Role of protein kinase C in the translational regulation of lipoprotein lipase in adipocytes

Increasing adipocyte lipoprotein lipase improves glucose metabolism in high fat diet-induced obesity

Lipid accumulation in liver and skeletal muscle contributes to co-morbidities associated with diabetes and obesity. We made a transgenic mouse in which the adiponectin (Adipoq) promoter drives expression of lipoprotein lipase (LPL) in adipocytes to potentially increase adipose tissue lipid storage. These mice (Adipoq-LPL) have improved glucose and insulin tolerance as well as increased energy expenditure when challenged with a high fat diet (HFD). To identify the mechanism(s) involved, we determined whether the Adipoq-LPL mice diverted dietary lipid to adipose tissue to reduce peripheral lipotoxicity, but we found no evidence for this. Instead, characterization of the adipose tissue of the male mice after HFD challenge revealed that the mRNA levels of peroxisome proliferator-activated receptor-γ (PPARγ) and a number of PPARγ-regulated genes were higher in the epididymal fat pads of Adipoq-LPL mice than control mice. This included adiponectin, whose mRNA levels were increased, leading to increased adiponectin serum levels in the Adipoq-LPL mice. In many respects, the adipose phenotype of these animals resembles thiazolidinedione treatment except for one important difference, the Adipoq-LPL mice did not gain more fat mass on HFD than control mice and did not have increased expression of genes in adipose such as glycerol kinase, which are induced by high affinity PPAR agonists. Rather, there was selective induction of PPARγ-regulated genes such as adiponectin in the adipose of the Adipoq-LPL mice, suggesting that increasing adipose tissue LPL improves glucose metabolism in diet-induced obesity by improving the adipose tissue phenotype. Adipoq-LPL mice also have increased energy expenditure.

PKC signaling is involved in the regulation of progranulin (acrogranin/PC-cell-derived growth factor/granulin-epithelin precursor) protein expression in human ovarian cancer cell lines

OBJECTIVE

Overexpression of progranulin (also named acrogranin, PC-cell-derived growth factor, or granulin-epithelin precursor) is associated with ovarian cancer, specifically with cell proliferation, malignancy, chemoresistance, and shortened overall survival. The objective of the current study is to identify the signaling pathways involved in the regulation of progranulin expression in ovarian cancer cell lines.

METHODS

We studied the relation of protein kinase C (PKC), phosphatidylinositol 3-kinase, protein kinase A, P38, extracellular signal-regulated kinase, and Akt pathways on the modulation of progranulin expression levels in NIH-OVCAR-3 and SK-OV-3 ovarian cancer cell lines. The different pathways were examined using pharmacological inhibitors (calphostin C, LY294002, H89, SB203580, PD98059, and Akt Inhibitor), and mRNA and protein progranulin expression were analyzed by reverse transcriptase polymerase chain reaction and Western blot techniques, respectively.

RESULTS

Inhibition of PKC signal transduction pathway by calphostin C decreased in a dose-dependent manner protein but not mRNA levels of progranulin in both ovarian cancer cell lines. LY294002 but not wortmannin, which are phosphatidylinositol 3-kinase inhibitors, also diminished the expression of progranulin in both cell lines. In addition, LY294002 treatment produced a significant reduction in cell viability. Inhibition of protein kinase A, P38, extracellular signal-regulated kinase, and Akt did not affect progranulin protein expression.

CONCLUSIONS

These results suggest that the PKC signaling is involved in the regulation of progranulin protein expression in 2 different ovarian cancer cell lines. Inhibiting these intracellular signal transduction pathways may provide a future therapeutic target for hindering the cellular proliferation and invasion in ovarian cancer produced by progranulin.

