DIFFERENTIAL REGULATION OF THE ACTIVITY OF THE 42 KD MITOGEN-ACTIVATED PROTEIN-KINASE (P42(MAPK)) DURING ENTEROCYTE DIFFERENTIATION IN-VIVO

The family of mitogen activated protein (MAP) kinases appear to play a central role in relaying signals generated by receptor protein tyrosi ne kinases (RPTK) from the cell surface to the nucleus, We previously demonstrated that undifferentiated and mitotically active crypt cells have high levels of tyrosine phosphorylated proteins (DR Burgess, W Ji ang, S Mamajiwalla and W Kinsey. 1989, J. Cell Biol., 109: 2139) possi bly due to the activation of RPTKs and also have high pp60(mapk) prote in tyrosine kinase activity (CA Cartwright, SN Mamajiwalla, SA Skolnik , W Eckhart and DR Burgess, 1993. Oncogene. 8: 1033) when compared to differentiated, non-mitotic villus cells. Since activation of RPTKs le ading to cell proliferation or differentiation involves activation of the Ras-MAP kinase pathway, we chose to determine in this study if the activity of the MAP kinases were also regulated during differentiatio n of normal adult enterocytes. Our data show that although the 42 kD M AP kinase (p42(mapk)) was expressed in both crypt and villus cells, it was phosphorylated on tyrosine and active only in the crypt cells, Ou r data further suggest that p42(mapk) is inactivated during differenti ation, possibly by a protein tyrosine phosphatase, Immunofluorescence studies revealed that p42(mapk) localized to the nuclei in both undiff erentiated and differentiated enterocytes and colocalized with phospho tyrosine containing proteins at the region of the junctional complex, These results suggest that p42(mapk) and its regulators are tightly co ntrolled during enterocyte differentiation in vivo and implicate p42(m apk) as a key regulatory molecule in the normal development of the epi thelium.