Biomedical subjects

Sohye Kang

Publications and source records attributed to Sohye Kang.

11 recordsLinked to original sources

PubMed2006

Coordinate transcriptional repression of liver fatty acid-binding protein and microsomal triglyceride transfer protein blocks hepatic very low density lipoprotein secretion without hepatosteatosis.

Unlike the livers of humans and mice, and most hepatoma cells, which accumulate triglycerides when treated with microsomal triglyceride transfer protein (MTP) inhibitors, L35 rat hepatoma cells do not express MTP and cannot secrete very low density lipoprotein (VLDL), yet they do not accumulate triglyceride. In these studies we show that transcriptional co-repression of the two lipid transfer proteins, liver fatty acid-binding protein (L-FABP) and MTP, which cooperatively shunt fatty acids into de novo synthesized glycerolipids and the transfer of lipids into VLDL, respectively, act together to maintain hepatic lipid homeostasis. FAO rat hepatoma cells express L-FABP and MTP and demonstrate the ability to assemble and secrete VLDL. In contrast, L35 cells, derived as a single cell clone from FAO cells, do not express L-FABP or MTP nor do they assemble and secrete VLDL. We used these hepatoma cells to elucidate how a conserved DR1 promoter element present in the promoters of L-FABP and MTP affects transcription, expression, and VLDL production. In FAO cells, the DR1 elements of both L-FABP and MTP promoters are occupied by peroxisome proliferator-activated receptor alpha-retinoid X receptor alpha (RXRalpha), with which PGC-1beta activates transcription. In contrast, in L35 cells the DR1 elements of both L-FABP and MTP promoters are occupied by chicken ovalbumin upstream promoter transcription factor II, and transcription is diminished. The combined findings indicate that peroxisome proliferator-activated receptor alpha-RXRalpha and PGC-1beta coordinately up-regulate L-FABP and MTP expression, by competing with chicken ovalbumin upstream promoter transcription factor II for the DR1 sites in the proximal promoters of each gene. Additional studies show that ablation of L-FABP prevents hepatic steatosis caused by treating mice with an MTP inhibitor. Our findings show that reducing both L-FABP and MTP is an effective means to reduce VLDL secretion without causing hepatic steatosis.

Animals↗

PubMed2006

PI 3-kinases: hidden potentials revealed.

The potential oncogenicity of PI 3-kinases is revealed by two principal mechanisms: mutations causing gain of function and overexpression of wild-type proteins. Cancer-specific mutations in PIK3CA, the gene coding for the catalytic subunit p110alpha of PI 3-kinase, are oncogenic in the animal. These mutations are therefore significant determinants of the oncogenic cellular phenotype in human tumors and are appropriate and promising targets for small molecule inhibitors. Overexpression of wild-type p110beta, gamma and delta induces oncogenic transformation in cell culture. Although these non-alpha isoforms of PI 3-kinase have not been found mutated in human cancer, deregulated expression could contribute to cellular oncogenic properties and deserves increased attention.

Animals↗

PubMed2006

Oncogenic transformation induced by the p110beta, -gamma, and -delta isoforms of class I phosphoinositide 3-kinase.

Class I phosphoinositide 3-kinase contains four isoforms of the catalytic subunit, p110alpha, -beta, -gamma, and -delta. At physiological levels of expression, the wild-type p110alpha isoform lacks oncogenic potential, but gain-of-function mutations and overexpression of p110alpha are correlated with oncogenicity. The p110beta, -gamma, and -delta isoforms induce transformation of cultured cells as wild-type proteins. This oncogenic potential requires kinase activity and can be suppressed by the target of rapamycin inhibitor rapamycin. The p110delta isoform constitutively activates the Akt signaling pathway; p110gamma activates Akt only in the presence of serum. The isoforms differ in their requirements for upstream signaling. The transforming activity of the p110gamma isoform depends on rat sarcoma viral oncogene homolog (Ras) binding; preliminary data suggest the same for p110beta and indicate Ras-independent oncogenic potential of p110delta. The surprising oncogenic potential of the wild-type non-alpha isoforms of class I phosphoinositide 3-kinase may explain the dearth of cancer-specific mutations in these proteins, because these non-alpha isoforms could contribute to the oncogenic phenotype of the cell by differential expression.

