Liver tyrosine kinase activation during early stages of chemical hepatocarcinogenesis

Protein phosphorylation at tyrosine residues is believed to he involved in several important cellular processes because tyrosine‐specific protein kinase activation is associated with stimulation of cellular proliferation by hormones and growth factors, embryogenesis, and retroviral cell transformation. Because cell proliferation is thought to be an essential component of chemical carcinogenesis. liver tyrosine‐specific protein kinase activity was examined during the early stages of the Solt and Farber chemical hepatocarcinogenesis model. Rats were given diethylnitrosamine in one dose (200 mg/kg, IP) followed by 2 weeks of dietary 0.02% 2‐acctylamino‐fluorene starting at day 14 after diethylnitrosamine, followed by partial hepatectomy on day 21. By day 32 this regimen produces a relatively synchronized population of hyperplastic liver nodules up to 1.5 mm in diameter. Rats were sacrificed on day 32, their livers were perfused with cold normal saline, homogenized, and centrifuged at 1,000g for 10 min. The resulting supernatant was centrifuged at 30,000g for 30 min and the pellet was assayed for tyrosine kinase activity using the synthetic peptide [Val 5 ]angiotensin II as substrate. Rats that received the complete regimen had a 2.6‐fold increase in their liver tyrosine kinase activity as compared to sham controls (2.4 pmoles/min/mg protein vs 6.4 pmoles/min/mg protein, P < .05). In contrast, rats that received a partial regimen (ie, partial hepatectomy, or 2‐acetylaminofluorene + partial hepatectomy, or diethylnitrosamine + 2‐acetylaminofluorene) did not have elevated tyrosine kinase activity nor did they have hyperplastic nodules. These preliminary data suggest that activation of liver tyrosine kinase is associated with the very early stages of chemical hepatocarcinogenesis.