Insulin regulates heregulin binding and ErbB3 expression in rat hepatocytes

The heregulin-ErbB system of ligands and receptors are newly described epidermal growth factor (EGF) and EGF receptor-related proteins that regulate growth, differentiation, and gene expression in numerous cell types. This study describes a receptor for heregulin beta-1 (HRGbeta1) on cultured rat hepatocytes and an inhibitory influence of insulin on HRGbeta1 binding. HRGbeta1 (30 nM) stimulated DNA synthesis 2-fold and was not augmented by insulin as is the case with EGF receptor ligands. A labeled peptide corresponding to the EGF domain of HRGbeta1 bound to a single population of 19,600 +/- 1,800 binding sites/cell with a Kd of 360 +/- 22 pM. Cross-linking experiments showed binding of HRGbeta1 to ErbB3 but not ErbB2 or ErbB4. HRGbeta1 induced phosphorylation of ErbB3 and decreased ErbB3 protein levels, suggesting that HRGbeta1 activates signaling through the ErbB3 receptor and influences receptor trafficking. Following plating, [125I]HRGbeta1 binding and ErbB3 protein levels increased 8- and 3-fold, respectively, over the first 12 h in culture. These increases required de novo protein synthesis and were inhibited with 50 nM insulin resulting in 3500 binding sites with a Kd of 265 pM. These data suggest that the heregulin-ErbB system can regulate liver functions and may be linked to the metabolic and nutritional status of the animal.

Liver regeneration and hepatocarcinogenesis in transforming growth factor-alpha-targeted mice

Transforming growth factor-alpha (TGF alpha), a member of the epidermal growth factor receptor ligand family, has been implicated in the regeneration and transformation of liver. Our recent development of mice that are homozygous for a disrupted TGF alpha gene allowed us to assess the requirement for this growth factor in these complex processes. We report here that although a 70% hepatectomy produced a significant increase in hepatic TGF alpha protein levels in wild-type mice, liver regeneration nevertheless proceeded normally in the absence of the growth factor. The hepatocyte labeling indices determined for homozygous targeted and wild-type mice at 36 and 48 h after hepatectomy were comparable, and the total liver DNA to body weight ratios 8 d after hepatectomy were essentially identical for the two genotypes. These results indicate that TGF alpha, is not necessary for liver regeneration. To test its requirement in liver carcinogenesis, young mice were administered single doses of diethylnitrosamine (DEN) with or without subsequent chronic treatment with the promoting agent phenobarbital (PB). Both wild-type and homozygous mutant male mice treated with DEN or DEN plus PB developed multiple preneoplastic foci or tumors by 9 mo of age with relatively high incidence. However, while five of 88 tumors in wild-type mice attained a diameter greater than 5 mm and were classified as hepatocellular carcinomas, none of 132 tumors in livers of targeted mice reached this size. Furthermore, three of these large wild-type tumors expressed significantly elevated levels of TGF alpha protein compared with normal liver. These results indicate that TGF alpha is not required for early events in chemically induced hepatocarcinogenesis but suggest that it could be important in the progression from small preneoplastic foci to large tumors.

Liver regeneration and hepatocarcinogenesis in transforming growth factor-alpha-targeted mice

Transforming growth factor-alpha (TGF alpha), a member of the epidermal growth factor receptor ligand family, has been implicated in the regeneration and transformation of liver. Our recent development of mice that are homozygous for a disrupted TGF alpha gene allowed us to assess the requirement for this growth factor in these complex processes. We report here that although a 70% hepatectomy produced a significant increase in hepatic TGF alpha protein levels in wild-type mice, liver regeneration nevertheless proceeded normally in the absence of the growth factor. The hepatocyte labeling indices determined for homozygous targeted and wild-type mice at 36 and 48 h after hepatectomy were comparable, and the total liver DNA to body weight ratios 8 d after hepatectomy were essentially identical for the two genotypes. These results indicate that TGF alpha, is not necessary for liver regeneration. To test its requirement in liver carcinogenesis, young mice were administered single doses of diethylnitrosamine (DEN) with or without subsequent chronic treatment with the promoting agent phenobarbital (PB). Both wild-type and homozygous mutant male mice treated with DEN or DEN plus PB developed multiple preneoplastic foci or tumors by 9 mo of age with relatively high incidence. However, while five of 88 tumors in wild-type mice attained a diameter greater than 5 mm and were classified as hepatocellular carcinomas, none of 132 tumors in livers of targeted mice reached this size. Furthermore, three of these large wild-type tumors expressed significantly elevated levels of TGF alpha protein compared with normal liver. These results indicate that TGF alpha is not required for early events in chemically induced hepatocarcinogenesis but suggest that it could be important in the progression from small preneoplastic foci to large tumors.