Lipoprotein lipase: from gene to obesity

Lipoprotein lipase (LPL) is a multifunctional enzyme produced by many tissues, including adipose tissue, cardiac and skeletal muscle, islets, and macrophages. LPL is the rate-limiting enzyme for the hydrolysis of the triglyceride (TG) core of circulating TG-rich lipoproteins, chylomicrons, and very low-density lipoproteins (VLDL). LPL-catalyzed reaction products, fatty acids, and monoacylglycerol are in part taken up by the tissues locally and processed differentially; e.g., they are stored as neutral lipids in adipose tissue, oxidized, or stored in skeletal and cardiac muscle or as cholesteryl ester and TG in macrophages. LPL is regulated at transcriptional, posttranscriptional, and posttranslational levels in a tissue-specific manner. Nutrient states and hormonal levels all have divergent effects on the regulation of LPL, and a variety of proteins that interact with LPL to regulate its tissue-specific activity have also been identified. To examine this divergent regulation further, transgenic and knockout murine models of tissue-specific LPL expression have been developed. Mice with overexpression of LPL in skeletal muscle accumulate TG in muscle, develop insulin resistance, are protected from excessive weight gain, and increase their metabolic rate in the cold. Mice with LPL deletion in skeletal muscle have reduced TG accumulation and increased insulin action on glucose transport in muscle. Ultimately, this leads to increased lipid partitioning to other tissues, insulin resistance, and obesity. Mice with LPL deletion in the heart develop hypertriglyceridemia and cardiac dysfunction. The fact that the heart depends increasingly on glucose implies that free fatty acids are not a sufficient fuel for optimal cardiac function. Overall, LPL is a fascinating enzyme that contributes in a pronounced way to normal lipoprotein metabolism, tissue-specific substrate delivery and utilization, and the many aspects of obesity and other metabolic disorders that relate to energy balance, insulin action, and body weight regulation.

Translational regulation of lipoprotein lipase in adipocytes: depletion of cellular protein kinase Calpha activates binding of the C subunit of protein kinase A to the 3'-untranslated region of the lipoprotein lipase mRNA

Adipose LPL (lipoprotein lipase) plays an important role in regulating plasma triacylglycerols and lipid metabolism. We have previously demonstrated that PKCalpha (protein kinase Calpha) depletion inhibits LPL translation in 3T3-F442A adipocytes. Using in vitro translation experiments, the minimum essential region on the 3'UTR (3'-untranslated region) of LPL mRNA required for the inhibition of translation was identified as the proximal 39 nt. These results were confirmed by RNase protection analysis using cytoplasmic proteins isolated from the adipocytes treated with PKCalpha antisense oligomers and the LPL 3'UTR transcript (LPL 3'UTR nt: 1512-1640). The protein components involved in this RNA-binding interaction from PKCalpha depletion were passed through an affinity column containing a sequence of the LPL 3'UTR and, after Western blotting, the RNA-binding proteins were identified as the catalytic and the regulatory subunits of PKA (protein kinase A), Calpha and RIIbeta, and AKAP (A-kinase-anchoring protein) 121. This RNA inhibitory complex consisted of the same RNA-binding proteins that have been identified previously as mediators of LPL translational inhibition by PKA activation, suggesting that PKCalpha depletion inhibits LPL translation through PKA activation. In additional experiments, PKC depletion by prolonged PMA treatment or PKCalpha antisense oligomers resulted in an increase in PKA activity in 3T3-F442A adipocytes, comparable with PKA activation with adrenaline (epinephrine) treatment. These results demonstrate that LPL translational inhibition occurs through an RNA-binding complex involving PKA subunits and AKAP121, and this complex can be activated either through traditional PKA activation methods or through the depletion of PKCalpha.

Protein kinase D is a key regulator of cardiomyocyte lipoprotein lipase secretion after diabetes

The diabetic heart switches to exclusively using fatty acid (FA) for energy supply and does so by multiple mechanisms including hydrolysis of lipoproteins by lipoprotein lipase (LPL) positioned at the vascular lumen. We determined the mechanism that leads to an increase in LPL after diabetes. Diazoxide (DZ), an agent that decreases insulin secretion and causes hyperglycemia, induced a substantial increase in LPL activity at the vascular lumen. This increase in LPL paralleled a robust phosphorylation of Hsp25, decreasing its association with PKCdelta, allowing this protein kinase to phosphorylate and activate protein kinase D (PKD), an important kinase that regulates fission of vesicles from the golgi membrane. Rottlerin, a PKCdelta inhibitor, prevented PKD phosphorylation and the subsequent increase in LPL. Incubating control myocytes with high glucose and palmitic acid (Glu+PA) also increased the phosphorylation of Hsp25, PKCdelta, and PKD in a pattern similar to that seen with diabetes, in addition to augmenting LPL activity. In myocytes in which PKD was silenced or a mutant form of PKCdelta was expressed, high Glu+PA were incapable of increasing LPL. Moreover, silencing of cardiomyocyte Hsp25 allowed phorbol 12-myristate 13-acetate to elicit a significant phosphorylation of PKCdelta, an appreciable association between PKCdelta and PKD, and a vigorous activation of PKD. As these cells also demonstrated an additional increase in LPL, our data imply that after diabetes, PKD control of LPL requires dissociation of Hsp25 from PKCdelta, association between PKCdelta and PKD, and vesicle fission. Results from this study could help in restricting cardiac LPL translocation, leading to strategies that overcome contractile dysfunction after diabetes.

