抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

One signal that is overactivated in a wide range of tumour types is the production of a phospholipid, phosphatidylinositol (3,4,5) trisphosphate, by phosphatidylinositol 3-kinase (PI3K) This lipid and the protein kinase that is activated by it — AKT — trigger a cascade of responses, from cell growth and proliferation to survival and motility, that drive tumour progression Small-molecule therapeutics that block PI3K signalling might deal a severe blow to cancer cells by blocking many aspects of the tumour-cell phenotype

The phosphatidylinositol 3-Kinase AKT pathway in human cancer.

The programmed cell death that occurs as part of normal mammalian development was first observed nearly a century ago (Collin 1906). It has since been established that approximately half of all neurons in the neuroaxis and >99.9% of the total number of cells generated during the course of a human lifetime go on to die through a process of apoptosis (for review, see Datta and Greenberg 1998; Vaux and Korsmeyer 1999). The induction of developmental cell death is a highly regulated process and can be suppressed by a variety of extracellular stimuli. The purification in the 1950s of the nerve growth factor (NGF), which promotes the survival of sympathetic neurons, set the stage for the discovery that peptide trophic factors promote the survival of a wide variety of cell types in vitro and in vivo (Levi-Montalcini 1987). The profound biological consequences of growth factor (GF) suppression of apoptosis are exemplified by the critical role of target-derived neurotrophins in the survival of neurons and the maintenance of functional neuronal circuits. (Pettmann and Henderson 1998). Recently, the ability of trophic factors to promote survival have been attributed, at least in part, to the phosphatidylinositide 38-OH kinase (PI3K)/c-Akt kinase cascade. Several targets of the PI3K/c-Akt signaling pathway have been recently identified that may underlie the ability of this regulatory cascade to promote survival. These substrates include two components of the intrinsic cell death machinery, BAD and caspase 9, transcription factors of the forkhead family, and a kinase, IKK, that regulates the NF-kB transcription factor. This article reviews the mechanisms by which survival factors regulate the PI3K/c-Akt cascade, the evidence that activation of the PI3K/c-Akt pathway promotes cell survival, and the current spectrum of c-Akt targets and their roles in mediating c-Akt-dependent cell survival.

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Cellular survival: a play in three Akts

A specific inhibitor of phosphatidylinositol 3-kinase, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002).

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Purification and Characterization

Since their discovery, protein tyrosine phosphatases have been speculated to play a role in tumor suppression because of their ability to antagonize the growth-promoting protein tyrosine kinases. Recently, a tumor suppressor from human chromosome 10q23, called PTEN or MMAC1, has been identified that shares homology with the protein tyrosine phosphatase family. Germ-line mutations in PTEN give rise to several related neoplastic disorders, including Cowden disease. A key step in understanding the function of PTEN as a tumor suppressor is to identify its physiological substrates. Here we report that a missense mutation in PTEN, PTEN-G129E, which is observed in two Cowden disease kindreds, specifically ablates the ability of PTEN to recognize inositol phospholipids as a substrate, suggesting that loss of the lipid phosphatase activity is responsible for the etiology of the disease. Furthermore, expression of wild-type or substrate-trapping forms of PTEN in HEK293 cells altered the levels of the phospholipid products of phosphatidylinositol 3-kinase and ectopic expression of the phosphatase in PTEN-deficient tumor cell lines resulted in the inhibition of protein kinase (PK) B/Akt and regulation of cell survival.

https://www.pnas.org/content/pnas/95/23/13513.full.pdf

The lipid phosphatase activity of PTEN is critical for its tumor supressor function

The second messenger phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] is generated by the action of phosphoinositide 3-kinase (PI 3-kinase), and regulates a plethora of cellular processes. An approach for dissecting the mechanisms by which these processes are regulated is to identify proteins that interact specifically with PtdIns(3,4,5)P(3). The pleckstrin homology (PH) domain has become recognized as the specialized module used by many proteins to interact with PtdIns(3,4,5)P(3). Recent work has led to the identification of a putative phosphatidylinositol 3,4,5-trisphosphate-binding motif (PPBM) at the N-terminal regions of PH domains that interact with this lipid. We have searched expressed sequence tag databases for novel proteins containing PH domains possessing a PPBM. Surprisingly, many of the PH domains that we identified do not bind PtdIns(3,4,5)P(3), but instead possess unexpected and novel phosphoinositide-binding specificities in vitro. These include proteins possessing PH domains that interact specifically with PtdIns(3,4)P(2) [TAPP1 (tandem PH-domain-containing protein-1) and TAPP2], PtdIns4P [FAPP1 (phosphatidylinositol-four-phosphate adaptor protein-1)], PtdIns3P [PEPP1 (phosphatidylinositol-three-phosphate-binding PH-domain protein-1) and AtPH1] and PtdIns(3,5)P(2) (centaurin-beta2). We have also identified two related homologues of PEPP1, termed PEPP2 and PEPP3, that may also interact with PtdIns3P. This study lays the foundation for future work to establish the phospholipid-binding specificities of these proteins in vivo, and their physiological role(s).

Identification of pleckstrin-homology-domain-containing proteins with novel phosphoinositide-binding specificities.

