Mikael Lerner

DLEU2, frequently deleted in malignancy, functions as a critical host gene of the cell cycle inhibitory microRNAs miR-15a and miR-16-1

The microRNAs miR-15a and miR-16-1 are downregulated in multiple tumor types and are frequently deleted in chronic lymphocytic leukemia (CLL), myeloma and mantle cell lymphoma. Despite their abundance in most cells the transcriptional regulation of miR-15a/16-1 remains unclear. Here we demonstrate that the putative tumor suppressor DLEU2 acts as a host gene of these microRNAs. Mature miR-15a/miR-16-1 are produced in a Drosha-dependent process from DLEU2 and binding of the Myc oncoprotein to two alterative DLEU2 promoters represses both the host gene transcript and levels of mature miR-15a/miR-16-1. In line with a functional role for DLEU2 in the expression of the microRNAs, the miR-15a/miR-16-1 locus is retained in four CLL cases that delete both promoters of this gene and expression analysis indicates that this leads to functional loss of mature miR-15a/16-1. We additionally show that DLEU2 negatively regulates the G1 Cyclins E1 and D1 through miR-15a/miR-16-1 and provide evidence that these oncoproteins are subject to miR-15a/miR-16-1-mediated repression under normal conditions. We also demonstrate that DLEU2 overexpression blocks cellular proliferation and inhibits the colony-forming ability of tumor cell lines in a miR-15a/miR-16-1-dependent way. Together the data illuminate how inactivation of DLEU2 promotes cell proliferation and tumor progression through functional loss of miR-15a/miR-16-1.

MiRNA-27a controls FBW7/hCDC4-dependent cyclin E degradation and cell cycle progression

The F-box protein FBW7/hCDC4 is a tumor suppressor that acts as the substrate recognition component of an SCF ubiquitin ligase that targets numerous oncoproteins for proteasomal degradation. In this study, we investigated whether FBW7 is regulated by microRNAs, using a screen combining bioinformatic analysis, luciferase reporters and microRNA libraries. The ubiquitous miR-27a was identified as a major suppressor of FBW7 and in line with this, miR-27a prohibited ubiquitylation and turnover of the key FBW7 substrate cyclin E. Notably, we found that miR-27a only suppresses FBW7 during specific cell cycle phases, relieving its negative impact at the G1 to S-phase transition, prior to cyclin E protein degradation. We also demonstrate that attenuation of FBW7 by miR-27a overexpression leads to improper cell cycle progression and DNA replication stress, consistent with dysregulation of cyclin E expression. Finally, in the context of human cancer, miR-27a was discovered to be generally overexpressed in pediatric B-ALL and its expression to be inversely correlated with that of FBW7 in hyperdiploid cases of B-ALL. These data provide evidence for microRNA-mediated regulation of FBW7, and highlight the role of miR-27a as a novel factor fine-tuning the periodic events regulating cell cycle progression.

CDK-mediated activation of the SCFFBXO28 ubiquitin ligase promotes MYC-driven transcription and tumourigenesis and predicts poor survival in breast cancer

SCF (Skp1/Cul1/F‐box) ubiquitin ligases act as master regulators of cellular homeostasis by targeting key proteins for ubiquitylation. Here, we identified a hitherto uncharacterized F‐box protein, FBXO28 that controls MYC‐dependent transcription by non‐proteolytic ubiquitylation. SCFFBXO28 activity and stability are regulated during the cell cycle by CDK1/2‐mediated phosphorylation of FBXO28, which is required for its efficient ubiquitylation of MYC and downsteam enhancement of the MYC pathway. Depletion of FBXO28 or overexpression of an F‐box mutant unable to support MYC ubiquitylation results in an impairment of MYC‐driven transcription, transformation and tumourigenesis. Finally, in human breast cancer, high FBXO28 expression and phosphorylation are strong and independent predictors of poor outcome. In conclusion, our data suggest that SCFFBXO28 plays an important role in transmitting CDK activity to MYC function during the cell cycle, emphasizing the CDK‐FBXO28‐MYC axis as a potential molecular drug target in MYC‐driven cancers, including breast cancer.

DLEU2 encodes an antisense RNA for the putative bicistronic RFP2/LEU5 gene in humans and mouse

Our group previously identified two novel genes, RFP2/LEU5 and DLEU2, within a 13q14.3 genomic region of loss seen in various malignancies. However, no specific inactivating mutations were found in these or other genes in the vicinity of the deletion, suggesting that a nonclassical tumor‐suppressor mechanism may be involved. Here, we present data showing that the DLEU2 gene encodes a putative noncoding antisense RNA, with one exon directly overlapping the first exon of the RFP2/LEU5 gene in the opposite orientation. In addition, the RFP2/LEU5 transcript can be alternatively spliced to produce either several monocistronic transcripts or a putative bicistronic transcript encoding two separate open‐reading frames, adding to the complexity of the locus. The finding that these gene structures are conserved in the mouse, including the putative bicistronic RFP2/LEU5 transcript as well as the antisense relationship with DLEU2, further underlines the significance of this unusual organization and suggests a biological function for DLEU2 in the regulation of RFP2/LEU5.

