Understanding the role of thyroid hormone in Sertoli cell development: a mechanistic hypothesis.
More than a decade of research has shown that Sertoli cell proliferation is regulated by thyroid hormone. Neonatal hypothyroidism lengthens the period of Sertoli cell proliferation, leading to increases in Sertoli cell number, testis weight, and daily sperm production (DSP) when euthyroidism is re-established. In contrast, the neonatal Sertoli cell proliferative period is shortened under hyperthyroid conditions, but the mechanism by which thyroid hormone is able to negatively regulate Sertoli cell proliferation has been unclear. Recent progress in the understanding of the cell cycle has provided the opportunity to dissect the molecular targets responsible for thyroid-hormone-mediated effects on Sertoli cell proliferation. In this review, we discuss recent results indicating a critical role for the cyclin-dependent kinase inhibitors (CDKI) p27(Kip1) and p21(Cip1) in establishing Sertoli cell number, testis weight, and DSP, and the ability of thyroid hormone to modulate these CDKIs. Based on these recent results, we propose a working hypothesis for the way in which thyroid hormone regulates the withdrawal of the cell cycle by controlling CDKI degradation. Finally, although Sertoli cells have been shown to have two biologically active thyroid hormone receptor (TR) isoforms, TRalpha1 and TRbeta1, experiments with transgenic mice lacking TRalpha or TRbeta illustrate that only one TR mediates thyroid hormone effects in neonatal Sertoli cells. Although significant gaps in our knowledge still remain, advances have been made toward appreciation of the molecular sequence of events that occur when thyroid hormone stimulates Sertoli cell maturation.
Holsberger
Denise R
DR
Department of Veterinary Biosciences, University of Illinois, 2001 S. Lincoln Avenue, Urbana, IL 61802, USA.
Cooke
Paul S
PS
D000818:Animals; D002453:Cell Cycle / Q000502:physiology; D049109:Cell Proliferation; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000378:metabolism; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D044786:S-Phase Kinase-Associated Proteins / Q000378:metabolism; D012671:Seminiferous Tubules / Q000166:cytology / Q000378:metabolism; D012708:Sertoli Cells / Q000502:physiology*; D013737:Testis / Q000033:anatomy & histology / Q000378:metabolism; D013961:Thyroid Gland / Q000502:physiology; D037021:Thyroid Hormone Receptors alpha / Q000378:metabolism; D013963:Thyroid Hormones / Q000378:metabolism*
D050759:Cyclin-Dependent Kinase Inhibitor p21; D044786:S-Phase Kinase-Associated Proteins; D037021:Thyroid Hormone Receptors alpha; D013963:Thyroid Hormones; D050760:Cyclin-Dependent Kinase Inhibitor p27