Suppression of AKT phosphorylation restores rapamycin-based synthetic lethality in SMAD4-defective pancreatic cancer cells

Current State and Future Challenges for PI3K Inhibitors in Cancer Therapy

The phosphoinositide 3 kinase (PI3K)-protein kinase B (PKB/AKT)-mammalian target of the rapamycin (mTOR) axis is a key signal transduction system that links oncogenes and multiple receptor classes which are involved in many essential cellular functions. Aberrant PI3K signalling is one of the most commonly mutated pathways in cancer. Consequently, more than 40 compounds targeting key components of this signalling network have been tested in clinical trials among various types of cancer. As the oncogenic activation of the PI3K/AKT/mTOR pathway often occurs alongside mutations in other signalling networks, combination therapy should be considered. In this review, we highlight recent advances in the knowledge of the PI3K pathway and discuss the current state and future challenges of targeting this pathway in clinical practice.

Cancer cells with defective RB and CDKN2A are resistant to the apoptotic effects of rapamycin

Inhibition of mammalian target of rapamycin complex 1 (mTORC1) with rapamycin in the absence of transforming growth factor-β (TGFβ) signaling induces apoptosis in many cancer cell lines. In the presence of TGFβ, rapamycin induces G₁ cell cycle arrest; however, in the absence of TGFβ, cells do not arrest in G₁ and progress into S-phase where rapamycin is cytotoxic rather than cytostatic. However, we observed that DU145 prostate and NCI-H2228 lung cancer cells were resistant to the cytotoxic effect of rapamycin. Of interest, the rapamycin-resistant DU145 and NCI-H2228 cells have mutations in the RB and CDKN2A tumor suppressor genes. The gene products of RB and CDKN2A (pRb and p14^ARF) suppress E2F family transcription factors that promote cell cycle progression from G₁ into S. Restoration of wild type RB or inhibition of E2F activity in DU145 and NCI-H2228 cells led to rapamycin sensitivity. These data provide evidence that the combination of mutant RB and mutant CDKN2A in cancer cells leads to rapamycin resistance, which has implications for precision medicine approaches to anti-cancer therapies.

A novel mechanism of plasminogen activation in epithelial and mesenchymal cells

Cancer dissemination is initiated by the movement of cells into the vasculature which has been reported to be triggered by EMT (epithelial to mesenchymal transition). Cellular dissemination also requires proteases that remodel the extracellular matrix. The protease, plasmin is a prominent player in matrix remodeling and invasion. Despite the contribution of both EMT and the plasminogen activation (PA) system to cell dissemination, these processes have never been functionally linked. We reveal that canonical Smad-dependent TGFβ1 signaling and FOXC2-mediated PI3K signaling in cells undergoing EMT reciprocally modulate plasminogen activation partly by regulating the plasminogen receptor, S100A10 and the plasminogen activation inhibitor, PAI-1. Plasminogen activation and plasminogen-dependent invasion were more prominent in epithelial-like cells and were partly dictated by the expression of S100A10 and PAI-1.

5-Aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR) enhances the efficacy of rapamycin in human cancer cells

mTOR - the mammalian/mechanistic target of rapamycin - has been implicated as a key signaling node for promoting survival of cancer cells. However, clinical trials that have targeted mTOR with rapamycin or rapamycin analogs have had minimal impact. In spite of the high specificity of rapamycin for mTOR, the doses needed to suppress key mTOR substrates have proved toxic. We report here that rapamycin when combined with AICAR - a compound that activates AMP-activated protein kinase makes rapamycin cytotoxic rather than cytostatic at doses that are tolerated clinically. AICAR by itself is able to suppress mTOR complex 1 (mTORC1), but also stimulates a feedback activation of mTORC2, which activates the survival kinase Akt. However, AICAR also suppresses production of phosphatidic acid (PA), which interacts with mTOR in a manner that is competitive with rapamycin. The reduced level of PA sensitizes mTORC2 to rapamycin at tolerable nano-molar doses leading reduced Akt phosphorylation and apoptosis. This study reveals how the use of AICAR enhances the efficacy of rapamycin such that rapamycin at low nano-molar doses can suppress mTORC2 and induce apoptosis in human cancer cells at doses that are clinically tolerable.

