Insulin-like growth factor-i regulation of hepatic scavenger receptor class BI

Maternal nutrient metabolism in the liver during pregnancy

The liver plays pivotal roles in nutrient metabolism, and correct hepatic adaptations are required in maternal nutrient metabolism during pregnancy. In this review, hepatic nutrient metabolism, including glucose metabolism, lipid and cholesterol metabolism, and protein and amino acid metabolism, is first addressed. In addition, recent progress on maternal hepatic adaptations in nutrient metabolism during pregnancy is discussed. Finally, the factors that regulate hepatic nutrient metabolism during pregnancy are highlighted, and the factors include follicle-stimulating hormone, estrogen, progesterone, insulin-like growth factor 1, prostaglandins fibroblast growth factor 21, serotonin, growth hormone, adrenocorticotropic hormone, prolactin, thyroid stimulating hormone, melatonin, adrenal hormone, leptin, glucagon-like peptide-1, insulin glucagon and thyroid hormone. Our vision is that more attention should be paid to liver nutrient metabolism during pregnancy, which will be helpful for utilizing nutrient appropriately and efficiently, and avoiding liver diseases during pregnancy.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

Global transcriptome analysis implicates cholesterol availability in the regulation of canine cyclic luteal function

Considering the key role of the corpus luteum in the regulation of the canine diestrus, the present study aimed to investigate changes in the luteal transcriptome of pseudopregnant dogs (n = 18) from days (D) 10, 20, 30, 40, 50 and 60 post-ovulation. After RNAsequencing was performed, data was analyzed by resorting to several informatic tools. A total of 3300 genes were differently expressed among all samples (FDR < 0.01). By comparing different time points, enriched biological processes as response to estradiol and lipids (D20 vs D10) and intracellular cholesterol transport (D40 vs D60) were observed. Moreover, LXR/RXR (liver X receptor- retinoid X receptor) signaling appeared as an overrepresented pathway in all comparisons. Thus, the expression of 19 genes involved in intracellular cholesterol availability was further evaluated; most were affected by time (P < 0.05). Adding to the deep transcriptomic analysis, presented data implies the importance of cholesterol regulation in luteal physiology of pseudopregnant dogs.

SR-BI as a target of natural products and its significance in cancer

Scavenger receptor class B type I (SR-BI) protein is an integral membrane glycoprotein. SR-BI is emerging as a multifunctional protein, which regulates autophagy, efferocytosis, cell survival and inflammation. It is well known that SR-BI plays a critical role in lipoprotein metabolism by mediating cholesteryl esters selective uptake and the bi-directional flux of free cholesterol. Recently, SR-BI has also been identified as a potential marker for cancer diagnosis, prognosis, or even a treatment target. Natural products are a promising source for the discovery of new drug leads. Multiple natural products were identified to regulate SR-BI protein expression. There are still a number of challenges in modulating SR-BI expression in cancer and in using natural products for modulation of such protein expression. In this review, our purpose is to discuss the relationship between SR-BI protein and cancer, and the molecular mechanisms regulating SR-BI expression, as well as to provide an overview of natural products that regulate SR-BI expression.

Skipping Breakfast Is Associated with an Atherogenic Lipid Profile in Overweight and Obese Prepubertal Children

BACKGROUND

Skipping breakfast has been associated with a higher risk of obesity and cardiovascular (CV) risk factors. However, it is not known if skipping breakfast is also correlated with CV risk factors independently from obesity. The mechanisms explaining the role of skipping breakfast on promoting fat accumulation as well as CV risk are not known. Hormones, in particular, insulin-like growth factor-1 (IGF-1), may potentially play a role in the metabolic profile of breakfast skippers.

AIM

This cross-sectional study aims to test, in a sample of overweight/obese children, the hypotheses that skipping breakfast is associated with a worse metabolic profile and that IGF-1 levels are associated with this unfavorable metabolic profile.

