Abstract
Properties of a pituitary gonadotrope include the capacity to regulate gonadotropin synthesis and secretion in response to a GnRH signal. Progress in identifying the steps involved in these processes has been impeded by the lack of a homogeneous in vitro model of gonadotropes. This paper presents functional characterization of a L beta T2 gonadotrope cell line generated by tumorigenesis in transgenic mice carrying the rat LH beta-subunit regulatory region linked to the SV40 T-antigen oncogene. This cell line expresses LH beta, alpha-subunit, and GnRH-receptor (GnRH-R) mRNAs (though not FSH beta), responds to glucocorticoid treatment with a reversible dampening of proliferation, and responds to pulsatile, concentration-dependent GnRH administration with LH secretion. L beta T2 cells presented with four GnRH pulses (10 nM, 90-min interpulse interval) on each of 4 days respond with incremental increases in LH secretion on successive days. This increase was greatest (15-fold) in the presence of estradiol and dexamethasone. Part of the enhanced responsiveness is apparently due to an increase in GnRH-R; pulsatile GnRH treatment alone as well as steroid treatment alone led to an increase in GnRH-R mRNA levels. When secretion was stimulated on day 4 with 54 mM [K+] pulses, bypassing the GnRH-R, the LH-secretory response indicated that the GnRH pulse history as well as estradiol and dexamethasone have actions on L beta T2-secretory capacity distinct from changes in the GnRH-R. This increase can be explained in part by the marked up-regulation of LH beta, but not alpha-subunit, mRNA observed in GnRH-pulsed cells. In summary, L beta T2 clonal gonadotropes exhibit functional characteristics consistent with those of normal pituitary gonadotropes such as LH secretion via a regulated pathway and changes in GnRH-R and LH beta gene expression in response to signaling by GnRH and steroid hormones and therefore should be a useful tool for dissecting the cellular and molecular events involved in these fundamental gonadotrope properties.
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