Analysis of intracellular PTEN signaling and secretion

Hypoxic Microenvironment-Induced Reduction in PTEN-L Secretion Promotes Non-Small Cell Lung Cancer Metastasis through PI3K/AKT Pathway

Objective

Lung cancer is the leading cause of cancer-related deaths worldwide. The aim of this study was to investigate the effects of hypoxic microenvironment on PTEN-L secretion and the effects of PTEN-L on the metastasis of non-small cell lung cancer (NSCLC) and the potential mechanisms.

Methods

The expression levels of PTEN-L in NSCLC tissues, cells, and cell culture media were detected. The transfection of PTEN-L overexpression construct or HIF-1α-siRNAs was conducted to manipulate the expression of PTEN-L or HIF-1α. NSCLC cells were introduced into 200 μM CoCl2 medium for 72 hours under 37°C to simulate hypoxia. The proliferation and apoptosis of the A549 cells were determined by the Cell Counting Kit-8 assay and Annexin V-FITC/PI-stained flow cytometry assay, respectively. Wound healing assay and transwell invasion assay were used to measure the migration and invasion of A549 cells. The protein expression of PTEN, PTEN-L, PI3K/AKT pathway-related proteins, and HIF-1α was detected by Western blot.

Results

PTEN and PTEN-L are downregulated in lung cancer tissues and cells. The protein expression of PTEN-L in the culture medium of lung cancer cell lines is decreased. The hypoxic microenvironment inhibits PTEN-L secretion. The low level of PTEN-L promotes cell proliferation, migration, and invasion, as well as inhibits apoptosis of A549 cells. The overexpression of PTEN-L attenuated the activation of the PI3K/AKT pathway by the hypoxic microenvironment. The knockdown of HIF-1α upregulates PTEN-L secretion under hypoxia.

Conclusions

The hypoxic microenvironment inhibits PTEN-L secretion and thus activates PI3K/AKT pathway to induce proliferation, migration, and invasion promotion, and apoptosis inhibition in NSCLC cells.

Negative cross talk between LIMK2 and PTEN promotes castration resistant prostate cancer pathogenesis in cells and in vivo

Androgen deprivation therapy (ADT) and androgen receptor (AR) signaling inhibitors are front-line treatments for highly aggressive prostate cancer. However, prolonged inhibition of AR triggers a compensatory activation of PI3K pathway, most often due to the genomic loss of tumor suppressor PTEN, driving progression to the castration-resistant prostate cancer (CRPC) stage, which has very poor prognosis. We uncovered a novel mechanism of PTEN downregulation triggered by LIMK2, which contributes significantly to CRPC pathogenesis. LIMK2 is a CRPC-specific target. Its depletion fully reverses tumorigenesis in vivo. LIMK2 phosphorylates PTEN at five sites, degrading and inhibiting its activity, thereby driving highly aggressive oncogenic phenotypes in cells and in vivo. PTEN also degrades LIMK2 in a feedback loop, which was confirmed in prostates from PTEN^-/- and PTEN^+/+ mice. LIMK2 is also the missing link between hypoxia and PTEN degradation in CRPC. This is the first study to show a feedback loop between PTEN and its regulator. Uncovering the LIMK2-PTEN loop provides a powerful therapeutic opportunity to retain the activity and stability of PTEN protein by inhibiting LIMK2, thereby halting the progression to CRPC, ADT-resistance and drug-resistance.

Insulin-PI3K signalling: an evolutionarily insulated metabolic driver of cancer

Cancer is driven by incremental changes that accumulate, eventually leading to oncogenic transformation. Although genetic alterations dominate the way cancer biologists think about oncogenesis, growing evidence suggests that systemic factors (for example, insulin, oestrogen and inflammatory cytokines) and their intracellular pathways activate oncogenic signals and contribute to targetable phenotypes. Systemic factors can have a critical role in both tumour initiation and therapeutic responses as increasingly targeted and personalized therapeutic regimens are used to treat patients with cancer. The endocrine system controls cell growth and metabolism by providing extracellular cues that integrate systemic nutrient status with cellular activities such as proliferation and survival via the production of metabolites and hormones such as insulin. When insulin binds to its receptor, it initiates a sequence of phosphorylation events that lead to activation of the catalytic activity of phosphoinositide 3-kinase (PI3K), a lipid kinase that coordinates the intake and utilization of glucose, and mTOR, a kinase downstream of PI3K that stimulates transcription and translation. When chronically activated, the PI3K pathway can drive malignant transformation. Here, we discuss the insulin-PI3K signalling cascade and emphasize its roles in normal cells (including coordinating cell metabolism and growth), highlighting the features of this network that make it ideal for co-option by cancer cells. Furthermore, we discuss how this signalling network can affect therapeutic responses and how novel metabolic-based strategies might enhance treatment efficacy for cancer.

