Overexpression of Akt/PKB modulates N-myristoyltransferase activity in cancer cells.

N-myristoyltransferase (NMT) catalyses the myristoylation reaction. Since NMT activity is elevated in various cancers and activated Akt/PKB leads to cell survival, we were interested in studying if activation of Akt/PKB has any effect on NMT. Overexpression of constitutively active Akt/PKB in HepG2 cells (HepG2-CA-Akt/PKB) led to an approximately 50% reduction of NMT compared with parental HepG2 cells. Reduced NMT activity in HepG2-CA-Akt/PKB was found to be due to the NMT1 phosphorylation. We determined NMT activity in various human breast cancer cell lines with differing metastatic potentials and pseudo-normal breast cells (HBL-100). Tumourigenic or metastatic breast cancer cell lines such as MDA-MB-231, MDA-MB-435, and Hs 578T displayed reduced NMT activity. Western blot analysis revealed that NMT1 is phosphorylated in these breast cancer cells. Furthermore, patients' breast cancer tissue array revealed strong positivity and high intensity for NMT in malignant breast tissues compared with normal breast cells. A gradation in the NMT staining was observed for grade I, II, and III infiltrating ductal carcinoma breast tissues. These studies demonstrate that overexpression of Akt/PKB results in NMT1 phosphorylation and that NMT1 is phosphorylated in breast cancer cells. Immunohistochemical analysis suggests that NMT may prove to be an added diagnostic biomarker for breast cancer.

Effects of rapamycin on cell proliferation and phosphorylation of mTOR and p70(S6K) in HepG2 and HepG2 cells overexpressing constitutively active Akt/PKB.

Mammalian target of rapamycin (mTOR) is a serine-threonine kinase that plays an important role in the regulation of cell proliferation and protein synthesis through the activation of its downstream target ribosomal p70 S6 kinase (p70(S6K)). The levels of p-mTOR are regulated by the protein kinase B (Akt/PKB). Therefore, the effects of insulin and rapamycin (an inhibitor of mTOR) on the phosphorylation of mTOR (Ser 2448) and p70(S6K) (Thr 389) as well as on cell proliferation in parental HepG2 cells and HepG2 cells overexpressing constitutively active Akt/PKB (HepG2-CA-Akt/PKB) were studied. Insulin increased the levels of phosphorylated mTOR and p70(S6K) in both the cell lines. Rapamycin treatment partially decreased the phosphorylation of mTOR but completely abolished the phosphorylation of p70(S6K) in the absence as well as presence of insulin in both cell lines. The effect of insulin and rapamycin on the cell proliferation in both cell lines was further studied. In the presence of serum, parental HepG2 cells and HepG2-CA-Akt/PKB showed an increase in cell proliferation until 120 and 168 h respectively. Rapamycin inhibited cell proliferation under all experimental conditions more evident under serum deprived conditions. Parental HepG2 cells showed decline in the cell proliferation after 48 h and the presence of insulin prolonged cell survival until 120 h and this effect were also inhibited by rapamycin under serum deprived conditions. On the contrary, HepG2-CA-Akt/PKB cells continued proliferation until 192 h. The effects of insulin on cell proliferation were more pronounced in parental HepG2 cells as compared to HepG2-CA-Akt/PKB cells. Long term effects of rapamcyin significantly decreased the levels of p-mTOR (Ser 2448) both in the presence and absence of insulin in these cells. A positive correlation between the levels of p-mTOR (Ser2448) and cell proliferation was observed (99% confidence interval, r(2)=0.525, p<0.0001). These results suggest that rapamycin causes a decline in the cell growth through the inhibition of mTOR.

Overexpression of Akt1 upregulates glycogen synthase activity and phosphorylation of mTOR in IRS-1 knockdown HepG2 cells.

Insulin receptor substrate (IRS) proteins are important docking proteins in mediating the insulin signaling cascade. We have investigated the effect of short interfering RNA (siRNA) mediated knockdown of IRS-1 on insulin signaling cascade in primary human hepatocellular carcinoma HepG2 cell line and HepG2 cells overexpressing Akt1/PKB-alpha (HepG2-CA-Akt/PKB). IRS-1 knockdown in both cell lines resulted in reduction of insulin stimulated Akt1 phosphorylation at Ser 473. In parental HepG2 cells, IRS-1 knockdown resulted in reduction (ca. 50%) in the basal level of phosphorylated mTOR (Ser 2448) irrespective of insulin treatment. In contrast, HepG2-CA-Akt/PKB cells showed an upregulation in the basal level of phosphorylated mTOR (Ser 2448) (ca. 40%). Insulin mediated phosphorylation of mTOR was reduced. IRS-1 knockdown also reduced the cell proliferation of parental HepG2 cells by ca. 30% in the presence/absence of insulin, whereas in HepG2-CA-Akt/PKB the cell proliferation was reduced by 15% and treatment of insulin further reduced it to ca. 50% (vs. control). IRS-1 knockdown also reduced the glycogen synthase (GS) activity in parental HepG2 cells, however, it was upregulated in HepG2-CA-Akt/PKB cells. These results suggest that knockdown of IRS-1 abolished basal as well as insulin mediated phosphorylation/activity of proteins involved in cell proliferation or glycogen metabolism in the parental Hep2 cells. IRS-1 knockdown in cells overexpressing constitutively active Akt1/PKB-alpha either did not change or upregulated the basal levels of phosphorylated/active proteins. However, insulin mediated response was either not altered or downregulated in these cells.