The lipogenic enzymes DGAT1, FAS, and LPL in adipose tissue: effects of obesity, insulin resistance, and TZD treatment

Acyl-coenzyme A:diacylglycerol transferase (DGAT), fatty acid synthetase (FAS), and LPL are three enzymes important in adipose tissue triglyceride accumulation. To study the relationship of DGAT1, FAS, and LPL with insulin, we examined adipose mRNA expression of these genes in subjects with a wide range of insulin sensitivity (SI). DGAT1 and FAS (but not LPL) expression were strongly correlated with SI. In addition, the expression of DGAT1 and FAS (but not LPL) were higher in normal glucose-tolerant subjects compared with subjects with impaired glucose tolerance (IGT) (P < 0.005). To study the effects of insulin sensitizers, subjects with IGT were treated with pioglitazone or metformin for 10 weeks, and lipogenic enzymes were measured in adipose tissue. After pioglitazone treatment, DGAT1 expression was increased by 33 +/- 10% (P < 0.05) and FAS expression increased by 63 +/- 8% (P < 0.05); however, LPL expression was not altered. DGAT1, FAS, and LPL mRNA expression were not significantly changed after metformin treatment. The treatment of mice with rosiglitazone also resulted in an increase in adipose expression of DGAT1 by 2- to 3-fold, as did the treatment of 3T3 F442A adipocytes in vitro with thiazolidinediones. These data support a more global concept suggesting that adipose lipid storage functions to prevent peripheral lipotoxicity.

Protein kinase C attenuates β-adrenergic receptor-mediated lipolysis, probably through inhibition of the β1-adrenergic receptor system

Lipolysis in rat white adipocytes is stimulated by beta-adrenergic agonists. Phorbol 12-myristate 13-acetate (PMA) attenuated the receptor-mediated lipolysis by causing a shift of the dose-response curve to the higher concentrations of norepinephrine and isoproterenol. Although the adipocytes possess beta1-, beta2-, and beta3-adrenergic receptor subtypes, the effect of PMA was observed only when a beta1-agonist (dobutamine) was used. No lipolysis-attenuating effect of PMA was found when cells were exposed to a beta2-agonist (procaterol) and beta3-agonists (BRL 37344 and CL 316243), or to forskolin and 8-bromo cAMP. CGP 20712A (beta1-antagonist) efficiently inhibited lipolysis by norepinephrine, isoproterenol, and dobutamine, but did not affect lipolysis by the beta2- and beta3-agonists. ICI 118551 (beta2-antagonist) had no significant effect on lipolysis by the beta-agonists examined. CGP 20712A abolished the lipolysis-attenuating effect of PMA, but ICI 118551 did not. The protein kinase C (PKC) inhibitors, GF 109203X or Gö 6976, suppressed the effect of PMA. Pretreatment of adipocytes with PMA for 6 h caused downregulation of conventional and novel PKCs in association with a decrease in the lipolysis-attenuating effect of PMA. These results indicate that conventional and novel PKCs attenuate lipolysis mediated by beta-adrenergic receptors, probably through inhibition of the beta1-adrenergic receptor system.

Translation modulation of acid beta-glucosidase in HepG2 cells: participation of the PKC pathway

Acid beta-glucosidase (GCase) is the enzyme deficient in Gaucher disease, a prototypical inherited metabolic error for enzyme and gene therapy. An 80 kDa mammalian cytoplasmic translational control protein (TCP80) modulates GCase translation in vitro and ex vivo by interacting with the 5' coding region of GCase RNA. Ten predicted PKC phosphorylation sites (Ser- or Thr-) are in the TCP80 protein. Phosphorylation of TCP80 in vitro by PKC greatly enhanced its translational inhibitory function using in vitro translation assays; binding of GCase mRNA to TCP80 was unaltered. Conversely, de-phosphorylation of TCP80 reduced its translational inhibitory function. Phosphorylation-related modulation of GCase mRNA translation also was studied in HepG2 cells. GCase expression (protein and activity levels) in HepG2 cells increased (>2-fold) in cells treated with bisindolylmaleimide (BIM), a highly selective PKC specific inhibitor. This correlated with a 90% reduction in TCP80 phosphorylation in the presence of BIM. The amount of TCP80 protein in cytoplasm and its RNA-binding activity were unchanged. These experiments indicate that GCase mRNA translation is modulated by PKC signaling pathways that are mediated through TCP80. These findings indicate potential broader impacts of the TCP/PKC system on expression of this and other genes of therapeutic interest.

