Sonic hedgehog signaling pathway in normal and adenomatous pituitary
This work investigates for the first time the expression and the role of Sonic hedgehog signaling pathway in adult pituitary and in pituitary tumors. Shh is a signaling protein, important in regulating patterning and proliferation in the embryo and the adult. It has a crucial role in pituitary development and Shh deficient mice do not even have a rudimentary Rathke's pouch (the development structure that gives rise to anterior pituitary). This study reveals the presence of an active Shh pathway in the post-developmental pituitary gland, with major impacts on hormone secretion and cell proliferation. After embryonic development, Shh continues to be expressed in the normal adult pituitary, being mainly co-localized in corticotrophs. These cells express also the receptor Ptc2 and the Shh induced transcription factor Gli1, being so Shh-producing and Shh-responsive cells. Shh acts in an autocrine way inside corticotrophs, inducing a major stimulation of ACTH secretion in the normal rat pituitary and in the AtT-20 cell line. The Shh induced ACTH secretion effect is synergistic with CRH. Shh stimulation increases CRH-R1 levels, up-regulating so the response of corticotrophs to CRH. At the same time, Gli1 is not only activated by Shh, but also by CRH and PKA. Gli1 itself activates POMC-transcription and acts in parallel upstream to CREB and AP-1. A major increase in Shh protein levels is seen as a result of CRH stimulation. All these results put in evidence a multiple cross-talk between these two important pathways acting at different levels to insure the final ACTH stimulation. Other types of hormone-secreting adenohypophysial cells possess one of Shh receptors (Ptc1 or Ptc2) and the transcription factor Gli1, so they have an active Shh pathway. Shh produced in the corticotrophs is a signaling protein, so it diffuses and acts also in distance. Shh increases GH secretion from the rat pituitary somatotrophs and from the GH3 cell line, while the effect on Prolactin is not statistically significant. The Sonic hedgehog pathway is downregulated in pituitary adenomas. Screening of 55 pituitary tumors reveals that they have a significantly reduced expression of Shh and Gli1. Although Shh in the normal pituitary is secreted by corticotroph cells, all the Cushing tumors screened had no Shh expression at all. Cell culture experiments performed in the AtT-20 corticotroph cell line in vitro show that Shh reduces cell proliferation by 50% and this effect is partially reducible by Cyclopamine. So Shh maintains the low proliferative capacity of corticotrophs in the normal pituitary gland and its loss may be one of the factors leading to tumor progression. It is concluded that Shh is produced in the anterior pituitary gland, is a major stimulant of ACTH and GH secretion, acts synergistically with CRH, opposes corticotroph cell proliferation and is downregulated in pituitary adenomas.
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