Targeting the USP8 protein, which is frequently mutated in Cushing’s disease patients, lowered the levels of adrenocorticotropic hormone, inhibited cell proliferation, and caused cell death in a mouse cell model of the disease, a study found.
As a result, USP8 may be a promising target for therapies to treat Cushing’s disease, researchers said.
Cushing’s disease is caused by abnormally high production of adrenocorticotropic hormone (ACTH) by a tumor that grows on the pituitary gland. In as many as two-thirds of Cushing’s disease cases, the gene that gives instructions for making USP8 is mutated.
These mutations cause the protein to be overactive, which promotes signaling via the EGFR, a receptor that drives the growth of some types of cancer cells. In a previous study, researchers found that blocking EGFR signaling reduced tumor growth and ACTH production in a mouse model of Cushing’s disease.
Aiming to determine if blocking USP8 activity might have a similar therapeutic effect, researchers in Japan treated a mouse cell model of Cushing’s disease (AtT-20 cells) with a potent inhibitor of USP8, called DUBs-IN-2.
Results showed that one day of treatment with DUBs-IN-2 significantly reduced ACTH levels in the culture medium (the fluid the cells were being grown in) to 67% of what was seen in untreated cells. It similarly reduced mRNA levels of the precursor to ACTH, proopiomelanocortin, to 37% of normal levels. Of note, mRNA is the intermediate molecule that allows a protein to be produced from a given gene.
This treatment also significantly reduced EGFR protein levels to 43% of what was seen in untreated cells, reduced cell division (i.e., tumor cell growth), and increased DNA fragmentation, which is a marker of cell death.
Researchers examined additional genes involved in tumor growth and found that the USP8 inhibitor significantly reduced the mRNA levels of Pttg1, a gene that helps drive tumor cell growth, and increased the expression of two tumor-suppressing genes, Gadd45β and Cables1. In other words, blocking USP8 “turned off” genes that drive tumor growth and “turned on” genes that stop it.
The researchers noted these experiments were done in mouse cells in dishes; as such, whether the findings hold true in living animals – including in humans – will require future investigation.
“In conclusion,” they wrote, “USP8 signaling contributes to ACTH production and cell proliferation in AtT-20 cells. Therefore, a USP8-targeting therapy offers a promising approach to treat Cushing’s disease.”