Autocrine stimulation after transfer of the granulocyte/macrophage colony-stimulating factor gene and autonomous growth are distinct but interdependent steps in the oncogenic pathway.

Autocrine stimulation of cells by aberrant synthesis of growth factor may lead to malignant transformation, either as a direct consequence of endogenous factor production or as a first step of a series of successive events. Introduction of the granulocyte/macrophage colony-stimulating factor (GM-CSF) cDNA clone into a vector based on the myeloproliferative sarcoma virus allowed efficient transfer and expression of GM-CSF in factor-dependent myeloid cell lines (FDC-P1 and FDC-P2). Factor-independent growth was acquired when the vector was introduced into the GM-CSF-responsive FDC-P1 cell line but not the multi-CSF-dependent FDC-P2 line. Nonlinear clonability in the absence of exogenous growth factor and growth inhibition by GM-CSF antiserum support a model of autocrine stimulation that requires interaction of factor and receptor at the outer membrane. However, many, but not all, infected FDC-P1 cells acquired subsequently a second mutation that abrogated the requirement of GM-CSF secretion and external interaction. The nature of the second step, which presumably leads to tumorigenicity of these cells, is not well understood, but its frequency could be correlated with the level of GM-CSF released by an individual cell clone.