The role of the membrane‐bound tumour antigen, melanotransferrin (p97), in iron uptake by the human malignant melanoma cell

Melanotransferrin (MTf) is a membrane‐bound transferrin (Tf) homologue with several characteristics in common with serum Tf. MTf is found at high levels in melanoma cells and previous studies have shown that MTf can bind Fe. In addition, Chinese hamster ovary cells transfected with MTf transport Fe from 59Fe‐citrate at greater rates than control cells. However, the role of MTf in the Fe uptake process of human melanoma cells remains unknown. In the present study we have characterized the role of MTf in Fe uptake by SK‐Mel‐28 melanoma cells in order to understand its function. Initial studies examined whether modulation of intracellular Fe levels using the Fe chelator desferrioxamine (DFO) or the Fe donor ferric ammonium citrate (FAC) could change MTf mRNA levels. In contrast to transferrin receptor (TfR) mRNA that increased after exposure to DFO and decreased after incubation with FAC, there was no change in MTf mRNA levels. In addition, compared to control cells, there was no alteration of 125I‐labelled anti‐MTf mAb‐binding in cells exposed to DFO or FAC, suggesting no change in the number of MTf sites. Further studies examined the ability of DFO and FAC to modulate Fe uptake from 59Fe‐citrate which is bound by MTf. In contrast to the effect of DFO or FAC at increasing and decreasing Fe uptake from 59Fe‐Tf, respectively, DFO had no influence on 59Fe‐citrate uptake, whereas FAC markedly increased it. Collectively, these studies suggest that MTf is not regulated in a manner similar to the TfR in response to cellular Fe levels. MTf can be removed from the membrane by phosphatidylinositol‐specific phospholipase C (PtdIns‐PLC). Preincubation of melanoma cells with PtdIns‐PLC reduced anti‐MTf mAb binding to 3% of the control, while PtdIns‐PLC only slightly reduced 59Fe uptake from 59Fe‐citrate. These results suggest that MTf played only a minor role in Fe uptake from 59Fe‐citrate by these cells. The expression of MTf mRNA (poly A+) was also examined in 50 human tissues and found to be markedly different to Tf mRNA or TfR mRNA. Surprisingly, MTf mRNA expression was widespread in normal tissues, and was observed at its highest levels in the salivary gland. In contrast to expectations, MTf mRNA expression was generally greater in adult than fetal tissues.