The pituitary is highly sensitive to cell cycle disruptions. Although CDK gene mutations have not readily been identified in human pituitary tumors, overexpression of cyclins and dysregulation of CDK inhibitors are encountered in pituitary adenomas, indicating the importance of CDK activation for potential therapeutic targeting. However, it is difficult to predict which CDK inhibitor(s) may be effective against particular tumor types in vivo. Therefore, animal models faithfully recapitulating human pituitary tumors, which enable rapid and efficient in vivo testing, are needed to identify small molecule CDK inhibitors with optimal potency. We generated germline transgenic zebrafish with overexpression of pituitary tumor transforming gene (PTTG/securin) targeted to the adenohypophyseal proopiomelanocortin (POMC) lineage. PTTG was originally isolated from rat pituitary tumor cells. Dysequilibrium of intracellular PTTG abundance leads to cell cycle disruption and neoplastic formation, causing chromosomal instability and aneuploidy, and also aberrant G1/S and G2/M transition by transcriptional dysregulation of cyclin expression. On the contrary, PTTG overexpression also triggers irreversible cell cycle arrest in pituitary growth hormone (GH)- and gonadotropin (e.g., luteinizing hormone, follicle-stimulating hormone)-expressing tumors by activating lineage-specific senescence pathways, contributing to the benign propensity of pituitary tumors. Adult Tg:Pomc-Pttg zebrafish develop neoplastic coticotrophs and pituitary cyclin E up-regulation, as well as metabolic disturbances mimicking hypercortisolism caused by Cushing disease. Furthermore, Tg:Pomc-Pttg embryos recapitulate early features pathognomonic of corticotroph adenomas including corticotroph expansion and partial glucocorticoid resistance. The early-observed corticotroph pathology, combined with pituitary POMC lineage-specific expression of a fluorescent reporter in live transparent larvae, facilitated drug testing in vivo, and lead to identification of a pharmacologic CDK2/cyclin E inhibitor, R-roscovitine (seliciclib; CYC202), targeting corticotroph tumor growth and hormone secretion.
Declaration of interest: There is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding: Declaration of Funding: This work was supported by National Institutes of Health Grants KO8 DK 064806 and R03 DK075757 (to Ning-Ai Liu), CA75979 (to Shlomo Melmed), and RR13227 (to Shuo Lin) and the Doris Factor Molecular Endocrinology Laboratory.