Activin A specifically suppresses the expression of annexin A5 mRNA and augments gonadotropin-releasing hormone stimulation of A1 expression in LβT2 gonadotrope cells.

Recently, we reported that gonadotropin-releasing hormone (GnRH) stimulates annexin A1 (Anxa1) and A5 (Anxa5) mRNA expression through the GnRH-receptor-mitogen-activated protein kinase cascade in LβT2 cells. As LβT2 cells respond to activin, we examined the effect of activin A on Anxa1 and a5 expression in LβT2 cells. Activin A (0.4 and 4 ng/mL) treatment decreased Anxa5 mRNA levels in a dose-dependent manner, but did not affect Anxa1 mRNA levels at concentrations up to 40 ng/mL. After activin A treatment (4 ng/mL), Anxa5 mRNA levels significantly decreased at 6 h, gradually declined until 24 h, and remained low until 48 h, whereas Anxa1 mRNA levels did not significantly change following treatment. Pretreatment with activin A for 24 h increased GnRH agonist (GnRHa)-induced Anxa1 increase by approximately 7-fold compared with GnRHa stimulation alone, but Anxa5 was not affected. As previously reported, these activin A treatments increased gonadotropin β subunit and GnRH receptor mRNA levels and slightly decreased common α-glycoprotein subunit mRNA levels. Furthermore, we examined the effect of activin A on Nr4a3, which is repressed by ANXA5 and which reduces Fshb expression, and found that activin A augmented Nr4a3 expression and slightly decreased the GnRHa-induced increase in Nr4a3. These results suggest that in gonadotrope cells, the mechanism regulating Anxa1 and a5 expression is differentially coupled with activin A signal transduction. Activin A suppresses Anxa5 expression under increased Nr4a3 expression, whereas activin A and GnRH synergistically stimulate Anxa1 expression. These GnRH-inducible annexins may have different specific functions in gonadotropes.