PTEN downregulates p75NTR expression by decreasing DNA-binding activity of Sp1

PTEN Physically Interacts with and Regulates E2F1-mediated Transcription in Lung Cancer

PTEN phosphorylation at its C-terminal (C-tail) serine/threonine cluster negatively regulates its tumor suppressor function. However, the consequence of such inhibition and its downstream effects in driving lung cancer remain unexplored. Herein, we ascertain the molecular mechanisms by which phosphorylation compromises PTEN function, contributing to lung cancer. Replacement of the serine/threonine residues with alanine generated PTEN-4A, a phosphorylation-deficient PTEN mutant, which suppressed lung cancer cell proliferation and migration. PTEN-4A preferentially localized to the nucleus where it suppressed E2F1-mediated transcription of cell cycle genes. PTEN-4A physically interacted with the transcription factor E2F1 and associated with chromatin at gene promoters with E2F1 DNA-binding sites, a likely mechanism for its transcriptional suppression function. Deletion analysis revealed that the C2 domain of PTEN was indispensable for suppression of E2F1-mediated transcription. Further, we uncovered cancer-associated C2 domain mutant proteins that had lost their ability to suppress E2F1-mediated transcription, supporting the concept that these mutations are oncogenic in patients. Consistent with these findings, we observed increased PTEN phosphorylation and reduced nuclear PTEN levels in lung cancer patient samples establishing phosphorylation as a bona fide inactivation mechanism for PTEN in lung cancer. Thus, use of small molecule inhibitors that hinder PTEN phosphorylation is a plausible approach to activate PTEN function in the treatment of lung cancer. Abbreviations AKT V-Akt Murine Thymoma Viral Oncogene CA Cancer adjacent CDK1 Cyclin dependent kinase 1 CENPC-C Centromere Protein C ChIP Chromatin Immunoprecipitation co-IP Co-immunoprecipitation COSMIC Catalog of Somatic Mutations In Cancer CREB cAMP Responsive Element Binding Protein C-tail Carboxy terminal tail E2F1 E2F Transcription Factor 1 ECIS Electric Cell-substrate Impedance Sensing EGFR Epidermal Growth Factor Receptor GSI Gamma Secretase Inhibitor HDAC1 Histone Deacetylase 1 HP1 Heterochromatin protein 1 KAP1/TRIM28 KRAB-Associated Protein 1/Tripartite Motif Containing 28 MAF1 Repressor of RNA polymerase III transcription MAF1 homolog MCM2 Minichromosome Maintenance Complex Component 2 miRNA micro RNA MTF1 Metal-Regulatory Transcription Factor 1 PARP Poly(ADP-Ribose) Polymerase PD-1 Programmed Cell Death 1 PD-L1 Programmed Cell Death 1 Ligand 1 PI3K Phosphatidylinositol-4,5-Bisphosphate 3-Kinase PLK Polo-like Kinase pPTEN Phosphorylated PTEN PTEN Phosphatase and Tensin Homolog deleted on chromosome ten PTM Post Translational Modification Rad51 RAD51 Recombinase Rad52 RAD52 Recombinase RPA1 Replication protein A SILAC Stable Isotope Labeling with Amino Acids in Cell Culture SRF Serum Response Factor TKI Tyrosine Kinase inhbitors TMA Tissue Microarray TOP2A DNA Topoisomerase 2A.

Differential AKT dependency displayed by mouse models of BRAFV600E-initiated melanoma

Malignant melanoma is frequently driven by mutational activation of v-raf murine sarcoma viral oncogene homolog B1 (BRAF) accompanied by silencing of the phosphatase and tensin homology (PTEN) tumor suppressor. Despite the implied importance of PI3K signaling in PTENNull melanomas, mutational activation of the gene encoding the catalytic subunit of PI3Kα (PIK3CA), is rarely detected. Since PTEN has both PI3-lipid phosphatase-dependent and -independent tumor suppressor activities, we investigated the contribution of PI3K signaling to BRAFV600E-induced melanomagenesis using mouse models, cultured melanoma cells, and PI3K pathway-targeted inhibitors. These experiments revealed that mutationally activated PIK3CAH1047R cooperates with BRAFV600E for melanomagenesis in mice. Moreover, pharmacological inhibition of PI3Ks prevented growth of BRAFV600E/PTENNull melanomas in vivo and in tissue culture. Combined inhibition of BRAFV600E and PI3K had more potent effects on the regression of established BRAFV600E/PTENNull melanomas and cultured melanoma cells than individual blockade of either pathway. Surprisingly, growth of BRAFV600E/PIK3CAH1047R melanomas was dependent on the protein kinase AKT; however, AKT inhibition had no effect on growth of BRAFV600E/PTENNull melanomas. These data indicate that PTEN silencing contributes a PI3K-dependent, but AKT-independent, function in melanomagenesis. Our findings enhance our knowledge of how BRAFV600E and PI3K signaling cooperate in melanomagenesis and provide preclinical validation for combined pathway-targeted inhibition of PI3K and BRAFV600E in the therapeutic management of BRAFV600E/PTENNull melanomas.

