Background: Mechanisms that redirect androgen receptor signaling to primarily support prostate tumor growth are poorly understood. Results: Prostate cancer cells were addicted to ELK1, which tethered AR to activate growth genes in hormone-dependent and castration-recurrent PC without ELK1 phosphorylation. Conclusion: ELK1 directs a critical arm of transcriptional growth signaling by AR that is preserved in CRPC. Significance: The ELK1-AR interaction offers a functionally tumor-selective drug target.
eThe integrated stress response (ISR) controls cellular adaptations to nutrient deprivation, redox imbalances, and endoplasmic reticulum (ER) stress. ISR genes are upregulated in stressed cells, primarily by the bZIP transcription factor ATF4 through its recruitment to cis-regulatory C/EBP:ATF response elements (CAREs) together with a dimeric partner of uncertain identity. Here, we show that C/EBP␥:ATF4 heterodimers, but not C/EBP:ATF4 dimers, are the predominant CARE-binding species in stressed cells. C/EBP␥ and ATF4 associate with genomic CAREs in a mutually dependent manner and coregulate many ISR genes. In contrast, the C/EBP family members C/EBP and C/EBP homologous protein (CHOP) were largely dispensable for induction of stress genes. Cebpg ؊/؊ mouse embryonic fibroblasts (MEFs) proliferate poorly and exhibit oxidative stress due to reduced glutathione levels and impaired expression of several glutathione biosynthesis pathway genes. Cebpg ؊/؊ mice (C57BL/6 background) display reduced body size and microphthalmia, similar to ATF4-null animals. In addition, C/EBP␥-deficient newborns die from atelectasis and respiratory failure, which can be mitigated by in utero exposure to the antioxidant N-acetylcysteine. Cebpg ؊/؊ mice on a mixed strain background showed improved viability but, upon aging, developed significantly fewer malignant solid tumors than WT animals. Our findings identify C/EBP␥ as a novel antioxidant regulator and an obligatory ATF4 partner that controls redox homeostasis in normal and cancerous cells.A variety of stresses, such as amino acid limitation, protein misfolding in the endoplasmic reticulum (ER), oxidative stress, hypoxia, and intracellular pathogens, activate gene expression programs collectively known as the integrated stress response (ISR) (1). Stress-induced genes are involved in multiple cellular processes that include nutrient uptake, amino acid synthesis, metabolic changes, antioxidant defenses, and cell survival, leading to cell recovery and alleviation of stress. However, prolonged or irresolvable stress can trigger cell death (2).Protein misfolding and ER stress are associated with several diseases, including diabetes and neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease, and Huntington's disease (3). The ISR also plays an important role in cancer as tumor cells frequently experience nutrient deprivation, hypoxia, and oxidative stress and require stress response regulators such as the transcription factor (TF) ATF4 to thrive under adverse conditions (4, 5). Elevated levels of reactive oxygen species (ROS) can initiate oncogenesis by causing DNA mutations and genome instability (6). However, recent studies have shown that, once established, tumor cells are reliant on antioxidant pathways for growth and survival (7,8). Moreover, radio-and chemotherapies induce death or senescence of cancer cells partly by increasing ROS. Thus, acquiring a detailed understanding of the pathways and mechanisms that regulate stress response genes may le...
Nuclear expression of CCAAT enhancer binding protein-α (C/EBPα), which supports tissue differentiation through several antiproliferative protein–protein interactions, augurs terminal differentiation of prostate epithelial cells. C/EBPα is also a tumor suppressor, but in many tumors its antiproliferative interactions may be attenuated by de-phosphorylation. C/EBPα acts as a corepressor of the classical androgen response element (ARE)-mediated gene activation by the androgen receptor (AR), but this is paradoxical as the genotropic actions of AR are crucial not only for the growth of the prostate but also for its maintenance and function. We show that DNA-bound C/EPBα recruits AR to activate transcription. C/EBPα-dependent trans-activation by AR also overrode suppression of AREs by C/EBPα elsewhere in a promoter. This mechanism was remarkable in that its androgen dependence was apparently for nuclear translocation of AR; it was otherwise androgen independent, flutamide insensitive and tolerant to disruption of AR dimerization. Gene response profiles and global chromatin associations in situ supported the direct bimodal regulation of AR transcriptional signaling by C/EBPα. This unique mechanism explains the functional coordination between AR and C/EPBα in the prostate and also shows that hormone-refractory AR signaling in prostate cancer could occur through receptor tethering.
