Persistent suppression of hepatic CYP2A1 expression and serum triiodothyronine levels by tamoxifen in intact female rats: dose-response analysis and comparison with 4-hydroxytamoxifen, fulvestrant (ICI 182,780), and 17beta-estradiol-3-benzoate

Xenopus laevis and human type 3 iodothyronine deiodinase enzyme cross-species sensitivity to inhibition by ToxCast chemicals

Deiodinase enzymes are critical for tissue-specific and temporal control of activation or inactivation of thyroid hormones during vertebrate development, including amphibian metamorphosis. We previously screened ToxCast chemicals for inhibitory activity toward human recombinant Type 3 iodothyronine deiodinase enzyme (hDIO3) and subsequently produced Xenopus laevis recombinant dio3 enzyme (Xldio3) with the goals to identify specific chemical inhibitors of Xldio3, to evaluate cross-species sensitivity and explore whether the human assay results are predictive of the amphibian. We identified a subset of 356 chemicals screened against hDIO3 to test against Xldio3, initially at a single concentration (200 μM), and further tested 79 in concentration-response mode. Most chemicals had IC₅₀ values lower for hDIO3 than for Xldio3 and many had steep Hill slopes (a potential indication of non-specific inhibition). However, eight of the most potent chemicals are likely specific inhibitors, with IC₅₀ values of 14 μM or less, Hill slopes near -1 and curves not significantly different between species likely due to conservation of catalytically active amino acids. Controlling for assay conditions, human in vitro screening results can be predictive of activity in the amphibian assay. This study lays the groundwork for future studies using recombinant non-mammalian proteins to test cross-species sensitivity to chemicals. DISCLAIMER: The views expressed in this paper are those of the authors and do not necessarily reflect the views or policies of the U.S. Environmental Protection Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

Estradiol-mediated suppression of CYP1B1 expression in mouse MA-10 Leydig cells is independent of protein kinase A and estrogen receptor

Estrogens have multifaceted roles in mammalian testis. In the present study, we focused on estradiol as a potential regulator of testicular cytochrome P450 1B1 (CYP1B1) expression and investigated the possible mechanisms involved in the estradiol-mediated suppression. CYP1B1 protein levels were measured in the testes of rats that were treated with 17β-estradiol benzoate (1.5 mg/kg) at different stages of development. In addition, CYP1B1 mRNA levels were measured in mouse MA-10 Leydig tumor cells treated with (a) various concentrations of 17β-estradiol benzoate, (b) 17β-estradiol benzoate in the presence of exogenous luteinizing hormone (LH), or (c) 17β-estradiol benzoate in the presence of ICI 182,780, a competitive steroidal antagonist of estrogen receptors (ERs). Treatment of neonatal, pubertal, or adult rats with 17β-estradiol benzoate was associated with a reduction of approximately 90% in testicular CYP1B1 protein content compared to age-matched controls. Treatment of MA-10 cells with 17β-estradiol benzoate (10-500 nM) produced a concentration- and time-dependent decrease in CYP1B1 mRNA levels, but had no effect on LH receptor mRNA levels or on protein kinase A (PKA) activity. However, 17β-estradiol benzoate (10-500 nM), regardless of the concentration tested, failed to attenuate the LH-elicited increase in CYP1B1 mRNA or PKA activity in MA-10 cells that were co-treated with LH and estradiol. Similarly, ICI 182,780 (10-1000 µM) did not reverse the suppressive effect of estradiol on CYP1B1 mRNA expression in MA-10 cells co-treated with estradiol and ICI 182,780. The results indicate that downregulation of testicular CYP1B1 by estradiol was independent of PKA activity and was not mediated by ERs in MA-10 cells.

Regulation of cytochrome P450 1B1 expression by luteinizing hormone in mouse MA-10 and rat R2C Leydig cells: role of protein kinase A

In the present study, we investigated the signaling pathway involved in luteinizing hormone (LH)-mediated regulation of testicular CYP1B1 in mouse MA-10 and rat R2C Leydig cells. CYP1B1 mRNA and protein levels were measured in MA-10 and R2C cells treated with LH and protein kinase activators or inhibitors. Treatment with LH or 8-bromo-cAMP, a protein kinase A (PRKA) activator, increased CYP1B1 expression and PRKA activity in a concentration-dependent manner in both cell lines, albeit to different extents. Treatment with 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphorothioate, Rp-isomer, a PRKA inhibitor, decreased basal CYP1B1 expression and attenuated LH-elicited increases in CYP1B1 mRNA and protein levels and PRKA activity. In contrast, treatment with a protein kinase G activator or an inhibitor of protein kinase C had no effect on basal or LH-induced CYP1B1 expression in MA-10 or R2C cells. Collectively, the results identify PRKA as the major signaling pathway involved in the LH-mediated regulation of testicular CYP1B1 expression in Leydig tumor cells.

Oestrogen receptors mediate oestrogen-induced increases in post-exercise rat skeletal muscle satellite cells

AIM

Our laboratory recently demonstrated that increases in post-exercise muscle satellite cell numbers are augmented by oestrogen. We investigated whether muscle oestrogen receptors (ORs) mediate this effect through administration of an OR antagonist, ICI 182,780.

