COLON ABSORPTIVE EPITHELIAL-CELLS LOSE PROLIFERATIVE RESPONSE TO TGF-ALPHA AS THEY DIFFERENTIATE

As colon epithelial cells migrate up the cylindrical colonic crypt, th ey terminally differentiate and lose their ability to divide. Elevated levels of the epithelial cell mitogen TGF alpha have been found at th e top of the crypt by other investigators, causing us to speculate tha t colon epithelial cells lose mitogenic response to TGF alpha as they differentiate. We tested this hypothesis by using the HT29 colon carci noma sublines U4 and U4H as models of one colonocyte lineage, fluid-tr ansporting enterocytes. TGF alpha was mitogenic for the U4 cells, but inhibited the growth of the more differentiated U4H cells. However, p4 4 MAP kinase was activated by TGF alpha in both U4 and U4H cells, as w ell as in two control undifferentiated HT29 sublines which showed no c hange in proliferation in response to TGF alpha. In addition, TGF alph a activated the EGF receptor in each line by increasing its tyrosine p hosphorylation. No relationship was found in these four lines between response to TGF alpha and level of expression of either the EGF recept or or two EGF receptor ligands, TGF alpha and amphiregulin. Activated EGF receptors initiate both growth-inhibitory and mitogenic signals in these cells since blocking some of the EGF receptors on TGF alpha-gro wth-inhibited U4H cells and TGF alpha-unresponsive U9 cells overrode t he inhibitory signals and made both U9 and U4H cells sensitive to mito genesis by added TGF alpha. These data imply that upon reaching stages of greater maturation, colon enterocytes lose proliferative response to TGF alpha because of changes in signaling by their EGF receptors. ( C) 1995 Academic Press, Inc.