FIGURE 109.3 Regulation of the HPT axis and control points for potential disruption.
Flutamide is an androgen receptor antagonist. It competes with testosterone and DHT for binding at the androgen receptor (Simard et al., 1986). As a result, the androgen signal to the hypothalamus and pituitary is reduced, which stimulates an increase in LH secretion to counter the decreased androgen levels (Cook et al., 1993; ViguierMartinez et al., 1983). Examples of other androgen receptor antagonists known to induce LCTs in rats include cimetidine (Leslie et al., 1981); procymidone (Murakami et al., 1995); and bicalutamide (Iswaran et al., 1997).
In a similar manner, 5α-reductase inhibitors, such as finasteride block the conversion of testosterone to DHT (Prahalada et al., 1994; Rittmaster et al., 1992). Dihydrotestosterone has a higher binding affinity to the androgen receptor than does testosterone (DeGroot et al., 1995). Thus a decrease in DHT reduces the net androgenic signal to the hypothalamus and pituitary resulting in a compensatory increase in LH. Interestingly, 5α-reductase inhibitors induce LCTs in mice and LC hyperplasia in rats (Cook et al., 1999).
Inhibition of testosterone synthesis also lowers the androgen signal resulting in increased LH levels. Interestingly, ketoconazole, which is the most widely known inhibitor of testosterone synthesis, did not induce LCTs but this seemingly was due to the design of the study, which exposed animals to lower levels and for shorter durations than similar bioassays (Cook et al., 1999). Examples of compounds that induce LCTs by inhibiting testosterone synthesis include lansoprazole (Fort et al., 1995); calcium channel blockers such metronidazole (Rustia and Shubik, 1979).
Formestane is an aromatase inhibitor and as such it blocks the conversion of testosterone to estradiol. Since estradiol provides negative feedback to the hypothalamus and pituitary (see Figure 109.3), inhibition of estradiol, in theory, would increase the levels of LH and induce LCTs. However, formestane induced LC hyperplasia in beagle dogs but not rodents (Junker-Walker and Nogues, 1994) and aminoglutethimide, the most well-known aromatase inhibitor, did not induce LC hyperplasia or LCTs (Salhanick, 1982; Shaw et al., 1988). The role of aromatase inhibitors in leading to LCTs, therefore, is equivocal.
Dopamine agonists such as muselergine decrease prolactin levels, which causes downregulation of LH receptors on LCs (Prentice et al., 1992). The decrease in LH receptors lowers the overall testosterone production and LH levels increase to compensate. An alternative mechanism was proposed based on work with oxolinic acid. Yamanda et al. suggested that dopamine agonists increase levels of GnRH and thereby increase LH levels (Yamanda et al., 1995). The exact mechanism has yet to be worked out.
In rats GnRH induces LCTs at low doses but not at higher doses. This is because the LC of the rat contains GnRH receptors and can stimulate the LC directly. Low levels of GnRH can also increase LH through direct stimulation of the pituitary (Figure 109.3). At higher doses, negative feedback inhibition from GnRH would lower LH levels and thus LCTs. (Cook et al., 1999) Mice and humans do not have GnRH receptors on their LCs and are thus not susceptible to LCTs by GnRH agonists (Hunter et al., 1982).
Estrogen agonists induce LCTs in the mouse but almost never in the rat (Cook et al., 1999). However, PPAR-α agonists are known to lead to LCTs in rats. In at least some of the cases of PPAR-α agonists there is an increase in estradiol (but not LH or testosterone) that corresponds with the potency of the compound (Biegel et al., 2001). PPAR-α agonists also can reduce testosterone levels (Cook et al., 1992; Gazouli et al., 2002). As shown previously, compounds that lower testosterone can cause a compensatory rise in LH and lead to LCTs. Most likely, both elevation of estrogen and decreased testosterone play a role in induction of LCTs by PPAR-α agonists (Klaunig et al., 2003).
It is clear from the forgoing discussion that many compounds induce LCT in rodents, especially rats. Since the rat is one of the primary species used in toxicologic and carcinogenic risk assessment, these findings may be of importance in establishing exposure thresholds or compound safety data. However, a number of factors, when examined closely, illustrate that the occurrence of LCTs in rats is not biologically relevant to humans. These differences are thoroughly discussed and referenced in Cook et al. (1999) and the reader is referred there for a more in depth analysis. This discussion will focus on differences in comparative biology, tumor incidence, as well as cases of human genetic disease and data from epidemiology studies.
The biology of the rat and human differ in a number of important ways and some of those potentially have an impact on the relevance of LCTs in humans. These include differences in serum proteins, response to hCG, number of LH and GnRH receptors, sensitivity to prolactin, and LH-related aging changes.
The sex hormone-binding globulin is absent in the rat. It is produced in the liver in humans and binds to the majority of testosteron