Ni Zeng

The Critical Role of AKT2 in Hepatic Steatosis Induced by PTEN Loss

Insulin signaling in the liver leads to accumulation of phosphatidylinositol (3,4,5)-trisphosphate (PIP3). Deletion of the phosphatase Pten (phosphatase and tensin homologue deleted on chromosome 10) reduces PIP3 levels and leads to fatty liver development. The purpose of this study was to investigate the mechanisms underlying lipogenesis that result from PIP3 accumulation using liver Pten-deletion mice. To explore the role of AKT2, the major liver AKT isoform in steatosis induced by deletion of Pten, we created mice lacking both Pten and Akt2 in hepatocytes and compared the effect of deleting Akt2 and Pten in the double mutants to the Pten deletion mice alone. Hepatic lipid accumulation was significantly reduced in mice lacking both PTEN and AKT2, as compared with Pten mutant mice alone. This effect was due to the role of AKT2 in maintaining expression of genes involved in de novo lipogenesis. We showed that lipid accumulation in the double mutant hepatocytes was partially reversed by expression of constitutive active FOXO1, a transcription factor downstream of AKT not dependent on inhibition of atypical protein kinase C. In summary, this study delineated regulation of lipid metabolism by PI3K signaling pathway by showing that AKT mediates PIP3 accumulation (mimicked by PTEN loss) induced lipid deposition in the liver and provided an important molecular mechanism for insulin-regulated hepatic lipogenesis.

Expansion of hepatic tumor progenitor cells in Pten-null mice requires liver injury and is reversed by loss of AKT2.

BACKGROUND & AIMS The tumor suppressor PTEN inhibits AKT2 signaling; both are aberrantly expressed in liver tumors. We investigated how PTEN and AKT2 regulate liver carcinogenesis. Loss of PTEN leads to spontaneous development of liver tumors from progenitor cells. We investigated how the loss of PTEN activates liver progenitor cells and induces tumorigenesis. METHODS We studied mice with liver-specific disruptions in Pten and the combination of Pten and Akt2 to investigate mechanisms of liver carcinogenesis. RESULTS PTEN loss leads to hepatic injury and establishes selective pressure for tumor-initiating cells (TICs), which proliferate to form mixed-lineage tumors. The Pten-null mice had increasing levels of hepatic injury before proliferation of hepatic progenitors. Attenuation of hepatic injury by deletion of Akt2 reduced progenitor cell proliferation and delayed tumor development. In Pten/Akt2-null mice given 3,5-diethoxycarbonyl-1,4 dihydrocollidine (DDC), we found that the primary effect of AKT2 loss was attenuation of hepatic injury and not inhibition of progenitor-cell proliferation in response to injury. CONCLUSIONS Liver carcinogenesis in Pten-null mice requires not only the transformation of TICs but selection pressure from hepatic injury and cell death, which activates TICs. Further research is required to elucidate the mechanism for hepatic injury and its relationship with TIC activation.

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) signaling regulates mitochondrial biogenesis and respiration via estrogen-related receptor α (ERRα).

Background: Aberrant PTEN/PI3K signaling and mitochondrial abnormalities are commonly associated with cancer. Results: PTEN loss and activation of PI3K/protein kinase B up-regulate ERRα, increase mitochondrial mass, and induce a metabolic pattern similar to the “Warburg effect.” Conclusion: PTEN/PI3K signaling controls mitochondrial mass and function by regulating ERRα through the AKT/CREB axis. Significance: This study establishes a novel link between oncogenic signaling and dysregulated mitochondrial metabolism. Mitochondrial abnormalities are associated with cancer development, yet how oncogenic signals affect mitochondrial functions has not been fully understood. In this study, we investigate the relationship between mitochondrial alterations and PI3K/protein kinase B (AKT) signaling activation using hepatocytes and liver tissues as our experimental models. We show here that liver-specific deletion of Pten, which leads to activation of PI3K/AKT, is associated with elevated oxidative stress, increased mitochondrial mass, and augmented respiration accompanied by enhanced glycolysis. Consistent with these observations, estrogen-related receptor α (ERRα), an orphan nuclear receptor known for its role in mitochondrial biogenesis, is up-regulated in the absence of phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Our pharmacological and genetic studies show that PI3K/AKT activity regulates the expression of ERRα and mitochondrial biogenesis/respiration. Furthermore, cAMP-response element-binding protein, as a downstream target of AKT, plays a role in the regulation of ERRα, independent of PKA signaling. ERRα regulates reactive oxygen species production, and ERRα knockdown attenuates proliferation and colony-forming potential in Pten-null hepatocytes. Finally, analysis of clinical datasets from liver tissues showed a negative correlation between expressions of ERRα and PTEN in patients with liver cancer. Therefore, this study has established a previously unrecognized link between a growth signal and mitochondrial metabolism.