METHODS AND RESULTS

We enrolled 112 overweight/obese prepubertal children (3-12 years). Anthropometric characteristics (height SDS, weight SDS, and body mass index (BMI) z-score) were measured. Blood samples were collected to evaluate glucose and lipid metabolisms and hormone profile (growth hormone (GH), IGF-1, insulin, and cortisol). The triglycerides/high-density lipoprotein (HDL) cholesterol ratio was calculated as a predictor of cardiovascular risk. Children were divided into two groups according to breakfast habits: consumers (≥5 weekly; N = 76) and skippers (≤4 weekly; N = 36). Glycaemia, total and low-density lipoprotein (LDL) cholesterol, triglycerides (p < 0.05), and triglycerides/HDL cholesterol ratio (p < 0.001) were higher, while HDL cholesterol was lower (p < 0.01) in skippers as compared to consumers. IGF-1 concentrations were inversely correlated with LDL cholesterol (r = -0.279, p=0.013) and directly correlated with HDL cholesterol (r = 0.226, p=0.047). IGF-1 correlated positively with HDL cholesterol (r = 0.266, p=0.045) in consumers and correlated negatively with LDL cholesterol (r = -0.442, p=0.024) in skippers. Breakfast consumption among prepubertal overweight/obese children showed a better lipid profile in comparison with those who skipped breakfast [OR: 0.165 (95% CI: 0.053-0.518), p=0.001]; these latter odds of the increased triglycerides/HDL cholesterol ratio was 6.1-fold higher.

CONCLUSIONS

Breakfast skippers show a worse lipid profile when compared to breakfast consumers. IGF-1 might play a role as an independent modulator of lipid metabolism.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

IGF1 suppresses cholesterol accumulation in the liver of growth hormone-deficient mice via the activation of ABCA1

Recently, several clinical studies have suggested that adult growth hormone (GH) deficiency that also has low concentration of IGF1 is associated with an increased prevalence of fatty liver (FL). ATP-binding cassette transporter A1 (ABCA1) is a pivotal regulator of lipid efflux from cells to apolipoproteins and plays an important role on formation of FL. In this study, we determined the effects of IGF1 on ABCA1 expression in GH-deficient mice to clarify its effects on FL. Western blotting, real-time PCR, and a luciferase assay were employed to examine the effect of IGF1. The binding of FoxO1 to the ABCA1 promoter was assessed by chromatin immunoprecipitation (ChIP) assay. Cholesterol accumulation was analyzed by Oil Red O stain and cholesterol content measurement. We confirmed that IGF1 upregulated the ABCA1 expression. The activity of a reporter construct containing the ABCA1 promoter was induced by IGF1, and this effect was blocked by LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K). Constitutively active Akt stimulated the ABCA1 promoter activity, and a dominant-negative mutant of Akt or mutagenesis of the FoxO1 response element abolished the effect of IGF1. A ChIP assay indicated that FoxO1 mediated IGF1 transcriptional activity by directly binding to the ABCA1 promoter region. For in vivo experiments, we used an inhibitor for the GH receptor (Pegvisomant) to reduce the IGF1 level. A high-fat diet induced FL in mice (C57BL/6J) given Pegvisomant treatment. IGF1 treatment stimulated ABCA1 expression to improve cholesterol accumulation in these mice. These results show that the PI3K/Akt/FoxO1 pathway contributes to the regulation of ABCA1 expression in response to IGF1 stimulation that suppressed FL in GH-deficient mice.

Low insulin-like growth factor 1 is associated with low high-density lipoprotein cholesterol and metabolic syndrome in Chinese nondiabetic obese children and adolescents: a cross-sectional study

Background

Low serum high-density lipoprotein cholesterol (HDL-C) is an independent risk factor for developing cardiovascular disease. Insulin-like growth factor 1(IGF-1) levels have been proven to be positively associated with HDL-C, but few studies were based on the dataset of children or adolescents. The aim of this study is to investigate the relationship among IGF-1, HDL-C and the metabolic syndrome in Chinese nondiabetic obese children and adolescents.

