Prohibitin 1 (PHB1) is a highly conserved, ubiquitously expressed protein that partici-pates in diverse processes including mitochondrial chaperone, growth and apoptosis. The role of PHB1 in vivo is unclear and whether it is a tumor suppressor is controversial. Mice lacking methionine adenosyltransferase 1A (MAT1A) have reduced PHB1 expression, impaired mitochondrial function, and spontaneously develop hepatocellular carcinoma (HCC). To see if reduced PHB1 expression contributes to the Mat1a knockout (KO) phe-notype, we generated liver-specific Phb1 KO mice. Expression was determined at the mes-senger RNA and protein levels. PHB1 expression in cells was varied by small interfering RNA or overexpression. At 3 weeks, KO mice exhibit biochemical and histologic liver injury. Immunohistochemistry revealed apoptosis, proliferation, oxidative stress, fibrosis, bile duct epithelial metaplasia, hepatocyte dysplasia, and increased staining for stem cell and preneoplastic markers. Mitochondria are swollen and many have no discernible cris-tae. Differential gene expression revealed that genes associated with proliferation, malig-nant transformation, and liver fibrosis are highly up-regulated. From 20 weeks on, KO mice have multiple liver nodules and from 35 to 46 weeks, 38% have multifocal HCC. PHB1 protein levels were higher in normal human hepatocytes compared to human HCC cell lines Huh-7 and HepG2. Knockdown of PHB1 in murine nontransformed AML12 cells (normal mouse hepatocyte cell line) raised cyclin D1 expression, increased E2F tran-scription factor binding to cyclin D1 promoter, and proliferation. The opposite occurred with PHB1 overexpression. Knockdown or overexpression of PHB1 in Huh-7 cells did not affect proliferation significantly or sensitize cells to sorafenib-induced apoptosis. Conclu-sion: Hepatocyte-specific PHB1 deficiency results in marked liver injury, oxidative stress, and fibrosis with development of HCC by 8 months. These results support PHB1 as a tu-mor suppressor in hepatocytes. (HEPATOLOGY 2010;52:2096-2108)’

We previously showed that S-adenosylmethionine (SAMe) and its metabolite methylthioadenosine (MTA) blocked lipopolysaccharide (LPS)-induced tumor necrosis factor α (TNFα) expression in RAW (murine macrophage cell line) and Kupffer cells at the transcriptional level without affecting nuclear factor κ B nuclear binding. However, the exact molecular mechanism or mechanisms of the inhibitory effect were unclear. While SAMe is a methyl donor, MTA is an inhibitor of methylation. SAMe can convert to MTA spontaneously, so the effect of exogenous SAMe may be mediated by MTA. The aim of our current work is to examine whether the mechanism of SAMe and MTA's inhibitory effect on proinflammatory mediators might involve modulation of histone methylation. In RAW cells, we found that LPS induced TNFα expression by both transcriptional and posttranscriptional mechanisms. SAMe and MTA treatment inhibited the LPS-induced increase in gene transcription. Using the chromatin immunoprecipitation assay, we found that LPS increased the binding of trimethylated histone 3 lysine 4 (H3K4) to the TNFα promoter, and this was completely blocked by either SAMe or MTA pretreatment. Similar effects were observed with LPS-mediated induction of inducible nitric oxide synthase (iNOS). LPS increased the binding of histone methyltransferases Set1 and myeloid/lymphoid leukemia to these promoters, which was unaffected by SAMe or MTA. The effects of MTA in RAW cells were confirmed in vivo in LPS-treated mice. Exogenous SAMe is unstable and converts spontaneously to MTA, which is stable and cell-permeant. Treatment with SAMe doubled intracellular MTA and S-adenosylhomocysteine (SAH) levels. SAH also inhibited H3K4 binding to TNFα and iNOS promoters. Conclusion: The mechanism of SAMe's pharmacologic inhibitory effect on proinflammatory mediators is mainly mediated by MTA and SAH at the level of histone methylation.

E2F transcription factors are thought to be key regulators of cell growth control. Here we use mutant mouse strains to investigate the function of E2F1 and E2F2 in vivo. E2F1/E2F2 compound-mutant mice develop nonautoimmune insulin-deficient diabetes and exocrine pancreatic dysfunction characterized by endocrine and exocrine cell dysplasia, a reduction in the number and size of acini and islets, and their replacement by ductal structures and adipose tissue. Mutant pancreatic cells exhibit increased rates of DNA replication but also of apoptosis, resulting in severe pancreatic atrophy. The expression of genes involved in DNA replication and cell cycle control was upregulated in the E2F1/E2F2 compound-mutant pancreas, suggesting that their expression is repressed by E2F1/E2F2 activities and that the inappropriate cell cycle found in the mutant pancreas is likely the result of the deregulated expression of these genes. Interestingly, the expression of ductal cell and adipocyte differentiation marker genes was also upregulated, whereas expression of pancreatic cell marker genes were downregulated. These results suggest that E2F1/E2F2 activity negatively controls growth of mature pancreatic cells and is necessary for the maintenance of differentiated pancreatic phenotypes in the adult.