Transcriptional and translational control of Mcl-1 during apoptosis

Mcl-1 is an antiapoptotic member of the Bcl-2 family whose protein and mRNA have a short half-life. In this report, we studied the changes in Mcl-1 protein and mRNA expression induced by staurosporine and aspirin. Both drugs induced apoptosis in Jurkat cells and reduced the levels of Mcl-1 protein. The caspase inhibitor Z-VAD.fmk and the proteasome inhibitor MG132 partially protected Mcl-1 from decay, indicating that both caspase-dependent and proteasome pathways are involved during apoptosis. Staurosporine also reduced Mcl-1 mRNA levels and this reduction was mostly caspase-dependent. In addition, staurosporine reduced the transcriptional activity of the Mcl-1 promoter fused to a luciferase gene reporter more than actinomycin D, a general inhibitor of transcription. Thus, we conclude that staurosporine down-regulates Mcl-1 mRNA levels by inhibiting transcription in a caspase-dependent manner and reduces Mcl-1 protein levels by a caspase-independent post-transcriptional mechanism. In contrast aspirin, at doses and times that induced loss of viability and decay of Mcl-1 protein, had no effect on Mcl-1 mRNA levels. Aspirin rapidly inhibited de novo protein synthesis before caspase activation. Moreover, the translational factor eIF2alpha was transiently phosphorylated and therefore inhibited very soon after aspirin treatment. Aspirin also inhibited the luciferase reporter activity of several attached promoter constructs, but it did not affect the luciferase activity of a construct containing an internal ribosome entry site (IRES) in its mRNA 5(')UTR. We conclude that staurosporine inhibits transcription and translation, whereas aspirin only inhibits cap-dependent translation. Treatment with cycloheximide, at doses that inhibit protein synthesis without affecting cell viability, also induced Mcl-1 protein decay. Mcl-1 disappearance might be necessary but not sufficient for the induction of apoptosis by staurosporine and aspirin. A model for the control of Mcl-1 during drug-induced apoptosis is presented.

Transgenic mice expressing lipoprotein lipase in adipose tissue. Absence of the proximal 3'-untranslated region causes translational upregulation

Lipoprotein lipase (LPL) is a key enzyme in lipoprotein and adipocyte metabolism. Defects in LPL can lead to hypertriglyceridemia and the subsequent development of atherosclerosis. The mechanisms of regulation of this enzyme are complex and may occur at multiple levels of gene expression. Because the 3'-untranslated region (UTR) is involved in LPL translational regulation, transgenic mice were generated with adipose tissue expression of an LPL construct either with or without the proximal 3'-UTR and driven by the aP2 promoter. Both transgenic mouse colonies were viable and expressed the transgene, resulting in a 2-fold increase in LPL activity in white adipose tissue. Neither mouse colony exhibited any obvious phenotype in terms of body weight, plasma lipids, glucose, and non-esterified fatty acid levels. In the mice expressing hLPL with an intact 3'-UTR, hLPL mRNA expression approximately paralleled hLPL activity. However in the mice without the proximal 3'-UTR, hLPL mRNA was low in the setting of large amounts of hLPL protein and LPL activity. In previous studies, the 3'-UTR of LPL was critical for the inhibitory effects of constitutively expressed hormones, such as thyroid hormone and catecholamines. Therefore, these data suggest that the absence of the 3'-UTR results in a translationally unrepressed LPL, resulting in a moderate overexpression of adipose LPL activity.

