PRIP promotes tumor formation through enhancing serum-responsive factor-mediated FOS expression

Steroid Hormone Receptor Coregulators in Endocrine Cancers

Coregulators span a broad and extensive domain in modulating cellular transcriptional activity. Studies have established a dynamic role for such coregulators in various endocrine cancers. Steroid hormone receptors (SHRs) play a pivotal role in such endocrine cancers, and interact abundantly with transcriptional coregulators in altering gene expression. Several families of coregulators have implications in propagating the development, progression and invasion of breast, prostate, and other hormone-responsive cancers. This mini-review aims to discuss different classes of coregulators involved in endocrine cancers and highlight unique information regarding each family with relevance to mechanism, intervention, and novel directions being investigated. © 2016 IUBMB Life, 68(7):504-515, 2016.

The nuclear import of oncoprotein hepatitis B X-interacting protein depends on interacting with c-Fos and phosphorylation of both proteins in breast cancer cells

Aberrant nuclear localization of oncogenic transcription factors and coactivators always leads to the development of cancer. We have reported that the oncoprotein hepatitis B X-interacting protein (HBXIP) acts as a novel transcriptional coactivator to promote proliferation and migration of breast cancer cells. However, the mechanism of regulating the nuclear import of HBXIP remains unclear. In the present study, we found that HBXIP interacted with c-Fos through their leucine zipper domains in vitro and in vivo. Interestingly, the leucine zipper mutant of HBXIP (or c-Fos) was unavailable to bind to c-Fos (or HBXIP), resulting in the disappearance of nuclear localization of HBXIP. Moreover, we revealed that the nuclear import of HBXIP was required for phosphorylation of c-Fos at Thr(232), Thr(325), Thr(331), and Ser(374) by ERK1/2. In addition, the mutant of HBXIP at the Ser(108) phosphorylation site failed to import into the nucleus. Strikingly, we found that the kinase ataxia telangiectasia mutated (ATM) phosphorylated HBXIP at Ser(108). The knockdown of ATM by siRNA remarkably decreased the levels of serine phosphorylation and blocked the nuclear import of HBXIP. Then, we identified that ATM could bind to HBXIP. Moreover, we validated that the nuclear import of HBXIP contributed to its nuclear function. Therefore, we conclude that the nuclear import of the oncoprotein HBXIP requires interaction with c-Fos through their leucine zipper domains and phosphorylation of both proteins in breast cancer cells. Thus, our findings provide new insights into the mechanism of the nuclear import of HBXIP. Therapeutically, the block of the nuclear import of HBXIP is significant in breast cancer.

Coactivators in PPAR-Regulated Gene Expression

Peroxisome proliferator-activated receptor (PPAR)alpha, beta (also known as delta), and gamma function as sensors for fatty acids and fatty acid derivatives and control important metabolic pathways involved in the maintenance of energy balance. PPARs also regulate other diverse biological processes such as development, differentiation, inflammation, and neoplasia. In the nucleus, PPARs exist as heterodimers with retinoid X receptor-alpha bound to DNA with corepressor molecules. Upon ligand activation, PPARs undergo conformational changes that facilitate the dissociation of corepressor molecules and invoke a spatiotemporally orchestrated recruitment of transcription cofactors including coactivators and coactivator-associated proteins. While a given nuclear receptor regulates the expression of a prescribed set of target genes, coactivators are likely to influence the functioning of many regulators and thus affect the transcription of many genes. Evidence suggests that some of the coactivators such as PPAR-binding protein (PBP/PPARBP), thyroid hormone receptor-associated protein 220 (TRAP220), and mediator complex subunit 1 (MED1) may exert a broader influence on the functions of several nuclear receptors and their target genes. Investigations into the role of coactivators in the function of PPARs should strengthen our understanding of the complexities of metabolic diseases associated with energy metabolism.

Lens fiber cell differentiation and denucleation are disrupted through expression of the N-terminal nuclear receptor box of NCOA6 and result in p53-dependent and p53-independent apoptosis

