PBRM1 Inactivation Promotes Upregulation of Human Endogenous Retroviruses in a HIF-Dependent Manner.

Clear cell renal cell carcinoma (ccRCC) is considered an immunotherapy-responsive disease; however, the reasons for this remain unclear. Studies have variably implicated PBRM1 mutations as a predictive biomarker of immune checkpoint blockade (ICB) response, and separate studies demonstrate that expression of human endogenous retroviruses (hERV) might be an important class of tumor-associated antigens. We sought to understand whether specific mutations were associated with hERV expression. Two large, annotated genomic datasets, TCGA KIRC and IMmotion150, were used to correlate mutations and hERV expression. PBRM1 mutations were consistently associated with increased hERV expression in primary tumors. In vitro silencing of PBRM1, HIF1A, and HIF2A followed by RNA sequencing was performed in UMRC2 cells, confirming that PBRM1 regulates hERVs in a HIF1α- and HIF2α-dependent manner and that hERVs of the HERVERI superfamily are enriched in PBRM1-regulated hERVs. Our results uncover a role for PBRM1 in the negative regulation of hERVs in ccRCC. Moreover, the HIF-dependent nature of hERV expression explains the previously reported ccRCC-specific clinical associations of PBRM1-mutant ccRCC with both a good prognosis as well as improved clinical outcomes to ICB. See related Spotlight by Labaki et al., p. 274.

MERTK mediated novel site Akt phosphorylation alleviates SAV1 suppression.

Akt plays indispensable roles in cell proliferation, survival and metabolism. Mechanisms underlying posttranslational modification-mediated Akt activation have been extensively studied yet the Akt interactome is less understood. Here, we report that SAV1, a Hippo signaling component, inhibits Akt, a function independent of its role in Hippo signaling. Binding to a proline-tyrosine motif in the Akt-PH domain, SAV1 suppresses Akt activation by blocking Akt's movement to plasma membrane. We further identify cancer-associated SAV1 mutations with impaired ability to bind Akt, leading to Akt hyperactivation. We also determine that MERTK phosphorylates Akt1-Y26, releasing SAV1 binding and allowing Akt responsiveness to canonical PI-3K pathway activation. This work provides a mechanism underlying MERTK-mediated Akt activation and survival signaling in kidney cancer. Akt activation drives oncogenesis and therapeutic resistance; this mechanism of Akt regulation by MERTK/SAV1 provides yet another complexity in an extensively studied pathway, and may yield prognostic information and therapeutic targets.

Bap1 and Pbrm1: Determinants of Tumor Grade and mTOR Activation in VHL-Deficient Mouse Models of Renal Cell Carcinoma.

Large genome sequencing efforts have identified frequent mutations in the histone-modifying and chromatin-remodeling genes BAP1 and PBRM1 in clear cell renal cell carcinoma (ccRCC). In this issue of Cancer Discovery, Gu and colleagues model these genetic events in mice and report that dual inactivation of Vhl with either Bap1 or Pbrm1 results in faithful genetically engineered murine models of ccRCC. Moreover, their work establishes that Bap1 and Pbrm1 are determinants of tumor grade and mTORC1 activation and provocatively suggests that the cell of origin of ccRCC may lie in PAX8-expressing Bowman capsule cells. Cancer Discov; 7(8); 802-4. ©2017 AACRSee related article by Gu et al., p. 900.

Sav1 Loss Induces Senescence and Stat3 Activation Coinciding with Tubulointerstitial Fibrosis.

Tubulointerstitial fibrosis (TIF) is recognized as a final phenotypic manifestation in the transition from chronic kidney disease (CKD) to end-stage renal disease (ESRD). Here we show that conditional inactivation of Sav1 in the mouse renal epithelium resulted in upregulated expression of profibrotic genes and TIF. Loss of Sav1 induced Stat3 activation and a senescence-associated secretory phenotype (SASP) that coincided with the development of tubulointerstitial fibrosis. Treatment of mice with the YAP inhibitor verteporfin (VP) inhibited activation of genes associated with senescence, SASPs, and activation of Stat3 as well as impeded the development of fibrosis. Collectively, our studies offer novel insights into molecular events that are linked to fibrosis development from Sav1 loss and implicate VP as a potential pharmacological inhibitor to treat patients at risk for developing CKD and TIF.

MYC activation cooperates with Vhl and Ink4a/Arf loss to induce clear cell renal cell carcinoma.

Renal carcinoma is a common and aggressive malignancy whose histopathogenesis is incompletely understood and that is largely resistant to cytotoxic chemotherapy. We present two mouse models of kidney cancer that recapitulate the genomic alterations found in human papillary (pRCC) and clear cell RCC (ccRCC), the most common RCC subtypes. MYC activation results in highly penetrant pRCC tumours (MYC), while MYC activation, when combined with Vhl and Cdkn2a (Ink4a/Arf) deletion (VIM), produce kidney tumours that approximate human ccRCC. RNAseq of the mouse tumours demonstrate that MYC tumours resemble Type 2 pRCC, which are known to harbour MYC activation. Furthermore, VIM tumours more closely simulate human ccRCC. Based on their high penetrance, short latency, and histologic fidelity, these models of papillary and clear cell RCC should be significant contributions to the field of kidney cancer research.

The regulator of sex-limitation gene, rsl, enforces male-specific liver gene expression by negative regulation.

