Precursor-product relationships in the biosynthesis and secretion of thyrotropin and its subunits by mouse thyrotropic tumor cells

Differential effect of inhibitors of oligosaccharide processing on the secretion of thyrotropin from dispersed rodent pituitary cells

We examined the effect of various inhibitors of oligosaccharide processing on the content and secretion of newly synthesized thyroid-stimulating hormone (TSH) from dispersed hypothyroid rodent pituitary cells. 1-deoxynojirimycin and N-methyl-1-deoxynojirimycin, both inhibitors of glucosidases I and II, decreased intracellular TSH (to 60-76% of control) and secreted TSH (to 60-63% of control) after a 1-hour incubation (pulse) with [35S]methionine and an 8-hour incubation (chase) in isotope-free media. In contrast, deoxymannojirimycin and swainsonine, inhibitors of mannosidase I and II, respectively, increased both intracellular TSH (to 267-309% of control) and secreted TSH (to 192% of control) at 8 hours. TSH oligosaccharides synthesized in the presence of these glucosidase and mannosidase inhibitors were largely sensitive to endo-beta-N-acetylglucosaminidase H (endo H), confirming inhibition of processing. Despite differences in oligosaccharide structure, the in vitro bioactivities of these secreted TSH isoforms were nearly identical. These data confirm and extend previous work performed with 1-deoxynojirimycin suggesting that glucosylated high mannose forms of TSH are more susceptible to intracellular degradation. The novel finding that deoxymannojirimycin and swainsonine increase secreted and total TSH above control levels suggests that non-glucosylated high mannose forms as well as hybrid-type oligosaccharides may facilitate secretion and direct TSH away from a natural degradation pathway.

Mechanisms and regulation of TSH glycosylation

Thyroid-stimulating hormone provides an interesting model to study the glycosylation and carbohydrate processing of a heterodimeric glycoprotein with a clear physiological function. The carbohydrate moiety on TSH is required for subunit combination, protection from intracellular proteolysis and aggregation, and for attainment of full biological activity. Recent work, summarized herein, has studied mechanisms and kinetics of TSH carbohydrate maturation and has contrasted processing rates and composition of free and combined subunits. Neuroendocrine factors, such as thyrotropin-releasing hormone, appear to modulate the carbohydrate structure of secreted TSH, which results in a change in the relative bioactivity of the circulating hormone. The biochemical mechanisms by which these carbohydrate alterations occur and how they affect hormone-receptor interaction are currently under investigation.