Liver regeneration and hepatocarcinogenesis in transforming growth factor-alpha-targeted mice

Transforming growth factor-alpha (TGF alpha), a member of the epidermal growth factor receptor ligand family, has been implicated in the regeneration and transformation of liver. Our recent development of mice that are homozygous for a disrupted TGF alpha gene allowed us to assess the requirement for this growth factor in these complex processes. We report here that although a 70% hepatectomy produced a significant increase in hepatic TGF alpha protein levels in wild-type mice, liver regeneration nevertheless proceeded normally in the absence of the growth factor. The hepatocyte labeling indices determined for homozygous targeted and wild-type mice at 36 and 48 h after hepatectomy were comparable, and the total liver DNA to body weight ratios 8 d after hepatectomy were essentially identical for the two genotypes. These results indicate that TGF alpha, is not necessary for liver regeneration. To test its requirement in liver carcinogenesis, young mice were administered single doses of diethylnitrosamine (DEN) with or without subsequent chronic treatment with the promoting agent phenobarbital (PB). Both wild-type and homozygous mutant male mice treated with DEN or DEN plus PB developed multiple preneoplastic foci or tumors by 9 mo of age with relatively high incidence. However, while five of 88 tumors in wild-type mice attained a diameter greater than 5 mm and were classified as hepatocellular carcinomas, none of 132 tumors in livers of targeted mice reached this size. Furthermore, three of these large wild-type tumors expressed significantly elevated levels of TGF alpha protein compared with normal liver. These results indicate that TGF alpha is not required for early events in chemically induced hepatocarcinogenesis but suggest that it could be important in the progression from small preneoplastic foci to large tumors.

Transforming growth factor-alpha (TGF alpha) concentrations increase in regenerating rat liver: evidence for a delayed accumulation of mature TGF alpha

Changes in the concentration of transforming growth factor-alpha (TGF alpha) protein were measured in regenerating liver. TGF alpha stimulates both DNA and protein synthesis in various liver-derived cells, and its mRNA levels increase in liver after partial hepatectomy (PH), suggesting that it may be an important autocrine regulator of liver regeneration. Using a sheep antiserum raised against mature rat TGF alpha, we developed a sensitive TGF alpha RIA. TGF alpha was extracted from livers in a detergent-containing buffer with protease inhibitors. Liver extracts, to a volume of 10 microliters/tube, produced a displacement curve of [125I]TGF alpha that was parallel to the pure standard. The TGF alpha content of normal liver was 57.04 +/- 26.25 pg/mg protein, 5.24 +/- 2.61 ng/mg DNA, and 10.33 +/- 4.47 ng/g liver (n = 5; mean +/- SD). Between 13-17 h after operation, TGF alpha concentrations in the livers of PH animals increased over those in sham-operated (SH) controls (P < 0.05) and remained twice those in SH controls for more than 96 h, returning to control values by 8 days. In unoperated liver, gel chromatography showed all TGF alpha immunoactivity to be in fractions corresponding to known TGF alpha precursors (15-30 kilodaltons). Mature 5.6-kilodalton TGF alpha was not detected until 48 h after PH and was still present at 96 h. These data support a role for TGF alpha in the response to PH in the rat. However, the presence of TGF alpha precursors in normal liver, the short (< 4-h) interval between the increase in TGF alpha concentrations and the onset of hepatocyte DNA synthesis, the sustained elevation of TGF alpha levels after DNA synthesis has ceased, and the lack of detectable processing to the mature form until DNA synthesis has subsided all suggest that the membrane-anchored precursor and the mature forms of TGF alpha may have different functions, cellular sources, or target cells in regenerating liver.

A procedure for large-scale plasmid isolation without using ultracentrifugation

An expedient procedure for large-scale plasmid isolation from Escherichia coli strains without using ultracentrifugation or special setups or reagents is described. The protocol, which utilizes a modified alkaline extraction procedure as well as differential precipitations by isopropanol and lithium chloride, is simple and rapid and yet produces plasmid DNA with a yield of about 2 mg/liter culture. The isolated plasmids consisted of mostly monomeric and dimeric covalently closed circular DNA. The plasmids could be digested by various restriction endonucleases and were compatible with gene cloning, transfection-gene expression, and viral production.