Protein kinase C isozyme-mediated cell cycle arrest involves induction of p21(waf1/cip1) and p27(kip1) and hypophosphorylation of the retinoblastoma protein in intestinal epithelial cells

The molecular mechanisms underlying protein kinase C (PKC) isozyme- mediated control of cell growth and cell cycle progression are poorly understood. Our previous analysis of PKC isozyme regulation in the intestinal epithelium in situ revealed that multiple members of the PKC family undergo changes in expression and subcellular distribution precisely as the cells cease proliferating in the mid-crypt region, suggesting that activation of one or more of these molecules is involved in negative regulation of cell growth in this system (Saxon, M. L., Zhao, X., and Black, J. D. (1994) J. Cell Biol. 126, 747-763). In the present study, the role of PKC isozyme(s) in control of intestinal epithelial cell growth and cell cycle progression was examined directly using the IEC-18 immature crypt cell line as a model system. Treatment of IEC-18 cells with PKC agonists resulted in translocation of PKC α, δ, and ε from the soluble to the particulate subcellular fraction, cell cycle arrest in G₁ phase, and delayed transit through S and/or G₂/M phases. PKC-mediated cell cycle arrest in G₁ was accompanied by accumulation of the hypophosphorylated, growth-suppressive form of the retinoblastoma protein and induction of the cyclin-dependent kinase inhibitors p21(waf1/cip1) and p27(kip1). Reversal of these cell cycle regulatory effects was coincident with activator-induced down-regulation of PKC α, δ, and ε. Differential down-regulation of individual PKC isozymes revealed that PKC a in particular is sufficient to mediate cell cycle arrest by PKC agonists in this system. Taken together, the data implicate PKC α in negative regulation of intestinal epithelial cell growth both in vitro and in situ via pathways which involve modulation of Cip/Kip family cyclin-dependent kinase inhibitors and the retinoblastoma growth suppressor protein.