In the present study we have determined membrane, cytosolic, and cytoskeleton-associated
tyrosine protein kinase (TPK) activity in human
colon cancer cell lines exposed to (i) the differentiation-promoting agents
sodium butyrate and 8-chloro-cyclic-adenosine 3',5'-monophosphate (8-Cl-cAMP), (ii)
tyrphostins, specific TPK inhibitors, or (iii) differentiation-inducing culture manipulations. Treatment of human
colon cancer cell lines, LS 174T, COLO 205, and SW620, with
sodium butyrate and
8-Cl-cAMP or
tyrphostins AG-30 and
AG-34, significantly attenuated TPK activity concomitantly with an increase in the activity of
alkaline phosphatase, an enzymatic marker of intestinal cell differentiation. The differentiated phenotype induced in Caco-2 and HT-29
colon cancer cells by culture manipulation was associated with a significant decrease in cytoskeleton-associated TPK activity and marked activity of
alkaline phosphatase (AP). Electron microscopy and freeze-fracturing analysis of HT-29 cells showed that the gradual transition from the undifferentiated to the differentiated phenotype resulted in the acquisition of a distinct polarized morphology. Immunocytochemical
phosphotyrosine analysis of cultured SW620 cells showed positive staining mostly localized in zones of focal contacts. A marked reduction in
phosphotyrosine staining with notable changes in cell morphology was observed in SW620 cells exposed to
tyrphostins. Cumulatively, the present results indicate that the induction of the differentiated phenotype in
colon cancer cells is associated with a marked decrease in TPK activity and
tyrosine phosphorylation.