FOXO1 Is Regulated by Insulin/IGF and GnRH via Independent Signaling Pathways in Pituitary Gonadotropes

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: SAT 134-163-GnRH & Gonadotroph Biology & Signaling
Bench to Bedside
Saturday, June 15, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board SAT-156
Danalea V. Skarra* and Varykina G. Thackray
University of California, San Diego, La Jolla, CA
Luteinizing hormone (LH) is required for mammalian fertility and is produced exclusively by gonadotrope cells located within the anterior pituitary. We have previously reported that the transcription factor FOXO1 is expressed in murine pituitary gonadotropes and LβT2 cells, an immortalized gonadotrope cell line. Furthermore, we reported that FOXO1 is a repressor of LH beta subunit (Lhb) transcription and that FOXO1 repression of basal and gonadotropin-releasing hormone (GnRH)-induced Lhb transcription localized to the proximal Lhb promoter containing PTX1, SF1 and EGR1 binding elements. FOXO1 is a well-described regulator of metabolism and its activity is inhibited by insulin. Given that FOXO1 can repress Lhb transcription and that insulin stimulation of primary pituitary and LβT2 cells increases LH synthesis and secretion, we investigated how metabolic hormone signals regulate FOXO1 activity in gonadotropes in the current study. In LβT2 cells, we determined by western analyses that insulin and insulin-like growth factor I (IGFI) induced Ser253 FOXO1 phosphorylation, causing FOXO1 inhibition and cytoplasmic localization as identified by immunofluorescence microscopy. In addition, inhibition of components of the canonical insulin signaling pathway, such as PI3K or AKT, blocked insulin and IGFI-induced phosphorylation of FOXO1, indicating that both insulin and IGFI regulation of FOXO1 occurs principally through the canonical pathway. As the in vivogonadotrope hormone milieu includes GnRH along with insulin, we next investigated the effect of GnRH on FOXO1 phosphorylation and subcellular localization. Stimulation of LβT2 cells with GnRH alone induced moderate cytoplasmic localization of FOXO1, although GnRH did not induce phosphorylation of FOXO1 Ser253. Moreover, the GnRH-induced shuttling of FOXO1 to the cytoplasm was not blocked by inhibition of PI3K activity. Furthermore, GnRH stimulation did not alter insulin inhibition of FOXO1. Additional experiments are in progress to identify the GnRH pathway mediating effects on FOXO1. In summary, we show that insulin and IGFI regulate FOXO1 activity through the canonical insulin & IGF/PI3K/AKT pathways, while GnRH regulation of FOXO1 appears to occur through an independent pathway. These data also suggest that FOXO1 may play an important role in controlling gonadotropin levels in response to metabolic cues.

Nothing to Disclose: DVS, VGT

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Sources of Research Support: Supported by NIH Grants T32 HD007203 to DS; R01 HD067448 and U54 HD012303 to VGT.