Kinetic parameters of Thymidine kinase and DNA synthesis during liver regeneration: role of thyroid hormones

Advances in the regulation of liver regeneration

Liver regeneration is known to be a process involving highly organized and ordered tissue growth triggered by the loss of liver tissue, and remains a fascinating topic. A large number of genes are involved in this process, and there exists a sequence of stages that results in liver regeneration, while at the same time inhibitors control the size of the regenerated liver. The initiation step is characterized by priming of quiescent hepatocytes by factors such as TNF-α, IL-6 and nitric oxide. The proliferation step is the step during which hepatocytes enter into the cell cycle's G1 phase and are stimulated by complete mitogens including HGF, TGF-α and EGF. Hepatic stimulator substance, glucagon, insulin, TNF-α, IL-1 and IL-6 have also been implicated in regulating the regeneration process. Inhibitors and stop signals of hepatic regeneration are not well known and only limited information is available. Furthermore, the effects of other factors such as VEGF, PDGF, hypothyroidism, proliferating cell nuclear antigen, heat shock proteins, ischemic-reperfusion injury, steatosis and granulocyte colony-stimulating factor on liver regeneration are also systematically reviewed in this article. A tissue engineering approach using isolated hepatocytes for in vitro tissue generation and heterotopic transplantation of liver cells has been established. The use of stem cells might also be very attractive to overcome the limitation of donor liver tissue. Liver-specific differentiation of embryonic, fetal or adult stem cells is currently under investigation.

Therapeutic ultrasound in fracture healing: The mechanism of osteoinduction

BACKGROUND

Ultrasound has been used therapeutically for accelerating fracture healing since many years. However, the controversy on the exact mechanism of osteoinduction still continues. In this study, we try to bring out the exact biomolecular mechanism by which ultrasound induces fracture healing.

MATERIALS AND METHODS

THE STUDY WAS CONDUCTED IN TWO PHASES: animal experiments and clinical study. In the first phase, we induced fractures on the left tibia of Wistar strain rats under anaesthesia. They were divided into two groups. One of the groups was given low-intensity, pulsed ultrasound (30 MW/cm(2)) 20 min a day for 10 days. Tissue samples and radiographs were taken weekly for 3 weeks from both the groups. In the second phase of our study, ten patients with fractures of the distal end of the radius (ten fractures) were included. Five of these were treated as cases, and five were treated as controls. Ultrasound was given 30 MW/cm(2) for 20 min every day for 2 weeks. The patients were assessed radiologically and sonologically before and after ultrasound therapy. Tissue samples were studied with thymidine incorporation test with and without adding various neurotransmitter combinations.

RESULTS

Radiological findings revealed that there was an increased callus formation in the ultrasound group. At the cellular level, there was an increased thymidine incorporation in the ultrasound group. When various neurotransmitters were added to the cells, there was an increased thymidine incorporation in the ultrasound group. In the second phase of the study, radiological and sonological assessments showed that there was an increased callus formation in the ultrasound group. In cytological study, thymidine incorporation was found to be increased in the ultrasound group.

CONCLUSIONS

The results of animal and clinical studies demonstrated an early and increased callus formation in the ultrasound group. Cytological studies revealed increased thymidine incorporation, suggesting increased osteoblastic activity.

Methimazole protects lungs during hepatic ischemia-reperfusion injury in rats: an effect not induced by hypothyroidism

BACKGROUND

Hepatic ischemia-reperfusion injury may lead to remote organ failure with mortal respiratory dysfunction. The aim of the present study was to analyze the possible protective effects of methimazole on lungs after hepatic ischemia-reperfusion injury.

METHODS

Forty male Wistar albino rats were randomized into five groups: a control group, in which bilateral pulmonary lobectomy was done; a hepatic ischemia-reperfusion group, in which bilateral pulmonary lobectomy was done after hepatic ischemia-reperfusion; a thyroidectomy-ischemia-reperfusion group (total thyroidectomy followed by, 7 days later, bilateral pulmonary lobectomy after hepatic ischemia-reperfusion); a methimazole-ischemia-reperfusion group (following methimazole administration for 7 days, bilateral pulmonary lobectomy was done after hepatic ischemia-reperfusion); and a methimazole +L-thyroxine-ischemia-reperfusion group (following methimazole and L-thyroxine administration for 7 days, bilateral pulmonary lobectomy was performed after hepatic ischemia-reperfusion). Pulmonary tissue specimens were evaluated histopathologically and for myeloperoxidase and malondialdehyde levels.