Methods

As a cross-sectional study, this study includes 120 obese Chinese children and adolescents and 120 healthy ones. The obese subjects were divided into two groups based on using 1.03 mmol/L as a threshold value for HDL-C. Clinical examination and laboratory examinations were assessed for all participants.

Results

Obese subjects had significantly lower IGF-1SDS and higher Height SDS than those in the control group. Among 120 obese children and adolescents, 22 (18.3 %) subjects had an HDL-C level <1.03 mmol/L. IGF-1SDS was significantly lower (P = 0.001) in obese subjects with low HDL-C. According to the results of multivariate logistic regression analysis, IGF-1 SDS is significantly associated with low HDL-C(OR 0.518, 95 % CI 0.292–0.916; P = 0.024), after being adjusted for age, gender, pubertal status, BMI SDS, SBP, DBP, HOMR-IR, total cholesterol, low density lipoprotein-cholesterol, triglycerides, ALT and uric acid. In addition, IGF-1 SDS is significantly correlated with the level of serum HDL-C in study population (r = 0.19, P = 0.003). Based on logistic regression analysis with adjustment for age, gender and pubertal status, the increased IGF-1 SDS was associated with a decreased probability of metabolic syndrome (OR 0.555, 95 % CI 0.385–0.801; P = 0.002) and hypertriglyceridemia (OR 0.582, 95 % CI 0.395–0.856; P = 0.006), but no significant correlation with hypertension.

Conclusion

Obese children had lower IGF-1SDS and taller stature compared with the control group. Low levels of IGF-1 SDS were associated with low levels of HDL-C in chinese nondiabetic obese children and adolescents, independent of insulin resistance, as well as other traditional cardiovascular disease risk markers.

Roles of lipoprotein receptors in the entry of hepatitis C virus

Infection by hepatitis C virus (HCV), a plus-stranded RNA virus that can cause cirrhosis and hepatocellular carcinoma, is one of the major health problems in the world. HCV infection is considered as a multi-step complex process and correlated with abnormal metabolism of lipoprotein. In addition, virus attacks hepatocytes by the initial attaching viral envelop glycoprotein E1/E2 to receptors of lipoproteins on host cells. With the development of HCV model system, mechanisms of HCV cell entry through lipoprotein uptake and its receptor have been extensively studied in detail. Here we summarize recent knowledge about the role of lipoprotein receptors, scavenger receptor class B type I and low-density lipoprotein receptor in the entry of HCV, providing a foundation of novel targeting therapeutic tools against HCV infection.

Lipid metabolism and hormonal interactions: impact on cardiovascular disease and healthy aging

Populations in developed nations are aging gradually; it is predicted that by 2050 almost a quarter of the world's population will be over 60 years old, more than twice the figure at the turn of the 20th century. Although we are living longer, this does not mean the extra years will be spent in good health. Cardiovascular diseases are the primary cause of ill health and their prevalence increases with age. Traditionally, lipid biomarkers have been utilized to stratify disease risk and predict the onset of cardiovascular events. However, recent evidence suggests that hormonal interplay with lipid metabolism could have a significant role to play in modulating cardiovascular disease risk. This review will explore recent findings which have investigated the role hormones have on the dynamics of lipid metabolism. The aim is to offer an insight into potential avenues for therapeutic intervention.

Positive association between plasma IGF1 and high-density lipoprotein cholesterol levels in adult nondiabetic subjects

AIMS

Low IGF1 levels have been associated with an increased cardiovascular risk. It is unknown however whether IGF1 mediates the atherosclerotic process by modulating high-density lipoprotein cholesterol (HDL-C) independently from confounders. To address this issue, we evaluated the association between IGF1 levels and HDL-C in nondiabetic subjects.

METHODS

A cross-sectional analysis was used in the context of the CAtanzaro MEtabolic RIsk factors Study. One thousand and four participants (aged 20-69 years), for whom HDL-C and IGF1 measurements were available, were eligible for the study.

