ABSTRACT
As the regulator of pituitary reproductive hormone synthesis, the hypothalamic neuropeptide GnRH is the central regulator of reproduction. A hallmark of GnRH action is the differential control of gene expression in pituitary gonadotropes through varied pulsatile stimulation. Among other signaling events, GnRH activation of the ERK family of MAPKs plays a significant role in the transcriptional regulation of the luteinizing hormone ß-subunit gene and regulation of cap-dependent translation. We evaluated the ERK response to different GnRH pulse amplitudes in the gonadotrope cell line LßT2. We found that low-amplitude stimulation with GnRH invokes a rapid and transient ERK activation, whereas high-amplitude stimulation invokes a prolonged activation specifically in the cytoplasm fraction of LßT2 cells. Nuclear and cytoplasmic targets of ERK, Ets-like gene 1, and eukaryotic initiation factor 4E, respectively, are similarly activated. Feedback control of ERK activation occurs mainly through the dual-specificity protein phosphatases (DUSPs). DUSP1 is localized to the nucleus in LßT2 cells but DUSP4, another member implicated in GnRH feedback, exists in both the nucleus and cytoplasm. Manipulation of nuclear DUSP activity through overexpression or knockdown of Dusp1 modulates the ERK response to low and high GnRH pulse amplitudes and activation of the Lhb promoter. Dusp1 overexpression abolishes sustained ERK activation and inhibits Lhb promoter activity induced by high amplitude pulses. Conversely, Dusp1 knockdown enhances ERK activation by low-amplitude stimulation and increases stimulation of Lhb promoter activity. We conclude that DUSP1 feedback activity modulates ERK activation and the transcriptional response to GnRH.