Autism-associated PTEN missense mutation leads to enhanced nuclear localization and neurite outgrowth in an induced pluripotent stem cell line

Germline mutation in the PTEN gene is the genetic basis of PTEN hamartoma tumor syndrome with the affected individuals harboring features of autism spectrum disorders. Characterizing a panel of 14 autism‐associated PTEN missense mutations revealed reduced protein stability, catalytic activity, and subcellular distribution. Nine out of 14 (64%) PTEN missense mutants had reduced protein expression with most mutations confined to the C2 domain. Selected mutants displayed enhanced polyubiquitination and shortened protein half‐life, but that did not appear to involve the polyubiquitination sites at lysine residues at codon 13 or 289. Analyzing their intrinsic lipid phosphatase activities revealed that 78% (11 out of 14) of these mutants had twofold to 10‐fold reduction in catalytic activity toward phosphatidylinositol phosphate substrates. Analyzing the subcellular localization of the PTEN missense mutants showed that 64% (nine out of 14) had altered nuclear‐to‐cytosol ratios with four mutants (G44D, H123Q, E157G, and D326N) showing greater nuclear localization. The E157G mutant was knocked‐in to an induced pluripotent stem cell line and recapitulated a similar nuclear targeting preference. Furthermore, iPSCs expressing the E157G mutant were more proliferative at the neural progenitor cell stage but exhibited more extensive dendritic outgrowth. In summary, the combination of biological changes in PTEN is expected to contribute to the behavioral and cellular features of this neurodevelopmental disorder.

Celastrol Promotes Weight Loss in Diet-Induced Obesity by Inhibiting the Protein Tyrosine Phosphatases PTP1B and TCPTP in the Hypothalamus

Celastrol is a natural pentacyclic triterpene used in traditional Chinese medicine with significant weight-lowering effects. Celastrol-administered mice at 100 μg/kg decrease food consumption and body weight via a leptin-dependent mechanism, yet its molecular targets in this pathway remain elusive. Here, we demonstrate in vivo that celastrol-induced weight loss is largely mediated by the inhibition of leptin negative regulators protein tyrosine phosphatase (PTP) 1B (PTP1B) and T-cell PTP (TCPTP) in the arcuate nucleus (ARC) of the hypothalamus. We show in vitro that celastrol binds reversibly and inhibits noncompetitively PTP1B and TCPTP. NMR data map the binding site to an allosteric site in the catalytic domain that is in proximity of the active site. By using a panel of PTPs implicated in hypothalamic leptin signaling, we show that celastrol additionally inhibited PTEN and SHP2 but had no activity toward other phosphatases of the PTP family. These results suggest that PTP1B and TCPTP in the ARC are essential for celastrol's weight lowering effects in adult obese mice.

miR-29 promotes osteosarcoma cell proliferation and migration by targeting PTEN

Osteosarcoma (OS) is an aggressive malignant neoplasm that arises from primitively transformed cells of mesenchymal origin, and that exhibits osteoblastic differentiation and produces malignant osteoid. MicroRNAs (miRNAs) have been widely reported to have important regulatory roles in various human tumors, including OS. However, the potential mechanism of miR-29 in OS remains largely unknown. miR-29 was highly expressed in OS and overexpression of miR-29 promoted OS cell proliferation, as well as proliferating cell nuclear antigen (PCNA) expression and migration, whereas lower expression of miR-29 inhibited OS cell proliferation, PCNA expression and migration. In the present study, a dual-luciferase reporter system supporting phosphatase and tensin homolog (PTEN) was a target of miR-29 and its expression was inhibited by miR-29 mimic, but increased by miR-29 inhibitor. Overexpression of PTEN inhibited OS cell proliferation and migration and it could attenuate miR-29 promotion effect on OS progression. Overall, the results revealed that miR-29, as a tumor promoter, is involved in OS progression and metastasis by targeting PTEN, indicating that the miR-29/PTEN pathway is a potential therapeutic target for the treatment of OS.

