Molecular mechanisms controlling Sertoli cell proliferation and differentiation.

Whereas spermatogonia stem cells continually divide to populate the testis with developing germ cells, the major supporter of this process, the Sertoli cell, ceases to proliferate shortly after birth in rodents and during puberty in higher primates including men. Because Sertoli cells are capable of supporting only a fixed number of germ cells, the timing of Sertoli cell exit from the cell cycle, and therefore the final number of these cells, sets the upper limit for testicular sperm production and thus influences the levels of male fertility (reviewed in Refs. 1 and 2). In rodents and men, Sertoli cells begin to proliferate during fetal development. During the 3 wk after birth, the number of Sertoli cells in the rat testis increases 30-fold, which corresponds to a series of approximately five divisions over this period (3). The rate of Sertoli cell proliferation decreases steadily in rats and mice from 5–15 d after birth, with very limited mitotic activity detectable after d 15–21, depending on the strain studied (4–7). During d 14–21, coincident with exiting the cell cycle, Sertoli cells also undergo a differentiation process that includes morphological changes, the production of secreted proteins that are required by germ cells, as well as the formation of specialized tight junctions between Sertoli cells that establish the blood-testis barrier (8). Previously, the mechanisms responsible for halting the proliferation of Sertoli cells and initiating their differentiation were not clear. In rodents, exit from the cell cycle does not appear to be due to limited proliferative signals because serum levels of the major hormone that supports Sertoli cell proliferation, FSH, rise continually during early postnatal development (9). The expression of FSH receptors on Sertoli cells also increases during this period (10). Furthermore, supraphysiological levels of FSH are capable of increasing the frequency of cell division and, thus, the final number of Sertoli cells, but such treatments do prolong the proliferation period (11). Based on these findings, studies have focused on the actions of negative factors that may be responsible for Sertoli cell exit from the cell cycle. In this issue of Endocrinology, the studies presented by Holsberger et al. (12) and Buzzard et al. (13) provide evidence for thyroid hormone being an initial signal required to limit Sertoli cell proliferation. Also, testosterone and retinoic acid are shown to inhibit the proliferation of immature Sertoli cells. More importantly, these studies are the first to address the molecular mechanisms that cause Sertoli cells to exit the cell cycle and initiate differentiation. Specifically, the cell cycle inhibitory proteins p27 and p21 are identified as being induced in response to stimulation by thyroid hormone, testosterone, or retinoic acid (13). p27 and p21 are expressed at higher levels in many cell types undergoing exit from the cell cycle and terminal differentiation (reviewed in Ref. 14). These proteins bind to and inhibit the activity of the cyclin-dependent kinases Cdk2 and Cdk4 that are required for cells to pass from the G1 to S phase of the cell cycle (15). The studies by Holsberger et al. (12) link thyroid hormone stimulation to the induction of cell cycle inhibitors, as Sertoli cell p27 protein levels are higher in hyperthyroid compared with euthyroid mice. Furthermore, p27 levels were found to increase in euthyroid mice from d 5 postpartum, when Sertoli cells are dividing, to 16 d after birth when Sertoli cells are exiting the cell cycle. Buzzard et al. (13), using cultured Sertoli cells isolated from 6-d-old rats, demonstrate that, compared with control treatments, thyroid hormone stimulation slows Sertoli cell proliferation by greater than 50 and 80% after 4 and 8 d, respectively. These studies also revealed that retinoic acid or testosterone similarly inhibit Sertoli cell proliferation. Interestingly, various combinations of thyroid hormone, retinoic acid, and testosterone did not result in additive effects. The lack of synergism displayed in culture conditions raises the possibility that the hormones act through the same pathway to signal the end of Sertoli cell expansion, although further studies will be required to rule out the possibility that the hormones might synergize to slow Sertoli proliferation in vivo. Additional data presented by Buzzard et al. (13) suggest that thyroid hormone, testosterone, and retinoic acid can impact regulation of the cell cycle by a similar mechanism, that being the induction of p21 and p27. Over a 4-d period, thyroid hormone stimulation elevated p21 and p27 levels in cultured Sertoli cells by 75 and 95%, respectively. This induction of p21 and p27 is similar to that observed in differentiating epiphyseal chondrocytes over 7 d of thyroid hormone stimulation (16). Testosterone or retinoic acid also induce the expression of p21 and p27 as well or better than thyroid hormone. Specifically, the levels of the cell cycle inhibitors are increased 80–475% by these hormones. With the caveat that comparisons to untreated cells over the 4-d study are not provided, these data suggest that thyroid hormone, testosterone, and retinoic acid are each capable of inducing cell cycle inhibitors in Sertoli cells and that this mechanism may be used to initiate terminal differentiation of the Sertoli cell. However, the question of which factor ultimately triggers Sertoli cell exit from the cell cycle in vivo still remains to be determined. Previously, there has been little evidence that testosterone or retinoic acid was capable of directly regulating Sertoli cell proliferation. In contrast, there is support for thyroid hormone as the signal for Sertoli cells to exit the cell cycle, including the findings that hyperthyroidism or administration of thyroid hormone to Sertoli cells in culture causes an early halt to their proliferation (5, 17). In addition, reduction of thyroid hormone levels by chemically induced hypothyroidism increases the percentage of dividing Sertoli cells in rats 10 d and older by 4-fold and extends the proliferation 0013-7227/03/$15.00/0 Endocrinology 144(9):3719–3721 Printed in U.S.A. Copyright © 2003 by The Endocrine Society doi: 10.1210/en.2003-0765