Expression of a broad array of proteins is sexually dimorphic in rodent liver, dependent on sex-specific patterns of GH secretion. Mice carrying rsl (regulator of sex limitation) alleles, discovered as trans-acting loci affecting the mouse sex-limited protein (Slp) gene, reveal an additional axis in male-specific gene regulation. Slp expresses in adult males, but in rsl homozygous mice, Slp is also expressed in females. In this study, we examined congenic rsl strains to determine rsl's site of action, breadth of targets, and interaction with hormonal induction. We show that rsl affects Slp in liver, but not kidney, and that Rsl acts on a spectrum of male-specific liver genes, including mouse urinary proteins and a cytochrome P450 expressed predominantly by males, Cyp 2d-9, but does not act on the female-prominent P450, Cyp 2a-4. Slp expression in hypophysectomized or Tfm/Y rsl mice reveals that Rsl action is independent of GH or androgen signaling. Further, parabiosis of Rsl and rsl mice does not alter expression patterns, consistent with rsl action being liver intrinsic. Finally, Slp expression initiates earlier in rsl mice, suggesting that Rsl operates before, as well as independently of, hormonal induction. This characterization suggests Rsl functions to repress transcription of a set of genes that have in common their hormonal induction in male liver, and thus accentuates sexual dimorphism of liver gene expression.

Stearoyl co-A desaturase 1 as a ccRCC therapeutic target: death by stress.

There is a need for the discovery of novel therapeutic strategies to effectively treat advanced clear cell renal cell carcinoma (ccRCC). Inhibition of stearoyl-coA desaturase 1 (SCD1) in ccRCC reveals antitumor activity, independently and in synergy with mTOR inhibition. SCD1 may be a potential novel therapeutic target in treating ccRCC.

E2F6 associates with BRG1 in transcriptional regulation.

The E2F6 protein functions as an Rb-independent repressor of gene transcription. We have previously provided evidence suggesting a role for E2F6 in repression of E2F-responsive genes at S phase. Here, we have identified BRG1, the ATPase subunit of the SWI/SNF chromatin-remodeling complex, as an E2F6 interacting protein. Immunoprecipitation experiments demonstrate that BRG1 binds specifically to E2F6 and E2F4 but not the activator E2Fs. E2F6 was also able to interact with BAF155, a BRG1-associated factor, in the SWI/SNF complex. Chromatin immunoprecipitation assays demonstrate the binding of BRG1 coincident with E2F6 on G1/S gene promoters during S phase. Collectively, our studies suggest that E2F6 may recruit BRG1 in transcriptional regulation of genes important for G1/S phase transition of the cell cycle.

MYC activity mitigates response to rapamycin in prostate cancer through eukaryotic initiation factor 4E-binding protein 1-mediated inhibition of autophagy.

Loss of PTEN and activation of phosphoinositide 3-kinase are commonly observed in advanced prostate cancer. Inhibition of mammalian target of rapamycin (mTOR), a downstream target of phosphoinositide 3-kinase signaling, results in cell cycle arrest and apoptosis in multiple in vitro and in vivo models of prostate cancer. However, single-agent use of mTOR inhibition has limited clinical success, and the identification of molecular events mitigating tumor response to mTOR inhibition remains a critical question. Here, using genetically engineered human prostate epithelial cells (PrEC), we show that MYC, a frequent target of genetic gain in prostate cancers, abrogates sensitivity to rapamycin by decreasing rapamycin-induced cytostasis and autophagy. Analysis of MYC and the mTOR pathway in human prostate tumors and PrEC showed selective increased expression of eukaryotic initiation factor 4E-binding protein 1 (4EBP1) with gain in MYC copy number or forced MYC expression, respectively. We have also found that MYC binds to regulatory regions of the 4EBP1 gene. Suppression of 4EBP1 expression resulted in resensitization of MYC-expressing PrEC to rapamycin and increased autophagy. Taken together, our findings suggest that MYC expression abrogates sensitivity to rapamycin through increased expression of 4EBP1 and reduced autophagy.

Activation of AXIN2 expression by beta-catenin-T cell factor. A feedback repressor pathway regulating Wnt signaling.

The Wnt pathway regulates cell fate, proliferation, and apoptosis, and defects in the pathway play a key role in many cancers. Although Wnts act to stabilize beta-catenin levels in the cytosol and nucleus, a multiprotein complex containing adenomatous polyposis coli, glycogen synthase kinase 3beta, and Axin1 or its homolog Axin2/Axil/conductin promotes beta-catenin phosphorylation and subsequent proteasomal degradation. We found that the rat Axil gene was strongly induced upon neoplastic transformation of RK3E cells by mutant beta-catenin or gamma-catenin or after ligand-induced activation of a beta-catenin-estrogen receptor fusion protein. Expression of Wnt1 in murine breast epithelial cells activated the conductin gene, and human cancers with defective beta-catenin regulation had elevated AXIN2 gene and protein expression. Expression of AXIN2/Axil was strongly repressed in cancer cells by restoration of wild type adenomatous polyposis coli function or expression of a dominant negative form of T cell factor (TCF)-4. TCF binding sites in the AXIN2 promoter played a key role in the ability of beta-catenin to activate AXIN2 transcription. In contrast to AXIN2/Axil, expression of human or rat Axin1 homologs was nominally affected by beta-catenin-TCF. Because Axin2 can inhibit beta-catenin abundance and function, the data implicate AXIN2 in a negative feedback pathway regulating Wnt signaling. Additionally, although Axin1 and Axin2 have been thought to have comparable functions, the observation that Wnt pathway activation elevates AXIN2 but not AXIN1 expression suggests that there may be potentially significant functional differences between the two proteins.