Costanza Cossu

Gadd45β is induced through a CAR‐dependent, TNF‐independent pathway in murine liver hyperplasia

We previously observed that Gadd45β/MyD118, a member of the Gadd45 family of inducible factors, showed the strongest immediate‐early induction common to two distinctive proliferation responses of the liver: (1) regeneration induced by surgical partial hepatectomy and (2) hyperplasia induced by the primary mitogen TCPOBOP, a ligand of the constitutive androstane receptor (CAR). Gadd45β is known to be stimulated by nuclear factor (NF) κB, which is activated by tumor necrosis factor alpha (TNFα) in the early response to partial hepatectomy. We therefore investigated whether TNFα and NFκB also stimulated Gadd45β as part of the response to CAR ligands, or whether activation occurred by an alternative pathway. TCPOBOP effects were characterized in three mouse genotypes: wild‐type, TNFR1−/−, and TNFR1−/−TNFR2−/−. The results showed that TCPOBOP did not activate NFκB in any of the mice, but a strong induction of Gadd45β messenger RNA was observed in all three genotypes, where TCPOBOP also induced CyP2b10, a classical target gene of activated CAR, and cyclin D1, a proliferation linked gene. Thus, the absence of TNFR signaling and induction of NFκB did not impair CAR‐mediated gene induction. Moreover, hepatocyte proliferation was strongly induced, and at significantly higher levels than wild type, in both TNFR1−/− and TNFR1−/−TNFR2−/− mice. Further studies evaluated TCPOBOP‐induced gene expression in CAR−/− mice, by microarray expression profiling and Northern blot. The induced changes in gene expression, including the stimulation of Gadd45β, were almost completely abolished—hence all were mediated via CAR activation. In conclusion, in the liver, Gadd45β can be induced by a distinctive pathway that requires CAR and is independent of TNFα‐NFκB. The greater induction of proliferation in TNFR‐null mice suggests negative cross‐talk between the CAR and TNFα‐NFκB controls that regulate proliferation. (HEPATOLOGY 2005.)

Gadd45beta is induced through a CAR-dependent, TNF-independent pathway in murine liver hyperplasia.

We previously observed that Gadd45β/MyD118, a member of the Gadd45 family of inducible factors, showed the strongest immediate‐early induction common to two distinctive proliferation responses of the liver: (1) regeneration induced by surgical partial hepatectomy and (2) hyperplasia induced by the primary mitogen TCPOBOP, a ligand of the constitutive androstane receptor (CAR). Gadd45β is known to be stimulated by nuclear factor (NF) κB, which is activated by tumor necrosis factor alpha (TNFα) in the early response to partial hepatectomy. We therefore investigated whether TNFα and NFκB also stimulated Gadd45β as part of the response to CAR ligands, or whether activation occurred by an alternative pathway. TCPOBOP effects were characterized in three mouse genotypes: wild‐type, TNFR1−/−, and TNFR1−/−TNFR2−/−. The results showed that TCPOBOP did not activate NFκB in any of the mice, but a strong induction of Gadd45β messenger RNA was observed in all three genotypes, where TCPOBOP also induced CyP2b10, a classical target gene of activated CAR, and cyclin D1, a proliferation linked gene. Thus, the absence of TNFR signaling and induction of NFκB did not impair CAR‐mediated gene induction. Moreover, hepatocyte proliferation was strongly induced, and at significantly higher levels than wild type, in both TNFR1−/− and TNFR1−/−TNFR2−/− mice. Further studies evaluated TCPOBOP‐induced gene expression in CAR−/− mice, by microarray expression profiling and Northern blot. The induced changes in gene expression, including the stimulation of Gadd45β, were almost completely abolished—hence all were mediated via CAR activation. In conclusion, in the liver, Gadd45β can be induced by a distinctive pathway that requires CAR and is independent of TNFα‐NFκB. The greater induction of proliferation in TNFR‐null mice suggests negative cross‐talk between the CAR and TNFα‐NFκB controls that regulate proliferation. (HEPATOLOGY 2005.)

Thyroid hormone induces cyclin D1 nuclear translocation and DNA synthesis in adult rat cardiomyocytes

Although mammalian cardiomyocytes lose their proliferative capacity after birth, there is evidence that postmitotic cardiomyocytes can proliferate provided that cyclin D1 accumulates in the nucleus. Here we show by Northern blot, Western analysis, and immunohistochemistry that 3,5,3′‐triiodothyronine (T3) treatment of adult rats caused an increase of cyclin D1 mRNA and protein levels. The increased cyclin D1 protein content was associated with its translocation into the nucleus of cardiomyocytes. These changes were accompanied by the re‐entry of cardiomyocytes into the cell cycle, as demonstrated by increased levels of cyclin A, PCNA, and incorporation of bromodeoxyuridine into DNA (labeling index was 30.2% in T3‐treated rats vs. 2.2% in controls). Entry into the S phase was associated with an increased mitotic activity as demonstrated by positivity of cardiomyocyte nuclei to antibodies anti‐phosphohistone‐3, a specific marker of the mitotic phase (mitotic index was 3.01/1000 cardiomyocte nuclei in hyperthyroid rats vs. 0.04 in controls). No biochemical or histological signs of tissue damage were observed in the heart of T3‐treated rats. These results demonstrated that T3 treatment is associated with a re‐entry of cardiomyocytes into the cell cycle and so may be important for the development of future therapeutic strategies aimed at inducing proliferation of cardiomyocytes.—Ledda‐Columbano, G. M., Molotzu, F., Pibiri, M., Cossu, C., Perra, A., Columbano, A. Thyroid hormone induces cyclin D1 nuclear translocation and DNA synthesis in adult rat cardiomyocytes. FASEB J. 20, 87–94 (2006)

Different Effects of the Liver Mitogens Triiodo-Thyronine and Ciprofibrate on the Development of Rat Hepatocellular Carcinoma

Previous work has shown that treatment with thyroid hormone (T3) decreased the incidence of rat hepatocellular carcinoma (HCC). The present study was designed to determine whether the inhibitory effect of T3 on HCC development was limited to early steps of the carcinogenetic process or, whether a similar effect could also be exerted by starting T3 treatment at later stages. Hepatic nodules were induced in Fischer rats by a single dose of DENA, followed by a 2-week exposure of the animals to 2-AAF and partial hepatectomy. Rats were then divided into 3 groups: group 1 was maintained on basal diet; group 2 was fed a diet containing 4 mg/kg T3 for a week, every month/7 months, starting 9 weeks after DENA administration; group 3 was exposed to cycles of T3 starting 8 months after initiation. Results demonstrate that inhibition of HCC development was essentially similar in rats exposed to T3 starting either 9 weeks or 8 months after initiation (50% inhibition compared to control rats). We have previously shown that T3-induced nodule regression and HCC inhibition occurred in spite of its mitogenic effect. Therefore, we next wished to determine whether a similar antitumoral effect could be exerted by other liver mitogens, such as peroxisome proliferators. Rats exposed to the initiation-promotion protocol described previously, were subjected to 11 cycles of a T3 or a ciprofibrate-supplemented diet, each cycle consisting of 7 days/month; the incidence of HCC and lung metastases was determined 13.5 months after initiation. Results showed that although treatment with T3 strongly inhibited HCC development (only 31% of T3+ rats showed HCC vs 91% of controls), rats given ciprofibrate developed the same number of HCC as T3-untreated rats. In conclusion, the results of this study showed that the anticarcinogenic effect of T3 is maintained also when treatment begins late in the process, and its antitumoral property appears to be specific and may not be shared by other liver mitogens.