METHODS

Ovariectomized female rats were divided into three groups: sham, oestrogen (0.25 mg pellet) and oestrogen plus OR blocker (ICI 182,780). Each group was divided into control and exercised groups. ICI 182,780 (5 mg kg(-1) sc) was administered 1 day prior to and 6 days following oestrogen pellet implantation. After 8 days of oestrogen exposure, animals ran downhill for 90 min (17 m min(-1), -13.5 degrees grade) on a treadmill. Soleus and white vastus muscles were removed 24 and 72 h post-exercise and immunostained for total (Pax7), activated (MyoD) and proliferating (BrdU) satellite cells. Muscle damage was indirectly assessed by measuring beta-glucuronidase activity. Two markers (His48 and ED1) of leucocyte infiltration were also examined.

RESULTS

beta-Glucuronidase activities and His48+ and ED1+ leucocytes increased post-exercise, and these increases were attenuated with oestrogen. ICI 182,780 did not influence the attenuating effect of oestrogen on leucocyte infiltration or beta-glucuronidase activities in muscle. Total (Pax7+), activated (MyoD+) and proliferating (BrdU+) satellite cells increased post-exercise, and these increases were augmented with oestrogen. Interestingly, ICI 182,780 abolished both exercise- and oestrogen-mediated increases in these satellite cell markers.

CONCLUSION

Oestrogen may augment increases in muscle satellite cells following exercise through OR-mediated mechanisms; furthermore, the attenuation of post-exercise muscle damage and leucocyte infiltration by oestrogen appears to be a non-OR-mediated process.

Influence of gender on tamoxifen-induced biochemical changes in serum of rats

Tamoxifen, the widely prescribed drug in the prevention and therapy of breast cancer, may cause side effects which may be influenced by gender. The present study was undertaken to investigate the impact of gender on tamoxifen-induced toxic and biochemical changes following oral administration of tamoxifen at high dose level of 20 mg/kg once daily for a 2-week period in both male and female rats. The results showed marked increases in serum activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in female rats. In contrast, treatment with tamoxifen in male animals significantly decreased the activity of ALT, with a tendency for a decrease in serum AST levels. In female rats, a significant reduction in the serum activity of acid phosphatase (ACP) was noted, compared with a non-significant decrease in males. Non-significant changes in serum levels of alkaline phosphatase (ALP) were seen in both sexes. Tamoxifen lowered serum contents of total lipid and total cholesterol in both male and female rats. Serum levels of triglycerides were reduced in female rats as compared to a non-significant decrease in male animals. The serum albumin concentration was decreased in both male and female rats, while total protein was decreased only in female animals. Tamoxifen markedly increased serum levels of creatinine in female rats, compared with a non-significant rise in males. Total serum contents of calcium were similarly reduced in both males and females. This is the first study which points to gender-related differences in tamoxifen-induced toxic and metabolic changes in rats. The results indicated that females are more susceptible than males to tamoxifen toxicity, probably due to the ability of tamoxifen to antagonize the action of estrogen in females.

4-Hydroxytamoxifen attenuates methamphetamine-induced nigrostriatal dopaminergic toxicity in intact and gonadetomized mice

The present study was undertaken to assess the ability of 4-hydroxytamoxifen (4-OHT) to alter methamphetamine-induced nigrostriatal dopaminergic toxicity. Three daily doses of 4-OHT (6 micro g/day) effectively attenuated methamphetamine-induced nigrostriatal dopamine depletions in both sexes of intact and gonadectomized C57BL/6 J mice. 4-OHT alone did not alter the dopamine content levels in the striatum. Both male and female mice exhibited similar Cu, Zn-superoxide dismutase protein levels in the striata whether after gonadectomy or 4-OHT treatment. Furthermore, basal body temperature and methamphetamine-induced hyperthermia were not affected by 4-OHT treatment in either sex of mice. Using a lucigenen-derived chemiluminescence assay, we found that 4-OHT by itself can serve as a potent superoxide anion radical scavenger in vitro. The protective effects of 4-OHT against methamphetamine-induced nigrostriatal dopamine depletion can be, in part, due to its antioxidative characteristics. The free radical-scavenging ability of 4-OHT calls for further investigations for its uses in clinical practice.

Lack of modifying effects of atrazine and/or tamoxifen on thyroid carcinogenesis in rats pretreated with N-bis(2-hydroxypropyl)nitrosamine (DHPN)

The modifying effects of atrazine, and/or tamoxifen, on thyroid carcinogenesis were investigated in a rat two-stage carcinogenesis model following N-bis(2-hydroxypropyl)nitrosamine (DHPN) initiation. Five-week-old male F344 rats were given a single subcutaneous injection of DHPN (2800 mg/kg, body weight) or vehicle alone. Starting 1 week later, the animals were fed a diet supplemented with 0, 5, 50 or 500 ppm of atrazine, 500 ppm atrazine plus 5 ppm tamoxifen, or 5 ppm tamoxifen in the DHPN-treated groups, and 0 or 500 ppm of atrazine in the DHPN-untreated groups for 24 weeks. At autopsy major organs, including the thyroid, pituitary, liver, kidney, testis, epididymis, and brain, were collected and histopathologically examined. Body weights were significantly (P<0.05) decreased by the high doses of atrazine or tamoxifen, the effect being enhanced in combination. Relative thyroid weights were significantly increased (P<0.05) only in the tamoxifen-treated group and pituitary weights were elevated with 500 ppm atrazine plus tamoxifen (P<0.05). Relative liver weights were increased by the high dose of atrazine. However, the atrazine and/or tamoxifen treatments did not induce significant histopathological changes in the major organs, including the thyroid, nor cause significant changes in serum TSH levels. These results suggest that neither atrazine nor tamoxifen may promote thyroid carcinogenesis, alone as well as in combination.