The results of this study show that GC-1 is a member of a new class of thyromimetic compounds that are more synthetically accessible than traditional thyromimetics and have potentially useful receptor binding and activation properties. The TR beta selectivity of GC-1 is particularly interesting and suggests that GC-1 might be a useful in vivo probe for studying the physiological roles of the different thyroid hormone receptor isoforms.

Proteolysis is a critical post-translational modification for regulation of cellular processes. Our lab has previously developed a technique for specifically labeling unmodified protein N termini, the ␣-aminome, using the engineered enzyme, subtiligase. Here we present a database, called the DegraBase (http://wellslab.ucsf.edu/degrabase/), which compiles 8090 unique N termini from 3206 proteins directly identified in subtiligase-based positive enrichment mass spectrometry experiments in healthy and apoptotic human cell lines. We include both previously published and unpublished data in our analysis, resulting in a total of 2144 unique ␣-amines identified in healthy cells, and 6990 in cells undergoing apoptosis. The N termini derive from three general categories of proteolysis with respect to cleavage location and functional role: translational N-terminal methionine processing (ϳ10% of total proteolysis), sites close to the translational N terminus that likely represent removal of transit or signal peptides (ϳ25% of total), and finally, other endoproteolytic cuts (ϳ65% of total). Induction of apoptosis causes relatively little change in the first two proteolytic categories, but dramatic changes are seen in endoproteolysis. For example, we observed 1706 putative apoptotic caspase cuts, more than double the total annotated sites in the CASBAH and MEROPS databases. In the endoproteolysis category, there are a total of nearly 3000 noncaspase nontryptic cleavages that are not currently reported in the MEROPS database. These studies significantly increase the annotation for all categories of proteolysis in human cells and allow public access for investigators to explore interesting proteolytic events in healthy and apoptotic human cells. Molecular & Cellular

Gain-of-function mutations of the isocitrate dehydrogenase 1 (IDH1) gene are among the most prevalent in low-grade gliomas and secondary glioblastoma. They lead to intracellular accumulation of the oncometabolite 2-hydroxyglutarate, represent an early pathogenic event, and are considered a therapeutic target. In this proof-of-concept study, we show that [1-13C] α-ketoglutarate can serve as a metabolic imaging agent for non-invasive, real-time, in vivo monitoring of mutant IDH1 activity, and can inform on IDH1 status. Using 13C magnetic resonance spectroscopy in combination with dissolution Dynamic Nuclear Polarization, the metabolic fate of hyperpolarized [1-13C] α-ketoglutarate is studied in isogenic glioblastoma cells that differ only in their IDH1 status. In lysates and tumors that express wild-type IDH1, only hyperpolarized [1-13C] α-ketoglutarate can be detected. In contrast, in cells that express mutant IDH1, hyperpolarized [1-13C] 2-hydroxyglutarate is also observed, both in cell lysates and in vivo in orthotopic tumors.

The recent development of hyperpolarized 13 C magnetic resonance spectroscopic imaging provides a novel method for in vivo metabolic imaging with potential applications for detection of cancer and response to treatment. Chemotherapy-induced apoptosis was shown to decrease the flux of hyperpolarized 13 C label from pyruvate to lactate due to depletion of NADH, the coenzyme of lactate dehydrogenase. In contrast, we show here that in PC-3MM2 tumors, inhibition of platelet-derived growth factor receptor with imatinib reduces the conversion of hyperpolarized pyruvate to lactate by lowering the expression of lactate dehydrogenase itself. This was accompanied by reduced expression of vascular endothelial growth factor and glutaminase, and is likely mediated by reduced expression of their transcriptional factors hypoxia-inducible factor-1 and c-Myc. Our results indicate that hyperpolarized 13 C MRSI could potentially detect the molecular effect of various cell signaling inhibitors, thus providing a radiation-free method to predict tumor response.