3-Phosphoinositide-dependent protein kinase-1 (PDK1) interacts stereoselectively with the d-enantiomer of PtdIns(3,4,5)P3 (KD 1.6 nM) and PtdIns(3,4)P2 (KD 5.2 nM), but binds with lower affinity to PtdIns3P or PtdIns(4,5)P2. The binding of PtdIns(3,4,5)P3 to PDK1 was greatly decreased by making specific mutations in the pleckstrin homology (PH) domain of PDK1 or by deleting it. The same mutations also greatly decreased the rate at which PDK1 activated protein kinase Balpha (PKBalpha) in vitro in the presence of lipid vesicles containing PtdIns(3,4,5)P3, but did not affect the rate at which PDK1 activated a PKBalpha mutant lacking the PH domain in the absence of PtdIns(3,4,5)P3. When overexpressed in 293 or PAE cells, PDK1 was located at the plasma membrane and in the cytosol, but was excluded from the nucleus. Mutations that disrupted the interaction of PtdIns(3,4,5)P3 or PtdIns(4,5)P2 with PDK1 abolished the association of PDK1 with the plasma membrane. Growth-factor stimulation promoted the translocation of transfected PKBalpha to the plasma membrane, but had no effect on the subcellular distribution of PDK1 as judged by immunoelectron microscopy of fixed cells. This conclusion was also supported by confocal microscopy of green fluorescent protein-PDK1 in live cells. These results, together with previous observations, indicate that PtdIns(3,4,5)P3 plays several roles in the PDK1-induced activation of PKBalpha. First, it binds to the PH domain of PKB, altering its conformation so that it can be activated by PDK1. Secondly, interaction with PtdIns(3,4,5)P3 recruits PKB to the plasma membrane with which PDK1 is localized constitutively by virtue of its much stronger interaction with PtdIns(3,4,5)P3 or PtdIns(4,5)P2. Thirdly, the interaction of PDK1 with PtdIns(3,4,5)P3 facilitates the rate at which it can activate PKB.

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Role of phosphatidylinositol 3,4,5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1.

The PTEN tumour suppressor is a lipid and protein phosphatase that inhibits phosphoinositide 3-kinase (PI3K)-dependent signalling by dephosphorylating phosphatidylinositol 3,4,5-trisphosphate (PtdInsP3). Here, we discuss the concept of PTEN as an ‘interfacial enzyme’, which exists in a high activity state when bound transiently at membrane surfaces containing its substrate and other acidic lipids, such as PtdIns(4,5)P2 and phosphatidylserine (PtdSer). This mechanism ensures that PTEN functions in a spatially restricted manner, and may explain its involvement in forming the gradients of PtdInsP3, which are necessary for generating and/or sustaining cell polarity during motility, in developing neurons and in epithelial tissues. Coordinating PTEN activity with alternative mechanisms of PtdInsP3 metabolism, by the tightly regulated SHIP 5-phoshatases, synthesizing the independent second messenger PtdIns(3,4)P2, may also be important for cellular polarization in some cell types. Superimposed on this interfacial mechanism are additional post-translational regulatory processes, which generally act to reduce PTEN activity. Oxidation of the active site cysteine residue by reactive oxygen species and phosphorylation of serine/threonine residues at sites in the C-terminus of the protein inhibit PTEN. These phosphorylation sites also appear to play a role in regulating both stability and localization of PTEN, as does ubiquitination of PTEN. Because genetic studies in mice show that the level of expression of PTEN in an organism profoundly influences tumour susceptibility, factors that regulate PTEN, localization, activity and turnover should be important in understanding its biological functions as a tumour suppressor.

Understanding PTEN regulation: PIP2, polarity and protein stability.

We have tested the binding specificities of the pleckstrin homology (PH) domains of protein kinase B (PKB) and GRP1 (general receptor for phosphoinositides-1), expressed as green fluorescent protein (GFP) fusion proteins [PH(PKB)GFP and PH(GRP1)GFP respectively] in HEK 293 cells and Swiss 3T3 cells, using confocal microscopy. Stimulation of HEK 293 cells with insulin caused a small, but sustained, increase in PtdIns(3,4,5)P(3) levels, detected using a radioligand displacement assay, which was mirrored by the translocation of PH(PKB)GFP and PH(GRP1)GFP from the cytosol to the plasma membrane of live, transfected cells. Similar results were obtained using Swiss 3T3 cells stimulated with platelet-derived growth factor (PDGF) and expressing either PH(PKB)GFP or PH(GRP1)GFP. Biochemical analyses confirmed the accumulation of both PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2) in response to PDGF, but only the latter was present at increased levels in Swiss 3T3 cells 30 min after an oxidative stress (1 mM H(2)O(2)). Concomitantly, only PH(PKB)GFP, and not PH(GRP1)GFP, was localized at plasma membranes after 30 min of treatment with H(2)O(2). The fusion proteins appear accurately to report the spatial and temporal distribution of PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2) in intact cells. These results establish the lipid selectivity of these PH domains in vivo, and further emphasize the overlapping, but distinct, second messenger roles of PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2).

The pleckstrin homology domains of protein kinase B and GRP1 (general receptor for phosphoinositides-1) are sensitive and selective probes for the cellular detection of phosphatidylinositol 3,4-bisphosphate and/or phosphatidylinositol 3,4,5-trisphosphate in vivo.

C P Downes

Papers

The lipid phosphatase activity of PTEN is critical for its tumor supressor function

Identification of pleckstrin-homology-domain-containing proteins with novel phosphoinositide-binding specificities.

Role of phosphatidylinositol 3,4,5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1.

Understanding PTEN regulation: PIP2, polarity and protein stability.

The pleckstrin homology domains of protein kinase B and GRP1 (general receptor for phosphoinositides-1) are sensitive and selective probes for the cellular detection of phosphatidylinositol 3,4-bisphosphate and/or phosphatidylinositol 3,4,5-trisphosphate in vivo.