Differential secretion of gonadotrophins: investigation of the role of secretogranin II and chromogranin A in the release of LH and FSH in LbetaT2 cells

This study investigated the role of the secretory granule proteins, secretogranin II (SgII) and chromogranin A (CgA), in the differential secretion of FSH and LH from LbetaT2 mouse gonadotroph cells. Exogenous activin, which synergises with GnRH, is essential for the release of FSH from these cells, but also has stimulatory effects on LH and enhances GnRH-induced LH secretion. Two experiments are reported. In experiment 1, cultures were supplemented with activin (0-50 ng/ml), with and without a daily 1 h treatment of 10 nM GnRH, for 3 days. Protein secretion and mRNA levels were measured. In experiment 2, cells were treated with activin (50 ng/ml) alone, a daily 1 h treatment of 10 nM GnRH, or a combination of both for 6 days. In addition, cells exposed to activin+GnRH for 3 days were subsequently left untreated or given activin or GnRH alone for a further 3 days for comparison with cells maintained in activin+GnRH for 6 days. Protein secretion, intracellular protein and mRNA levels were measured. FSH secretion was stimulated, dose dependently, by activin and this effect increased synergistically in the presence of GnRH. The close correlation between secreted and intracellular FSH and FSHbeta mRNA levels was maintained in cells that had undergone treatment withdrawal after previous exposure to activin+GnRH, but there was no correlation between FSH and the granins. These results are consistent with the view that FSH released in response to activin/GnRH is constitutively secreted via a granin-independent pathway. SgII secretion mirrored the GnRH-induced secretion of LH, but was unaffected by activin, which stimulated LH secretion and had a detrimental effect on CgA mRNA transcription. This confirms previous observations that the LH released in response to GnRH is co-released with SgII via a regulated, granin-dependent pathway, and, in addition, suggests that activin may stimulate LH secretion through a constitutive, granin-independent pathway.