Animals↗

PubMed2006

Cancer-specific mutations in PIK3CA are oncogenic in vivo.

The PIK3CA gene, coding for the catalytic subunit p110alpha of class IA phosphatidylinositol 3-kinases (PI3Ks), is frequently mutated in human cancer. Mutated p110alpha proteins show a gain of enzymatic function in vitro and are oncogenic in cell culture. Here, we show that three prevalent mutants of p110alpha, E542K, E545K, and H1047R, are oncogenic in vivo. They induce tumors in the chorioallantoic membrane of the chicken embryo and cause hemangiosarcomas in the animal. These tumors are marked by increased angiogenesis and an activation of the Akt pathway. The target of rapamycin inhibitor RAD001 blocks tumor growth induced by the H1047R p110alpha mutant. The in vivo oncogenicity of PIK3CA mutants in an avian species strongly suggests a critical role for these mutated proteins in human malignancies.

Animals↗

PubMed2006

Phosphoinositide 3-kinase: from viral oncoprotein to drug target.

The catalytic subunit p110alpha of the phosphoinositide 3-kinase (PI3K) and the serine-threonine protein kinase Akt have been extensively studied as retroviral oncoproteins. The experimental tools developed with the retroviral vectors are now being applied to PI3K mutations in human cancer. The most frequently occurring mutants of p110alpha are oncogenic in vitro and in vivo, show gain of enzymatic function, activate Akt, and their oncogenic activity is sensitive to rapamycin. The related isoforms p110beta, gamma and delta induce oncogenic transformation as wild-type proteins. Mutated p110alpha proteins are ideal drug targets. Identification of small molecule inhibitors that specifically target these mutant proteins is a realistic and urgent goal.

Animals↗

PubMed2006

Kinase inhibitors: vice becomes virtue.

In this issue of Cancer Cell, Fan and coworkers describe a novel inhibitor of PI3 kinase (PI3K) that potently interferes with the growth of glioma cells. They show that the efficacy of this inhibitor results from dual, synergistic activity against the p110alpha subunit of PI3K and against TOR. Although p110alpha and TOR belong to the same signaling pathway, they both must be inactivated because of the need to silence the regulatory feedback loop that remains unaffected by monospecific inhibitors. The new PI3K inhibitor achieves the effects of combination therapy as a single agent by fortuitously hitting two critical targets.

Animals↗

PubMed2005

Mutated PI 3-kinases: cancer targets on a silver platter.

The PI3K signaling pathway is upregulated in numerous cancers. The catalytic subunit p110alpha of PI3K shows hot spot mutations in nearly 30% of several types of solid tumors. The most prominent of these mutations result in gain of enzymatic function, activate Akt signaling and induce oncogenic cellular transformation. The mutated p110alpha proteins are ideal targets for specific small molecule inhibitors that discriminate between the oncogenic and the wild-type forms of the enzyme. Such inhibitors could become highly effective anti-cancer drugs.

Animals↗

PubMed2005

Phosphatidylinositol 3-kinase mutations identified in human cancer are oncogenic.

Mutations in genes that encode components of the phosphatidyl-inositol 3-kinase (PI3-kinase) signaling pathway are common in human cancer. The recent discovery of nonrandom somatic mutations in the PIK3CA gene of many human tumors suggests an oncogenic role for the mutated enzyme. We have determined the growth-regulatory and signaling properties of the three most frequently observed PI3-kinase mutations: E542K, E545K, and H1047R. Expressed in chicken embryo fibroblasts, all three mutants induce oncogenic transformation with high efficiency. This transforming ability is correlated with elevated catalytic activity in in vitro kinase assays. The mutant-transformed cells show constitutive phosphorylation of Akt, of p70 S6 kinase, and of the 4E-binding protein 1. Phosphorylation of S6 kinase and of 4E-binding protein 1 is regulated by the target of rapamycin (TOR) kinase and affects rates of protein synthesis. The inhibitor of TOR, rapamycin, strongly interferes with cellular transformation induced by the PI3-kinase mutants, suggesting that the TOR and its downstream targets are essential components of the transformation process. The oncogenic transforming activity makes the mutated PI3-kinase proteins promising targets for small molecule inhibitors that could be developed into effective and highly specific anticancer drugs.

