Autocrine secretion of peptide growth factors by tumor cells.

Transforming growth factors (TGF) are growth-promoting polypeptides that cause phenotypic transformation and anchorage-independent growth of normal cells. They have been isolated from several human and animal carcinoma and sarcoma cells. One TGF is sarcoma growth factor (SGF) which is released by murine sarcoma virus-transformed cells. Whereas the TGF interacts with epidermal growth factor (EGF) cell membrane receptors, it is not detectable in culture fluids from cells which contain high numbers of free EGF cell membrane receptors. The SGF acts as a tumor promoter in cell culture systems, and its effect on the transformed phenotype is blocked by retinoids (vitamin A and synthetic analogs). The production of TGF by transformed cells and the responses of normal cells to the addition of TGF to the culture medium raise the possibility that cells "autostimulate" their growth by releasing factors that rebind at the cell surface. The term "autocrine secretion" has been proposed for this type of situation in which a cell secretes a hormone-like substance for which it has external cell membrane receptors. The autocrine concept may provide a partial explanation for some aspects of tumor cell progression.