Tyrosinase gene expression in the Kupffer cells of Rana esculenta L

The liver of Amphibia and Reptilia shows a dark-brown pigmentation due to the presence of particular melanin-containing cells that are different from melanocytes and derive from cells of macrophage lineage (Kupffer Cells), which have been shown to have an autonomous capacity to synthesize melanins. To date, as far as we know, there are no reports in the literature about the genetic system of tyrosinase as regards these melanin-synthesizing cells; we carried out the present study to analyze how the tyrosinase gene may function. We showed that the Kupffer cells of Rana esculenta L. do indeed have a transcriptionally active tyrosinase gene. Evidence of this was obtained by reverse transcription polymerase chain reaction analysis carried out on both the liver tissue and the Kupffer cells in culture. Moreover, analysis of the cells in culture enabled us to observe that, by increasing the culture time from 0 to 72 hr, an appreciable increase occurred in the amplification products of the tyrosinase gene, as well as in the level of dopa oxidase activity and in the quantity of melanin in the cells. The results of the present study demonstrate that frog Kupffer cells possess an active tyrosinase gene and that the increase of the tyrosinase mRNA accumulation closely correlates with phenotypic differentiation, in terms of increased dopa oxidase activity and melanosome content. This provides further strong support of the hypothesis that amphibian Kupffer cells possess an endogenous ability to synthesize melanin and suggests the involvement of the transcriptional level of control in the modulation of their melanogenic activity.