SAT-311 Role of SREBP in Regulation of Corticotroph Tumor ACTH Secretion and Cell Proliferation

Cushing Disease (CD) is a life-threatening condition with suboptimal medical treatment. To identify drugs that not only inhibit ACTH secretion to attain eucortisolemia but also inhibit tumor growth, we conducted a high throughput screen employing a novel “gain of signal” ACTH AlphaLISA assay. From a kinase inhibitor library containing 430 compounds, we identified the dual PI3K/HDAC inhibitor, CUDC-907, as a potent inhibitor of both in vitro and in vivo corticotroph tumor ACTH secretion and growth. By stepwise comparison of CUDC-907 with mono-functional PI3K and HDAC inhibitors, we demonstrated that CUDC-907 exerts its inhibitory effect on ACTH secretion primarily through its inhibition of HDAC activity at the POMC transcriptional level; while PI3K-mediated inhibition of corticotroph cell viability further contributes to reduced ACTH secretion. We also used RNA-seq to characterize the global transcriptiome changes associated with CUDC-907 treatment. Hierarchical clustering showed that 1432 differentially expressed genes (DEGs, p≤0.05 and fold-change≥1.5) were altered by CUDC-907 treatment in comparison to the vehicle-treated control cells. Gene ontology (GO) analysis of 456 downregulated and 976 upregulated DEGs revealed that the most enriched biological processes were cholesterol biosynthesis (GO:0006695, p=1.977e-17) and the type I interferon signaling pathway (GO: 0060337, p=4.928e-7) respectively. Further analysis demonstrated downregulation of the membrane-bound transcription factor sterol regulatory element binding proteins (SREBPs). Downregulation of SREBP-2 by CUDC-907 as well as the several other target enzymes in the cholesterol biosynthesis and uptake pathway including IDI2, NSDHL, MVD, and HMGCR, was confirmed by real-time PCR. To further characterize a role for SREBP-2 in regulation of corticotroph tumor ACTH secretion and proliferation, we employed siRNA targeting endogenous SREBP-2 (SREBP-2 mRNA, Control vs. siRNA 1±0.03 vs. 0.6±0.08, p<0.05), and demonstrated that knockdown of SREBP-2 not only inhibited POMC mRNA expression (POMC mRNA, 1±0.03 vs. 0.7±0.01, p<0.01), and ACTH secretion (ACTH (ng/mL) 29±0.4 vs. 23±0.3, p<0.005), but also suppressed cell proliferation (Relative Proliferation Rate, 1±0.01 vs. 0.7±0.01, p<0.005). This was further confirmed by overexpression of cleaved mature SREBP-2, which led to increased POMC expression and cell proliferation. We demonstrate for the first time the role of the SREBP-mediated cholesterol biosynthesis pathway in regulation of corticotroph tumor POMC regulation and growth. Our studies identify SREBP and cholesterol biosynthesis as a therapeutic target in CD.