RESULTS

All of the ischemia-reperfusion intervention groups had higher pulmonary injury scoring indices than the control group (P < 0.001). Pulmonary injury index of the ischemia-reperfusion group was higher than that of both the methimazole-supplemented hypothyroid and euthyroid groups (P = 0028; P = 0,038, respectively) and was similar to that of the thyroidectomized group. Pulmonary tissue myeloperoxidase and malondialdehyde levels in the ischemia-reperfusion group were similar with that in the thyroidectomized rats but were significantly higher than that in the control, and both the methimazole-supplemented hypothyroid and euthyroid groups.

CONCLUSION

Methimazole exerts a protective role on lungs during hepatic ischemia-reperfusion injury, which can be attributed to its anti-inflammatory and anti-oxidant effects rather than hypothyroidism alone.

The influence of hypothyroidism on liver regeneration: an experimental study in rats

BACKGROUND: The influence of hypothyroidism in liver regeneration has been a controversial opinions. PURPOSE: The aim of this study is to identify the relationship between hypothyroidism and liver regeneration in rats. METHODS: Forty male Wistar rats divided into two groups of 20 specimens each. One group (C) consisted of euthyroid rats, and the other (H) of hypothyroid rats. All the animals were anesthetized with xylazine and ketamine and subjected to a longitudinal incision in the anterior cervical region. The thyroid was completely resected in group H and left intact in group C. Ten days after the first surgery, both groups of rats were weighed and submitted to partial hepatectomy, in which the left lateral and median lobes were resected and weighed. Examinations were carried out after 24 hours and, on day 7, using 3 methods: KWON et al.'s formula to identify increase in volume; mitotic figure count in five fields; and the percentage of PCNA-positive nuclei in five fields. RESULTS: Using KWON's formula, the regeneration rate for Group C after 24 hours was 58.49% whereas that for Group H was 50.42% (p=0.0165). After 7 days, the regeneration rate for Group C was 93.04% and Group H 93.74% (p=0.2165). The average number of mitotic figures after 24 hours was 14 ± 1.5 for Group C and 9.8 ± 2.2 for Group H (p=0,00016). After 7 days the corresponding figures were 5.4 ± 1.1 and 5.1 ± 1.2 (p=0,6343). The average number of PCNA-positive nuclei after 24 hours was 13.55 ± 3.84 in Group C and 7.7 ± 2.11 in Group H (p =0,0006)). The corresponding figures after 7 days were 3.5 ± 2.39 for Group C and 4.11 ± 1.90 for Group H (p>0.05). CONCLUSION: We conclude that hypothyroidism in rats causes a delay in hepatic regeneration in the first 24 hours, but that after seven days the rate of regeneration is equal to that in euthyroid rats.

Thyroid Status Affects Rat Liver Regeneration After Partial Hepatectomy by Regulating Cell Cycle and Apoptosis

In rats, various growth factors and hormones, as well as partial hepatectomy (PH) are able to trigger the proliferative response of hepatocytes. Although recent evidence highlights the important role of thyroid hormones and thyroid status in regulating the growth of liver cells in vitro and in vivo models, the mechanism involved in the pro-proliferative effects of thyroid hormones is still unclear. Here we have investigated how in rats made hypo- and hyperthyroid after prolonged treatment respectively with propylthiouracil (PTU) and triiodothyronine (T3), the thyroid status affects liver regeneration after PH by regulating cell cycle and apoptosis proteins. Our results show that both in control and partially hepatectomized animals hyperthyroidism increases the cyclin D1, E and A levels and the activity of cyclin-cdk complexes, and decreases the levels of cdk inhibitors such as p16 and p27. On the contrary hypothyroidism induces a down-regulation of the activity of cyclin cdk complexes decreasing cyclin levels. Thyroid hormones control also p53 and p73, two proteins involved in apoptosis and growth arrest which are induced by PH. In particular, hypothyroidism increases and T3 treatment decreases p73 levels. The analysis of the phosphorylated forms of p42/44 and p38 MAPK revealed that they are induced during hepatic regeneration in euthyroid and hyperthyroid rats whereas they are negatively regulated in hypothyroid rats. In conclusion our data demonstrate that thyroid status can affects liver regeneration, altering the expression and the activity of the proteins involved in the control of cell cycle and growth arrest.

