Involvement of p21(WAF1/Cip1) and p27(Kip1) in intestinal epithelial cell differentiation

Using the conditionally immortalized human cell line tsFHI, we have investi gated the role of cyclin-dependent kinase inhibitors (CKIs) in intestinal e pithelial cell differentiation. Expression of cyclins, cyclin-dependent kin ases (Cdk), and CKIs was examined under conditions promoting growth, growth arrest, or expression of differentiated traits. Formation of complexes amo ng cell cycle regulatory proteins and their kinase activities were also inv estigated. The tsFHI cells express three CKIs: p16, p21, and p27. With diff erentiation, p21 and p27 were strongly induced, but with different kinetics : the p21 increase was rapid but transient and the p27 increase was delayed but sustained. Our results suggest that the function of p16 is primarily t o inhibit cyclin D-associated kinases, making tsFHI cells dependent on cycl in E-Cdk2 for pRb phosphorylation and G(1)/S progression. Furthermore, they indicate that p21 is the main CKI involved in irreversible growth arrest d uring the early stages of cell differentiation in association with D-type c yclins, cyclin E, and Cdk2, whereas p27 may induce or stabilize expression of differentiated traits acting independently of cyclin-Cdk function.