SAT625 The EGF-TBX19-EGFR Positive Feedback Loop Regulates The Expression Of The CD44v9/SLC7A11 Antioxidant System Within Corticotroph Pituitary Neuroendocrine Tumors

Disclosure: J. Chakrabarti: None. J. Eschbacher: None. K. Yuen: None. A.S. Little: None. Y. Zavros: None. Background: Stereotactic radiosurgery is a therapeutic modality considered in the adjuvant setting of patients with failed transsphenoidal surgery or recurrence of corticotroph pituitary neuroendocrine tumors (PitNETs), with variable biological remission rates ranging between 17% and 87%. Generated by alternative mRNA splicing (CD44v), CD44v9 is expressed in cancer stem cells and promotes resistance to reactive oxygen species (ROS) through the stabilization of the SLC7A11 (xCT) cystine glutamate transporter, but the role of this antioxidant system in PitNETs is unknown. TBX19 belongs to the T subfamily of TBX genes and is known to promote cancer cell metastasis. TBX19 is expressed within the pituitary gland and acts as a component of the proopiomelanocortin (POMC) gene activation program that works as a transcriptional activator. Epidermal Growth Factor Receptor (EGFR) via binding to its ligand Epidermal Growth Factor (EGF) signals to upregulate TBX19 expression via the ERK/NF-κB pathway. Within the pituitary, EGFR and its downstream pathway components (AKT, mTORC) are significantly increased in corticotroph PitNETs are associated with elevated ACTH secretion. Hypothesis: EGF/EGFR induces an EGF-TBX19-EGFR positive feedback loop that subsequently leads to the expression of the CD44v9/SLC7A11 antioxidant system within radiation-therapy resistant SOX2+ corticotroph PitNET cells. Methods: Human PitNET tissue was harvested during transsphenoidal pituitary surgery from CD patients to generate organoids (PitNETorg). Patient hPitNETs(org) were pretreated with either Vandetanib (Vand), Rapamycin (Rapa, mTOR inhibitor), Erastin (xCT inhibitor), Mifepristone (Mife, glucocorticoid inhibitor) or Cabergoline (Caber, D2 receptor inhibitor) prior to 0, 4, 8 and 16Gy radiation doses. Functional analyses will be performed by spectral flow cytometry (Cytek™ Aurora), hormone secretion and high content confocal microscopy. Results: Decreased expression of CD44v9 and xCT correlated with increased apoptosis in hPitNET(org) cultures in response to Vand, Rapa, and Erastin when compared to Veh, Mife and Caber pretreatments at an optimal radiation dose of 8Gy. Pretreatment of hPitNETorg with Rapa resulted in decreased expression CD44v9, a response mediated by of HIF-1α. Inhibition of TBX19 using a MEK inhibitor resulted in decreased expression of EGFR and downstream signaling components AKT and mTORC that correlated with decreased ACTH hPitNETorg secretion. Conclusions: The EGF-TBX19-EGFR positive feedback loop regulates CD44v9/xCT resistance to radiation therapy. Therefore, pretreatment of CD patients with drugs targeting the CD44v9/xCT antioxidant system may increase the efficacy, shorten latency time to remission, and decrease toxicity of stereotactic radiosurgery for the treatment of residual or recurrent functional corticotroph PitNETs. Presentation: Saturday, June 17, 2023