Private and public autocrine loops in neoplastic cells

Autocrine growth factor loops ensure the continued growth of neoplastic cells. According to the traditional view of such autocrine loops, receptor binding and transduction of a mitogenic signal occur when a growth factor is secreted and subsequently interacts with its receptor on the surface of the secreting or neighboring cells. For several growth factors there is now evidence that the mitogenic signal may be transduced without factor secretion. In these instances, the growth factor appears to interact with its receptor intracellularly, creating in effect a "private" autocrine loop. We will discuss three growth factors that may operate by the latter mechanism, as well as two that rely instead on classical "public" autocrine loops, where the growth factor must be secreted and is therefore accessible to neighboring cells. We also consider the properties of these growth factor/receptor systems that may determine their involvement in either type of autocrine loop.