A RAS-CaMKKβ-AMPKα2 pathway promotes senescence by licensing post-translational activation of C/EBPβ through a novel 3'UTR mechanism

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.

Abstract PR02: C/EBPG: A critical stress response regulator with a pro-oncogenic role

Stress signals such as amino acid deprivation and redox imbalances activate the integrated stress response (ISR), which allows cells to alleviate the stress or to undergo apoptosis if the stress is unresolved. The ISR is also important for the survival of cancer cells, since they experience stress in the form of nutrient and oxygen deprivation and hence frequently activate stress-response pathways. Cellular stresses trigger up-regulation of the transcription factor ATF4, which subsequently activates stress response genes through recruitment to cis-regulatory sites known as C/EBP:ATF response elements (CAREs). Although the role of ATF4 in regulating stress response is well established, the identity of the C/EBP partner that heterodimerizes with ATF4 to execute this crucial function remains obscure. Here we show that the transcription factor C/EBPG is a critical partner of ATF4 and that C/EBPG:ATF4 heterodimers are the predominant CARE-binding species in stressed cells. Similar to ATF4, C/EBPG is necessary for resistance of MEFs to oxidative stress. MEFs lacking C/EBPG show increased levels of reactive oxygen species (ROS) as a consequence of impaired glutathione biosynthesis, as was also seen in ATF4-deficient MEFs. C/EBPG is required for stress-induced association of ATF4 with CAREs and the subsequent activation of several critical stress-responsive genes, including those with known pro-oncogenic functions. Accordingly, resistance to stress conferred by C/EBPG also facilitates the growth of cancer cells. Depletion of C/EBPG impairs the proliferation of cancer cells in vitro and elevates ROS levels. These effects can be suppressed by addition of the antioxidant, N-acetylcysteine. Mice lacking C/EBPG are smaller in size and show defective eye lens formation, similar to ATF4-deficient mice. The absence of C/EBPG also causes perinatal mortality due to pulmonary atelectasis and respiratory failure. This mortality can be rescued by in utero exposure to N-acetylcysteine. Accordingly, gene expression analysis suggests the presence of increased oxidative stress and impaired expression of stress-responsive genes in the lungs of Cebpg-/- newborn mice. Interestingly, Cebpg-/- mice on a mixed strain background, which do not show perinatal lethality, are resistant to the development of solid malignant tumors. These findings suggest that the role of C/EBPG as a stress response regulator may be important for tumor development/progression. A pro-oncogenic function for C/EBPG is also suggested by the observation that elevated C/EBPG levels are associated with poor patient prognosis in several clinical cancer studies. Activation of stress-responsive genes through upregulation of C/EBPG could be a mechanism deployed by cancer cells to mitigate the high levels of ROS and metabolic stresses that they experience. The importance of C/EBPG and its targets in tumor cells could potentially be exploited to devise novel anti-cancer therapies.

Citation Format: Manasi K. Mayekar, Christopher J. Huggins, Nancy Martin, Karen L. Saylor, Mesfin Gonit, Parthav Jailwala, Manjula Kasoji, Diana C. Haines, Octavio A. Quinones, Peter F. Johnson. C/EBPG: A critical stress response regulator with a pro-oncogenic role. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr PR02.

The ETS domain transcription factor ELK1 directs a critical component of growth signaling by the androgen receptor in prostate cancer cells

The androgen receptor (AR) is essential for diverse aspects of prostate development and function. Molecular mechanisms by which prostate cancer (PC) cells redirect AR signaling to genes that primarily support growth are unclear. A systematic search for critical AR-tethering proteins led to ELK1, an ETS transcription factor of the ternary complex factor subfamily. Although genetically redundant, ELK1 was obligatory for AR-dependent growth and clonogenic survival in both hormone-dependent PC and castration-recurrent PC cells but not for AR-negative cell growth. AR required ELK1 to up-regulate a major subset of its target genes that was strongly and primarily enriched for cell growth functions. AR functioned as a coactivator of ELK1 by association through its A/B domain, bypassing the classical mechanism of ELK1 activation by phosphorylation and without inducing ternary complex target genes. The ELK1-AR synergy per se was ligand-independent, although it required ligand for nuclear localization of AR as targeting the AR A/B domain to the nucleus recapitulated the action of hormone; accordingly, Casodex was a poor antagonist of the synergy. ELK3, the closest substitute for ELK1 in structure/function and genome recognition, did not interact with AR. ELK1 thus directs selective and sustained gene induction that is a substantial and critical component of growth signaling by AR in PC cells. The ELK1-AR interaction offers a functionally tumor-selective drug target.

