DAXX drives de novo lipogenesis and contributes to tumorigenesis.

Cancer cells exhibit elevated lipid synthesis. In breast and other cancer types, genes involved in lipid production are highly upregulated, but the mechanisms that control their expression remain poorly understood. Using integrated transcriptomic, lipidomic, and molecular studies, here we report that DAXX is a regulator of oncogenic lipogenesis. DAXX depletion attenuates, while its overexpression enhances, lipogenic gene expression, lipogenesis, and tumor growth. Mechanistically, DAXX interacts with SREBP1 and SREBP2 and activates SREBP-mediated transcription. DAXX associates with lipogenic gene promoters through SREBPs. Underscoring the critical roles for the DAXX-SREBP interaction for lipogenesis, SREBP2 knockdown attenuates tumor growth in cells with DAXX overexpression, and DAXX mutants unable to bind SREBP1/2 have weakened activity in promoting lipogenesis and tumor growth. Remarkably, a DAXX mutant deficient of SUMO-binding fails to activate SREBP1/2 and lipogenesis due to impaired SREBP binding and chromatin recruitment and is defective of stimulating tumorigenesis. Hence, DAXX's SUMO-binding activity is critical to oncogenic lipogenesis. Notably, a peptide corresponding to DAXX's C-terminal SUMO-interacting motif (SIM2) is cell-membrane permeable, disrupts the DAXX-SREBP1/2 interactions, and inhibits lipogenesis and tumor growth. These results establish DAXX as a regulator of lipogenesis and a potential therapeutic target for cancer therapy.

ßArrestin1 regulates glucocorticoid receptor mitogenic signaling in castration-resistant prostate cancer.

BACKGROUND: Prostate cancer (PC) is the most commonly diagnosed malignancy and the second leading cause of cancer-related deaths in males. The disease is initially treated with methods that inhibit androgen receptor (AR) signal transduction. Laboratory-based and clinical studies have identified alternative pathways that cause the failure of AR signal inhibition and consequent development of castration-resistant prostate cancer (CRPC). Glucocorticoid receptor (GR) signaling is activated in certain PC patients and promotes the emergence of CRPC, although by as yet incompletely understood mechanisms. We have previously demonstrated that ubiquitous ßarrestin1 (ßArr1) expression levels are linked to PC progression. Here, we consider the possibility that ßArr1 interacts with and activates GR in model CRPC cells. METHODS: Bioinformatic analysis of tumor xenograft and human PC datasets was used to correlate the expression of ßArr1 and GR. Western blot, immunohistochemistry and immunofluorescence microscopy, and subcellular fractionation were used to determine protein expression level and localization. Immunoprecipitation was applied to detect protein-protein interactions. RNA expression levels were determined using quantitative reverse transcription-polymerase chain reaction. Prostate sphere analysis was used to assess the rate of growth and invasion. The xenograft tumor implantation method was used to determine the tumor growth rate, local invasion, and metastasis. RESULTS: Elevated expression of ßArr1 positively correlated with increased GR expression and function in CRPC xenograft and in human PC patients. ßArr1 is expressed in the cell cytosol and nucleus, and it formed a complex with GR in the nucleus and not cytosol. Depletion of ßArr1 in AR-null CRPC cells inhibited GR function and CRPC growth and invasion in both in vitro and in vivo settings. CONCLUSIONS: ßArr1 binds GR that initiates mitogenic signaling cascades involved in the progression of PC to CRPC. The targeting of the ßArr1-GR axis may provide a new opportunity to better manage the CRPC disease.

Nuclear ßArrestin1 regulates androgen receptor function in castration resistant prostate cancer.

Progression of prostate cancer (PC) to terminal castration-resistant PC (CRPC) involves a diverse set of intermediates, and androgen receptor (AR) is the key mediator of PC initiation and progression to CRPC. Hence, identification of factors involved in the regulation of AR expression and function is a necessary first-step to improve disease outcome. In this study, we identified ubiquitous ßArrestin 1 (ßArr1) as a regulator of AR function in CRPC. Unbiased gene expression analysis of public datasets revealed increased levels of ARRB1 (the gene encoding ßArr1) in CRPC when compared to normal tissue. Further, ßArr1 expression correlated with enhanced AR transcriptional function in these datasets. The ßArr1 partitions to both nucleus and cytosol and mechanistic studies showed that nuclear, and not cytosolic, ßArr1 formed a complex with AR and AR-coregulator ßCatenin and that the heterotrimeric protein complex was recruited to androgen-response elements of AR-regulated genes. Functionally, we demonstrate that depletion of ßArr1 attenuates PC cell and tumor growth and metastasis, and rescued expression of nuclear, but not cytosolic, ßArr1 restores the PC colony growth and invasion of Matrigel in vitro and tumor growth and metastasis in mice. The targeting of ßArr1-regulated AR transcriptional function may be used in the development of new drugs to treat lethal CRPC.

ßArrestin2 Mediates Renal Cell Carcinoma Tumor Growth.

Renal Cell Carcinoma (RCC) is one of the most lethal urological cancers worldwide. The disease does not present early clinical symptoms and is commonly diagnosed at an advanced stage. Limited molecular drivers have been identified for RCC, resulting in the lack of effective treatment for patients with progressive disease. Ubiquitous ßArrestin2 (ßArr2) is well established for its function in the desensitization and trafficking of G protein-coupled receptors. More recently, ßArr2 has been implicated in the regulation of fundamental cellular functions, including proliferation and invasion. We used bioinformatic and genetic approaches to determine role of ßArr2 in RCC tumor growth. Analysis of published human datasets shows that ARRB2 (gene encoding ßArr2) expression is increased in RCC tumor compared to normal tissue and that high levels of ARRB2 correlate with worse patient survival. Experimentally, we show that knockout of ARRB2 decreases rate of RCC cell proliferation and migration in vitro and xenograft tumor growth in animals. Mechanistically, ßArr2 regulates c-Src activity, Cyclin A expression and cell cycle progression that are involved in tumor growth. These results show that ßArr2 is a critical regulator of RCC tumor growth and suggest its utility as a potential marker and drug target to treat advanced disease.

Maintenance of androgen receptor inactivation by S-nitrosylation.

Antiandrogens target ligand-binding domain of androgen receptor (AR) and are used as first-line therapeutics to treat patients diagnosed with locally advanced and metastatic prostate cancer. Although initially beneficial as judged with actual tumor mass shrinkage, this therapy invariably fails and the cancer reappears as castration-resistant disease. Here, we report that increased intracellular nitric oxide (NO) levels lead to growth inhibition of both androgen-dependent and castration-resistant prostate tumors through a mechanism that involves AR function inactivation by S-nitrosylation of a single C601 residue present in the DNA-binding domain. AR S-nitrosylation does not impact its subcellular distribution but attenuates its ability to bind AR-responsive elements in promoter region of target genes. Mechanistically, AR is transnitrosylated by its partner HSP90 protein. Ubiquitous small-molecule NO donors promote the AR S-nitrosylation and inhibit growth of castration-resistant prostate tumors. These findings reveal a new mechanism of regulating AR function and suggest that sequential targeting of distinct domains of AR may extend therapeutic efficacy for patients with advanced prostate cancer.