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 (TGFalpha) protein were measured in regenerating liver. TGFalpha 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 TGFalpha, we developed a sensitive TGFalpha RIA. TGFalpha was extracted from livers in a detergent-containing buffer with protease inhibitors. Liver extracts, to a volume of 10 mul/tube, produced a displacement curve of [I-125]TGFalpha that was parallel to the pure standard. The TGFalpha 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, TGFalpha 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 TGFalpha immunoactivity to be in fractions corresponding to known TGFalpha precursors (15-30 kilodaltons). Mature 5.6-kilodalton TGFalpha was not detected until 48 h after PH and was still present at 96 h. These data support a role for TGFalpha in the response to PH in the rat. However, the presence of TGFalpha precursors in normal liver, the short (<4-h) interval between the increase in TGFalpha concentrations and the onset of hepatocyte DNA synthesis, the sustained elevation of TGFalpha 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 TGFalpha may have different functions, cellular sources, or target cells in regenerating liver.