PTEN CONTROLS β-CELL REGENERATION IN AGED MICE BY REGULATING CELL CYCLE INHIBITOR P16INK4A

Tissue regeneration diminishes with age, concurrent with declining hormone levels including growth factors such as insulin‐like growth factor‐1 (IGF‐1). We investigated the molecular basis for such decline in pancreatic β‐cells where loss of proliferation occurs early in age and is proposed to contribute to the pathogenesis of diabetes. We studied the regeneration capacity of β‐cells in mouse model where PI3K/AKT pathway downstream of insulin/IGF‐1 signaling is upregulated by genetic deletion of Pten (phosphatase and tensin homologue deleted on chromosome 10) specifically in insulin‐producing cells. In this model, PTEN loss prevents the decline in proliferation capacity in aged β‐cells and restores the ability of aged β‐cells to respond to injury‐induced regeneration. Using several animal and cell models where we can manipulate PTEN expression, we found that PTEN blocks cell cycle re‐entry through a novel pathway leading to an increase in p16ink4a, a cell cycle inhibitor characterized for its role in cellular senescence/aging. A downregulation in p16ink4a occurs when PTEN is lost as a result of cyclin D1 induction and the activation of E2F transcription factors. The activation of E2F transcriptional factors leads to methylation of p16ink4a promoter, an event that is mediated by the upregulation of polycomb protein, Ezh2. These analyses establish a novel PTEN/cyclin D1/E2F/Ezh2/p16ink4a signaling network responsible for the aging process and provide specific evidence for a molecular paradigm that explain how decline in growth factor signals such as IGF‐1 (through PTEN/PI3K signaling) may control regeneration and the lack thereof in aging cells.

PI3K/AKT signaling regulates bioenergetics in immortalized hepatocytes

Regulation of cellular bioenergetics by PI3K/AKT signaling was examined in isogenic hepatocyte cell lines lacking the major inhibitor of PI3K/AKT signaling, PTEN (phosphatase and tensin homolog deleted on chromosome 10). PI3K/AKT signaling was manipulated using the activator (IGF-1) and the inhibitor (LY 294002) of the PI3K/AKT pathway. Activation of PI3K/AKT signaling resulted in an enhanced anaerobic glycolysis and mitochondrial respiration. AKT, when phosphorylated and activated, translocated to mitochondria and localized within the membrane structure of mitochondria, where it phosphorylated a number of mitochondrial-resident proteins including the subunits α and β of ATP synthase. Inhibition of GSK3β by either phosphorylation by AKT or lithium chloride resulted in activation of pyruvate dehydrogenase, i.e., a decrease in its phosphorylated form. AKT-dependent phosphorylation of ATP synthase subunits α and β resulted in an increased complex activity. AKT translocation to mitochondria was associated with an increased expression and activity of complex I. These data suggest that the mitochondrial signaling pathway AKT/GSK3β/PDH, AKT-dependent phosphorylation of ATP synthase, and upregulation of mitochondrial complex I expression and activity are involved in the control of mitochondrial bioenergetics by increasing substrate availability and regulating the mitochondrial catalytic/energy-transducing capacity.

Adaptive Basal Phosphorylation of eIF2α Is Responsible for Resistance to Cellular Stress–Induced Cell Death in Pten-Null Hepatocytes

The α-subunit of eukaryotic initiation factor 2 (eIF2α) is a key translation regulator that plays an important role in cellular stress responses. In the present study, we investigated how eIF2α phosphorylation can be regulated by a tumor suppressor PTEN (phosphatase and tensin homolog deleted on chromosome 10) and how such regulation is used by PTEN-deficient hepatocytes to adapt and cope with oxidative stress. We found that eIF2α was hyperphosphorylated when Pten was deleted, and this process was AKT dependent. Consistent with this finding, we found that the Pten-null cells developed resistance to oxidative glutamate and H2O2-induced cellular toxicity. We showed that the messenger level of CReP (constitutive repressor of eIF2α phosphorylation), a constitutive phosphatase of eIF2α, was downregulated in Pten-null hepatocytes, providing a possible mechanism through which PTEN/AKT pathway regulates eIF2α phosphorylation. Ectopic expression of CReP restored the sensitivity of the Pten mutant hepatocytes to oxidative stress, confirming the functional significance of the downregulated CReP and upregulated phospho-eIF2α in the resistance of Pten mutant hepatocytes to cellular stress. In summary, our study suggested a novel role of PTEN in regulating stress response through modulating the CReP/eIF2α pathway. Mol Cancer Res; 9(12); 1708–17. ©2011 AACR.

Crosstalk Between Activated Myofibroblasts and β Cells in Injured Mouse Pancreas.

