Physiological role of glucocorticoids on rat serum and liver metallothionein in basal and stress conditions

Renal effects of metallothionein induction by zinc in vitro and in vivo

BACKGROUND

Metallothionein (MTT) is an endogenous antioxidant that can be induced by both zinc (Zn) and ischemia. In kidneys, increased MTT expression exerts a putative protective role in diabetes and hypoxia. Our goal was to further investigate the behavior of MTT under the influence of Zn and hypoxia in vitro and in vivo.

METHODS

MTT expression was measured in vitro in cell cultures of proximal tubular cells (LCC-PK1) by immune-histochemistry and real-time PCR after incubation with increasing concentrations of Zn under hypoxic and non-hypoxic conditions. In addition, in vivo studies were carried out in 54 patients to study MTT induction through Zn. This is a sub-study of a prospective, randomized, double-blind trial on prevention of contrast-media-induced nephropathy using Placebo, Zn and N-Acetylcysteine. Blood samples were obtained before and after 2 days p.o. treatment with or without Zn (60 mg). ELISA-based MTT level measurements were done to evaluate the effects of Zn administration. For in vivo analysis, we considered the ratio of MTT to baseline MTT (MTT₁/MTT₀) and the ratio of eGFR (eGFR₁/eGFR₀), correspondingly.

RESULTS

In vitro quantitative immuno-histochemical analysis (IHC) and real-time PCR showed that at increasing levels of Zn (5, 10, and 15 μg/ml) led to a progressive increase of MTTs: Median (IQR) expression of IHC also increased progressively from 0.10 (0.09-0.12), 0.15 (0.12-0.18), 0.25 (0.25-0.27), 0.59 (0.48-0.70) (p < 0.0001). Median (IQR) expression of PCR: 0.59 (0.51-1.72), 1.62 (1.38-4.70), 3.58 (3.06-10.42) and 10.81 (9.24-31.47) (p < 0.0001). In contrast, hypoxia did not change MTT-levels in vitro (p > 0.05). In vivo no significant differences (p = 0.96) occurred in MTT-levels after 2 days of Zn administration compared with no Zn intake. Nevertheless, there was a significant correlation between MTT (MTT₁/MTT₀) and eGFR (eGFR₁/eGFR₀) in case of Zn administration (rho = -0.49; 95%-CI: -0.78 to -0.03; p = 0.04).

CONCLUSIONS

We found that Zn did induce MTTs in vitro, whereas hypoxia had no significant impact. In contrast, no significant increase of MTTs was detected after in vivo administration of Zn. However, there was a significant negative correlation between MTT and eGFR in vivo in case of Zn administration, this could indicate a protective role of MTTs in a setting of reduced kidney function, which is possibly influenced by Zn.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00399256 . Retrospectively registered 11/13/2006.

Metallothionein-IIA promotes initial neurite elongation and postinjury reactive neurite growth and facilitates healing after focal cortical brain injury

Metallothioneins (MTs) are small, cysteine-rich, metal binding proteins. Their function has often been considered as stress-related proteins capable of protecting cells from heavy metal toxicity and oxidative free radicals. However, recent interest has focused on the brain-specific MT-III isoform, which has neurite-inhibitory properties. To investigate the effect of another MT isoform, human MT-IIA, on neurite growth, we used rat cortical neuron cultures. MT-IIA promoted a significant increase in the rate of initial neurite elongation of individually plated neurons. We also investigated the effect of MT-IIA on the neuronal response to axonal transection in vitro. MT-IIA promoted reactive axonal growth after injury, and, by 18 hr after transection, MT-IIA had promoted axonal growth across the injury tract. Exogenous application of MT-IIA after cortical brain injury promoted wound healing, as observed by a significant decrease in cellular degradation at 4 d after injury. Furthermore, MT-IIA-treated rats exhibited numerous SMI-312-immunoreactive axonal processes within the injury tract. This was in contrast to vehicle-treated animals, in which few axonal sprouts were observed. By 7 d after injury, MT-IIA treatment resulted in a total closing over of the injury tract by microglia, astrocytes, and reactive axonal processes. However, although some reactive axonal processes were observed within the injury tract of vehicle-treated rats, the tract itself was almost never entirely enclosed. These results are discussed in relation to a possible physiological role of metallothioneins in the brain, as well as in a therapeutic context.

Metallothionein induction in response to restraint stress. Transcriptional control, adaptation to stress, and role of glucocorticoid

Metallothioneins (MT) have been implicated in the protection of cells from oxidative stress. We studied the molecular mechanism of induction of MT-I and MT-II in response to restraint stress using a mouse model system in which the animals were restrained in well ventilated polypropylene tubes for 12 h each day (one cycle). Here, we show that MT-I and MT-II mRNA levels were elevated as much as 10-20-fold after just one cycle of this simple stress. Stress-mediated MT induction occurred at the transcriptional level. The level of MT mRNA correlated with the stress-induced increase, and not with the diurnal variation, in the level of serum glucocorticoid. Treatment of the mice with RU 486, a glucocorticoid receptor antagonist, prior to restraint stress inhibited MT induction by at least 50%. Furthermore, the glucocorticoid responsive element-binding activity in the liver nuclear extracts from the stressed mice was significantly higher than that in the control mice. The complex formations between the transcription factor Sp1, MTF1, or MLTF/ARE and the respective specific oligonucleotides were not altered in the liver from the stressed mouse. The MT mRNA levels returned to the basal level at the end of nine cycles of stress, indicating habituation of the animals to restraint stress. At this stage, exposure of the animals to another type of stress, treatment with heavy metals, resulted in further induction of MT. These data indicate that glucocorticoid is the primary physiological factor responsible for MT induction following restraint stress, and the glucocorticoid receptor is the major transcription factor involved in this process.

Testosterone-dependent induction of metallothionein in genital organs of male rats

Metallothioneins (MTs) are a group of cysteine-rich heavy-metal-binding proteins. We have investigated MT gene expression in the ventral and dorsolateral lobes of the prostate and coagulating gland of male Wistar rats. In intact rats, both MT mRNA and MT were present in the dorsolateral lobe and coagulating gland but not in the ventral lobe. Orchidectomy caused involution of the above organs, and both MT mRNA and MT were considerably decreased or become undetectable. An injection of testosterone propionate into orchidectomized rats restored not only the size of these organs, but also MT mRNA and MT concentrations, particularly in the dorsolateral lobe and coagulating gland. In the dorsolateral lobe, no selective uptake of Zn2+ preceding the increase in MT was observed, suggesting that Zn2+ ions are not associated with the increased expression of the MT gene. The present result suggests that of the male auxiliary genital organs, the dorsolateral lobe and coagulating gland, but not the ventral lobe, contain MT, the biosynthesis of which is regulated by testosterone.