Cellular processing of follicle-stimulating hormone by Sertoli cells in serum-free culture.

Specific binding of radioiodinated human follicle-stimulating hormone [( 125I]hFSH) to receptors on the cell surface of immature rat Sertoli cells in serum-free culture (4-7 days) was concentration-, time- and temperature-dependent with an apparent steady state of binding stained after 2 h at 4, 18 or 32 degrees C. Analysis of [125I]hFSH binding data indicated an apparent Ka of 3 X 10(9) M-1 with approximately 670 fmoles binding sites/mg protein. The radioiodinated hFSH (60-70 microCi/micrograms) bound to receptor and stimulated estradiol production, thus confirming its biological activity and Sertoli cell viability. Approximately 50% of the specifically bound radioactivity was lost from the cell surface after 0.25 h at 32 degrees C, 1 h at 18 degrees C or 4 h at 4 degrees C. Most (greater than 90%) of the receptor-bound hormone was removed from the cell surface and degraded to [125I]mono- and diiodotyrosine within 4 h at 32 degrees C. FSH degradation was inhibited at low temperatures (4 degrees C) and by the lysosomotropic agents chloroquine, NH4Cl or concanavalin A. Bacitracin and methylamine, inhibitors of internalization, were effective in blocking degradation but not loss of membrane-bound [125I]hFSH. Thus it appears that, as with small peptide and simple protein hormones, complex glycoprotein hormone-receptor complexes, such as FSH Sertoli cell receptors, are endocytosed by normal target cells and subsequently degraded, most probably within the lysosomes.