Stable Production of a Recombinant Single-Chain Eel Follicle-Stimulating Hormone Analog in CHO DG44 Cells

This study aimed to produce single-chain recombinant eel follicle-stimulating hormone (rec-eel FSH) analogs with high activity in Chinese hamster ovary DG44 (CHO DG44) cells. We recently reported that an O-linked glycosylated carboxyl-terminal peptide (CTP) of the equine chorionic gonadotropin (eCG) β-subunit plays contributes on high activity and time-dependent secretion in mammalian cells. We constructed a mutant (FSH-M) in which a linker including the eCG β-subunit CTP region (amino acids 115 to149) was inserted between the β-subunit and α-subunit of wild-type single chain eel FSH (FSH-wt). Plasmids containing eel FSH-wt and eel FSH-M were transfected into CHO DG44 cells. Single cells expressing each protein were isolated from 10 and 7 clones, respectively. Secretion increased gradually during the cultivation period and peaked at 4,000–5,000 ng/mL on day 9. The molecular weight of eel FSH-wt was 34–40 kDa, while that of eel FSH-M increased substantially, with two bands at 39–46 kDa. Treatment with PNGase F to remove the N glycosylation sites decreased the molecular weight remarkably to approximately 8 kDa. The EC50 value and maximal responsiveness of eel FSH-M were approximately 1.23- and 1.06-fold higher than those of eel FSH-wt, indicating that the mutant showed slightly higher biological activity. Phosphorylated extracellular regulated kinase (pERK1/2) activation exhibited a sharp peak at 5 min, followed by a rapid decline. These findings indicate that the new rec-eel FSH molecule with the eCG β-subunit CTP linker showed potent activity and could be produced in massive quantities using the stable CHO DG44 cell system.