A chimeric protein PTEN-L-p53 enters U251 cells to repress proliferation and invasion

PTEN, a well-known tumor suppressor, dephosphorylates PIP3 and inhibits AKT activity. A translational variant of PTEN has been identified and termed PTEN-Long (PTEN-L). The additional 173 amino acids (PTEN-L leader) at the N-terminal constitute a potential signal peptide. Differing from canonical PTEN, PTEN-L is secreted into the extracellular fluid and re-enters recipient cells, playing the similar roles as PTEN in vivo and in vitro. This character confers the PTEN-L a therapeutic ability via directly protein delivering instead of traditional DNA and RNA vector options. In the present study, we employed PTEN-L leader to assemble a fusion protein, PTEN-L-p53, inosculated with the transcriptional regulator TP53, which is another powerful tumor suppressor. We overexpressed PTEN-L-p53 in HEK293T cells and detected it in both the cytoplasm and nucleus. Subsequently, we found that PTEN-L-p53 was secreted outside of the cells and detected in the culture media by immunoblotting. Furthermore, we demonstrated that PTEN-L-p53 freely entered the cells and suppressed the viability of U251cells (p53^R273H, a cell line with p53 R273H-mutation). PTEN-L-p53 is composed of endogenous protein/peptide bearing low immunogenicity, and only the junction region between PTEN-L leader and p53 can act as a new immune epitope. Accordingly, this fusion protein can potentially be used as a therapeutic option for TP53-abnormality cancers.

MicroRNA-1908 is upregulated in human osteosarcoma and regulates cell proliferation and migration by repressing PTEN expression

Osteosarcoma is a high-grade malignant bone neoplasm. Although the introduction of chemotherapy has reduced its mortality, >50% of patients develop chemoresistance and have an extremely poor prognosis due to pulmonary metastasis. Several molecular pathways contributing to osteosarcoma development and progression have recently been identified. Various studies have addressed the genes involved in the metastasis of osteosarcoma. However, the highly complex molecular mechanisms of metastasis remain to be elucidated. Recent studies have emphasized causative links between aberrant microRNA expression patterns and osteosarcoma progression. miR-1908 is dysregulated in certain human types of cancer. The expression pattern, clinical significance and biological role of miR-1908 in osteosarcoma, however, remain largely undefined. In the present study, we showed that miR-1908 was markedly upregulated in osteosarcoma cells and tissues compared with normal bone tissues using RT-qPCR. miR-1908 upregulation in osteosarcoma tissues was significantly associated with cell proliferation, invasion, advanced TNM stage and tumor growth. Both gain- and loss-of-function studies showed that miR-1908 markedly increased the ability of osteosarcoma cells to proliferate and to invade through Matrigel in vitro. Analyses using mouse xenograft model revealed that xenografts of miR-1908 stable-expressing osteosarcoma cells exhibited a significant increase in tumor volume and weight, compared with the control group. Subsequent investigations revealed that miR-1908 directly inhibited the expression of phosphatase and tensin homolog deleted on chromosome ten (PTEN). Using a luciferase reporter carrying the 3'-untranslated region (3'-UTR) of PTEN, we identified PTEN as a direct target of miR-1908. Collectively, the results showed that, miR-1908 promotes proliferation and invasion of osteosarcoma cells by repressing PTEN expression.

PI3kα and STAT1 Interplay Regulates Human Mesenchymal Stem Cell Immune Polarization

The immunomodulatory capacity of mesenchymal stem cells (MSCs) is critical for their use in therapeutic applications. MSC response to specific inflammatory cues allows them to switch between a proinflammatory (MSC1) or anti-inflammatory (MSC2) phenotype. Regulatory mechanisms controlling this switch remain to be defined. One characteristic feature of MSC2 is their ability to respond to IFNγ with induction of indoleamine 2,3-dioxygenase (IDO), representing the key immunoregulatory molecule released by human MSC. Here, we show that STAT1 and PI3Kα pathways interplay regulates IFNγ-induced IDO production in MSC. Chemical phosphoinositide 3-kinase (PI3K) pan-inhibition, PI3Kα-specific inhibition or shRNA knockdown diminished IFNγ-induced IDO production. This effect involved PI3Kα-mediated upregulation of STAT1 protein levels and phosphorylation at Ser727. Overexpression of STAT1 or of a constitutively active PI3Kα mutant failed to induce basal IDO production, but shifted MSC into an MSC2-like phenotype by strongly enhancing IDO production in response to IFNγ as compared to controls. STAT1 overexpression strongly enhanced MSC-mediated T-cell suppression. The same effect could be induced using short-term pretreatment of MSC with a chemical inhibitor of the counter player of PI3K, phosphatase and tensin homolog. Finally, downregulation of STAT1 abrogated the immunosuppressive capacity of MSC. Our results for the first time identify critical upstream signals for the induced production of IDO in MSCs that could be manipulated therapeutically to enhance their immunosuppressive phenotype.