The Enigma of Rapamycin Dosage

The mTOR pathway is a critical regulator of cell growth, proliferation, metabolism, and survival. Dysregulation of mTOR signaling has been observed in most cancers and, thus, the mTOR pathway has been extensively studied for therapeutic intervention. Rapamycin is a natural product that inhibits mTOR with high specificity. However, its efficacy varies by dose in several contexts. First, different doses of rapamycin are needed to suppress mTOR in different cell lines; second, different doses of rapamycin are needed to suppress the phosphorylation of different mTOR substrates; and third, there is a differential sensitivity of the two mTOR complexes mTORC1 and mTORC2 to rapamycin. Intriguingly, the enigmatic properties of rapamycin dosage can be explained in large part by the competition between rapamycin and phosphatidic acid (PA) for mTOR. Rapamycin and PA have opposite effects on mTOR whereby rapamycin destabilizes and PA stabilizes both mTOR complexes. In this review, we discuss the properties of rapamycin dosage in the context of anticancer therapeutics.

Apoptotic effects of high-dose rapamycin occur in S-phase of the cell cycle

Mutations in genes encoding regulators of mTOR, the mammalian target of rapamycin, commonly provide survival signals in cancer cells. Rapamycin and analogs of rapamycin have been used with limited success in clinical trials to target mTOR-dependent survival signals in a variety of human cancers. Suppression of mTOR predominantly causes G1 cell cycle arrest, which likely contributes to the ineffectiveness of rapamycin-based therapeutic strategies. While rapamycin causes the accumulation of cells in G1, its effect in other cell cycle phases remains largely unexplored. We report here that when synchronized MDA-MB-231 breast cancer cells are allowed to progress into S-phase from G1, rapamycin activates the apoptotic machinery with a concomitant increase in cell death. In Calu-1 lung cancer cells, rapamycin induced a feedback increase in Akt phosphorylation at Ser473 in S-phase that mitigated rapamycin-induced apoptosis. However, sensitivity to rapamycin in S-phase could be reestablished if Akt phosphorylation was suppressed. We recently reported that glutamine (Gln) deprivation causes K-Ras mutant cancer cells to aberrantly arrest primarily in S-phase. Consistent with observed sensitivity of S-phase cells to rapamycin, interfering with Gln utilization sensitized both MDA-MB-231 and Calu-1 K-Ras mutant cancer cells to the apoptotic effect of rapamycin. Importantly, rapamycin induced substantially higher levels of cell death upon Gln depletion than that observed in cancer cells that were allowed to progress through S-phase after being synchronized in G1. We postulate that exploiting metabolic vulnerabilities in cancer cells such as S-phase arrest observed with K-Ras-driven cancer cells deprived of Gln, could be of great therapeutic potential.

Rapamycin-induced G1 cell cycle arrest employs both TGF-β and Rb pathways

The mammalian target of rapamycin complex 1 (mTORC1) is a critical regulator of G1 cell cycle progression. Two key substrates of mTORC1 are ribosomal subunit S6 kinase (S6K) and eukaryotic initiation factor 4E (eIF4E) binding protein-1 (4E-BP1). We reported previously that simultaneous knockdown of S6K and eIF4E causes a transforming growth factor-β (TGF-β)-dependent G1 cell cycle arrest in MDA-MB-231 human breast cancer cells. Rapamycin inhibits the phosphorylation of S6K at nano-molar concentrations in MDA-MB-231 cells; however, micro-molar concentrations of rapamycin are required to inhibit phosphorylation of 4E-BP1 - the phosphorylation of which liberates eIF4E to initiate translation. Micro-molar doses of rapamycin are required for complete G1 cell cycle arrest - indicating that 4E-BP1 is a critical target of mTOR for promoting cell cycle progression. Data are provided demonstrating that G1 cell cycle arrest induced by rapamycin is due to up-regulation of TGF-β signaling and down-regulation of Rb phosphorylation via phosphorylation of the mTORC1 substrates S6K and 4E-BP1 respectively. These findings enhance the current understanding of the cytostatic effects of mTORC1 suppression with therapeutic implications.

