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To explore the biologic significance of the membrane-anchored form of macrophage colony-stimulating factor (M-CSF), we examined whether interaction between membrane-bound M-CSF and its receptor mediates intercellular adhesion as well as cell proliferation and differentiation. Human M-CSF receptors were expressed on a murine interleukin-2 (IL-3)-dependent cell line, FDC-P2, by DNA transfection with the c-fms gene (FDC-P2-MCSFR). A human bone marrow-derived stromal cell line, KM102, was used in the cell adhesion assay. The expression of membrane-bound M-CSF on KM102 cells was demonstrated by flow cytometry and immunoblot analysis. After the incubation of parent and transformed FDC-P2 cells on confluent KM102 cells, nonadherent cells were removed and the cells attached to KM102 cells were examined microscopically. Almost all parent FDC-P2 cells were nonadherent, whereas a significant number of FDC-P2-MCSFR cells bound to KM102 cells. The addition of anti-M-CSF or anti-M-CSF receptor neutralizing antibodies dose-dependently inhibited the binding of [3H]-thymidine- labeled FDC-P2-MCSFR cells to KM102 cells. An excess amount of M-CSF also inhibited the binding. On the other hand, the addition of antibodies against some representative adhesion molecules (vitronectin receptor, Pgp-1/CD44, and VLA-4) did not inhibit the adhesion between FDC-P2-MCSFR cells and KM102 cells. These results support the idea that membrane-anchored M-CSF and its receptor may function as mediators of cell-cell adhesion.

Since the ras family of proto-oncogenes is supposed to be involved in leukemogenesis by point-mutational activation, we studied the effect of the activated ras gene on the growth of a murine interleukin-3 (IL-3)- dependent cell line, FDC-P2. The human activated c-H-ras gene was transfected into FDC-P2 cells by electroporation using a high-level expression vector, BMGhph, which contains a partial DNA sequence from bovine papillomavirus (BPV) and a hygromycin B (hmB)-resistant gene as a selectable marker. The transformed FDC-P2 cells showed a high incidence of IL-3-independent growth and tumorigenicity in nude mice. These clones did not express or secrete IL-3, suggesting the acquisition of IL-3 independence by a nonautocrine mechanism. The high incidence of autonomous growth may be due to the use of the BMG vector, because (1) the activated ras gene in pBR322 vector (pHs-49) was not so efficient in the induction of IL-3 independence, (2) the c-H-ras genome copies per cell increased in number up to about 50 copies by using the BMG vector, and (3) cotransfection with the activated ras gene and the BPV gene in separate plasmids partly enhanced the incidence of autonomous growth without increasing the copy number of the ras gene compared with transfection with the activated ras gene alone. The present study supports the idea that the activation of ras gene is an important step in malignant transformation of hematopoietic cells and suggests that the BPV gene products may cooperate with ras gene activation probably by affecting the cellular genes that may be involved in multistep leukemogenesis. The BMG vector may be useful to test the transforming ability of oncogenes whose oncogenic potential is relatively low.

The c-Abl gene encodes a widely expressed tyrosine kinase that is important for development, particularly of the nervous system.[1][1] But the signaling pathways that employ c-Abl in normal cells are not well defined. Abl protein is located in both the nucleus and the cytoplasm, and recent studies