and 4 or 23-27 are commercially available, but other positions require de novo biosynthesis. Small amounts of low-enrichment cholesterol have been obtained in vivo by skin injection of enriched mevalonate in rat ( 2 ) or feeding mammals and humans with 13 C enriched precursors ( 3, 4 ). An in vitro alternative based on human hepatoma Hep G2 cells cultures ( 5 ) produces small amounts of cholesterol with higher enrichment levels, but if these methods can produce samples for MS analysis and 14 C labeling, they are not effi cient enough for NMR applications. We have previously engineered a Saccharomyces cerevisiae strain by deleting the ERG5 and ERG6 genes and introducing plasmids that express the DHCR7 and DHCR24 genes from Danio rerio , leading to the synthesis of cholesterol ( 6 ). Subsequently, we integrated cassettes expressing DHCR7 and DHCR24 into the ERG5 and ERG6 loci, creating deletions of the latter genes, to create a more stable strain that effi ciently produces cholesterol as its major (<95%) sterol ( 7 ). The metabolically engineered yeast use the same biosynthesis pathway as animals ( Fig. 1 ) and effi ciently produce 13 C-enriched cholesterol with different labeling pat terns with a yield of ‫ف‬ 1 mg of cholesterol per gram of glucose in 100 ml of culture medium. EXPERIMENTAL PROCEDURES MaterialsThe Saccharomyces cervisiae strain used was RH6829 (MATa ura3 leu2 his3 trp1 bar1 erg5 ⌬ ::HIS5-GPD-DHCR24 erg6 ⌬ ::TRP1-GPD-DHCR7 ( 7 ). Yeast nitrogen base and yeast extracts were obtained from US Biological and Difco, respectively. D-glucose (99%), leucine (95%), and uracil (99%) were obtained from Aristar, Fluka, and Sigma, respectively. The pyrogallol and petroleum ether Sterols are important lipids in most eukaryotes. In particular, cholesterol has attracted a lot of attention because of its involvement in cardiovascular diseases in humans and because it has been suggested to play an important

dMacrophages can undergo cell-cell fusion, leading to the formation of multinucleated giant cells and osteoclasts. This process is believed to promote the proteolytic activity of macrophages toward pathogens, foreign bodies, and extracellular matrices. Here, we examined the role of PTP-PEST (PTPN12), a cytoplasmic protein tyrosine phosphatase, in macrophage fusion. Using a macrophage-targeted PTP-PEST-deficient mouse, we determined that PTP-PEST was not needed for macrophage differentiation or cytokine production. However, it was necessary for interleukin-4-induced macrophage fusion into multinucleated giant cells in vitro. It was also needed for macrophage fusion following implantation of a foreign body in vivo. Moreover, in the RAW264.7 macrophage cell line, PTP-PEST was required for receptor activator of nuclear factor kappa-B ligand (RANKL)-triggered macrophage fusion into osteoclasts. PTP-PEST had no impact on expression of fusion mediators such as ␤-integrins, E-cadherin, and CD47, which enable macrophages to become fusion competent. However, it was needed for polarization of macrophages, migration induced by the chemokine CC chemokine ligand 2 (CCL2), and integrin-induced spreading, three key events in the fusion process. PTP-PEST deficiency resulted in specific hyperphosphorylation of the protein tyrosine kinase Pyk2 and the adaptor paxillin. Moreover, a fusion defect was induced upon treatment of normal macrophages with a Pyk2 inhibitor. Together, these data argue that macrophage fusion is critically dependent on PTP-PEST. This function is seemingly due to the ability of PTP-PEST to control phosphorylation of Pyk2 and paxillin, thereby regulating cell polarization, migration, and spreading.

e PTPN12 is a cytoplasmic protein tyrosine phosphatase (PTP) reported to be a tumor suppressor in breast cancer, through its capacity to dephosphorylate oncogenic receptor protein tyrosine kinases (PTKs), such as ErbB2. However, the precise molecular and cellular impact of PTPN12 deficiency in breast cancer progression remains to be fully clarified. Here, we addressed this issue by examining the effect of PTPN12 deficiency on breast cancer progression in vivo, in a mouse model of ErbB2-dependent breast cancer using a conditional PTPN12-deficient mouse. Our studies showed that lack of PTPN12 in breast epithelial cells accelerated breast cancer development and lung metastases in vivo. PTPN12-deficient breast cancer cells displayed enhanced tyrosine phosphorylation of the adaptor Cas, the adaptor paxillin, and the kinase Pyk2. They exhibited no detectable increase in ErbB2 tyrosine phosphorylation. PTPN12-deficient cells were more resistant to anoikis and had augmented migratory and invasive properties. Enhanced migration was corrected by inhibiting Pyk2. PTPN12-deficient breast cancer cells also acquired partial features of epithelial-to-mesenchymal transition (EMT), a feature of more aggressive forms of breast cancer. Hence, loss of PTPN12 promoted tumor progression in a mouse model of breast cancer, supporting the notion that PTPN12 is a tumor suppressor in human breast cancer. This function was related to the ability of PTPN12 to suppress cell survival, migration, invasiveness, and EMT and to inhibit tyrosine phosphorylation of Cas, Pyk2, and paxillin. These findings enhance our understanding of the role and mechanism of action of PTPN12 in the control of breast cancer progression. P rotein tyrosine phosphatases (PTPs) play a key role in normal cellular processes, such as proliferation, migration, adhesion, differentiation, and immune cell activation (1-3). Depending on the phosphatase and the cell type, they also have the ability either to suppress or to promote malignant transformation (3). PTPN12, also referred to as PTP-PEST (PTP-proline, glutamic acid, serine, and threonine rich), is a cytoplasmic PTP expressed in a wide spectrum of cell types (4). Studies of PTPN12-deficient mice showed that PTPN12 is a critical positive regulator of migration and adhesion in embryonic fibroblasts, endothelial cells, T cells, macrophages, and dendritic cells (5-11). This function relates to the capacity of PTPN12 to dephosphorylate cytoskeletonassociated substrates such as protein tyrosine kinases (PTKs) Pyk2 and FAK or the adaptors Cas, paxillin, and PSTPIP-1. These substrates are components of the cellular machinery controlling migration and adhesion. PTPN12 can also regulate other substrates, including receptor PTKs, such as ErbB2 and the adaptor . Proteomic data suggested that the ability of PTPN12 to regulate Shc might be critical for conversion of Shc-dependent proliferative signals into migratory signals (15). Several PTPN12 substrates, and in particular, Cas, paxillin, and Shc, also directly associate with PTPN12....