Protein kinase Cα signaling inhibits cyclin D1 translation in intestinal epithelial cells

Cyclin D1 is a key regulator of cell proliferation, acting as a mitogen sensor and linking extracellular signaling to the cell cycle machinery. Strict control of cyclin D1 levels is critical for maintenance of tissue homeostasis. We have reported previously that protein kinase C alpha (PKC alpha), a negative regulator of cell growth in the intestinal epithelium, promotes rapid down-regulation of cyclin D1 ( Frey, M. R., Clark, J.A., Leontieva, O., Uronis, J.M., Black, A. R., and Black, J.D. (2000) J. Cell Biol. 151, 763-778). The current study explores the mechanisms underlying PKC alpha-induced loss of cyclin D1 protein in non-transformed intestinal epithelial cells. Our findings exclude several mechanisms previously implicated in down-regulation of cyclin D1 during cell cycle exit/differentiation, including alterations in cyclin D1 mRNA expression and protein turnover. Instead, we identify PKC alpha as a novel repressor of cyclin D1 translation, acting at the level of cap-dependent initiation. Inhibition of cyclin D1 translation initiation is mediated by PKC alpha-induced hypophosphorylation/activation of the translational suppressor 4E-BP1, association of 4E-BP1 with the mRNA cap-binding protein eIF4E, and sequestration of cyclin D1 mRNA in 4E-BP1-associated complexes. Together, these post-transcriptional effects ensure rapid disappearance of the potent mitogenic molecule cyclin D1 during PKC alpha-induced cell cycle withdrawal in the intestinal epithelium.