(-)-Oleocanthal rapidly and selectively induces cancer cell death via lysosomal membrane permeabilization

(-)-Oleocanthal (OC), a phenolic compound present in extra-virgin olive oil (EVOO), has been implicated in the health benefits associated with diets rich in EVOO. We investigated the effect of OC on human cancer cell lines in culture and found that OC induced cell death in all cancer cells examined as rapidly as 30 minutes after treatment in the absence of serum. OC treatment of non-transformed cells suppressed their proliferation but did not cause cell death. OC induced both primary necrotic and apoptotic cell death via induction of lysosomal membrane permeabilization (LMP). We provide evidence that OC promotes LMP by inhibiting acid sphingomyelinase (ASM) activity, which destabilizes the interaction between proteins required for lysosomal membrane stability. The data presented here indicate that cancer cells, which tend to have fragile lysosomal membranes compared to non-cancerous cells, are susceptible to cell death induced by lysosomotropic agents. Therefore, targeting lysosomal membrane stability represents a novel approach for the induction of cancer-specific cell death.

MicroRNA-26b inhibits epithelial-mesenchymal transition in hepatocellular carcinoma by targeting USP9X

BACKGROUND

Metastasis is responsible for the rapid recurrence and poor survival of malignancies. Epithelial-mesenchymal transition (EMT) has a critical role in metastasis. Increasing evidence indicates that EMT can be regulated by microRNAs (miRNAs). The aim of this study was to investigate the role of miR-26b in modulating epithelial-mesenchymal transition (EMT) in hepatocellular carcinoma (HCC), as well as to identify its underlying mechanism of action.

METHODS

The expression level of miR-26b was assessed in multiple HCC cell lines (HepG2, MHCC97H, Hep3B, MHCC97L, HCCC9810, BEL-7402, Huh7 and QGY-7703), as well as in liver tissue from patients with HCC. Follow-up studies examined the effects of a miR-26b mimic (increased expression) and a miR-26b inhibitor (decreased expression) on markers of EMT, wound healing and cell migration. The molecular target of miR-26b was also identified using a computer algorithm and confirmed experimentally.

RESULTS

MiR-26b expression was decreased in HCC cell lines and was inversely correlated with the grade of HCC. Increased expression of miR-26b inhibited the migration and invasiveness of HCC cell lines, which was accompanied by decreased expression of the epithelial marker E-cadherin and increased expression of the mesenchymal marker vimentin, at both the mRNA and protein expression levels. A binding site for miR-26b was theoretically identified in the 3'UTR of USP9X. Further studies revealed that overexpression of miR-26b repressed the endogenous level of USP9X protein expression. Overexpression of miR-26b also repressed Smad4 expression, whereas its inhibition elevated Smad4 expression.

CONCLUSIONS

Taken together, our results indicate that miR-26b were inhibited in HCC. In HCC cell lines, miR-26b targeted the 3'UTR of USP9X, which in turn affects EMT through Smad4 and the TGF-β signaling pathway. Our analysis of clinical HCC samples verifies that miR-26b also targets USP9X expression to inhibit the EMT of hepatocytes. Thus, miR-26b may have some effects on the EMT of HCC cells.