Animals↗

PubMed2005

Oncogenic PI3K deregulates transcription and translation.

There have long been indications of a role for PI3K (phosphatidylinositol 3-kinase) in cancer pathogenesis. Experimental data document a requirement for deregulation of both transcription and translation in PI3K-mediated oncogenic transformation. The recent discoveries of cancer-specific mutations in PIK3CA, the gene that encodes the catalytic subunit p110alpha of PI3K, have heightened the interest in the oncogenic potential of this lipid kinase and have made p110alpha an ideal drug target.

Catalytic Domain↗

PubMed2003

ARP-1/COUP-TF II determines hepatoma phenotype by acting as both a transcriptional repressor of microsomal triglyceride transfer protein and an inducer of CYP7A1.

L35 and FAO cells were derived as single cell isolates from H35 cells. Whereas L35 cells do not express microsomal triglyceride transfer protein (MTP), which regulates lipoprotein secretion, they express CYP7A1, which regulates bile acid synthesis from cholesterol. FAO cells display the opposite phenotype (i.e. expression of MTP but not CYP7A1). We examined the molecular basis of the transcriptional inactivation of the MTP gene in L35 cells. Nested deletion and mutagenesis studies show that a conserved DR1 element within the 135-bp proximal MTP promoter is responsible for differential expression by L35 and FAO cells. Yeast one-hybrid screening identified apolipoprotein A1 regulatory protein-1/chicken ovalbumin upstream promoter transcription factor II (ARP-1/COUP-TFII) and retinoid X receptor (RXRalpha) as the protein factors that can bind to the conserved DR1 element. Nuclear extracts from L35 cells contained 2-fold more ARP-1/COUP-TFII and 50% less RXRalpha than those from FAO cells. Immunologic studies show that in L35 cells, ARP-1/COUP-TFII is bound to the DR1 element, whereas in FAO cells, a complex containing RXRalpha is bound to the DR1 element. Co-transfection studies show that ARP-1/COUP-TFII repressed MTP promoter activity by approximately 70% in FAO hepatoma cells, whereas RXRalpha and its ligand 9-cis-retinoic acid increased MTP promoter activity by 6-fold in L35 cells. The combined data suggest that in the context of the MTP promoter, ARP-1/COUP-TFII (repressor) and a complex containing RXRalpha (inducer) compete for the DR1 element. Analysis of the CYP7A1 promoter revealed that it is approximately 5-fold more active in L35 cells than in FAO cells. Co-transfection of an ARP-1/COUP-TFII expression vector showed that it enhances CYP7A1 promoter activity by 6-fold in FAO cells. These combined findings indicate that ARP-1/COUP-TFII acts as both a transcriptional repressor (of MTP) and as a transcription activator (of CYP7A1). This dual function of ARP-1/COUP-TFII may play an important role in determining the metabolic phenotype of individual liver cells.

Animals↗

PubMed2002

Microsomal triglyceride transfer protein is essential for hepatic secretion of apoB-100 and apoB-48 but not triglyceride.

Despite a complete lack of microsomal triglyceride transfer protein (MTP), L35 rat hepatoma cells secrete triglyceride-containing lipoproteins, albeit at a rate 25% of that of parental FAO hepatoma cells, which express high levels of MTP. The inability to express MTP was associated with a complete block in the secretion of both apolipoprotein (apo)B-100 and apoB-48. Stable expression of a MTP transgene restored the secretion of both apoB-100 and apoB-48 in L35 cells, indicating that MTP is essential for the secretion of both forms of apoB. Treatment with the MTP inhibitor BMS-200150 reduced the secretion of triglyceride by 70% in FAO cells, whereas the inhibitor did not affect the secretion of triglycerides by L35 cells. Thus, in the presence of the MTP inhibitor, both cell types secreted triglycerides at similar rates. Essentially, all of the triglycerides secreted by L35 cells were associated with HDL containing apoA-IV and apoE but devoid of apoB-100 or apoB-48. These results suggest that these triglyceride-containing lipoproteins are assembled and secreted via a pathway that is independent of both apoB and MTP. Our findings support the concept that apoB and MTP co-evolved and provided a means to augment the secretion of triglyceride through the formation of lipoproteins containing large hydrophobic cores enriched with triglycerides.

Animals↗