Atypical protein kinase C mediates activation of NF-E2-related factor 2 in response to oxidative stress

Transcription factor NF-E2-related factor 2 (Nrf2) regulates the induction of antioxidative proteins, including heme oxygenase-1 (HO-1). Nrf2 is sequestered in the cytoplasm by Keap1 under unstimulated conditions but translocates into the nucleus and transactivates the antioxidant responsive element (ARE) upon exposure to oxidative insults. It has recently been demonstrated that in vitro phosphorylation of Nrf2 on Ser40 by protein kinase C (PKC) facilitates the dissociation of Nrf2 from the Keap1 complex (Huang HC, Nguyen T, and Pickett CB. J Biol Chem 277: 42769-42774, 2002). The present study was designed to examine whether PKC is involved in oxidative stress-mediated nuclear translocation of Nrf2 in vivo and, if so, which PKC isoforms are involved. Induction of HO-1 gene expression by phorone, a glutathione depletor, and 4-hydroxy-2,3-nonenal (4-HNE), an end product of lipid peroxidation, was suppressed by a specific PKC inhibitor, Ro-31-8220, at concentrations that inhibit all isoforms in WI-38 cells. The induction of HO-1 was not affected by prolonged exposure of the cells to 12-O-tetradecanoylphorbol-13 acetate (TPA), suggesting that TPA-insensitive atypical PKC (aPKC) isoforms are involved. An immunocomplex kinase assay revealed that phorone and 4-HNE increased aPKCiota activity. In COS-7 cells, 4-HNE induced nuclear translocation of the Nrf2-green fluorescent protein (GFP) fusion protein, but not the Nrf2(S40A)-GFP mutant. In the absence of oxidative insults, the Nrf2(S40E)-GFP mutant was distributed in the nucleus. The Nrf2-GFP accumulation in the nucleus was induced by coexpression of aPKCiota, but not by a kinase inactive mutant aPKCiota(K274W). The activity of an ARE-driven reporter was increased by coexpression of aPKCiota, and this effect was eliminated by Ro-31-8220 in HepG2 cells. The reporter activity induced by 4-HNE was inhibited by coexpression of aPKCiota(K274W). These results suggest that phosphorylation of Nrf2 Ser40 by aPKC(s) is involved in the nuclear translocation and ARE transactivation of Nrf2 by oxidative stress.

The translational regulation of lipoprotein lipase by epinephrine involves an RNA binding complex including the catalytic subunit of protein kinase A

The balance of lipid flux in adipocytes is controlled by the opposing actions of lipolysis and lipogenesis, which are controlled primarily by hormone-sensitive lipase and lipoprotein lipase (LPL), respectively. Catecholamines stimulate adipocyte lipolysis through reversible phosphorylation of hormone-sensitive lipase, and simultaneously inhibit LPL activity. However, LPL regulation is complex and previous studies have described translational regulation of LPL in response to catecholamines because of an RNA-binding protein that interacts with the 3'-untranslated region of LPL mRNA. In this study, we identified several protein components of an LPL RNA binding complex. Using an LPL RNA affinity column, we identified two of the RNA-binding proteins as the catalytic (C) subunit of cAMP-dependent protein kinase (PKA), and A kinase anchoring protein (AKAP) 121/149, one of the PKA anchoring proteins, which has known RNA binding activity. To determine whether the C subunit was involved in LPL translation inhibition, the C subunit was depleted from the cytoplasmic extract of epinephrine-stimulated adipocytes by immunoprecipitation. This resulted in the loss of LPL translation inhibition activity of the extract, along with decreased RNA binding activity in a gel shift assay. To demonstrate the importance of the AKAPs, inhibition of PKA-AKAP binding with a peptide competitor (HT31) prevented epinephrine-mediated inhibition of LPL translation. C subunit kinase activity was necessary for LPL RNA binding and translation inhibition, suggesting that the phosphorylation of AKAP121/149 or other proteins was an important part of RNA binding complex formation. The hormonal activation of PKA results in the reversible phosphorylation of hormone-sensitive lipase, which is the primary mediator of adipocyte lipolysis. These studies demonstrate a dual role for PKA to simultaneously inhibit LPL-mediated lipogenesis through inhibition of LPL translation.

Regulation of lipoprotein lipase by protein kinase C alpha in 3T3-F442A adipocytes