Nuclear receptor coactivator 6 (NCOA6) is a multifunctional protein implicated in embryonic development, cell survival, and homeostasis. An 81-amino acid fragment, dnNCOA6, containing the N-terminal nuclear receptor box (LXXLL motif) of NCOA6, acts as a dominant-negative (dn) inhibitor of NCOA6. Here, we expressed dnNCOA6 in postmitotic transgenic mouse lens fiber cells. The transgenic lenses showed reduced growth; a wide spectrum of lens fiber cell differentiation defects, including reduced expression of gamma-crystallins; and cataract formation. Those lens fiber cells entered an alternate proapoptotic pathway, and the denucleation (karyolysis) process was stalled. Activation of caspase-3 at embryonic day (E)13.5 was followed by double-strand breaks (DSBs) formation monitored via a biomarker, gamma-H2AX. Intense terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) signals were found at E16.5. Thus, a window of approximately 72 h between these events suggested prolonged though incomplete apoptosis in the lens fiber cell compartment that preserved nuclei in its cells. Genetic experiments showed that the apoptotic-like processes in the transgenic lens were both p53-dependent and p53-independent. Lens-specific deletion of Ncoa6 also resulted in disrupted lens fiber cell differentiation. Our data demonstrate a cell-autonomous role of Ncoa6 in lens fiber cell differentiation and suggest novel insights into the process of lens fiber cell denucleation and apoptosis.

Cooperation between the thyroid hormone receptor TRalpha1 and the WNT pathway in the induction of intestinal tumorigenesis

BACKGROUND & AIMS

Colorectal tumorigenesis is a multistep process involving the alteration of oncogenes and tumor suppressor genes, leading to the deregulation of molecular pathways that govern intestinal homeostasis. We have previously shown that the thyroid hormone receptor alpha1 (TRalpha1) controls intestinal development and homeostasis through the WNT pathway. More precisely, TRalpha1 directly enhances the transcription of several components of this pathway, allowing increased expression of beta-catenin/Tcf4 target genes and stimulation of cell proliferation. Because the WNT pathway is a major player in controlling intestinal homeostasis, we addressed whether the TRalpha1 receptor has tumor-inducing potential.

METHODS

We generated mice overexpressing TRalpha1 specifically in the intestinal epithelium in a wild-type (vil-TRalpha1) or a WNT-activated (vil-TRalpha1/Apc(+/1638N)) genetic background.

RESULTS

The intestine of vil-TRalpha1 mice presents aberrant intestinal mucosal architecture and increased cell proliferation and develops adenoma at a low rate. However, TRalpha1 overexpression is unable to induce cancer development. On the contrary, we observed accelerated tumorigenesis in vil-TRalpha1/Apc(+/1638N) mice compared with the Apc(+/1638N) mutants.

CONCLUSION

Our results suggest that this phenotype is due to cooperation between the activated TRalpha1 and WNT pathways. This is the first report describing the tumor-inducing function of TRalpha1 in the intestine.

Transcription coactivator PBP/MED1-deficient hepatocytes are not susceptible to diethylnitrosamine-induced hepatocarcinogenesis in the mouse

Nuclear receptor coactivator [peroxisome proliferator-activated receptor-binding protein (PBP)/mediator subunit 1 (MED1)] is a critical component of the mediator transcription complex. Disruption of this gene in the mouse results in embryonic lethality. Using the PBP/MED1 liver conditional null (PBP/MED1^ΔLiv) mice, we reported that PBP/MED1 is essential for liver regeneration and the peroxisome proliferator-activated receptor α ligand Wy-14,643-induced receptor-mediated hepatocarcinogenesis. We now examined the role of PBP/MED1 in genotoxic chemical carcinogen diethylnitrosamine (DEN)-induced and phenobarbital-promoted hepatocarcinogenesis. The carcinogenic process was initiated by a single intraperitoneal injection of DEN at 14 days of age and initiated cells were promoted with phenobarbital (PB) (0.05%) in drinking water. PBP/MED1^ΔLiv mice, killed at 1, 4 and 12 weeks, revealed a striking proliferative response of few residual PBP/MED1-positive hepatocytes that escaped Cre-mediated deletion of PBP/MED1 gene. No proliferative expansion of PBP/MED1 null hepatocytes was noted in the PBP/MED1^ΔLiv mouse livers. Multiple hepatocellular carcinomas (HCCs) developed in the DEN-initiated PBP/MED1^fl/fl and PBP/MED1^ΔLiv mice, 1 year after the PB promotion. Of interest is that all HCC developing in PBP/MED1^ΔLiv mice were PBP/MED1 positive. None of the tumors was PBP/MED1 negative implying that hepatocytes deficient in PBP/MED1 are not susceptible to neoplastic conversion. HCC that developed in PBP/MED1^ΔLiv mouse livers were transplantable in athymic nude mice and these maintained PBP/MED1^fl/fl genotype. PBP/MED1^fl/fl HCC cell line derived from these tumors expressed PBP/MED1 and deletion of PBP/MED1^fl/fl allele by adeno-Cre injection into tumors caused necrosis of tumor cells. These results indicate that PBP/MED1 is essential for the development of HCC in the mouse.