RESULTS

After adjusting for gender and age, IGF1 levels were positively correlated with HDL-C, and negatively correlated with body mass index (BMI), waist circumference, blood pressure (BP), triglyceride, fasting insulin, and homeostasis model assessment (HOMA). In a logistic regression model adjusted for age and gender, IGF1 in the lowest tertile (<125 ng/ml) was associated with an increased risk of having low HDL-C (odds ratio (OR) 2.14, 95% confidence interval (CI) 1.4-3.0; P=4x10(-5)) compared with the highest tertile (>186 ng/ml). When BMI, waist circumference, total cholesterol, triglyceride, and HOMA index were added to the model, IGF1 remained significantly associated with increased risk of low HDL-C (OR 1.52, 95% CI 1.01-2.31; P=0.04). A stepwise multivariate regression analysis in a model including age, gender, BMI, total cholesterol, triglycerides, IGF1, HOMA, and BP showed that the variables significantly associated with HDL-C were gender (P<0.0001), triglycerides (P<0.0001), total cholesterol (P<0.0001), BMI (P<0.0001), IGF1 levels (P<0.0001), and HOMA (P=0.001), accounting for 32.6% of its variation.

CONCLUSIONS

These data provide evidence that IGF1 may be an independent modulator for HDL-C in nondiabetic individuals.

Scavenger receptor class BI mediates the anti-apoptotic effect of erythropoietin

A scavenger receptor of the B class (SR-BI)/human homolog of SR-BI, CD36, and LIMP II analogous-1 (CLA-1), has been identified as a receptor for high-density lipoprotein (HDL). Mice lacking SR-B1 develop anemia, plausibly explained by the observation that the erythrocyte life-span in these animals is reduced. Erythropoietin (EPO) is known to promote survival of erythroid cells, in large part through protection from apoptosis. We have examined the role of EPO on hSR-BI/CLA-1 expression and erythrocyte apoptosis. Endogenous expression of hSR-BI/CLA-1 was increased by exposure to EPO. EPO increased transcriptional activity of hSR-BI/CLA-1 promoter. The stimulatory effect of EPO on hSR-BI/CLA-1 promoter activity was abrogated by LY294002, specific inhibitor of phosphatidylinositol-3 kinase (PI3K). Constitutively active Akt stimulates the activity of the hSR-BI/CLA-1 promoter and a dominant-negative mutant of Akt abolished the ability of EPO to stimulate promoter activity. Finally, EPO in combination with HDL protected the cell from apoptosis, which suggests that hSR-BI/CLA-1 induced by EPO might contribute to the erythrocyte life-span.

The transcriptional factor prolactin regulatory element-binding protein mediates the gene transcription of adrenal scavenger receptor class B type I via 3',5'-cyclic adenosine 5'-monophosphate

Prolactin regulatory element-binding (PREB) protein is a transcription factor that regulates prolactin promoter activity in the rat anterior pituitary. The PREB protein is not only expressed in the anterior pituitary but also in the adrenal gland. However, the role of PREB in the adrenal gland is not clearly understood. Scavenger receptor class B type I (SR-BI) is a receptor for high-density lipoprotein that mediates the cellular uptake of high-density lipoprotein-cholesteryl ester and is a major route for cholesterol delivery to the steroidogenic pathway in the adrenal gland. In the present study, we have examined the role of PREB in regulating SR-BI. SR-BI expression was found to be regulated by cAMP, which stimulated the expression of PREB in a dose-dependent manner. Conversely, overexpression of PREB using a PREB-expressing adenovirus increased the expression of the SR-BI protein in the adrenocortical cell line Y-1. In addition, PREB induced the expression of the luciferase reporter protein that was under the control of the SR-BI promoter. EMSA showed that PREB mediates its transcriptional effect by binding to the PREB-responsive cis-element of the SR-BI promoter. Finally, we used small interfering RNA to inhibit PREB expression in the Y-1 cells and demonstrated that the knockdown of PREB expression attenuated the effects of cAMP on SR-BI expression. In summary, our data showed that in the adrenal gland, PREB regulates the transcription of the SR-BI gene via cAMP.