Lipoprotein lipase (LPL) is an important enzyme in adipocyte and lipid metabolism with complex cellular regulation. Previous studies demonstrated an inhibition of LPL activity and synthesis following depletion of protein kinase C (PKC) isoforms with long term treatment of 3T3-F442A adipocytes with 12-O-tetradecanoylphorbol-13-acetate. To identify the specific PKC isoforms involved, we treated cells with antisense oligonucleotides that block expression of specific PKC isoforms. An antisense oligonucleotide to PKC alpha inhibited LPL activity by 78 +/- 8%, whereas antisense oligonucleotides directed against PKC delta or PKC epsilon had no effect on LPL activity. The change in LPL activity was maximal at 72 h and was accompanied by a decrease in LPL protein and LPL synthetic rate but no change in LPL mRNA, suggesting regulation at the level of translation. However, PKC depletion resulted in no change in the polysome profile, indicating that translation initiation was not affected. However, the addition of cytoplasmic extracts from adipocytes treated with 12-O-tetradecanoylphorbol-13-acetate or PKC alpha antisense oligomers inhibited LPL translation in vitro. This inhibition of LPL translation in vitro was lost when the LPL mRNA transcript did not contain nucleotides 1599-3200, thus implicating the 3'-untranslated region of LPL in the regulation of translation by PKC depletion. Both LPL activity and Raf1 activity were decreased in parallel following depletion of either total PKC or specific inhibition of PKC alpha. An antisense oligonucleotide to RAF1, which inhibited RAF1 activity, also inhibited LPL activity by 48 +/- 10%, and this decrease in LPL activity was not accompanied by a change in LPL mRNA. Cells were treated with U0126, a specific inhibitor of the ERK-activating kinases MEK1 and MEK2. Although U0126 inhibited ERK1 and ERK2 phosphorylation, U0126 had no effect on LPL activity, indicating that MEK/ERK pathways were not involved in this mechanism of LPL regulation. Together, these data indicate that PKC alpha and RAF1 are important in the translational regulation of LPL in adipocytes and that the mechanism of regulation is probably through an ERK-independent pathway.

Lipoprotein lipase: the regulation of tissue specific expression and its role in lipid and energy metabolism

PURPOSE OF REVIEW

The aim of this review is to summarize and discuss recent advances in the understanding of the physiological role of lipoprotein lipase in lipid and energy metabolism.

RECENT FINDINGS

Studies on the transcriptional and the posttranscriptional level of lipoprotein lipase expression have provided new insights into the complex mechanisms that are involved in the regulation of the enzyme. Additionally a large body of evidence from both human studies and animal models suggests that the level of lipoprotein lipase expression in a given tissue is the rate limiting process for the uptake of triglyceride derived fatty acids. Imbalances in the partitioning of fatty acids among peripheral tissues have major metabolic consequences. For example, in mice both decreased lipoprotein lipase activities in adipose tissue and increased activity in muscle are associated with resistance to obesity; lack of lipoprotein lipase activity in macrophages is correlated with a decreased susceptibility to develop atherosclerotic lesions and overexpression of the enzyme in muscle is associated with increased blood glucose levels and insulin resistance.

SUMMARY

Considering the central role of lipoprotein lipase in energy metabolism it is a reasonable goal to discover and develop new drugs that affect the tissue specific expression pattern of the enzyme.

Anti-proliferative effects of new staurosporine derivatives isolated from a marine ascidian and its predatory flatworm

Nine indolocarbazole alkaloids of the staurosporine type, including three new derivatives, were evaluated for their potential as inhibitors of cell proliferation and macromolecule synthesis. Four derivatives were tested as inhibitors of cell proliferation with twelve human leukemia cell lines and demonstrated powerful antiproliferative activities, with 3-hydroxystaurosporine being the most potent. IC(50) values were determined using the cell line MONO-MAC-6 and with an IC(50) of 13 ng/ml, 3-hydroxystaurosporine turned out to be one of the most active staurosporine-type inhibitors described so far. All derivatives, except 3-hydroxy-3'-demethoxy-3'-hydroxystaurosporine and 4'-N-methylstaurosporine very strongly reduced RNA and DNA synthesis with 3-hydroxystaurosporine again being the strongest inhibitor. Analysis of structure-activity relationships demonstrated that hydroxylation of staurosporine at position 3 of the indolocarbazole moiety caused an increase in anti-proliferative activity, while hydroxylation at carbon 11 resulted in a decrease in activity. Our results suggest that not only the presence or absence of hydrophilic substitutions, but also the position of the alteration within the molecule, is important in the antiproliferative properties of the various staurosporine analogues.

