Endocytosis and degradation of pegvisomant and a potential new mechanism that inhibits the nuclear translocation of GHR.

Context: Pegvisomant, a growth hormone receptor (GHR) antagonist, is a well-known drug that was designed to treat acromegaly. However, recent studies have indicated that the GHR is a "moonlighting" protein that may exhibit dual functions based on its localization in the plasma membrane and nucleus. In light of this finding, we explored whether pegvisomant is a potential "moonlighting" GHR antagonist. In addition, the mechanisms of the endocytosis, post-endocytic sorting and degradation of pegvisomant are not fully understood.

Objective: This study investigates whether pegvisomant is a "moonlighting" antagonist, and explores the mechanisms of the endocytosis, post-endocytic sorting and degradation of pegvisomant.

Methods: Indirect immunofluorescence and Western blot coupled with pharmacological inhibitors and gene silencing (SiRNA) were used to explore the mechanisms of the endocytosis, post-endocytic sorting and degradation of pegvisomant. Western blot, immunohistochemistry and indirect immunofluorescence coupled with subcellular fractionation analysis were used to determine the effect of pegvisomant on GHR's nuclear localization in vitro and in vivo.

Results: Here, we show that the endocytosis of pegvisomant is mainly mediated though the clathrin pathway. Further study of the post-endocytic sorting of pegvisomant shows that pegvisomant enters into different types of endosomes under GHR mediation. In addition, GHR is slightly downregulated by pegvisomant; further study indicates that both proteasomes and lysosomes maybe cooperate to regulate pegvisomant/GHR degradation. Most importantly, here we show that pegvisomant inhibits the nuclear localization of GHR.

Conclusion: Our study showed that pegvisomant is a "moonlighting" antagonist. In addition, we revealed the mechanisms of the endocytosis, post-endocytic sorting, and degradation of pegvisomant.