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

Upregulation of C/EBPα contributes to colorectal cancer growth, metastasis and indicates poor survival outcome

The function and clinical implication of transcription factor CCAAT/enhancer-binding protein α (C/EBPα) in colorectal cancer (CRC) still remains undefined. In fact, C/EBPα has long been considered as a tumor suppressor in hematopoietic system and also found lowly expressed in numerous solid tumors. However, our results here for the first time showed that C/EBPα was unexpectedly upregulated and was an independent prognostic marker for patients with CRC. We therefore aimed to explore the detailed role and mechanisms of C/EBPα in CRC. Our investigation demonstrated that C/EBPα promoted tumor growth both in vitro and in vivo. In addition, suppression of C/EBPα inhibited cell proliferation by inducing G1 phase arrest and inducing apoptosis. Also, C/EBPα enhances CRC cells migration and invasion in vitro as well as metastasis in vivo through regulating EMT. Mechanistically, C/EBPα exerts its oncogenic role by targeting c-Myc/cyclin D1 mediated by β-catenin involved pathway and we provide evidence indicating that cytoplasmic exclusion of C/EBPα might contribute to its oncogenic function in tumor progression. In conclusion, C/EBPα acts as an oncogene in CRC and could be a potential biomarker of colon carcinogenesis.

A tumor suppressor role for C/EBPα in solid tumors: more than fat and blood

The transcription factor CCAAT/enhancer-binding protein alpha (C/EBPα) plays a critical role during embryogenesis and is thereafter required for homeostatic glucose metabolism, adipogenesis and myeloid development. Its ability to regulate the expression of lineage-specific genes and induce growth arrest contributes to the terminal differentiation of several cell types, including hepatocytes, adipocytes and granulocytes. CEBPA loss of-function mutations contribute to the development of ~10% of acute myeloid leukemia (AML), stablishing a tumor suppressor role for C/EBPα. Deregulation of C/EBPα expression has also been reported in a variety of additional human neoplasias, including liver, breast and lung cancer. However, functional CEBPA mutations have not been found in solid tumors, suggesting that abrogation of C/EBPα function in non-hematopoietic tissues is regulated by alternative mechanisms. Here we review the function of C/EBPα in solid tumors and focus on the molecular mechanisms underlying its tumor suppressive role.

Oral Norovirus Infection Is Blocked in Mice Lacking Peyer's Patches and Mature M Cells

A critical early step in murine norovirus (MNV) pathogenesis is crossing the intestinal epithelial barrier to reach the target cells for replication, i.e., macrophages, dendritic cells, and B cells. Our previous work showed that MNV replication decreases in the intestines of mice conditionally depleted of microfold (M) cells. To define the importance of Peyer's patch (PP) M cells during MNV pathogenesis, we used a model of BALB/c mice deficient in recombination-activating gene 2 (Rag2) and the common gamma chain (γc) (Rag-γc(-/-)), which lack gut-associated lymphoid tissues (GALT), such as Peyer's patches, and mature GP2(+) M cells. Rag-γc(-/-) mice were infected intraperitoneally or perorally with MNV-1 or CR3 for 24 or 72 h. Although the intestinal laminae propriae of Rag-γc(-/-) mice have a higher frequency of certain MNV target cells (dendritic cells and macrophages) than those of wild-type mice and lack others (B cells), Rag-γc(-/-) and wild-type BALB/c mice showed relatively similar viral loads in the intestine following infection by the intraperitoneal route, which provides direct access to target cells. However, Rag-γc(-/-) mice were not productively infected with MNV by the oral route, in which virions must cross the intestinal epithelial barrier. These data are consistent with a model whereby PP M cells are the primary route by which MNV crosses the intestinal epithelia of BALB/c mice.

IMPORTANCE

Noroviruses (NoVs) are prevalent pathogens that infect their hosts via the intestine. Identifying key factors during the initial stages of virus infection in the host may provide novel points of intervention. Microfold (M) cells, antigen-sampling cells in the intestine, were previously shown to provide a gateway for murine NoV (MNV) into the host, but the relative importance of this uptake pathway remained unknown. Here we show that the absence of gut-associated lymphoid tissues (GALT), such as Peyer's patches, which contain high numbers of mature M cells, renders BALB/c mice refractory to oral infection with MNV. These findings are consistent with the model that M cells represent the primary route by which MNV crosses the intestinal epithelial barrier and infects underlying immune cells during a productive infection.

