A novel mechanism for mitogenic signaling via pro-transforming growth factor α within hepatocyte nuclei

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Abstract
Transforming growth factor (TGF) α, an important mediator of growth stimulation, is known to act via epidermal growth factor receptor (EGF-R) binding in the cell membrane. Here we show by immunohistology, 2-dimensional immunoblotting, and mass spectrometry of nuclear fractions that the pro-protein of wild-type TGF-α occurs in hepatocyte nuclei of human, rat, and mouse liver. Several findings show a close association between nuclear pro-TGF-α and DNA synthesis. (1) The number of pro-TGF-α+ nuclei was low in resting liver and increased dramatically after partial hepatectomy and after application of hepatotoxic chemicals or the primary mitogen cyproterone acetate (CPA); in any case, S phase occurred almost exclusively in pro-TGF-α+ nuclei. The same was found in human cirrhotic liver. (2) In primary culture, 7% of hepatocytes synthesized pro-TGF-α, which then translocated to the nucleus; 70% of these nuclei subsequently entered DNA replication, whereas only 2% of pro-TGF-α− hepatocytes were in S phase. (3) The frequency of hepatocytes coexpressing pro-TGF-α and DNA synthesis was increased by the hepatomitogens CPA or prostaglandin E2 and was decreased by the growth inhibitor TGF-β1. (4) Treatment with mature TGF-α increased DNA synthesis exclusively in pro-TGF-α− hepatocytes, which was abrogated by the EGF-R tyrosine kinase inhibitor tyrphostin A25. In conclusion, TGF-α gene products may exert mitogenic effects in hepatocytes via 2 different signaling mechanisms: (1) the “classic” pathway of mature TGF-α via EGF-R in the membrane and (2) a novel pathway involving the presence of pro-TGF-α in the nucleus.
Funding Information
  • Supported by Herzfelder'sche Familienstiftung.