Inflammation and pancreatic cancer: molecular and functional interactions between S100A8, S100A9, NT-S100A8 and TGFβ1

BACKGROUND

In order to gain further insight on the crosstalk between pancreatic cancer (PDAC) and stromal cells, we investigated interactions occurring between TGFβ1 and the inflammatory proteins S100A8, S100A9 and NT-S100A8, a PDAC-associated S100A8 derived peptide, in cell signaling, intracellular calcium (Cai2+) and epithelial to mesenchymal transition (EMT). NF-κB, Akt and mTOR pathways, Cai2+ and EMT were studied in well (Capan1 and BxPC3) and poorly differentiated (Panc1 and MiaPaCa2) cell lines.

RESULTS

NT-S100A8, one of the low molecular weight N-terminal peptides from S100A8 to be released by PDAC-derived proteases, shared many effects on NF-κB, Akt and mTOR signaling with S100A8, but mainly with TGFβ1. The chief effects of S100A8, S100A9 and NT-S100A8 were to inhibit NF-κB and stimulate mTOR; the molecules inhibited Akt in Smad4-expressing, while stimulated Akt in Smad4 negative cells. By restoring Smad4 expression in BxPC3 and silencing it in MiaPaCa2, S100A8 and NT-S100A8 were shown to inhibit NF-κB and Akt in the presence of an intact TGFβ1 canonical signaling pathway. TGFβ1 counteracted S100A8, S100A9 and NT-S100A8 effects in Smad4 expressing, not in Smad4 negative cells, while it synergized with NT-S100A8 in altering Cai2+ and stimulating PDAC cell growth. The effects of TGFβ1 on both EMT (increased Twist and decreased N-Cadherin expression) and Cai2+ were antagonized by S100A9, which formed heterodimers with TGFβ1 (MALDI-TOF/MS and co-immuno-precipitation).

CONCLUSIONS

The effects of S100A8 and S100A9 on PDAC cell signaling appear to be cell-type and context dependent. NT-S100A8 mimics the effects of TGFβ1 on cell signaling, and the formation of complexes between TGFβ1 with S100A9 appears to be the molecular mechanism underlying the reciprocal antagonism of these molecules on cell signaling, Cai2+ and EMT.

BxPC3 pancreatic cancer cells express a truncated Smad4 protein upon PI3K and mTOR inhibition

Smad4 is a critical regulator of transforming growth factor (TGF)-β signaling and is defective in numerous human cancers. In total, 30% of pancreatic cancers harbor a homozygous deletion of Smad4. The human pancreatic cancer cell line, BxPC3, has been reported to be Smad4-null due to a homozygous deletion and has been widely used as a Smad4-null model. The present study reports that Smad4 DNA is present in BxPC3 cells, and under conditions of suppressed mammalian target of rapamycin complex 1 (mTORC1) and phosphatidylinositol-3-kinase, a truncated Smad4 protein is expressed. While a high level of Smad4 protein can be expressed in these cells, the cells do not respond to TGF-β. The Smad4 defect in BxPC3 cells likely occurs via translocation rather than deletion as previously reported.

Mutant ras elevates dependence on serum lipids and creates a synthetic lethality for rapamycin

The conversion of normal cells to cancer cells involves a shift from catabolic to anabolic metabolism involving increased glucose uptake and the diversion of glycolytic intermediates into nucleotides, amino acids, and lipids needed for cell growth. An underappreciated aspect of nutrient uptake is the utilization of serum lipids. We investigated the dependence of human cancer cells on serum lipids and report here that Ras-driven human cancer cells are uniquely dependent on serum lipids for both proliferation and survival. Removal of serum lipids also sensitizes Ras-driven cancer cells to rapamycin-indicating that the enhanced need for serum lipids creates a synthetic lethal phenotype that could be exploited therapeutically. Although depriving humans of serum lipids is not practical, suppressing uptake of lipids is possible. Suppressing macropinocytosis in Ras-driven cancer cells also created sensitivity to suppression of the mammalian/mechanistic target of rapamycin complex 1 (mTORC1). It is speculated that this property displayed by Ras-driven cancer cells represents an Achilles' heel for the large number of human cancers that are driven by activating Ras mutations.