Bone Morphogenetic Protein Signaling Agonist SB4 (BMPSB4) Inhibits Corticotroph Pituitary Neuroendocrine Tumors by Activation of Autophagy via a BMP4/SMADs-Dependent Pathway

Corticotroph pituitary neuroendocrine tumors (PitNETs), associated with Cushing's disease (CD), have limited treatment options other than surgical resection. Bone morphogenetic protein 4 (BMP4), a potential therapeutic target, is decreased in patients with CD. Previous studies have identified BMPSB4 as a potent agonist of the BMP4 signaling pathway. Here, we investigated the effect of BMPSB4 on the corticotroph PitNET cell line AtT20/D16v-F2 and explored the underlying mechanisms and therapeutic potential. We verified the low expression patterns of BMP4 and downstream p-SMAD1/5/9 in CD samples at the transcriptional and protein levels. In addition, BMPSB4 activated SMAD1/5/9 in a time- and concentration-dependent manner, with concomitant inhibitory effects on AtT20/D16v-F2 cells. Further RNA sequencing, transmission electron microscopy (TEM), and transfection with the mRFP-EGFP-LC3 adenoviral vector revealed that BMPSB4 induced cellular autophagy, which was the basis for the inhibitory effect of BMPSB4. Moreover, we demonstrated that autophagy induced by BMPSB4 was achieved through the SMADs-dependent pathway. In vivo, BMPSB4 inhibited tumor growth and significantly reduced adrenocorticotrophin (ACTH) and corticosterone (CORT) secretion, thereby alleviating the CD phenotype. In conclusion, this study identified BMPSB4 as an effective therapeutic agent for CD. BMPSB4 activates autophagy through a SMADs-dependent pathway, which in turn promotes autophagy-mediated cell death. Our work further elucidates the mechanism of the BMP4 signaling pathway in CD and suggests broad prospects for the development and application of BMPSB4 in CD therapy.