MDM2 restrains estrogen-mediated AKT activation by promoting TBK1-dependent HPIP degradation

Abstract

Restoration of p53 tumor suppressor function through inhibition of its interaction and/or enzymatic activity of its E3 ligase, MDM2, is a promising therapeutic approach to treat cancer. However, because the MDM2 targetome extends beyond p53, MDM2 inhibition may also cause unwanted activation of oncogenic pathways. Accordingly, we identified the microtubuleassociated HPIP, a positive regulator of oncogenic AKT signaling, as a novel MDM2 substrate. MDM2-dependent HPIP degradation occurs in breast cancer cells on its phosphorylation by the estrogen-activated kinase TBK1. Importantly, decreasing Mdm2 gene dosage in mouse mammary epithelial cells potentiates estrogen-dependent AKT activation owing to HPIP stabilization. In addition, we identified HPIP as a novel p53 transcriptional target, and pharmacological inhibition of MDM2 causes p53-dependent increase in HPIP transcription and also prevents HPIP degradation by turning off TBK1 activity. Our data indicate that p53 reactivation through MDM2 inhibition may result in ectopic AKT oncogenic activity by maintaining HPIP protein levels. Cell Death and Differentiation (2014) 21, 811-824; doi:10.1038/cdd.2014.2; published online 31 January 2014Restoration of p53 tumor suppressor function in cancer cells expressing wild-type (WT) p53 is a promising therapeutic approach. 1 Reactivation of p53 activity can be achieved by small molecular inhibitors that disrupt the interaction between p53 and its main E3 ligase MDM2. As a result, targeted cells undergo cell cycle arrest and apoptosis through p53 stabilization. 2 A potential drawback associated with this approach is that, besides p53, MDM2 targets other substrates for degradation. 3 In this context, accumulative evidence show that MDM2 promotes the degradation of FOXO3a, a tumor-suppressing transcription factor as well as the apoptosome activator CAS and the ubiquitin E3 ligase HUWE1. 4,5 Although it is currently unclear whether MDM2 targets positive regulators of oncogenic pathways, an exhaustive characterization of MDM2 substrates will help to anticipate undesired side effects of MDM2 inhibitors used in cancer therapy.Oncogenic pathways include AKT-dependent signaling cascades. Indeed, AKT promotes cell proliferation, survival, migration and angiogenesis by targeting numerous substrates ranging from anti-apoptotic transcription factors to regulators of protein synthesis. 6,7 Mutations or altered expressions of various AKT-activating signaling molecules have been described in human malignancies, thereby defining AKT as a hallmark of tumor development and progression. 8,9 AKT activation by estrogens requires the microtubule-binding protein hematopoietic PBX-interaction protein (HPIP). 10 Initially identified as a corepressor of pre-B-cell leukemia homeobox protein 1 (PBX1), 11 HPIP assembles a signaling complex that connects the p85 subunit of PI3K and ERa to microtubules in order to properly activate AKT. 10 Likewise, HPIP also promotes the growth and differentiation of hematopoietic cells through AKT. 12 Because correct reg...

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“…Collected beads were washed five times in the same nondenaturant buffer and boiled in Laemmli blue sample buffer. Immunofluorescence (IF) analyses also were conducted as described (39). Briefly, to reveal the colocalization of endogenous TBK1 with GFP-APPL1 in endosome structures, RAW 264.7 cells were seeded on coverslips in six-well plates, transfected with GFP-APPL1, fixed 24 h later with 4% paraformaldehyde, and permeabilized with 0.3% Triton X-100 for 10 min at room temperature.…”

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HPIP: a predictor of lymph node metastasis and poor survival in cervical cancer

“…Another study has inferred that HPIP may be an important modulator of tamoxifen resistance in p53-deficient MCF7 cells through the regulation of AKT-activating proteins. 16 HPIP sensitized estrogen-receptor-positive MCF-7 cells as well as triple-negative MDA-MB-231 cells to paclitaxel; however, it had no effects on the sensitivity of breast cancer cells to the tubulin polymerization inhibitor, vinblastine, which implying that HPIP may function through microtubule stabilization instead of microtubule catastrophe. 28 In addition, HPIP has been described to activate some cell cycle regulators that may promote tumor proliferation and progression.…”