Effect of hydrostatic pressure and cholesterol depletion on the expression of a tumor-associated antigen

The molecular events related to the expression of three tumor-associated epitopes, Ca-MOv17, Ca-MOv18 and Ca-MOv19 have been addressed. The epitopes are carried by a 38-kDa glycoprotein (gp38), recently cloned and identified as a human folate-binding protein. They were found to be coexpressed on the surface of the ovarian carcinoma cell line OVCA432, while they are not coordinately expressed on other adenocarcinoma cell lines (IGROV1, HT-29). This lack of coexpression was investigated from a molecular point of view. We studied three carcinoma cell lines, characterized by a different reactivity with the three relevant monoclonal antibodies MOv17, MOv18 and MOv19. The epitope expression was examined after modifying the membrane properties by using hydrostatic pressure and/or the variation of cholesterol content. Measurement of the expression after cell labelling by mAbs was performed by indirect immunofluorescence, using both fluorescence microscopy and flow cytometry. At variance with HT-29 cells, treatment of ovarian carcinoma IGROV1 cells with hydrostatic pressure failed to exert any effect. On IGROV1, instead, cholesterol depletion affected the expression Ca-MOv17, increasing, in the indirect immunofluorescence tests, the proportion of positive cells from 0 to 66 +/- 9%. Moreover, restoring the cholesterol content of the plasma membrane did not reverse the induced epitope expression. In parallel, immunoprecipitation experiments confirmed that, on IGROV1 surface, gp38 was recognized by all three mAbs. The data presented suggest that in IGROV1 cells the selective lacking of the epitope expression is related to the physical state of the plasma membrane. An explanation is provided by the model of membrane microdomains in which epitope expression may be influenced by the cholesterol level of different plasma membrane regions.