PARENT SESSION
GROWTH FACTORS

Wednesday, August 4, 2004
10:30 AM–12:30 PM
Buchanan Courtyard

(670) REGULATION OF ACTIVIN SIGNALING BY VITAMIN A, FOLLISTATIN & FOLLISTATIN-LIKE PROTEIN.

Garland, Alaina¹, O'Brien, Deborah^{1, 2}, Tsuruta, James¹, ¹ University of North Carolina, Chapel Hill, NC² University of North Carolina, Chapel Hill, NC

ABSTRACT- Depleting the testis of vitamin A causes an arrest in spermatogonial proliferation. Restoring vitamin A levels to the VAD testis re-initiates spermatogonial proliferation. In the VAD testis, only Sertoli cells are known to express receptors for retinoic acid, the active metabolite of vitamin A. Thus, vitamin A cannot directly affect the mitoses of spermatogonia; these effects must be mediated by the Sertoli cell. Our current studies focus on the role activin plays in regulating the proliferation of spermatogonia. This member of the transforming growth factor superfamily is a potent paracrine regulator of mitosis and is negatively regulated by follistatin & follistatin-like protein. We previously reported that mRNA levels for follistatin-like protein were undetectable in VAD testes by Northern analyses but were dramatically up-regulated after treating Sertoli cells or VAD testes with vitamin A. We extend our original findings by reporting on follistatin mRNA levels. We failed to detect follistatin mRNA using Northern analyses of total RNA. However, real-time PCR analyses showed a slight (∼30%) decrease in follistatin mRNA levels after treating VAD testes for 12 hours with vitamin A. Activin A mRNA levels were also measured by real-time PCR analyses. Sertoli cells treated with 1 M vitamin A showed ∼40% decrease in activin A levels. Activin A message levels were more variable in VAD testes than in Sertoli cell cultures. However, they consistently decreased after vitamin A treatment, showing a four-fold decrease over levels in VAD testes. On balance, the combined levels of the two binding proteins (as measured by their mRNA levels) was low in VAD testes and increased dramatically after treating VAD testes with vitamin A. Since follistatin & follistatin-like protein both sequester and neutralize the signaling of activin, the resumption of spermatogonial proliferation in the VAD testis coincides with the abrogation of activin signaling. Thus, it appears that activin signaling may play a key regulatory role in arresting spermatogonial proliferation in the VAD testis, and down-regulating activin signaling may be a necessary requirement for re-initiating spermatogonial proliferation.

KEY WORDS: follistatin-like protein, vitamin A, follistatin, activin