No abstract

The deiodinase types 1 (D1) and 2 (D2) catalyze the activation of T4 to T3, whereas type 3 deiodinase (D3) catalyzes the inactivation of T3 and T4. D3 plays a key role in controlling thyroid hormone bioavailability. It is highly expressed during fetal development, but also in other processes with increased cell proliferation, e.g. in vascular tumors. Because tissue regeneration is dependent on cellular proliferation and is associated with activation of fetal genes, we evaluated deiodinase activities and mRNA expression in rat and mouse liver, as well as the local and systemic thyroid hormone status after partial hepatectomy (PH). We observed that in rats, D3 activity was increased 10-fold at 20 h and 3-fold at 48 h after PH; D3 mRNA expression was increased 3-fold at 20 h. The increase in D3 expression was associated with maximum 2- to 3-fold decreases of serum and liver T3 and T4 levels at 20 to 24 h after PH. In mice, D3 activity was increased 5-fold at 12 h, 8-fold at 24 h, 40-fold at 36 h, 15-fold at 48 h, and 7-fold at 72 h after PH. In correlation with this, D3 mRNA was highest (6-fold increase), and serum T3 and T4 were lowest at 36 h. Furthermore, as a measure for cell proliferation, 5-bromo-2'-deoxyuridine incorporation peaked at 20-24 h after PH in rats and at 36 h in mice. No significant effect on D1 activity or mRNA expression was found after PH. D2 activity was always undetectable. In conclusion, we found a large induction of hepatic D3 expression after PH that was correlated with an increased cellular proliferation and decreased serum and liver T3 and T4 levels. Our data suggest that D3 is important in the modulation of thyroid hormone levels in the regenerating liver, in which a decrease in cellular T3 permits an increase in proliferation.

The serotonin transporter is implicated in the uptake of the vasoconstrictor serotonin from the circulation into the platelets, where 95% of all blood serotonin is stored and released in response to vascular injury. In vivo studies indicated that platelet-derived serotonin mediates liver regeneration after partial hepatectomy. We have recently generated serotonin transporter knockout rats and demonstrated that their platelets were almost completely depleted of serotonin. Here we show that these rats exhibit impaired hemostasis and contain about 1-6% of wild-type serotonin levels in the blood. Despite the marked reduction of serotonin levels in blood and platelets, efficient liver regeneration and collagen-induced platelet aggregation occur in rats lacking the serotonin transporter. These results provide evidence that liver regeneration is not dependent on the release of serotonin from platelets. Our findings indicate that very low levels of serotonin in blood are sufficient for liver regeneration.

The presence of polyploid cells in the endocrine and exocrine pancreas has been reported for four decades. In rodents, pancreatic polyploidization is initiated after weaning and the number of polyploid cells increases with age. Surprisingly the molecular regulators and biological functions of polyploidization in the pancreas are still unknown. We discovered that atypical E2f activity is essential for polyploidization in the pancreas, using an inducible Cre/LoxP approach in new-born mice to delete ubiquitously the atypical E2f transcription factors, E2f7 and E2f8. In contrast to its critical role in embryonic survival, conditional deletion of both of both atypical E2fs in newborn mice had no impact on postnatal survival and mice lived until old age. However, deficiency of E2f7 or E2f8 alone was sufficient to suppress polyploidization in the pancreas and associated with only a minor decrease in blood serum levels of glucose, insulin, amylase and lipase under 4 hours starvation condition compared to wildtype littermates. In mice with fewer pancreatic polyploid cells that were fed ad libitum, no major impact on hormones or enzymes levels was observed. In summary, we identified atypical E2fs to be essential for polyploidization in the pancreas and discovered that postnatal induced loss of both atypical E2fs in many organs is compatible with life until old age.

The long term prognosis of liver cancer patients remains unsatisfactory because of cancer recurrence after surgical interventions, particularly in patients with viral infections. Since hepatitis B and C viral proteins lead to inactivation of the tumor suppressors p53 and Retinoblastoma (Rb), we hypothesize that surgery in the context of p53/Rb inactivation initiate de novo tumorigenesis.We, therefore, generated transgenic mice with hepatocyte and cholangiocyte/liver progenitor cell (LPC)-specific deletion of p53 and Rb, by interbreeding conditional p53/Rb knockout mice with either Albumin-cre or Cytokeratin-19-cre transgenic mice.We show that liver cancer develops at the necrotic injury site after surgical resection or radiofrequency ablation in p53/Rb deficient livers. Cancer initiation occurs as a result of specific migration, expansion and transformation of cytokeratin-19+-liver (CK-19+) cells. At the injury site migrating CK-19+ cells formed small bile ducts and adjacent cells strongly expressed the transforming growth factor β (TGFβ). Isolated cytokeratin-19+ cells deficient for p53/Rb were resistant against hypoxia and TGFβ-mediated growth inhibition. CK-19+ specific deletion of p53/Rb verified that carcinomas at the injury site originates from cholangiocytes or liver progenitor cells.These findings suggest that human liver patients with hepatitis B and C viral infection or with mutations for p53 and Rb are at high risk to develop tumors at the surgical intervention site.