Modulation of insulin effects on phosphorylation of protein kinase B and glycogen synthesis by tumor necrosis factor-alpha in HepG2 cells.

The effects of tumor necrosis factor-alpha (TNF-alpha) on insulin-induced phosphorylation of protein kinase B-alpha (PKB-alpha) and downstream enzyme glycogen synthase kinase-3 beta (GSK-3 beta) was examined in HepG2 liver cells. The exogenous treatment of HepG2 cells with TNF-alpha for 1 h caused phosphorylation of Ser473 and Thr308 residues of PKB-alpha. The maximal phosphorylation (approximately 4-fold) was obtained with 1 ng/ml TNF-alpha and no further increase was observed with higher concentrations of this cytokine. The cells pretreated with TNF-alpha for 1 h followed by incubation with insulin (10 nM) showed near additive effect on phosphorylation of PKB-alpha and downstream enzyme GSK-3 beta. The long-term (4, 8, 24 h) exogenous treatment of cells with optimal (1 ng/ml) concentration of TNF-alpha also caused phosphorylation of PKB-alpha, albeit to a lesser degree. However, long-term pretreatments of cells with TNF-alpha reduced insulin-stimulated phosphorylation of PKB-alpha and GSK-3 beta. Short- and long-term preincubation of HepG2 cells with TNF-alpha also resulted in parallel changes in glycogen synthesis in the presence of insulin. In fact, long-term preincubation with TNF-alpha completely abolished the insulin-induced glycogen synthesis. These results suggest that short-term exposure to TNF-alpha augments insulin effects whereas long-term exposure causes insulin resistance in HepG2 cells.

Decreased activity and impaired induction of nitric oxide synthase by lipopolysaccharides in streptozotocin-induced diabetic rats.

The effect of diabetes was determined on nitric oxide synthase (NOS) activity in rat heart and liver. The diabetes was induced by streptozotocin (STZ) and NOS activity was determined after 1 or 12 weeks post-STZ injection. In both tissues, the majority of NOS activity was associated with endothelial constitutive calcium-sensitive NOS (ecNOS) isoform and found in the particulate (100,000xg pellet) fraction in young rats. The diabetes as well as age reduced this activity significantly in heart, whereas only the age caused a decrease in ecNOS activity in liver tissue. Lipopolysaccharides (LPS) induced calcium-insensitive iNOS activity in both young and old rats. The induction was significantly higher (up to 10-fold) in liver as compared to heart. Although the maximum induction of iNOS in young rats was almost similar in diabetic tissues as compared to control animals, there was a lag period for induction of iNOS in diabetic tissues. In old diabetic rats, the induction by LPS was almost completely abolished. These results suggest that diabetes causes either no change or a decrease in ecNOS activity and impairment in the induction of iNOS by LPS in rat heart and liver.

Long-term effects of rapamycin treatment on insulin mediated phosphorylation of Akt/PKB and glycogen synthase activity.

Protein kinase B (Akt/PKB) is a Ser/Thr kinase that is involved in the regulation of cell proliferation/survival through mammalian target of rapamycin (mTOR) and the regulation of glycogen metabolism through glycogen synthase kinase 3beta (GSK-3beta) and glycogen synthase (GS). Rapamycin is an inhibitor of mTOR. The objective of this study was to investigate the effects of rapamycin pretreatment on the insulin mediated phosphorylation of Akt/PKB phosphorylation and GS activity in parental HepG2 and HepG2 cells with overexpression of constitutively active Akt1/PKB-alpha (HepG2-CA-Akt/PKB). Rapamycin pretreatment resulted in a decrease (20-30%) in the insulin mediated phosphorylation of Akt1 (Ser 473) in parental HepG2 cells but showed an upregulation of phosphorylation in HepG2-CA-Akt/PKB cells. Rictor levels were decreased (20-50%) in parental HepG2 cells but were not significantly altered in the HepG2-CA-Akt/PKB cells. Furthermore, rictor knockdown decreased the phosphorylation of Akt (Ser 473) by 40-60% upon rapamycin pretreatment. GS activity followed similar trends as that of phosphorylated Akt and so with rictor levels in these cells pretreated with rapamycin; parental HepG2 cells showed a decrease in GS activity, whereas as HepG2-CA-Akt/PKB cells showed an increase in GS activity. The changes in the levels of phosphorylated Akt/PKB (Ser 473) correlated with GS and protein phoshatase-1 activity.