Leptin is an adiopokine that plays a pivotal role in the progression of liver fibrogenesis and carcinogenesis. Recently, leptin was shown to be mitogenic in human liver cancer cell lines HepG2 and Huh7. Whether leptin can act as a mitogen in normal hepatocytes is unclear. Methionine adenosyltransferase (MAT) is an essential enzyme that catalyzes the formation of S-adenosylmethionine (SAMe), the principal methyl donor and precursor of polyamines. Two genes ( MAT1A and MAT2A ) encode for the catalytic subunit of MAT, whereas a third gene ( MAT2β ) encodes for a regulatory subunit that modulates the activity of MAT2A -encoded isoenzyme. The aims of this study were to examine whether leptin's mitogenic activity involves MAT2A and MAT2β and whether this can be modulated. We found that leptin is mitogenic in HepG2 cells but not in primary human or mouse hepatocytes. Leptin induced the expression of MAT2A and MAT2β in HepG2 cells and normal human and mouse hepatocytes, but although it increased SAMe level in HepG2 cells, it had no effect on SAMe level in normal hepatocytes. Leptin-mediated induction of MAT genes and growth in HepG2 cells required activation of extracellular signal-regulated kinase and phosphatidylinositol-3-kinase signaling pathways. Treatment with SAMe or its metabolite methylthioadenosine (MTA) lowered expression of MAT2A and MAT2β and blocked leptin-induced signaling, including an increase in MAT gene expression and growth. Increased expression of MAT2A and MAT2β is required for leptin to be mitogenic, although by entirely different mechanisms. Conclusion: Leptin induces MAT2A and MAT2β expression in HepG2 cells and normal hepatocytes but is mitogenic only in HepG2 cells. Pharmacological doses of SAMe or MTA lower expression of both MAT2A and MAT2β and interfere with leptin signaling. (Hepatology 2007.)

Background & Aims: Methionine adenosyltransferase (MAT) catalyzes S-adenosylmethionine biosynthesis. Two genes (MAT1A and MAT2A) encode for the catalytic subunit of MAT, while a third gene (MAT2β) encodes for a regulatory subunit that modulates the activity of MAT2A-encoded isoenzyme. We uncovered multiple splicing variants while characterizing its 5′-flanking region. The aims of our current study are to examine the expression pattern, regulation, and functions of the 2 major variants: V1 and V2. Methods: Studies were conducted using RNA from normal human tissues, resected hepatocellular carcinoma specimens, and cell lines. Gene expression, promoter and nuclear binding activities, growth, and apoptosis were measured by routine assays. Results:MAT2β is expressed in most but not all tissues, and the 2 variants are differentially expressed. The messenger RNA levels of both variants are markedly increased in hepatocellular carcinoma. Tumor necrosis factor (TNF)-α, which induces MAT2A in HepG2 cells, also induced V1 (but not V2) expression. TNF-α induced the promoter activity of MAT2β V1, likely via nuclear factor κB and activator protein 1. Both variants regulate growth, but only V1 regulates apoptosis. Reduced expression of V1 led to c-Jun-N-terminal kinase (JNK) activation, apoptosis, and sensitized HepG2 cells to TNF-α–induced apoptosis, while overexpression of V1 was protective. However, blocking JNK1 or JNK2 activation did not prevent apoptosis induced by V1 knockdown. V1 (but not V2) knockdown also leads to apoptosis in a colon cancer cell line, suggesting these variants play similar roles in many cell types. Conclusions: Different variants of MAT2β regulate growth and death, which broadens their importance in biology.

BHMT (betaine–homocysteine methyltransferase) remethylates homocysteine to form methionine. SAM (S-adenosylmethionine) inhibits BHMT activity, but whether SAM modulates BHMT gene expression is unknown. Transcriptional regulation of the human BHMT is also unknown. The present study examined regulation of the human BHMT gene by SAM and its metabolite, MTA (5′-methylthioadenosine). To facilitate these studies, we cloned the 2.7 kb 5′-flanking region of the human BHMT gene (GenBank® accession number AY325901). Both SAM and MTA treatment of HepG2 cells resulted in a dose- and time-dependent decrease in BHMT mRNA levels, which paralleled their effects on the BHMT promoter activity. Maximal suppression was observed with the BHMT promoter construct −347/+33, which contains a number of NF-κB (nuclear factor κB) binding sites. SAM and MTA treatment increased NF-κB nuclear binding and NF-κB-driven luciferase activities, and increased nuclear binding activity of multiple histone deacetylase co-repressors to the NF-κB sites. Overexpression of p50 and p65 decreased BHMT promoter activity, while blocking NF-κB activation increased BHMT expression and promoter activity, and prevented SAM but not MTA's ability to inhibit BHMT expression. The NF-κB binding site at −301 is responsible, at least in part, for this effect. Lower BHMT expression can impair homocysteine metabolism, which can induce ER (endoplasmic reticulum) stress. Indeed, MTA treatment resulted in increased expression ER stress markers. In conclusion, SAM and MTA down-regulate BHMT expression in HepG2 cells in part by inducing NF-κB, which acts as a repressor for the human BHMT gene. While SAM's mechanism is NF-κB-dependent, MTA has both NF-κB-dependent and -independent mechanisms.