Abstract
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 alpha, delta, and epsilon 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 alpha, delta, and epsilon. Differential down-regulation of individual PKC isozymes revealed that PKC alpha in particular is sufficient to mediate cell cycle arrest by PKC agonists in this system. Taken together, the data implicate PKC alpha 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.
Highlights
The growth-regulatory consequences of protein kinase C (PKC) activation suggest a link between PKC signaling and control of the cell cycle machinery
We have extended the characterization of PKC isozyme expression and activation in the rat intestinal epithelium in situ and, based on this analysis, have explored the role of PKC isozyme(s) in control of cell growth and cell cycle progression using the non-transformed IEC-18 immature crypt cell line as a complementary in vitro model system
Previous morphological and biochemical analysis of the expression and subcellular distribution of individual PKC isozymes in the rat intestinal epithelium revealed that PKC ␣, II, ␦, ⑀, and , but not PKC ␥ or I, are present in cells of the crypt-villus unit and are differentially regulated with respect to cell growth and differentiation [27]
Summary
The growth-regulatory consequences of PKC activation suggest a link between PKC signaling and control of the cell cycle machinery.
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