Shedding of Virus-Free MMTV gp52 by Nonproducer Mammary Tumor Cells: A Source of Tumor Marker in the Presence and Absence of Dexamethasone

Abstract

The envelope glycoprotein of the mouse mammary tumor virus (MMTV gp52), a useful plasma marker for mammary tumors in mice, was measured in extracellular medium by radioimmunoassay to quantitatively compare gp52 levels released by a C3H MMTV producer and its subcloned B9 nonproducer cell line. Experiments analyzed levels of viral gp52 released from nonproducer cells both in the presence and absence of dexamethasone (DXS) to compare levels with those of producer cells and to assess quantitative changes in nonproducer release of both soluble and virion-associated gp52. Basal gp52 levels of producer cells were 2.4- to 7.0-fold higher than nonproducer cells. Very little virion-associated gp52 was detected in nonproducer cultures, whereas soluble gp52 was abundant and easily detectable in both cultures. DXS stimulated virus production in B9 cultures, converting nonproducer cells into producers. However, under identical conditions of DXS stimulation, B9 cells never reached the same elevated levels of gp52 as producer cells (90 ng/l06 cells versus 216 ng/l06 cells). Significant soluble gp52 release by B9 cells in the absence of DXS and enhancement in the presence of DXS argues that nonproducer cells can inñuence extracellular levels of viral antigen. By analogy to in vivo studies, results suggest that nonproducer cells in a tumor might also contribute to the pool of gp52 which serves as a systemic plasma marker for tumor.