Maintenance Of Cell Growth Research Articles

Monoclonal antibodies (GB16, GB18, GB19, GB22) raised against human placental microvilli recognize the transferrin receptor

The placental syncytial trophoblasts perform a variety of transport and secretory functions during pregnancy. It has long been established that these trophoblasts possess specific membrane receptors for transferrin (Faulk and Galbraith, 1979; Seligman, Schleicher and Allen, 1979; Wada, Hass and Sussman, 1979), an iron-binding plasma protein. This liganddreceptor interaction is similar to that of reticulocytes in which it serves the function for haemoglobin synthesis (Jandl and Katz, 1963). Transferrin is an indispensable nutrient for the maintenance of cell growth in serum-free media (Iscove and Melchers, 1978). However, the mechanism(s) by which transferrin exerts its effect on cell growth has yet to be determined; it has been suggested that iron transport may represent one of its complex functions (Faulk and Galbraith, 1979; Ekblom et al, 1983; Hill et al, 1985). A few polyclonal antibodies raised against placental syncytiotrophoblastic microvillous membrane were found to react with the transferrin receptor (Enns et al, 1981; Booth and Booth, 1982). However, to date, none of the monoclonal antibodies produced in the same fashion were directed against the receptor for transferrin (Bulmer and Johnson, 1985). During our programme of screening monoclonal antibodies to human extraembryonic membranes, we have found that four antibodies (GB16, GBI~, GBI~, GBaa), raised against syncytiotrophoblastic microvilli, immunoprecipitated a r8o-kilodalton (K) dimer. This molecule had the ability to bind human transferrin, and it could not be detected on terminally differentiated HI,-60 cells. The epitope of GBI~ was different from that recognized by GBI~, GBI~ and GBaa. In addition, GBaa inhibited the cell growth of activated lymphocytes and Daudi cells, but not that of HL-60 or Jurkat cells.

Effect of polyamine depletion on c-myc expression in human colon carcinoma cells.

Polyamine depletion generally results in an inhibition of cell growth. However, the precise role of polyamines in the regulation of cell proliferation is unknown. In the present study, we demonstrate that polyamine depletion induced by 2-difluoromethylornithine in COLO 320 human colon carcinoma cells results in a greater than 90% decrease in expression of a key gene in the maintenance of cell growth, the c-myc protooncogene. The decrease in c-myc expression accompanying polyamine depletion appears to occur at the transcriptional level. It is not simply a result of decreased growth rate since growth-inhibited cells at confluence maintain a high level of c-myc expression. It is also due to a change in cell cycle phase distribution and is not a reflection of a generalized decrease in gene expression which accompanies a decrease in cellular proliferation. Thus, the expression of the histone H2A gene was similar to and temporally paralleled the growth status of both treated and untreated cells, while the beta-actin and ornithine decarboxylase genes actually had increased expression during polyamine depletion. These studies demonstrate that polyamines may be critical to the expression of c-myc and suggest one mechanism by which they modulate cell growth.