AUTOACTIVATION OF COLON CANCER CELL BY INTRACELLULAR TGF

The ability of a cell to produce and respond to its own growth
factors has been referred to as autocrine growth. The events
involved are generally viewed as consisting of the secretion of the
growth factor followed by its binding back to the appropriate
surface receptor on the secreting cell. More recently, limited
numbers of autocrine loops have been described in which the growth
factor is not secreted, but rather binds to the receptor in the
secretory compartment and carries out its stimulatory activity
intracellularly. This is referred to as an internal autocrine loop
rather than the extracellular autocrine loop described above. Using a well-characterized bank of human colon carcinoma
cells we have found that transforming growth factor alpha
(TGF-alpha) is an autocrine growth factor. Furthermore, we have
identified specific growth regulatory phenotypes which show
different autocrine mechanisms in colon cancer.
Well-differentiated GEO cells show a classical external autocrine
loop while poorly-differentiated HCT 116 cells show an internal
autocrine loop. This is the first description of an internal
autocrine loop for TGF-alpha, the first description of an internal
autocrine loop in a human cell line, and the first "native"
internal loop in which the growth factor has not been transfected
into the cell line expressing this autocrine mechanism. It is hypothesized that the internal autocrine mechanism
results in the loss of cell cycle controls such that the cells are
constitutively stimulated and lose the ability to respond to normal
controls for growth and differentiation. These hypotheses will be
tested by carrying out the following specific aims. (1) Determine
the extent to which internal and external loops are expressed in
a well-characterized bank of colon cancer cell lines. (2) Determine
the effects of uncoupling the internal TGF-alpha loop and restoring
response to external controls on the expression of cell cycle
related molecules. (3) Create internal TGF-alpha loops in
untransformed cells and cancer cells which currently utilize
external loops. (4) Determine the effects of carrying out specific
aim #3 on growth controls. (5) Determine the relationship between
differentiation and internal vs. external loop mechanisms for
TGF-alpha.