Response: Cellular Uptake of Sex Steroid Hormones

of the 3H-label in their experiments is associated with the cells is difficult to interpret. Although some of the experiments by Hammes et al. are consistent with their “bound steroid” hypothesis (see, for example, Figure 2A), others are not (for example, see Figures 2B and 2C). At a concentration of 1 nM DHT plus 1 nM SHBG, 50% of the steroid exists in the free form. One nanomole of DHTSHBG and 1 nM DHT are equally effective at entering cells and activating an androgen-responsive reporter gene (Figure 2B); SHBG alone is about 25% as effective at activating the reporter gene as DHT alone. The most reasonable interpretation is that free DHT is the active species and that SHBG itself has a modest effect (through mechanisms not explained by these experiments). RAP almost completely inhibits activation of the reporter gene by 1 nM DHT-SHBG, despite the fact that half of the DHT is unbound at this concentration. In contrast, 1 nM DHT alone is not blocked by RAP and yields the same increase in reporter gene activation as 1 nM DHT-SHBG. It is clear that SHBG is not necessary for DHT to enter the cell and to activate the reporter gene. Thus, the conclusion by Hammes et al. that there is a megalin-mediated endocytic pathway for the uptake of androgens and estrogens bound to their SHBG proteins does not seem to be sufficiently supported by the evidence presented.