An acidic fibroblast growth factor-like factor secreted into the brain cell culture medium upregulates apoE synthesis, HDL secretion and cholesterol metabolism in rat astrocytes

Lipoprotein Particles in Cerebrospinal Fluid

The brain is the most lipid-rich organ in the body, and the intricate interplay between lipid metabolism and pathologies associated with neurodegenerative disorders is being increasingly recognized. The brain is bathed in cerebrospinal fluid (CSF), which, like plasma, contains lipid-protein complexes called lipoproteins that are responsible for extracellular lipid transport. Multiple CSF lipoprotein populations exist, some of which are produced de novo in the central nervous system and others that appear to be generated from protein constituents that are produced in the periphery. These CSF lipoproteins are thought to play key roles in maintaining lipid homeostasis in the central nervous system, while little else is known due to their limited accessibility and their low abundance in CSF. Recent work has provided new insights into the compositional complexity of CSF lipoprotein families and their metabolism in cerebral circulation. The purpose of this review is to summarize our current state of knowledge on the composition, origin, and metabolism of CSF lipoproteins.

Impact of Methyl-β-Cyclodextrin and Apolipoprotein A-I on The Expression of ATP-Binding Cassette Transporter A1 and Cholesterol Depletion in C57BL/6 Mice Astrocytes

Objective

Dysregulation of cholesterol metabolism in the brain is responsible for many lipid storage disorders, including Niemann-Pick disease type C (NPC). Here, we have investigated whether cyclodextrin (CD) and apolipoprotein A-I (apoA-I) induce the same signal to inhibit cell cholesterol accumulation by focusing on the main proteins involved in cholesterol homeostasis in response to CD and apoA-I treatment.

Materials and Methods

In this experimental study, astrocytes were treated with apoA-I or CD and then lysed in RIPA buffer. We used Western blot to detect protein levels of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR) and ATP-binding cassette transporter A1 (ABCA1). Cell cholesterol content and cholesterol release in the medium were also measured.

Results

ApoA-I induced a significant increase in ABCA1 and a mild increase in HMGCR protein level, whereas CD caused a significant increase in HMGCR with a significant decrease in ABCA1. Both apoA-I and CD increased cholesterol release in the medium. A mild, but not significant increase, in cell cholesterol content was seen by apoA-I; however, a significant increase in cell cholesterol was detected when the astrocytes were treated with CD.

Conclusion

CD, like apoA-I, depletes cellular cholesterol. This depletion occurs in a different way from apoA-I that is through cholesterol efflux. Depletion of cell cholesterol with CDs led to reduced protein levels of ABCA1 along with increased HMGCR and accumulation of cell cholesterol. This suggested that CDs, unlike apoA-I, could impair the balance between cholesterol synthesis and release, and interfere with cellular function that depends on ABCA1.

Subcellular Changes in Bridging Integrator 1 Protein Expression in the Cerebral Cortex During the Progression of Alzheimer Disease Pathology

Genome-wide association studies have established BIN1 (Bridging Integrator 1) as the most significant late-onset Alzheimer disease (AD) susceptibility locus after APOE We analyzed BIN1 protein expression using automated immunohistochemistry on the hippocampal CA1 region in 19 patients with either no, mild, or moderate-to-marked AD pathology, who had been assessed by Clinical Dementia Rating and CERAD scores. We also examined the amygdala, prefrontal, temporal, and occipital regions in a subset of these patients. In non-demented controls without AD pathology, BIN1 protein was expressed in white matter, glia, particularly oligodendrocytes, and in the neuropil in which the BIN1 signal decorated axons. With increasing severity of AD, BIN1 in the CA1 region showed: 1) sustained expression in glial cells, 2) decreased areas of neuropil expression, and 3) increased cytoplasmic neuronal expression that did not correlate with neurofibrillary tangle load. In patients with AD, both the prefrontal cortex and CA1 showed a decrease in BIN1-immunoreactive (BIN1-ir) neuropil areas and increases in numbers of BIN1-ir neurons. The numbers of CA1 BIN1-ir pyramidal neurons correlated with hippocampal CERAD neuritic plaque scores; BIN1 neuropil signal was absent in neuritic plaques. Our data provide novel insight into the relationship between BIN1 protein expression and the progression of AD-associated pathology and its diagnostic hallmarks.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