Influence of the d3-growth hormone (GH) receptor isoform on short-term and long-term treatment response to GH replacement in GH-deficient adults

OBJECTIVE

Recombinant human GH (rhGH) replacement in adults is aimed at improving signs and symptoms of the adult GH deficiency (GHD) syndrome. In children, a common polymorphism of the GH receptor (exon-3 deletion, d3GHR) increases the response to rhGH replacement. The aim of this study was to assess the effects of this polymorphism on the response to rhGH replacement in adults.

DESIGN

Prospective intervention with rhGH during 1 yr (n = 99) and in a subset during 5 yr (n = 53).

PATIENTS AND METHODS

The presence of the d3GHR variant was established in GHD patients and linked to short-term and long-term effects of rhGH replacement on IGF-I, lipid metabolism, anthropometric parameters, and bone mineral density.

RESULTS

Fifty-five patients had two wild-type alleles (56%), whereas 38 patients (38%) had one allele and six patients (6%) had two alleles coding the d3GHR isoform. During short-term rhGH replacement, the increase in IGF-I was higher in patients bearing at least one d3GHR allele, compared with those with two wild-type alleles (at an identical mean dose of rhGH). The decrease in total cholesterol and low-density lipoprotein cholesterol was lower in the group bearing at least one d3GHR allele, whereas the increase in high-density lipoprotein cholesterol was higher, compared with patients with the wild-type genotype. In contrast, these differential responses of GHR genotype could not be demonstrated during long-term rhGH replacement.

CONCLUSION

The d3GHR genotype contributes, at least for some parameters, to the interindividual differences in efficacy of short-term, but not long-term, rhGH replacement in adults with GHD.

Hyperglycemia suppresses hepatic scavenger receptor class B type I expression

Hyperglycemia is a major risk factor for atherosclerotic disease. Hepatic scavenger receptor class B type I (SR-BI) binds HDL particles that mediate reverse cholesterol transport and thus lowers the risk of atherosclerosis. Here we examined glucose regulation of SR-BI gene expression in both HepG2 cells and whole animals. Results showed that hepatic SR-BI mRNA, protein, and uptake of cholesterol from HDL were halved following 48 h of exposure to 22.4 vs. 5.6 mM glucose. As in the case of the cell culture model, hepatic expression of SR-BI was lower in diabetic rats than in euglycemic rats. Transcriptional activity of the human SR-BI promoter paralleled endogenous expression of the gene, and this activity was dependent upon the dose of glucose. Next, we used inhibitors of select signal transduction pathways to demonstrate that glucose suppression of SR-BI was sensitive to the p38 MAPK inhibitor. Expression of a constitutively active p38 MAPK inhibited SR-BI promoter activity in the presence or absence of glucose. A dominant-negative p38 MAPK abolished the inhibitory effect of glucose on promoter activity. Deletional analysis located a 50-bp fragment of the promoter that mediated the effects of glucose. Within this DNA fragment there were several specificity protein-1 (Sp1) binding sites, and cellular knockdown of Sp1 abrogated its suppression by glucose. Together, these results indicate that the glucose suppression of SR-B1 expression is partially mediated by the activation of the p38 MAPK-Sp1 pathway and raise the possibility that the inhibition of hepatic SR-BI expression under high-glucose conditions provides a mechanism for accelerated atherosclerosis in diabetics.

