c-MYC (MYC) overexpression or hyperactivation is one of the most common drivers of human cancer. Despite intensive study, the MYC oncogene remains recalcitrant to therapeutic inhibition. MYC is a transcription factor, and many of its pro-tumorigenic functions have been attributed to its ability to regulate gene expression programs1–3. Notably, oncogenic MYC activation has also been shown to increase total RNA and protein production in many tissue and disease contexts4–7. While such increases in RNA and protein production may endow cancer cells with pro-tumor hallmarks, this elevation in synthesis may also generate new or heightened burden on MYC-driven cancer cells to properly process these macromolecules8. Herein, we discover the spliceosome as a new target of oncogenic stress in MYC-driven cancers. We identify BUD31 as a MYC-synthetic lethal gene, and demonstrate that BUD31 is a component of the core spliceosome required for its assembly and catalytic activity. Core spliceosomal factors (SF3B1, U2AF1, and others) associated with BUD31 are also required to tolerate oncogenic MYC. Notably, MYC hyperactivation induces an increase in total pre-mRNA synthesis, suggesting an increased burden on the core spliceosome to process pre-mRNA. In contrast to normal cells, partial inhibition of the spliceosome in MYC-hyperactivated cells leads to global intron retention, widespread defects in pre-mRNA maturation, and deregulation of many essential cell processes. Importantly, genetic or pharmacologic inhibition of the spliceosome in vivo impairs survival, tumorigenicity, and metastatic proclivity of MYC-dependent breast cancers. Collectively, these data suggest that oncogenic MYC confers a collateral stress on splicing and that components of the spliceosome may be therapeutic entry points for aggressive MYC-driven cancers.

Metabolic profiling of cancer cells has recently been established as a promising tool for the development of therapies and identification of cancer biomarkers.Here we characterized the metabolomic profile of human breast tumors and uncovered intrinsic metabolite signatures in these tumors using an untargeted discovery approach and validation of key metabolites. The oncometabolite 2-hydroxyglutarate (2HG) accumulated at high levels in a subset of tumors and human breast cancer cell lines. We discovered an association between increased 2HG levels and MYC pathway activation in breast cancer, and further corroborated this relationship using MYC overexpression and knockdown in human mammary epithelial and breast cancer cells. Further analyses revealed globally increased DNA methylation in 2HG-high tumors and identified a tumor subtype with high tissue 2HG and a distinct DNA methylation pattern that was associated with poor prognosis and occurred with higher frequency in African-American patients. Tumors of this subtype had a stem cell-like transcriptional signature and tended to overexpress glutaminase, suggestive of a functional relationship between glutamine and 2HG metabolism in breast cancer. Accordingly, 13C-labeled glutamine was incorporated into 2HG in cells with aberrant 2HG accumulation, whereas pharmacologic and siRNAmediated glutaminase inhibition reduced 2HG levels. Our findings implicate 2HG as a candidate breast cancer oncometabolite associated with MYC activation and poor prognosis.

Myeloid-derived suppressor cells (MDSCs) play critical roles in primary and metastatic cancer progression. While MDSC regulation is widely variable even within patients harboring the same type of malignancy, the mechanisms governing such heterogeneity are largely unknown. Here, integrating human tumor genomics and syngeneic mammary tumor models, we demonstrate that mTOR signaling in cancer cells dictates a mammary tumor’s ability to stimulate MDSC accumulation through regulating G-CSF. Inhibiting this pathway or its activators (e.g., FGFR) impairs tumor progression, which is partially rescued by restoring MDSCs or G-CSF. Tumor-initiating cells (TICs) exhibit elevated G-CSF. MDSCs reciprocally increase TIC frequency through activating Notch in tumor cells, forming a feed-forward loop. Analyses of primary breast cancers and patient-derived xenografts (PDXs) corroborate these mechanisms in patients. These findings establish a non-canonical oncogenic role of mTOR signaling in recruiting pro-tumorigenic MDSCs and show how defined cancer subsets may evolve to promote and depend upon a distinct immune microenvironment.

Background: Current staging systems for cutaneous squamous cell carcinoma (cSCC) have limited positive predictive value for identifying patients who will experience metastasis.Objective: To develop and validate a gene expression profile (GEP) test for predicting risk for metastasis in localized, high-risk cSCC with the goal of improving risk-directed patient management.Methods: Archival formalin-fixed paraffin-embedded primary cSCC tissue and clinicopathologic data (n = 586) were collected from 23 independent centers in a prospectively designed study. A GEP signature

The transforming growth factor-B (TGF-B) superfamily of growth factors is responsible for a variety of physiologic actions, including cell cycle regulation. Activin is a member of the TGF-B superfamily that inhibits the proliferation of breast cancer cells. Activin functions by interacting with its type I and type II receptors to induce phosphorylation of intracellular signaling molecules known as Smads. Smads regulate transcription of many genes in a cell-and tissue-specific manner. In this study, the role of activin A in growth regulation of breast cancer cells was investigated. Activin stimulated the Smad-responsive promoter, p3TP, 2-fold over control in T47D breast cancer cells. Activin inhibited cellular proliferation of T47D breast cancer cells after 72 hours, an effect that could be abrogated by incubation with the activin type I receptor inhibitor, SB431542. Activin arrested T47D cells in the G 0 -G 1 cell cycle phase. Smad2 and Smad3 were phosphorylated in response to activin and accumulated in the nucleus of treated T47D cells. Infection of T47D cells with adenoviral Smad3 resulted in cell cycle arrest and activation of p3TP-luciferase, whereas a adenoviral dominant-negative Smad3 blocked activin-mediated cell cycle arrest and gene transcription. Activin maintained expression of p21 and p27 cyclin-dependent kinase inhibitors involved in cell cycle control, enhanced expression of p15, reduced cyclin A expression, and reduced phosphorylation of the retinoblastoma (Rb) protein. Smad3 overexpression recapitulated activininduced p15 expression and repression of cyclin A and Rb phosphorylation. These data indicate that activin A inhibits breast cancer cellular proliferation and activates Smads responsible for initiating cell cycle arrest. (Cancer Res 2005; 65(17): 7968-75)