Profiling of Unfolded Protein Response Markers and Effect of IRE1α-specific Inhibitor in Pituitary Neuroendocrine Tumor.

Context: Inositol-requiring enzyme 1α (IRE1α) and PKR-like ER kinase (PERK), which are endoplasmic reticulum (ER) membrane proteins, regulate the unfolded protein response (UPR). These molecules have recently gained attention as a novel therapeutic target in secretory tumors. The roles of the UPR in pituitary neuroendocrine tumors (PitNETs) are unclear.

Objective: To clarify UPR profiling of PitNETs and to investigate the effect of pharmacological modulation of UPR by KIRA8, a newly developed IRE1α-specific inhibitor.

Methods: In 131 patients with PitNETs, we evaluated RNA expression of UPR markers in PitNETs and their clinical phenotypes. Using GH3 cells, we examined the effects of KIRA8 and its combination with octreotide on UPR profiling, cell growth, and apoptosis.

Results: Cytoprotective adaptive-UPR (A-UPR) markers were more increased in functioning PitNETs (FPitNETs, n = 112) than in nonfunctioning PitNETs (NFPitNETs, n = 19), while there was no difference in proapoptotic terminal-UPR (T-UPR) markers. Similarly, overt somatotroph tumors (STs, acromegaly, n = 11) increased A-UPR compared with silent STs (n = 10). In STs, serum IGF-1 levels were inversely correlated with Txnip mRNA expression, a representative T-UPR marker. KIRA8 inhibited cell growth and facilitated apoptosis in GH3 cells with increased expressions of T-UPR markers, which was enhanced by the combination with octreotide. Octreotide increased mRNA expression of Txnip and Chop, but decreased spliced Xbp1 under ER stress. Octreotide is suggested to inhibit activation of IRE1α but to reciprocally induce T-UPR under PERK.

Conclusion: UPR markers in FPitNETs are implicated as dominant A-UPR but blunted T-UPR. KIRA8, enhanced with octreotide, unbalances the UPR, leading to antitumor effects. Targeting IRE1α may provide a novel strategy to treat PitNETs.