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

Abstract

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 L T2 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 FSH 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.

Molecular characterisation and functional interrogation of a local natriuretic peptide system in rodent pituitaries, αT3-1 and LβT2 gonadotroph cells

“…Interestingly, in cardiac tissues, ANP and BNP are colocalised with chromogranin A (CgA) in secretory granules (Tota et al 2008). As CgA and secretogranin II are implicated in gonadotrophin secretion (Nicol et al 2002(Nicol et al , 2004, it is tempting to speculate that CNP may well be cosecreted with gonadotrophins in response to GNRH.…”

Acute regulation of murine follicle-stimulating hormone β subunit transcription by activin A

“…In rats, increases in Fshb mRNA levels in the pituitary parallel and temporally precede increases in FSH release during the secondary surge (Ortolano et al 1988, Halvorson et al 1994. At the same time there is no increase in intra-pituitary FSH content, suggesting that the newly synthesized Fshb mRNA is quickly translated, linked to the glycoprotein hormone subunit and constitutively released as mature hormone (Nicol et al 2004). Arguably, intra-pituitary activins provide the major stimulus for FSH synthesis during this stage of the rodent estrous cycle.…”

Secretoneurin stimulates the production and release of luteinizing hormone in mouse LβT2 gonadotropin cells

“…We hypothesize that the well-conserved SN peptide also plays a role in regulating LH secretion in mammals. The mouse L␤T2 gonadotroph cell line was chosen because it is a highly amenable model system that exhibits the major functional characteristics consistent with those of normal mammalian pituitary gonadotrophs (38) and produces SgII (28,29), and intracellular signal transduction pathways are well described (14,19,20,32,40,43). Our results show that SN simulates intracellular signaling pathways similar to those regulated by GnRH.…”