FGF-1 induces expression of LXRalpha and production of 25-hydroxycholesterol to upregulate the apoE gene in rat astrocytes

Fibroblast growth factor 1 (FGF-1) enhances apolipoprotein E (apoE) expression and apoE-HDL biogenesis in autocrine fashion in astrocytes (Ito, J., Y. Nagayasu, R. Lu, A. Kheirollah, M. Hayashi, and S. Yokoyama. Astrocytes produce and secrete FGF-1, which promotes the production of apoE-HDL in a manner of autocrine action. J. Lipid Res. 2005. 46: 679-686) associated with healing of brain injury (Tada,T., J-i. Ito, M. Asai, and S. Yokoyama. Fibroblast growth factor 1 is produced prior to apolipoprotein E in the astrocytes after cryo-injury of mouse brain. Neurochem. Int. 2004. 45: 23-30). FGF-1 stimulates mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) to increase cholesterol biosynthesis and phosphatidylinositol 3-OH kinase (PI3K)/Akt to enhance apoE-HDL secretion (Ito, J., Y. Nagayasu, K. Okumura-Noji, R. Lu, T. Nishida, Y. Miura, K. Asai, A. Kheirollah, S. Nakaya, and S. Yokoyama. Mechanism for FGF-1 to regulate biogenesis of apoE-HDL in astrocytes. J. Lipid Res. 2007. 48: 2020-2027). We investigated the mechanism for FGF-1 to upregulate apoE transcription. FGF-1 increased apoE and liver X receptor alpha (LXRalpha) mRNAs in rat astrocytes. Increase of LXRalpha mRNA was suppressed by inhibition of the FGF-1 receptor-1 and MEK/ERK but not by inhibition of PI3K/Akt. The increases of apoE mRNA and apoE-HDL secretion were both inhibited by downregulation or inhibition of LXRalpha, while they were partially suppressed by inhibiting cholesterol biosynthesis. We identified the liver X receptor element responsible for activation of the rat apoE promoter by FGF-1 located between -450 and -320 bp, and the direct repeat 4 (DR4) element in this region (-448 to -433 bp) was responsible for the activation. Chromatin immunoprecipitation analysis supported that FGF-1 enhanced association of LXR with the rat apoE promoter. FGF-1 partially activated the apoE promoter even in the presence of an MEK inhibitor that inhibits the FGF-1-mediated enhancement of cholesterol biosynthesis. On the other hand, FGF-1 induced production of 25-hydroxycholesterol by MEK/ERK as an sterol regulatory element-dependent reaction besides cholesterol biosynthesis. We concluded that FGF-1-induced apoE expression in astrocytes depends on LXRalpha being mediated by both LXRalpha expression and an LXRalpha ligand biosynthesis.

Mechanism for FGF-1 to regulate biogenesis of apoE-HDL in astrocytes

Fibroblast growth factor-1 (FGF-1) is secreted by astrocytes and stimulates apolipoprotein E (apoE)-HDL biogenesis by an autocrine mechanism to help in recovery from brain injury. In apoE-deficient mouse astrocytes, FGF-1 stimulated cholesterol biosynthesis without enhancing its release, indicating a signaling pathway independent of apoE biosynthesis upregulation. SU5402, an inhibitor of FGF receptor, inhibited FGF-1-induced phosphorylation of MEK, ERK, and Akt, as well as all the apoE-HDL biogenesis-related events in rat astrocytes. LY294002, an inhibitor of phosphatidylinositide 3-OH kinase (PI3K) and of Akt phosphorylation, inhibited apoE-HDL secretion but not cholesterol biosynthesis, whereas U0126, an inhibitor of MEK and of ERK phosphorylation, inhibited cholesterol biosynthesis but not apoE-HDL secretion. Increase of apoE-mRNA by FGF-1 was not influenced by either inhibitor. When rat apoE/pcDNA3.his was transfected to transformed rat astrocyte GA-1 cells that otherwise do not synthesize apoE (GA-1/25), FGF-1 did not influence apoE-mRNA, but did increase the apoE secretion and Akt phosphorylation that were suppressed by LY294002. Lipid biosynthesis was increased by FGF-1 in GA-1/25 cells and suppressed by U0126. FGF-1 upregulates apoE-HDL biogenesis by three independent signaling pathways. The PI3K/Akt pathway upregulates secretion of apoE/apoE-HDL, the MEK/ERK pathway stimulates cholesterol biosynthesis, and an unknown pathway enhances apoE transcription.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