Hypoxia regulates the expression and localization of CCAAT/enhancer binding protein α by hypoxia inducible factor-1α in bladder transitional carcinoma cells

Hypoxia inducible factor-1α (HIF-1α) is overexpressed in various types of solid tumor in humans, including bladder cancer. HIF-1α regulates the expression of a series of genes, which are involved in cell proliferation, differentiation, apoptosis, angiogenesis, migration and invasion and represents a potential therapeutic target for the treatment of human cancer. Despite extensive investigation of the effects of HIF-1α in the progression and metastasis of bladder cancer, the possible regulatory mechanisms underlying the effects of HIF-1α on bladder cancer cell proliferation and differentiation remain to be elucidated. It has been suggested that the transcription factor CCAAT/enhancer binding protein α (C/EBPα) acts as a tumor suppressor in several types of cancer cell, which are involved in regulating cell differentiation, proliferation and apoptosis. The present study confirmed that, in bladder cancer cells, the expression and localization of C/EBPα was regulated by hypoxia through an HIF-1α -dependent mechanism, which may be significant in bladder cancer cell proliferation and differentiation. The 5637 and T24 bladder cancer cell lines were incubated under normoxic and hypoxic conditions. The expression levels of HIF-1α and C/EBPα were detected by reverse transcription-quantitative polymerase chain reaction, western blotting and immunofluorescence analysis. The results revealed that, under hypoxic conditions, the protein expression levels of HIF-1α were markedly upregulated, but the mRNA levels were not altered. However, the mRNA and protein levels of C/EBPα were significantly reduced. The present study further analyzed the subcellular localization of C/EBPα, which was markedly decreased in the nuclei under hypoxic conditions. Following HIF-1α small interference RNA silencing of HIF-1α, downregulation of C/EBPα was prevented in the bladder cancer cells cultured under hypoxic conditions. In addition, groups of cells treated with 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole, which inhibits the expression of HIF-1α in hypoxia, contributed to the inhibited expression of HIF-1α and enhanced expression of C/EBPα in hypoxic bladder cancer cells. These results suggested that C/EBPα was a downstream effector regulated by HIF-1α in hypoxic bladder cancer cells and that this regulatory pathway may represent a potential therapeutic target in the treatment of bladder cancer.

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.

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.

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.

Complexity of CEBPA dysregulation in human acute myeloid leukemia

The transcription factor CCAAT enhancer binding protein alpha (CEBPA) is crucial for normal development of granulocytes. Various mechanisms have been identified how CEBPA function is dysregulated in patients with acute myeloid leukemia (AML). In particular, dominant-negative mutations located either at the N- or the C terminus of the CEBPA gene are observed in roughly 10% of AML patients, either in the combination on separate alleles or as sole mutation. Clinically significant complexity exists among AML with CEBPA mutations, and patients with double CEBPA mutations seem to have a more favorable course of the disease than patients with a single mutation. In addition, myeloid precursor cells of healthy carriers with a single germ-line CEBPA mutation evolve to overt AML by acquiring a second sporadic CEBPA mutation. This review summarizes recent reports on dysregulation of CEBPA function at various levels in human AML and therapeutic concepts targeting correction of CEBPA activity. The currently available data are persuasive evidence that impaired CEBPA function contributes directly to the development of AML, whereas restoring CEBPA function represents a promising target for novel therapeutic strategies in AML.

In prostate cancer C/EBPalpha promotes cell growth by the loss of interactions with CDK2, CDK4, and E2F and by activation of AKT

BACKGROUND

The CCAAT/Enhancer binding protein alpha (C/EBPalpha) is an important transcription factor for granulopoiesis and adipogenesis. While decreased expression and mutation of C/EBPalpha has been found in several types of tumors, the role of C/EBPalpha in prostate cancer has not been well characterized.

METHODS

We quantitatively analyzed the immunochemical staining of prostate cancer tissue and examined the growth properties of prostate cancer cells stably expressing C/EBPalpha by measure growth curve, cell cycle, and anchorage independent colony formation, investigated the association of C/EBPalpha with E2Fs and CDKs by co-immunoprecipitation and examined the expression of CDKs and activation of AKT by Western blot analysis.

RESULTS

The ratio of C/EBPalpha expression between cancer cells close to the pseudolumen of glands and those nearer the basal cell layer was more than threefold greater than that seen in the normal prostate epithelium. Further, this ratio increased with increased Gleason score of the prostate cancer. Forced expression of C/EBPalpha in prostate cancer cell lines accelerated cell growth, stimulated cells into the S and G2 phases of cell cycle, and enhanced anchorage-independent colony formation. Simultaneously, forced expression of C/EBPalpha increased expression of CDK2/CDK4 and nuclear PP2A, and activated AKT. In addition, C/EBPalpha was no longer found associated with E2F1/E2F4 and CDK2/CDK4. AKT and PPA2 inhibitors restored both the anti-proliferation function of C/EBPalpha and the interaction between C/EBPalpha and E2F1/E2F4.

CONCLUSION

In prostate cancer cells C/EBPalpha cannot function as a tumor suppressor.