MiR-200c regulates Noxa expression and sensitivity to proteasomal inhibitors.

The pro-apoptotic p53 target Noxa is a BH3-only protein that antagonizes the function of selected anti-apoptotic Bcl-2 family members. While much is known regarding the transcriptional regulation of Noxa, its posttranscriptional regulation remains relatively unstudied. In this study, we therefore investigated whether Noxa is regulated by microRNAs. Using a screen combining luciferase reporters, bioinformatic target prediction analysis and microRNA expression profiling, we identified miR-200c as a negative regulator of Noxa expression. MiR-200c was shown to repress basal expression of Noxa, as well as Noxa expression induced by various stimuli, including proteasomal inhibition. Luciferase reporter experiments furthermore defined one miR-200c target site in the Noxa 3′UTR that is essential for this direct regulation. In spite of the miR-200c:Noxa interaction, miR-200c overexpression led to increased sensitivity to the clinically used proteasomal inhibitor bortezomib in several cell lines. This apparently contradictory finding was reconciled by the fact that in cells devoid of Noxa expression, miR-200c overexpression had an even more pronounced positive effect on apoptosis induced by proteasomal inhibition. Together, our data define miR-200c as a potentiator of bortezomib-induced cell death. At the same time, we show that miR-200c is a novel negative regulator of the pro-apoptotic Bcl-2 family member Noxa.

Abstract 1849: High nuclear expression of the p53 target Wig-1 is associated with poor prognosis in cervical carcinoma

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Cervical cancer is the third most common cancer in women worldwide, and is the most common cancer in Eastern Africa, South-Central Asia and Melanesia. Persistent HPV infection is an important feature of cervical cancer. The central role of chromosome 3q for progression from low-grade dysplastic lesions to higher grades and to invasive carcinomas has been well established. Gain of 3q can occur in morphologically normal cytology of women who developed cervical carcinomas after only a short latency. The Wig-1 gene (wt p53-induced gene 1) is located at the frequently gained chromosomal region 3q. We examined Wig-1 in eight cervical cancer cell lines (Ca Ski, ME-180, MS751, SiHa, SW 756, C-4I, C-33A, and HT-3) by FISH, Southern, Northern, qRT-PCR, and Western blotting. We did not detect any Wig-1 gene alterations in these cell lines. C33-A and HT-3 with strong Wig-1 mRNA expression were HPV negative while the HPV positive cells only exhibited low / moderate Wig-1 expression by Northern blotting. However, no association was found between Wig-1 protein expression and HPV infection. We also assessed Wig-1 expression in a series of 38 cervical tumors, including both squamous cell carcinoma (SCC) and adenocarcinoma (ADCA), by immunohistochemistry. Interestingly, patients who showed moderate nuclear Wig-1 expression levels and positive cytoplasmic Wig-1 staining in their tumors had significantly better prognosis than patients with high nuclear Wig-1 expression and no cytoplasmic staining (Log Rank P value = 0.042). Moreover, higher nuclear Wig-1 expression levels were associated with HPV negative cervical tumors (P=0.003). These results suggest that a marker like Wig-1 could have considerable impact on prediction of the prognosis in women with cervical cancer. Our previous results indicate that Wig-1 acts as a survival factor. Thus, high levels of nuclear Wig-1 may promote tumor growth and survival. The exact role of Wig-1 at the molecular level during cervical carcinoma development needs further investigation. Citation Format: Lidi Xu, Susanne Muller, Mikael Lerner, Svetlana Lagercrantz, Dan Grander, Keng-Ling Wallin, Klas G. Wiman, Sonia Andersson, Catharina Larsson. High nuclear expression of the p53 target Wig-1 is associated with poor prognosis in cervical carcinoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1849. doi:10.1158/1538-7445.AM2014-1849

Targeting autophagy by small molecule inhibitors of vacuolar protein sorting 34 (Vps34) improves the sensitivity of breast cancer cells to Sunitinib

Resistance to chemotherapy is a challenging problem for treatment of cancer patients and autophagy has been shown to mediate development of resistance. In this study we systematically screened a library of 306 known anti-cancer drugs for their ability to induce autophagy using a cell-based assay. 114 of the drugs were classified as autophagy inducers; for 16 drugs, the cytotoxicity was potentiated by siRNA-mediated knock-down of Atg7 and Vps34. These drugs were further evaluated in breast cancer cell lines for autophagy induction, and two tyrosine kinase inhibitors, Sunitinib and Erlotinib, were selected for further studies. For the pharmacological inhibition of autophagy, we have characterized here a novel highly potent selective inhibitor of Vps34, SB02024. SB02024 blocked autophagy in vitro and reduced xenograft growth of two breast cancer cell lines, MDA-MB-231 and MCF-7, in vivo. Vps34 inhibitor significantly potentiated cytotoxicity of Sunitinib and Erlotinib in MCF-7 and MDA-MB-231 in vitro in monolayer cultures and when grown as multicellular spheroids. Our data suggests that inhibition of autophagy significantly improves sensitivity to Sunitinib and Erlotinib and that Vps34 is a promising therapeutic target for combination strategies in breast cancer.