Astrocytes produce and secrete FGF-1, which promotes the production of apoE-HDL in a manner of autocrine action

The astrocytes prepared by 1 week secondary culture after 1 month primary culture of rat brain cells (M/W cells) synthesized and secreted apolipoprotein E (apoE) and cholesterol more than the astrocytes prepared by conventional 1 week primary and 1 week secondary culture (W/W cells) (Ueno, S., J. Ito, Y. Nagayasu, T. Furukawa, and S. Yokoyama. 2002. An acidic fibroblast growth factor-like factor secreted into the brain cell culture medium upregulates apoE synthesis, HDL secretion and cholesterol metabolism in rat astrocytes. Biochim. Biophys. Acta. 1589: 261-272). M/W cells also highly expressed fibroblast growth factor-1 (FGF-1) mRNA. FGF-1 was identified in the cell lysate of both cell types, but M/W cells released more of it into the medium. Immunostaining of FGF-1 and apoE revealed that both localized in the cells that produce glial fibrillary acidic protein. The conditioned media of M/W cells and FGF-1 stimulated W/W cells to release apoE and cholesterol to generate more HDL. Pretreatment with a goat anti-FGF-1 antibody or heparin depleted the stimulatory activity of M/W cell-conditioned medium. The presence of the anti-FGF-1 antibody in the medium suppressed apoE secretion by M/W cells. Differential inhibition of signaling pathways suggested that FGF-1 stimulates apoE synthesis via the phosphoinositide 3-OH kinase for PI3K/Akt pathway. Thus, astrocytes release FGF-1, which promotes apoE-HDL production by an autocrine mechanism. These results are consistent with our in vivo observation that astrocytes produce FGF-1 before the increase of apoE in the postinjury lesion of the mouse brain (Tada, T., J. Ito, M. Asai, and S. Yokoyama. 2004. Fibroblast growth factor 1 is produced prior to apolipoprotein E in the astrocytes after cryo-injury of mouse brain. Neurochem. Int. 45: 23-30).

Apolipoprotein A-I induces translocation of protein kinase Cα to a cytosolic lipid-protein particle in astrocytes

Apolipoprotein A-I (apoA-I) induces the translocation of newly synthesized cholesterol as well as caveolin-1 to the cytosolic lipid-protein particle (CLPP) fraction in astrocytes before its appearance in high density lipoprotein generated in the medium (Ito, J., Y. Nagayasu, K. Kato, R. Sato, and S. Yokoyama. 2002. Apolipoprotein A-I induces translocation of cholesterol, phospholipid, and caveolin-1 to cytosol in rat astrocytes. J. Biol. Chem. 277: 7929-7935). We here report the association of signal-related molecules with CLPP. ApoA-I induces rapid translocation of protein kinase Calpha to the CLPP fraction and its phosphorylation in astrocytes. ApoA-I also induces the translocation of phospholipase Cgamma to CLPP. Diacylglyceride (DG) production is increased by apoA-I in the cells, with a maximum at 5 min after the stimulation, and the increase takes place also in the CLPP fraction. An inhibitor of receptor-coupled phospholipase C, U73122, inhibited all the apoA-I-induced events, such as DG production, cholesterol translocation to the cytosol, release of cholesterol, and translocation of protein kinase Calpha into the CLPP fraction. CLPP may thus be involved in the apoA-I-initiated signal transduction in astrocytes that is related to intracellular cholesterol trafficking for the generation of high density lipoprotein in the brain.