NFATc1 regulation of TRAIL expression in human intestinal cells

TNF-related apoptosis-inducing ligand (TRAIL; Apo2) has been shown to promote intestinal cell differentiation. Nuclear factor of activated T cells (NFAT) participates in the regulation of a variety of cellular processes, including differentiation. Here, we examined the role of NFAT in the regulation of TRAIL in human intestinal cells. Treatment with a combination of phorbol 12-myristate 13-acetate (PMA) plus the calcium ionophore A23187 (Io) increased NFAT activation and TRAIL expression; pretreatment with the calcineurin inhibitor cyclosporine A (CsA), an antagonist of NFAT signaling, diminished NFAT activation and TRAIL induction. In addition, knockdown of NFATc1, NFATc2, NFATc3, and NFATc4 blocked PMA/Io increased TRAIL protein expression. Expression of NFATc1 activated TRAIL promoter activity and increased TRAIL mRNA and protein expression. Deletion of NFAT binding sites from the TRAIL promoter did not significantly abrogate NFATc1-increased TRAIL promoter activity, suggesting an indirect regulation of TRAIL expression by NFAT activation. Knockdown of NFATc1 increased Sp1 transcription factor binding to the TRAIL promoter and, importantly, inhibition of Sp1, by chemical inhibition or RNA interference, increased TRAIL expression. These studies identify a novel mechanism for TRAIL regulation by which activation of NFATc1 increases TRAIL expression through negative regulation of Sp1 binding to the TRAIL promoter.

PTEN level in tumor suppression: how much is too little?

The importance of PTEN (phosphatase and tensin homolog located on chromosome 10) in cancer has surpassed all predictions and expectations from the time it was discovered and has qualified this gene as one of the most commonly mutated and deleted tumor suppressors in human cancer. PTEN levels are frequently found downregulated in cancer, even in the absence of genetic loss or mutation. PTEN is heavily regulated by transcription factors, microRNAs, competitive endogenous RNAs (such as the PTEN pseudogene), and methylation, whereas the tumor suppressive activity of the PTEN protein can be altered at multiple levels through aberrant phosphorylation, ubiquitination, and acetylation. These regulatory cues are presumed to play a key role in tumorigenesis through the alteration of the appropriate levels, localization, and activity of PTEN. The identification of all these levels of PTEN regulation raises, in turn, a key corollary question: How low should PTEN level(s) or activity drop in order to confer cancer susceptibility at the organismal level? Our laboratory and others have approached this question through the genetic manipulation of Pten in the mouse. This work has highlighted the exquisite and tissue-specific sensitivity to subtle reductions in Pten levels toward tumor initiation and progression with important implications for cancer prevention and therapy.

PTEN inhibition to facilitate intrinsic regenerative outgrowth of adult peripheral axons

In vivo regeneration of peripheral neurons is constrained and rarely complete, and unfortunately patients with major nerve trunk transections experience only limited recovery. Intracellular inhibition of neuronal growth signals may be among these constraints. In this work, we investigated the role of PTEN (phosphatase and tensin homolog deleted on chromosome 10) during regeneration of peripheral neurons in adult Sprague Dawley rats. PTEN inhibits phosphoinositide 3-kinase (PI3-K)/Akt signaling, a common and central outgrowth and survival pathway downstream of neuronal growth factors. While PI3-K and Akt outgrowth signals were expressed and activated within adult peripheral neurons during regeneration, PTEN was similarly expressed and poised to inhibit their support. PTEN was expressed in neuron perikaryal cytoplasm, nuclei, regenerating axons, and Schwann cells. Adult sensory neurons in vitro responded to both graded pharmacological inhibition of PTEN and its mRNA knockdown using siRNA. Both approaches were associated with robust rises in the plasticity of neurite outgrowth that were independent of the mTOR (mammalian target of rapamycin) pathway. Importantly, this accelerated outgrowth was in addition to the increased outgrowth generated in neurons that had undergone a preconditioning lesion. Moreover, following severe nerve transection injuries, local pharmacological inhibition of PTEN or siRNA knockdown of PTEN at the injury site accelerated axon outgrowth in vivo. The findings indicated a remarkable impact on peripheral neuron plasticity through PTEN inhibition, even within a complex regenerative milieu. Overall, these findings identify a novel route to propagate intrinsic regeneration pathways within axons to benefit nerve repair.