Lung vitamin E transport processes are affected by both age and environmental oxidants in mice

Despite the physiological importance of alpha-tocopherol (AT), the molecular mechanisms involved in maintaining cellular and tissue tocopherol levels remain to be fully characterized. Scavenger receptor B1 (SRB1), one of a large family of scavenger receptors, has been shown to facilitate AT transfer from HDL to peripheral tissues via apo A-1-mediated processes and to be important in the delivery of AT to the lung cells. In the present studies the effects of age and two environmental oxidants ozone (O(3)) (0.25 ppm 6 h/day) and cigarette smoke (CS) (60 mg/m(3) 6 h/day) for 4 days on selected aspects of AT transport in murine lung tissues were assessed. While AT levels were 25% higher (p<0.05) and 15% lower (p<0.05) in plasma and lung tissue, respectively, in aged versus young mice, acute environmental exposure to O(3) or CS at the doses used had no effect. Gene expression levels, determined by RT-PCR of AT transport protein (ATTP), SRB1, CD36, ATP binding cassette 3 (ABCA3) and ABCA1 and protein levels, determined by Western blots for SRB1, ATTP and ABCA1 were assessed. Aged mouse lung showed a lower levels of ATTP, ABCA3 and SRB1 and a higher level CD36 and ABCA1. Acute exposure to either O(3) or CS induced declines in ATTP and SRB1 in both aged and young mice lung. CD36 increased in both young and aged mice lung upon exposure to O(3) and CS. These findings suggest that both age and environmental oxidant exposure affect pathways related to lung AT homeostasis and do so in a way that favors declines in lung AT. However, given the approach taken, the effects cannot be traced to changes in these pathways or AT content in any specific lung associated cell type and thus highlight the need for further follow-up studies looking at specific lung associated cell types.

Induction of glutathione transferase in insulin-like growth factor type I receptor-overexpressed hepatoma cells

Insulin-like growth factor type I receptor (IGF-IR) is frequently overexpressed in human hepatocellular carcinoma cells (HCC), and this overexpression has been correlated with increased tumor growth. The protective response of HCC to reactive oxygen species (ROS) produced by chemotherapeutic agents is mediated with the induction of phase II detoxifying genes including glutathione transferase (GST). To understand the roles of IGF-IR overexpression in HCC in terms of its detoxifying effect on ROS and conferred resistance to chemotherapy, we analyzed whether IGF-IR overexpressions affect IGF-1-inducible GST expression. GSTalpha was induced by exposure to IGF-1 in IGF-IR cells but not in cells expressing normal levels of IGF-IR. Furthermore, IGF-IR-overexpressed HCCs (IR-HCC) are more resistant to doxorubicin than control HCC cells, which was associated with the increased GST induction by IGF-1. Molecular analyses using GSTA2 promoter supported the involvement of xenobiotic response element (XRE) in GSTalpha induction. IGF-1 caused the nuclear translocation of CCAAT/enhancer-binding protein beta (C/EBPbeta), which might be responsible for XRE activation. In addition, IGF-1 increased the activities of phosphatidylinositol 3-kinase (PI3-kinase) and extracellular signal-regulated kinase in IR-HCCs. Moreover, the inhibition of PI3-kinase completely abolished the nuclear translocation of C/EBPbeta and the up-regulation of GSTalpha protein in IR-HCC treated with IGF-1. However, specific inhibitors against extracellular signal-regulated kinase, c-Jun N-terminal kinase, or p38 kinase did not alter IGF-1-inducible GSTalpha expression. These results provide evidence that one of the pathological consequences of IGF-IR overexpression in HCCs is the potentiation of GSTalpha inducibility by IGF-1. Moreover, this potentiation of GST may be associated with decreased susceptibility to chemotherapeutic agents such as doxorubicin.