Abstract B12: The androgen receptor is a critical coactivator of Elk1 in prostate cancer cells

The androgen receptor (AR) not only plays an essential role in the physiology of the normal prostate but is also essential for the growth of most prostate tumors. The physiological actions of AR are primarily through its role as a transcription factor. AR mediates androgen-stimulated growth in the early stages of prostate cancer. AR also supports the growth of androgen ablation resistant advanced tumors that are either hypersensitized to androgen or that can grow in the complete absence of androgen through overexpression of AR, post-translational modifications of AR or AR mutations. The current paradigm for adjuvant therapy in prostate cancer is total and ubiquitous attenuation of AR signaling which has many undesirable side effects. Therefore it is desirable to develop functionally selective interventions by first identifying aspects of AR signaling that are only required by the cancer cells. In the classical mechanism of gene activation by AR the receptor, in response to hormone binding, localizes to the nucleus and binds as a homodimer to androgen response elements (AREs) to activate target genes. However, this laboratory has recently demonstrated that in prostate cancer cells in which AR is localized in the nucleus independent of hormone, the AR apo-protein can support growth without binding to ARE and likely by tethered associations with a distinct set of growth genes. In the present study, we used ChlP-chip and AR interactome data together with promoter activation assays to screen for DNA binding transcription factors that may serve to tether AR to target genes and are critical for AR dependent growth. Several candidate proteins were identified and further studies focused on Elk1. Elk1 belongs to the large Ets family of transcription factors and several signaling pathways impinge on this protein to regulate early growth response genes. Elk1 however is genetically and functionally redundant in normal tissues. Nevertheless, in our studies Elk1 was essential for the androgen or androgen receptor (AR)-dependent growth of cell line models of hormone-dependent early stage (LNCaP, VCaP) prostate cancer as well as advanced prostate cancer models, including both the AR overexpressing C4-2 cells and the non-overexpressing C81 cells. Elk1 supported a gene set strongly enriched for cell division functions in an androgen-dependent (in hormone-dependent cells) or AR-dependent manner. Using several approaches including promoter and gene activation assays, co-immunoprecipitation assays, mammalian two-hybrid assays and chromatin immunoprecipitation we established that AR associates with Elk1 as a coactivator. Elk1 could therefore serve as a functionally selective target for intervention in both early and advanced prostate cancer without a need for total attenuation of AR signaling.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr B12.

Hormone depletion-insensitivity of prostate cancer cells is supported by the AR without binding to classical response elements

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.

Androgen activation of the folate receptor α gene through partial tethering of the androgen receptor by C/EBPα

The folate receptor α (FRα) is critical for normal embryonic and fetal development. The receptor has a relatively narrow tissue specificity which includes the visceral endoderm and the placenta and mediates delivery of folate, inadequacy of which results in termination of pregnancy or developmental defects. We have previously reported that the FRα gene is negatively and directly regulated by estrogen and positively but indirectly by progesterone and glucocorticoid. To further investigate hormonal control of this gene and in view of the growing evidence for the importance of the androgen receptor (AR) in endometrial and placental functions, we examined the response of the FRα gene to androgen. Here we demonstrate that the FRα gene is directly activated by androgen. The P4 promoter of the FRα gene is the target of hormone-dependent activation by the androgen receptor (AR) in a manner that is co-activator-dependent. The site of functional association of AR in the FRα gene maps to a 35bp region occurring ∼1500bp upstream of the target promoter. The functional elements within this region are an androgen response element (ARE) half-site and a non-canonical C/EBP element that cooperate to recruit AR in a manner that is dependent on the DNA-bound C/EBPα. Since the placenta is rich in C/EBPα, the findings underscore the multiplicity of mechanisms by which the FRα gene is under the exquisite control of steroid hormones.

C/EBPalpha redirects androgen receptor signaling through a unique bimodal interaction

Nuclear expression of CCAAT enhancer binding protein-alpha (C/EBPalpha), which supports tissue differentiation through several antiproliferative protein-protein interactions, augurs terminal differentiation of prostate epithelial cells. C/EBPalpha is also a tumor suppressor, but in many tumors its antiproliferative interactions may be attenuated by de-phosphorylation. C/EBPalpha 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/EPBalpha recruits AR to activate transcription. C/EBPalpha-dependent trans-activation by AR also overrode suppression of AREs by C/EBPalpha 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/EBPalpha. This unique mechanism explains the functional coordination between AR and C/EPBalpha in the prostate and also shows that hormone-refractory AR signaling in prostate cancer could occur through receptor tethering.

Expression and sub-cellular localization of the CCAAT/enhancer binding protein alpha in relation to postnatal development and malignancy of the prostate

BACKGROUND

C/EBPalpha is a critical mediator of terminal differentiation and a tumor suppressor through its strong antiproliferative actions on cell cycle regulatory proteins. C/EBPalpha also appears to regulate androgen receptor (AR) AR signaling. There, is a paucity of information on the expression and sub-cellular localization of C/EBPalpha 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/EBPalpha 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/EBPalpha mRNA was expressed at all ages. In humans, C/EBPalpha showed prominent nuclear localization from peripubescence up to middle age but was sequestered in the cytosol in older individuals; the mRNA level for C/EBPalpha remained essentially unchanged. Most prostate adenocarcinomas expressed a range of levels of C/EBPalpha mRNA and protein that were relatively high in metastatic tumors in a manner that correlated with AR expression; however, most cells showed C/EBPalpha sequestered in the cytosol.

CONCLUSIONS

Temporal changes in sub-cellular localization of C/EBPalpha are consistent with a role in prostate differentiation and as a prostate tumor suppressor; the cytoplasmic sequestration of C/EBPalpha, unique to older human prostates, is arguably a permissive condition for the greater frequency of proliferative disorders of the prostate. In malignant prostate C/EBPalpha may be available to regulate AR signaling through transient changes in its sub-cellular localization.