Long-term effects of tumor necrosis factor-alpha treatment on insulin signaling pathway in HepG2 cells and HepG2 cells overexpressing constitutively active Akt/PKB.

Tumor necrosis factor-alpha (TNF-alpha) mediated attenuation of insulin signaling pathway is an important cause in several disorders like obesity, obesity linked diabetes mellitus. TNF-alpha actions vary depending upon concentration and time of exposure in various cells. In the present study, the effects of long-term TNF-alpha (1 ng/ml) exposure on the components of insulin signaling pathway in HepG2 and HepG2 cells overexpressing constitutively active Akt1/PKB-alpha (HepG2-CA-Akt/PKB) have been investigated. In parental HepG2 cells, TNF-alpha treatment for 24 h reduced the phosphorylation of Akt1/PKB-alpha and GSK-3beta and under these conditions cells also showed reduced insulin responsiveness in terms of Akt1/PKB-alpha and GSK-3beta phosphorylation. TNF-alpha pre-incubated HepG2-CA-Akt/PKB cells showed lower reduction in Akt1/PKB-alpha and GSK-3beta phosphorylation and insulin responsiveness after 24 h as compared to parental HepG2 cells. We report that the long-term TNF-alpha pre-incubation in both parental HepG2 and HepG2-CA-Akt/PKB-alpha cells leads to the reduction in the levels of IRS-1 without altering the levels of IRS-2. In order to understand the reason for the differential insulin resistance in both the cell types, the effect of long-term TNF-alpha treatment on the proteins upstream to Akt/PKB was investigated. TNF-alpha pre-incubation also showed reduced insulin-stimulated Tyr phosphorylation of insulin receptor (IR-beta) in both the cell types, moreover hyperphosphorylation of IRS-1 at Ser 312 residue was observed in TNF-alpha pre-incubated cells. As hyperphosphorylation of IRS-1 at Ser 312 can induce its degradation, it is possible that reduced insulin responsiveness after long-term TNF-alpha pre-incubation observed in this study is due to the decrease in IRS-1 levels.

Unique ability of troglitazone to up-regulate peroxisome proliferator-activated receptor-gamma expression in hepatocytes.

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a nuclear receptor that is activated by the binding of an appropriate ligand. Several studies have demonstrated that certain ligands can also induce the expression of PPAR-gamma. In the present study, we examined the mechanism whereby this induction occurs by specifically addressing whether potentiation of the transactivation function of PPAR-gamma per se leads to induction of expression. We observed that thiazolidinediones, a group of insulin-sensitizing drugs, had differential effects, with troglitazone inducing protein levels of PPAR-gamma, while rosiglitazone, englitazone, and ciglitazone were without effect. Similarly, the prostaglandin metabolite 15-deoxy-Delta(12,14)-prostaglandin J(2) and the potent synthetic ligand GW1929 (N-(2-benzoyl phenyl)-L-tyrosine) also had no effect, as did ligands for other isoforms of PPAR. Since troglitazone has antioxidant properties, we also examined the effect of alpha-tocopherol and observed that it induced PPAR-gamma expression in a dose-dependent fashion. Finally, we found that mice fed troglitazone as a dietary admixture displayed an up-regulation of hepatic PPAR-gamma mRNA and protein, indicating that the mechanism of action is at the level of gene expression and not protein stability. These data indicate that 1) up-regulation of the transactivation function of PPAR-gamma does not alone account for the induction of expression of PPAR-gamma by troglitazone, and 2) an antioxidant-related mechanism may be involved.

Effect of overexpression and nuclear translocation of constitutively active PKB-alpha on cellular survival and proliferation in HepG2 cells.

Protein kinase B (Akt/PKB) is a key component in the PI 3-kinase mediated cell survival pathway and has oncogenic transformation potential. Although the over-expression of PKB-alpha can prevent cell death following growth factor withdrawal, the long-term effects of stable over-expression of PKB-alpha on cell survival in the absence of growth factors remain to be resolved. In the present study, we generated HepG2 cells with stable expression of active PKB-alpha and compared its characteristics with HepG2 cells. Basal as well as insulin-stimulated levels of Ser(473) and Thr(308) phosphorylation in PKB-alpha transfected HepG2 cells were much higher than HepG2 cells. Constitutive expression of active PKB-alpha enabled HepG2 cells to survive up to 96 h without serum in growth media while HepG2 cells fail to survive after 48 h of serum withdrawal. A strong positive correlation (R(2) = 0.71) between cell proliferation and phosphorylated form of PKB-alpha at Thr(308) was observed along with higher levels of phosphorylated 3'-phosphoinositide-dependent kinase-1 (PDK-1). HepG2 cells with constitutive expression of active PKB-alpha also showed higher levels of phosphorylated p65 subunit of nuclear factor-kappaB (NFkappaB) in comparison with HepG2 cells. Predominant nuclear localization of phosphorylated PKB-alpha was observed in stably transfected HepG2 cells. These results indicate that constitutive expression of active PKB-alpha renders HepG2 cells independent of serum based growth factors for survival and proliferation.