Pancreatic glucokinase is activated by insulin-like growth factor-I

Glucokinase (GK) plays a key role in the regulation of glucose use and glucose-stimulated insulin secretion in pancreatic islet cells. Gene targeting of the IGF-I receptor down-regulated pancreatic islet GK activity. That finding prompted us to examine the potential mechanism that may control GK gene activity using an islet cell line, INS-1, known to express IGF-I receptor. Exposure of these cells to IGF-I induced GK protein expression and activity of the enzyme in a dose-dependent manner. In addition, IGF-I induced activity of a reporter construct containing the GK promoter in parallel with the effect on endogenous GK mRNA levels. The stimulatory effect of IGF-I on GK promoter activity was abrogated by wortmannin and LY294002, specific inhibitors of phosphatidylinositol 3-kinase. Exposure of cells to IGF-I elicited a rapid phosphorylation of Akt and FoxO1, a known target of Akt signaling. Constitutively active Akt stimulates the activity of the GK promoter, and a dominant-negative mutant of Akt or mutagenesis of a FoxO1 response element in the GK promoter abolished the ability of IGF-I to stimulate the promoter activity. Furthermore, cell knockdown of FoxO1 with small interfering RNA disrupted the effect of IGF-I on GK expression. These results demonstrate that the phosphatidylinositol 3-kinase/Akt/FoxO1 pathway contributes to the regulation of GK gene expression in response to IGF-I stimulation.

Platelet derived growth factor regulates ABCA1 expression in vascular smooth muscle cells

The ATP-binding cassette transporter A1 (ABCA1) regulates lipid efflux from peripheral cells to High-density lipoprotein. The platelet-derived growth factor (PDGF) is a potent mitogen that enables vascular smooth muscle cells to participate in atherosclerosis. In this report, we showed that PDGF suppressed endogenous expression of ABCA1 in cultured vascular smooth muscle cells. Exposure of CRL-208 cells to PDGF elicited a rapid phosphorylation of a kinase downstream from PI3-K, Akt. The constitutively active form of both p110, a subunit of PI3-K, and Akt inhibited activity of the ABCA1 promoter. In conclusion, PI3-K-Akt pathways participate in PDGF-suppression of ABCA1 expression.

High-density lipoprotein is a potential growth factor for adrenocortical cells

The entry of cholesterol contained within high-density lipoprotein (HDL) into adrenocortical cells is mediated by a human homologue of SR-BI, CD36, and LIMPII Analogous-1 (CLA-1) and thus augmenting their growth. To address the role of CLA-1, we created a mutant mCLA that lacked the C-terminal tail. HDL CE selective uptake by cells carrying the mCLA-1 receptor was fully active and equivalent to those transfected with full-length CLA-1 (fCLA-1). Expression of mCLA inhibited the proliferation of an adrenocortical cell line and the incorporation of [(3)H]thymidine into the cells. This effect was sensitive to wortmannin, an inhibitor of phosphoinositide 3-kinase (PI3K). Our transcriptional studies revealed that the inhibitory action of mCLA required the transcriptional factor AP-1 and the effect of HDL on AP-1 activation was also abrogated by wortmannin. These findings raise the possibility that the inhibitors of the effects of HDL may be of therapeutic value for adrenocortical tumor.

IGF-1 induces rat glomerular mesangial cells to accumulate triglyceride

Rat glomerular mesangial cells (MC) become lipid-laden foam cells when they are exposed to IGF-1. IGF-1 increased accumulation of triglyceride (TG) 2.5-fold in MC after 7 days. TG accumulation resulted from enhanced macropinocytosis and decreased efflux secondary to a 40-50% reduction in peroxisome proliferator-activated receptor (PPAR)-delta (PPARdelta). There was no evidence of primary or secondary changes in cholesterol or TG synthesis, increased uptake by LDL or scavenger receptors, or reduced efflux via ATP-binding cassette A-1. Although the lipid moiety taken up can be influenced by the concentration of cholesterol or TG in the medium, in standard medium MC preferentially accumulate TG. TG-rich MC foam cells fail to contract in response to angiotensin II (Berfield AK, Andress DL, and Abrass CK. Kidney Int 62: 1229-1237, 2002); however, their migratory response to IGF binding protein-5 is unaffected. This differs from cholesterol loading, which impairs both phagocytosis and migration. These findings have important implications for understanding the mechanisms that contribute to lipid accumulation in MC and the functional consequences of different forms of foam cells. These observations are relevant to understanding vascular disease and progressive renal diseases that are accelerated by abnormalities in lipid metabolism.