Glucose and insulin stimulate heparin-releasable lipoprotein lipase activity in mouse islets and INS-1 cells. A potential link between insulin resistance and beta-cell dysfunction

Lipoprotein lipase (LpL) provides tissues with triglyceride-derived fatty acids. Fatty acids affect beta-cell function, and LpL overexpression decreases insulin secretion in cell lines, but whether LpL is regulated in beta-cells is unknown. To test the hypothesis that glucose and insulin regulate LpL activity in beta-cells, we studied pancreatic islets and INS-1 cells. Acute exposure of beta-cells to physiological concentrations of glucose stimulated both total cellular LpL activity and heparin-releasable LpL activity. Glucose had no effect on total LpL protein mass but instead promoted the appearance of LpL protein in a heparin-releasable fraction, suggesting that glucose stimulates the translocation of LpL from intracellular to extracellular sites in beta-cells. The induction of heparin-releasable LpL activity was unaffected by treatment with diazoxide, an inhibitor of insulin exocytosis that does not alter glucose metabolism but was blocked by conditions that inhibit glucose metabolism. In vitro hyperinsulinemia had no effect on LpL activity in the presence of low concentrations of glucose but increased LpL activity in the presence of 20 mm glucose. Using dual-laser confocal microscopy, we detected intracellular LpL in vesicles distinct from those containing insulin. LpL was also detected at the cell surface and was displaced from this site by heparin in dispersed islets and INS-1 cells. These results show that glucose metabolism controls the trafficking of LpL activity in beta-cells independent of insulin secretion. They suggest that hyperglycemia and hyperinsulinemia associated with insulin resistance may contribute to progressive beta-cell dysfunction by increasing LpL-mediated delivery of lipid to islets.

Direct regulatory effect of fatty acids on macrophage lipoprotein lipase: potential role of PPARs

Atherosclerosis is a major complication of type 2 diabetes. The pathogenesis of this complication is poorly understood, but it clearly involves production in the vascular wall of macrophage (Mo) lipoprotein lipase (LPL). Mo LPL is increased in human diabetes. Peripheral factors dysregulated in diabetes, including glucose and free fatty acids (FAs), may contribute to this alteration. We previously reported that high glucose stimulates LPL production in both J774 murine and human Mo. In the present study, we evaluated the direct effect of FAs on murine Mo LPL expression and examined the involvement of peroxisome proliferator-activated receptors (PPARs) in this effect. J774 Mo were cultured for 24 h with 0.2 mmol/l unsaturated FAs (arachidonic [AA], eicosapentaenoic [EPA], and linoleic acids [LA]) and monounsaturated (oleic acid [OA]) and saturated FAs (palmitic acid [PA] and stearic acid [SA]) bound to 2% bovine serum albumin. At the end of this incubation period, Mo LPL mRNA expression, immunoreactive mass, activity, and synthetic rate were measured. Incubation of J774 cells with LA, PA, and SA significantly increased Mo LPL mRNA expression. In contrast, exposure of these cells to AA and EPA dramatically decreased this parameter. All FAs, with the exception of EPA and OA, increased extra- and intracellular LPL immunoreactive mass and activity. Intracellular LPL mass and activity paralleled extracellular LPL mass and activity in all FA-treated cells. In Mo exposed to AA, LA, and PA, an increase in Mo LPL synthetic rate was observed. To evaluate the role of PPARs in the modulatory effect of FAs on Mo LPL gene expression, DNA binding assays were performed. Results of these experiments demonstrate an enhanced binding of nuclear proteins extracted from all FA-treated Mo to the peroxisome proliferator-response element (PPRE) consensus sequence of the LPL promoter. PA-, SA-, and OA-stimulated binding activity was effectively diminished by immunoprecipitation of the nuclear proteins with anti-PPAR-alpha antibodies. In contrast, anti-PPAR-gamma antibodies only significantly decreased AA-induced binding activity. Overall, these results provide the first evidence for a direct regulatory effect of FAs on Mo LPL and suggest a potential role of PPARs in the regulation of Mo LPL gene expression by FAs.

The translational regulation of lipoprotein lipase in diabetic rats involves the 3'-untranslated region of the lipoprotein lipase mRNA

