[In situ biosynthesis of LH in cultured anterior pituitary cells: effects of GnRH].

In order to investigate the biosynthetic pathway of LH subunits, anterior pituitary cells were cultured in the presence of [35S]Met and polypeptides immunologically-related (i.r.) to the alpha and LH beta subunits were isolated from cells and media using specific antisera. SDS-polyacrylamide gel electrophoresis allowed us to identify 3 forms of alpha polypeptide differing in their apparent Mr: 21 K, 23 K and 25 K. Pulse-chase experiments showed the 21 K peptide (partially glycosylated) being successively converted into 23 K (authentic alpha) and 25 K ("hyperglycosylated") peptides, with spontaneous release of the 2 larger forms into the medium. With regard to LH beta, a single polypeptide of Mr 19 K was resolved by electrophoresis from material immuno-precipitated with antiserum to LH beta. This LH beta-related polypeptide was primarily present in cells, and only in trace amounts in the medium. When the incubation of cells was performed in the presence of tunicamycin (an inhibitor of glycosylation), a 16 K i.r.-form of alpha was observed, corresponding to the apopeptide alpha. The use of hydroxynorvaline, which substitutes for threonine thereby blocking glycosylation sites, induced accumulation of the 21 K-form. In the presence of GnRH (10-40 nM), the rate of synthesis of the polypeptide chains of the LH subunits increased, whether or not glycosylation was blocked, suggesting that GnRH has no direct effect on glycosylation. GnRH also readily induced the release of the newly synthesized LH subunits. This stimulatory effect of GnRH was more pronounced for the highly glycosylated forms of alpha (23 and 25 K) compared to the 16 and 21 K forms.(ABSTRACT TRUNCATED AT 250 WORDS)