INDUCTION OF THE DIFFERENTIATED PHENOTYPE IN HUMAN COLON-CANCER CELLSIS ASSOCIATED WITH THE ATTENUATION OF SUBCELLULAR TYROSINE PHOSPHORYLATION

In the present study we have determined membrane, cytosolic, and cytos keleton-associated tyrosine protein kinase (TPK) activity in human col on cancer cell lines exposed to (i) the differentiation-promoting agen ts sodium butyrate and 8-chloro-cyclic-adenosine 3',5'-monophosphate ( 8-Cl-cAMP), (ii) tyrphostins, specific TPK inhibitors, or (iii) differ entiation-inducing culture manipulations. Treatment of human colon can cer cell lines, LS 174T, COLO 205, and SW620, with sodium butyrate and 8-Cl-cAMP or tyrphostins AG-30 and AG-34, significantly attenuated TP K activity concomitantly with an increase in the activity of alkaline phosphatase, an enzymatic marker of intestinal cell differentiation. T he differentiated phenotype induced in Caco-2 and HT-29 colon cancer c ells by culture manipulation was associated with a significant decreas e in cytoskeleton-associated TPK activity and marked activity of alkal ine phosphatase (AP). Electron microscopy and freeze-fracturing analys is of HT-29 cells showed that the gradual transition from the undiffer entiated to the differentiated phenotype resulted in the acquisition o f a distinct polarized morphology. Immunocytochemical phosphotyrosine analysis of cultured SW620 cells showed positive staining mostly local ized in zones of focal contacts. A marked reduction in phosphotyrosine staining with notable changes in cell morphology was observed in SW62 0 cells exposed to tyrphostins. Cumulatively, the present results indi cate that the induction of the differentiated phenotype in colon cance r cells is associated with a marked decrease in TPK activity and tyros ine phosphorylation.