A need for androgen response elements (AREs) for androgen receptor (AR)-dependent growth of hormone depletion-insensitive prostate cancer is generally presumed. In such cells, androgen-independent activation by AR of certain genes has been attributed to selective increases in basal associations of AR with putative enhancers. We examined the importance of AR binding to DNA in prostate cancer cells in which proliferation in the absence of hormone was profoundly (∼ 90%) dependent on endogenous AR and where the receptor was not up-regulated or mutated but was predominantly nuclear. Here, ARE-mediated promoter activation and the binding of AR to a known ARE in the chromatin remained entirely androgen dependent, and the cells showed an androgen-responsive gene expression profile with an unaltered sensitivity to androgen dose. In the same cells, a different set of genes primarily enriched for cell division functions was activated by AR independently of hormone and significantly overlapped the signature gene overexpression profile of hormone ablation-insensitive clinical tumors. After knockdown of endogenous AR, hormone depletion-insensitive cell proliferation and AR apoprotein-dependent gene expression were rescued by an AR mutant that was unable to bind to ARE but that could transactivate through a well-established AR tethering protein. Hormone depletion-insensitive AR binding sites in the chromatin were functional, binding, and responding to both the wild-type and the mutant AR and lacked enrichment for canonical or noncanonical ARE half-sites. Therefore, a potentially diverse set of ARE-independent mechanisms of AR interactions with target genes must underlie truly hormone depletion-insensitive gene regulation and proliferation in prostate cancer.
Oncogene-induced senescence (OIS) is an intrinsic tumor suppression mechanism that requires the p53 and RB pathways and post-translational activation of C/EBPβ through the RAS-ERK cascade. We previously reported that in transformed/proliferating cells, C/EBPβ activation is inhibited by G/U-rich elements (GREs) in its 3′UTR. This mechanism, termed “3′UTR regulation of protein activity” (UPA), maintains C/EBPβ in a low-activity state in tumor cells and thus facilitates senescence bypass. Here we show that C/EBPβ UPA is overridden by AMPK signaling. AMPK activators decrease cytoplasmic levels of the GRE binding protein HuR, which is a key UPA component. Reduced cytoplasmic HuR disrupts 3′UTR-mediated trafficking of Cebpb transcripts to the peripheral cytoplasm – a fundamental feature of UPA – thereby stimulating C/EBPβ activation and growth arrest. In primary cells, oncogenic RAS triggers a Ca++-CaMKKβ-AMPKα2-HuR pathway, independent of AMPKα1, that is essential for C/EBPβ activation and OIS. This axis is disrupted in cancer cells through down-regulation of AMPKα2 and CaMKKβ. Thus, CaMKKβ-AMPKα2 signaling constitutes a key tumor suppressor pathway that activates a novel UPA-cancelling mechanism to unmask the cytostatic and pro-senescence functions of C/EBPβ.
The glycosyl-phosphatidylinositol anchored folate receptor (FR) mediates selective delivery of a broad range of experimental drugs to the receptor-rich tumors, but molecular mechanisms controlling FR internalization have not been adequately studied. FR quantitatively recycles between the cell surface and endocytic compartments via a Cdc42-dependent pinocytic pathway. Protein kinase C (PKC) activators including diacylglycerol and phorbol ester have previously been reported to increase the proportion of FR on the cell surface. Here we identify the alpha-subtype of PKC as the mediator of phorbol ester action on FR recycling and provide evidence that activated PKCalpha is recruited to FR-rich membrane microdomains where, in association with its receptor RACK1, it inhibits FR internalization; the activation state of Cdc42 remains unaltered. We also show that the PKC substrate, annexin II, is required for FR internalization. The studies clarify a molecular mechanism for the regulation of FR recycling through PKC which could potentially be exploited for effective drug delivery.
BACKGROUND C/EBPα is a critical mediator of terminal differentiation and a tumor suppressor through its strong antiproliferative actions on cell cycle regulatory proteins. C/EBPα also appears to regulate androgen receptor (AR) AR signaling. There, is a paucity of information on the expression and sub-cellular localization of C/EBPα in normal mouse and human prostate and in prostate cancer. METHODS Immunohistochemistry of tissues including tissue arrays, quantitative polymerase chain reaction and mRNA expression database mining. RESULTS In the mouse prostate epithelium, C/EBPα was present at 1 week postnatal localized in the cytosol, began to show nuclear localization at 8 weeks and continued to show prominent nuclear expression at 10 weeks and beyond; C/EBPα mRNA was expressed at all ages. In humans, C/EBPα showed prominent nuclear localization from peripubescence up to middle age but was sequestered in the cytosol in older individuals; the mRNA level for C/EBPα remained essentially unchanged. Most prostate adenocarcinomas expressed a range of levels of C/EBPα mRNA and protein that were relatively high in metastatic tumors in a manner that correlated with AR expression; however, most cells showed C/EBPα sequestered in the cytosol. CONCLUSIONS Temporal changes in sub-cellular localization of C/EBPα are consistent with a role in prostate differentiation and as a prostate tumor suppressor; the cytoplasmic sequestration of C/EBPα, unique to older human prostates, is arguably a permissive condition for the greater frequency of proliferative disorders of the prostate. In malignant prostate C/EBPα may be available to regulate AR signaling through transient changes in its sub-cellular localization.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.