Promoter polymorphism in fibroblast growth factor 1 gene increases risk of definite Alzheimer's disease

Fibroblast growth factor 1 (FGF1, also known as acidic FGF) protects selective neuronal populations against neurotoxic effects such as those in Alzheimer's disease (AD) and HIV encephalitis. The FGF1 gene is therefore a strong candidate gene for AD. Using the promoter polymorphism of the FGF1 gene, we examined the relationship between AD and the FGF1 and apolipoprotein E (APOE) genes in 100 Japanese autopsy-confirmed late-onset AD patients and 106 age-matched non-demented controls. The promoter polymorphism (-1385 A/G) was significantly associated with AD risk. The odds ratio for AD associated with the GG vs non-GG genotype was 2.02 (95% CI = 1.16-3.52), while that of s4 vs non-ł4 in APOE4 gene was 5.19 (95% CI = 2.68-10.1). The odds ratio for APOEP4 and FGF1 GG carriers was 20.5 (95% CI = 6.88-60.9). The results showed that the FGF1 gene is associated with autopsy-confirmed AD.

Fibroblast growth factor 1 is produced prior to apolipoprotein E in the astrocytes after cryo-injury of mouse brain

We recently reported that fibroblast growth factor 1 (FGF-1) upregulates apolipoprotein E (apoE) synthesis and its secretion as high density lipoprotein (HDL) in cultured astrocytes potentially by an autocrine or paracrine mechanism [Biochim. Biopys. Acta 1589 (2002) 261]. In order to examine pathophysiological relevance of this reaction, we studied association of the production of FGF-1 and apoE in the post-injury mouse brain. After the spot-injury of the brain by liquid nitrogen, the surface size of the wound shrunk more rapidly in the C57BL/6 wild-type mice than the apoE-knock out C57BL/6 mice. Immunohistochemical analysis of the lesions revealed that production of FGF-1 was identified in the reactive astrocytes by the day 2 after the injury in both types of mouse, prior to the production of apoE confirmed by the day 4 in the wild-type. These findings were consistent with our in-vitro observations and hypothesis that FGF-1 upregulates apoE synthesis and subsequently HDL production in the reactive astrocytes by an autocrine or paracrine manner. FGF-1 thus would exert its effect after the CNS damage through apoE secretion.

Apolipoprotein-mediated cellular lipid release requires replenishment of sphingomyelin in a phosphatidylcholine-specific phospholipase C-dependent manner

When sphingomyelin is digested by sphingomyelinase in the plasma membrane of rat astrocytes, productions of sphingomyelin, diacylglycerol, and phosphatidylcholine are stimulated. D609, an inhibitor of phosphatidylcholine-specific phospholipase C, suppressed these effects. Similarly, when apolipoprotein A-I removed cellular cholesterol, phosphatidylcholine, and sphingomyelin to generate high density lipoprotein, cholesterol synthesis from acetate subsequently increased, and sphingomyelin synthesis from acetate and serine also increased. D609 inhibited these effects again. D609 also inhibited the cholesterol removal by apoA-I not only from the astrocytes but also from BALB/3T3 and RAW264 cells. D609 decreased cholesterol synthesis, although D609 did not directly inhibit hydroxymethylglutaryl-CoA reductase. ApoA-I-stimulated translocation of newly synthesized cholesterol to cytosol was also decreased by D609. A diacylglycerol analog increased the apoA-I-mediated cholesterol release, whereas ceramide did not influence it. We concluded that removal of cellular sphingomyelin by apolipoproteins is replenished by transfer of phosphorylcholine from phosphatidylcholine to ceramide, and this reaction may limit the removal of cholesterol by apoA-I. This reaction also produces diacylglycerol that potentially triggers subsequent cellular signal cascades and regulates intracellular cholesterol trafficking.

Fibroblast growth factors in cancer: therapeutic possibilities

The fibroblast growth factor (FGF) family of signalling molecules and its receptors (FGFRs) contribute to normal developmental and physiological processes. However, the subversion of this powerful growth stimulatory pathway has been implicated in the generation of a variety of pathological conditions. This review focuses on the role of FGF/FGFRs in cancer. The case will be made that this signalling pathway is associated with and functionally important for the growth of some human tumours. As such, FGF/FGFRs can be viewed as rational therapeutic oncology targets and strategies used to inhibit these molecules are discussed. The therapeutic exploitation of tumour-associated FGFR expression to deliver toxins or antiproliferative signals to tumour cells is also reviewed, as is the use of FGFs as protein therapeutics to alleviate the side effects of cancer therapy.