Metallothioneins (I+II) and thyroid-thymus axis efficiency in old mice: role of corticosterone and zinc supply

Thymic atrophy or thymus absence causes depressed thyroid-thymus axis (TTA) efficiency in old, young propyl-thiouracil (PTU) (experimental hypothyroidism) and in young-adult thymectomised (Tx) mice, respectively. Altered zinc turnover may be also involved in depressed TTA efficiency. Zinc turnover is under the control of zinc-bound metallothioneins (Zn-MTs) synthesis. Thyroid hormones, corticosterone and nutritional zinc affect Zn-MT induction. Zn-MT releases zinc in young-adult age during transient oxidative stress for prompt immune response. In constant oxidative stress (ageing and liver regeneration after partial hepatectomy), high liver Zn-MTs, low zinc ion bioavailability and depressed TTA efficiency appear. This last finding suggested that MT might not release zinc during constant oxidative stress leading to impaired TTA efficiency. The aim of this work/study is to clarify the role of Zn-MTs (I+II) in TTA efficiency during development and ageing. The main results are (1) Old and PTU mice display high corticosterone, enhanced liver MTmRNA, low zinc and depressed TTA efficiency restored by zinc supply. Increased survival and no significant increments in basal liver Zn-MTs proteins occur in old and PTU mice after zinc supply. (2) Lot of zinc ions bound with MT in the liver from old mice than young (HPLC). (3) Young-adult Tx mice, evaluated at 15 days from thymectomy, display high MTmRNA and nutritional-endocrine-immune damage restored by zinc supply or by thymus grafts from old zinc-treated mice. (4) Young-adult Tx mice, but evaluated at 40 days from thymectomy, display natural normalisation in MTmRNA and nutritional-endocrine-immune profile with survival similar to normal mice. (5) Stressed (constant dark for 10 days) mice overexpressing MT display low zinc, depressed immunity, reduced thymic cortex, high corticosterone, altered thyroid hormones turnover showing a likeness with old mice. These findings, taken altogether, show that corticosterone is pivotal in MTs induction under stress. MTs bind preferentially zinc ions in constant oxidative stress, but with no release of zinc from MT leading to impaired TTA efficiency. Zinc supply restores the defect because zinc has no interference in affecting pre-existing Zn-MTs protein concentrations in old and PTU mice. Therefore, free zinc ions are available for TTA efficiency after zinc supply. Thymus from old zinc-treated mice induces the same restoring effect when transplanted in Tx recipients. However, Tx mice display natural normalisation in MTmRNA and in nutritional-endocrine-immune profile in the long run. Therefore, Zn-MTs (I+II) are crucial in zinc homeostasis for endocrine-immune efficiency during the entire life assuming a role of potential and novel 'biological clock of ageing'.

Poly(ADP-ribose) polymerase is affected early by thyroid state during liver regeneration in rats

Poly(ADP-ribose) polymerase (PARP), a nuclear enzyme involved in DNA synthesis, DNA repair, and cell replication and transformation, also plays a role in the early steps of liver regeneration induced by partial hepatectomy (PH). PARP and DNA topoisomerase I (Topo I) activities and de novo DNA synthesis were studied during liver regeneration in rats with altered thyroid state. Hepatic PARP activity, evaluated as [(32)P]NAD incorporated into isolated liver nuclei, was inhibited in hyperthyroid rats and increased in hypothyroid animals. In both euthyroid and hyperthyroid rats PARP activity was rapidly stimulated, peaking 6 h after PH. In hypothyroid animals, an early decrease in activity was found, at a minimum of 6 h after PH, followed by an early onset of DNA synthesis. An inverse relationship between PARP and Topo I activities was a shared feature among euthyroid, hypothyroid, and hyperthyroid rats. Together these data show that, in replicating hepatocytes, thyroid hormones exert a regulatory role on PARP activity, which reflects the control of a number of nuclear proteins involved in DNA metabolism.

Induction of DNA synthesis in primary cultures of rat hepatocytes by serotonin: possible involvement of serotonin S2 receptor

The involvement of serotonin and its receptor subtype in the induction of hepatocyte DNA synthesis was investigated in primary cultures of adult rat hepatocytes. Serotonin caused a dose-dependent increase in DNA synthesis in primary cultures of rat hepatocytes in the presence of epidermal growth factor (EGF) and insulin, as measured by [3H]thymidine incorporation. The serotonin S2 receptor antagonists, ketanserin (10(-6) mol/L) and spiperone (10(-6) mol/L), blocked stimulation of DNA synthesis by serotonin. Displacement studies on [3H]5-hydroxytryptamine (5-HT) binding to crude membranes from control and regenerating liver tissue, using cold ketanserin and spiperone, showed an increased involvement of S2 receptors of serotonin in the regenerating liver during the DNA-synthetic phase. Serotonin enhanced the phosphorylation of a 40-kd substrate protein of protein kinase C (PKC) in the regenerating liver during the DNA synthetic phase of the hepatocyte cell cycle. This was blocked by ketanserin, indicating that serotonin S2 receptor activates PKC, an important second messenger in cell growth and division, during rat liver regeneration. Our results show that serotonin can act as a potent hepatocyte comitogen and induce DNA synthesis in primary cultures of rat hepatocytes, which is suggested to be mediated through the serotonin S2 receptors of hepatocytes.