A possible new signal trunsduction mechanism of anterior pituitary cells

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Physiological importance of a novel calcium-phospholipid binding protein, armexin V.in functions of the anterior pituitary gland was investigated. We synthesized recombinant rat annexin V, and purified it by the affinity to phospholipid membranes and the reverse phase HPLC.When annexin V was applied to the primary culture of rat anterior pituitary cells, it dramatically increased LH release but inhibited prolactin release. These diverse effects were driven by different mechanisms. As anti annexin V itself augmented prolactin release and Indomethacin reversed this effect, annexin V was thought to inhibit prolactin release by inhibiting phospholipase A2 and reducing prostaglandin synthesis in the primary culture. Prostaglandins are shown to stimulate prolactin release in vitro. On the other hand, the stimulating effect of annexin V on LH secretion is specific to gonadotropes. By using semi-quantitative RT-PCR, annexin V was also shown to stimulate LH beta subunit synthesis. Hence, annexin V was shown to enhance the all process of LH secretion, namely from synthesis to release. As we had observed annexin V distributed on gonadotropes by immunocytochemistry, we examined whether gonadoiropes themselves could synthesize annexin V.For this purpose, we utilized clonal cell line of the gonadotropes, alphaT3-1, to examine the expression of annexin V mRNA and found this cell line synthesizes annexin V.This result suggests the pituitary gonadotropes synthesize annexin V.Getting together, annexin V is suggested to function as a factor of the signal transduction to enhance the synthesis and the release of LH at the gonadotropes, and it is supposed to work simultaneously as a paracrine factor to inhibit prolactin cell function.