EGFR as a therapeutic target for human, canine, and mouse ACTH-secreting pituitary adenomas

Abstract

Cushing disease is a condition in which the pituitary gland releases excessive adrenocorticotropic hormone (ACTH) as a result of an adenoma arising from the ACTH-secreting cells in the anterior pituitary. ACTH-secreting pituitary adenomas lead to hypercortisolemia and cause significant morbidity and mortality. Pituitarydirected medications are mostly ineffective, and new treatment options are needed. As these tumors express EGFR, we tested whether EGFR might provide a therapeutic target for Cushing disease. Here, we show that in surgically resected human and canine corticotroph cultured tumors, blocking EGFR suppressed expression of proopiomelanocortin (POMC), the ACTH precursor. In mouse corticotroph EGFR transfectants, ACTH secretion was enhanced, and EGF increased Pomc promoter activity, an effect that was dependent on MAPK. Blocking EGFR activity with gefitinib, an EGFR tyrosine kinase inhibitor, attenuated Pomc expression, inhibited corticotroph tumor cell proliferation, and induced apoptosis. As predominantly nuclear EGFR expression was observed in canine and human corticotroph tumors, we preferentially targeted EGFR to mouse corticotroph cell nuclei, which resulted in higher Pomc expression and ACTH secretion, both of which were inhibited by gefitinib. In athymic nude mice, EGFR overexpression enhanced the growth of explanted ACTH-secreting tumors and further elevated serum corticosterone levels. Gefitinib treatment decreased both tumor size and corticosterone levels; it also reversed signs of hypercortisolemia, including elevated glucose levels and excess omental fat. These results indicate that inhibiting EGFR signaling may be a novel strategy for treating Cushing disease.

Decoding the genetic basis of Cushing's disease: USP8 in the spotlight

“…EGF treatment increased Pomc transcription and ACTH secretion in EGFR-overexpressing corticotrophinoma cells and in corticotroph tumors from dogs and human patients with CD (51). EGFR overexpression in the mouse corticotrophinoma AtT-20 cells (that do not express the receptor) triggered basal Pomc transcription (28).…”

“…EGFR overexpression in the mouse corticotrophinoma AtT-20 cells (that do not express the receptor) triggered basal Pomc transcription (28). All these effects were mediated through Erk1/2 and acted on the proximal to initiation site of the Pomc promoter (28,51). Treatment with the EGFR tyrosine kinase inhibitor gefitinib decreased ACTH synthesis and secretion in human corticotrophinomas in vitro and decreased tumor growth and corticosterone levels in a xenograft corticotroph tumor animal model, where it ameliorated the symptoms of CD (51).…”

The USP8 mutational status may predict drug susceptibility in corticotroph adenomas of Cushing's disease

“…The positive expression of dopamine receptor D2 (DRD2), somatostatin receptors (SSTRs), and EGFR have been shown to predict drug susceptibility to dopamine agonists, somatostatin analogs, and TK inhibitors respectively (12,13,17). In contrast, the negative expression of O 6 -methylguanine DNA methyltransferase (MGMT) and the DNA mismatch repair protein MSH6 have been associated with tumor responsiveness (18,19) and resistance to temozolomide (20) respectively.…”

Nur77 gene expression levels were involved in different ACTH-secretion autonomy between Cushing’s disease and subclinical Cushing’s disease

“…POMC promoter activity by EGF required a -323 to -200 bp element that includes Tpit/Pitx sites [18]. Recently Reincke et al reported that ubiquitin-specific peptidase 8 (USP8) gene mutations were found in a high percentage of corticotroph adenomas [19,20].…”