Objectives In injury conditions, myofibroblasts are induced to lay down matrix proteins and support the repair process. In this study, we investigated the role of myofibroblasts, particularly stellate cells, in the growth and regeneration of pancreatic &bgr; cells. Methods We used both in vitro and in vivo approaches to address whether stellate cells may promote the growth of &bgr; cells. Results Our experiments demonstrated that activated stellate cells support the proliferation of &bgr; cells in vitro. In vivo, mesenchymals surrounding the pancreatic islets are activated (induced to proliferate) in the islet regeneration model of Pten null mice. These mesenchymals display markers of pancreatic stellate cells, such as desmin and to a lesser extent, smooth muscle actin &agr;. We have shown previously that targeted &bgr;-cell deletion of Pten lead to a significant increase in total islet mass. This phenotype was accompanied by an increase in peri-islet mitotic activity, particularly in islets injured by streptozotocin, a &bgr; cell–specific toxin. Conclusions Together with the in vitro observations, our data, here, suggest that that these mesenchymal cells may support the regeneration of the islets. Identifying how the communication occurs may provide clinically relevant mechanism for inducing &bgr;-cell regeneration.

The Role of PTEN in β-Cell Growth.

This paper describes the biological functions of PTEN and the PTEN regulated signaling pathway in pancreatic β-cells. PTEN has been shown to regulate the regeneration of β-cells. We review the pathways that are controlled by PTEN signaling and their functions in β-cell regeneration. In particular, we describe the unique effect of Pten deletion in β-cells. Unlike its effect in other tissues, Pten deletion does not lead to tumor formation but does enhance β-cell proliferation and function. In addition to the literature review, we also report new results exploring PTEN loss in adult β-cells. We demonstrate that inducing PTEN loss in adult cells has the same regenerative effects previously found for prenatal deletion.

R1 Regulates Prostate Tumor Growth and Progression By Transcriptional Suppression of the E3 Ligase HUWE1 to Stabilize c-Myc

Prostate cancer is a prevalent public health problem, especially because noncutaneous advanced malignant forms significantly affect the lifespan and quality of life of men worldwide. New therapeutic targets and approaches are urgently needed. The current study reports elevated expression of R1 (CDCA7L/RAM2/JPO2), a c-Myc–interacting protein and transcription factor, in human prostate cancer tissue specimens. In a clinical cohort, high R1 expression is associated with disease recurrence and decreased patient survival. Overexpression and knockdown of R1 in human prostate cancer cells indicate that R1 induces cell proliferation and colony formation. Moreover, silencing R1 dramatically reduces the growth of prostate tumor xenografts in mice. Mechanistically, R1 increases c-Myc protein stability by inhibiting ubiquitination and proteolysis through transcriptional suppression of HUWE1, a c-Myc–targeting E3 ligase, via direct interaction with a binding element in the promoter. Moreover, transcriptional repression is supported by a negative coexpression correlation between R1 and HUWE1 in a prostate cancer clinical dataset. Collectively, these findings, for the first time, characterize the contribution of R1 to prostate cancer pathogenesis. Implications: These findings provide evidence that R1 is a novel regulator of prostate tumor growth by stabilizing c-Myc protein, meriting further investigation of its therapeutic and prognostic potential.

Abstract 3900: Pten deletion in SOX9+ cells leads to tumor initiating cell expansion and tumor development in mouse liver

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Liver cancer is one of the most aggressive malignancies with a five-year survival rate less than 10%. There is thus an urgent unmet need to understand the process of liver tumorigenesis in order to develop early diagnosis and effective therapeutic treatments. Clinical studies have suggested that tumor initiating cell (TIC) activation is observed in over 70% of liver cancer samples and is closely linked with poor survival outcome after treatment. Consistent with these observations, our previous studies utilizing a Pten (loxp/loxp); Albumin-Cre+ mouse liver cancer model also confirmed the expansion of TICs during liver tumorigenesis. However, how TICs are activated and what mechanisms contribute to liver tumor development remains unclear. Here, we studied the role of tumor suppressor PTEN in TICs using the Pten (loxp/loxp); SOX9-CreER+ mouse model. Lineage tracing results indicated that in the liver SOX9+ cells possess the progenitor cell characteristics and give rise to bile duct cells and peri-portal hepatocytes later on. Pten deletion in SOX9+ cells at 4 weeks of age leads to expansion of both CK+ duct cells and HNF4+ hepatocytes in the peri-portal areas. Interestingly, we also observed tumor development at 12 months old mutant mice and close examinations revealed that the tumors are heterogeneous, indicating the contribution of TICs during this process. Together, our study suggested that the PTEN signaling is critical for TICs regulation and that TICs activation plays a key role in liver tumorigenesis. Citation Format: Ni Zeng, Janel Kopp, Lina He, Maike Sander, Bangyan Stiles. Pten deletion in SOX9+ cells leads to tumor initiating cell expansion and tumor development in mouse liver. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3900. doi:10.1158/1538-7445.AM2014-3900