Adipose tissue lipoprotein lipase (LPL) activity is decreased in patients with poorly controlled diabetes, and this contributes to the dyslipidemia of diabetes. To study the mechanism of this decrease in LPL, we studied adipose tissue LPL expression in male rats with streptozotocin-induced diabetes. Heparin releasable and extractable LPL activity in the epididymal fat decreased by 75-80% in the diabetic group and treatment of the rats with insulin prior to sacrifice reversed this effect. Northern blot analysis indicated no corresponding change in LPL mRNA levels. However, LPL synthetic rate, measured using [(35)S]methionine pulse labeling, was decreased by 75% in the diabetic adipocytes, and insulin treatment reversed this effect. These results suggested regulation of LPL at the level of translation. Diabetic adipocytes demonstrated no change in the distribution of LPL mRNA associated with polysomes, suggesting no inhibition of translation initiation. Addition of cytoplasmic extracts from control and diabetic adipocytes to a reticulocyte lysate system demonstrated the inhibition of LPL translation in vitro. Using different LPL mRNA transcripts in this in vitro translation assay, we found that the 3'-untranslated region (UTR) of the LPL mRNA was important in controlling translation inhibition by the cytoplasmic extracts. To identify the specific region involved, gel shift analysis was performed. A specific shift in mobility was observed when diabetic cytoplasmic extract was added to a transcript containing nucleotides 1818-2000 of the LPL 3'-UTR. Thus, inhibition of translation is the predominant mechanism for the decreased adipose tissue LPL in this insulin-deficient model of diabetes. Translation inhibition involves the interaction of a cytoplasmic factor, probably an RNA-binding protein, with specific sequences of the LPL 3'-UTR.

Estrogen suppresses transcription of lipoprotein lipase gene. Existence of a unique estrogen response element on the lipoprotein lipase promoter

Estrogen exerts a variety of effects not only on female reproductive organs but also on nonreproductive organs, including adipose tissue. Estrogen inhibits obesity triggered by ovariectomy in rodents. We studied the mechanism underlying this estrogen-dependent inhibition of obesity. Estrogen markedly decreased the amounts of fat accumulation and lipoprotein lipase (LPL) mRNA as well as triglyceride accumulation in genetically manipulated 3T3-L1 adipocytes stably expressing the estrogen receptor (ER). A pLPL(1980)-CAT construct, along with an ER expression vector, was introduced into differentiated 3T3-L1 cells, and CAT activities were determined. ER, mostly ligand-dependently, inhibited the basal LPL promoter activity by 7-fold. We searched the LPL promoter for an estrogen-responsive suppressive element by employing a set of 5'-deletion mutants of the pLPL-CAT reporter. Although there was no classical estrogen response element, it was demonstrated that an AP-1-like TGAATTC sequence located at (-1856/-1850) was responsible for the suppression of the LPL gene transcription by estrogen. An electrophoretic mobility shift assay probed with the TGAATTC sequence demonstrated formation of a specific DNA-nuclear protein complex. Interestingly, this complex was not affected by the addition of any antibodies against ER, c-Jun, c-Fos, JunB, or JunD. Because this TGAATTC element responded to phorbol ester and overexpression of CREB-binding protein abrogated the suppressive effect of estrogen on the LPL promoter, we conclude that a unique protein that is related to the AP-1 transcription factor families may be involved in the complex that binds to the TGAATTC element.

The effect of protein kinase C activation on colonic epithelial cellular integrity

We have investigated whether activation of protein kinase C has a direct cytotoxic effect on colonic mucosal epithelial cells and whether oxidant-induced damage to colonocytes is mediated by activation of cellular protein kinase C. Incubation of freshly harvested cells from rat colon with the protein kinase C activator, phorbol 12-myristate, resulted in a concentration-dependent increase in the extent of cell injury. Phorbol 12-myristate acetate (0.1-10 microM) also increased cellular protein kinase C activity and this was reduced significantly by treating cells with the antagonists staurosporine or 2-[1-(3-dimethylaminopropyl)-indol-3-yl]3-(-indol-3-yl)maleimide (GF 109203X; 10 microM). Phorbol 12-myristate acetate treatment also resulted in increased translocation of proteins for protein kinase C isoforms alpha, delta and epsilon from cytosol to membrane particulate fractions. The antagonists reduced the extent of cell damage in response to phorbol 12-myristate acetate. Furthermore, cell injury in response to the phorbol acetate was also inhibited by the addition of the oxidant scavengers, superoxide dismutase or catalase to the cell suspension. Addition of H(2)O(2) to the incubation medium (0.1-100 microM) resulted in an increase in cellular protein kinase C activity, an increase in the expression of the alpha, beta and zeta isoforms and a reduction in cell integrity. The cellular damaging actions of H(2)O(2) were significantly reduced by the protein kinase C antagonists, staurosporine or 2-[1-(3-dimethylaminopropyl)-indol-3-yl]-3-(-indol-3-yl)maleimide (GF 109203X). These findings suggest that protein kinase C activation results in colonic cellular injury and this damage is mediated, at least in part, by release of reactive oxidants. Furthermore, oxidant-mediated damage to these cells also involves protein kinase C activation.