Protein Kinase Cα Signaling Regulates Inhibitor of DNA Binding 1 in the Intestinal Epithelium

Increasing evidence supports a role forPKC alpha in growth arrest and tumor suppression in the intestinal epithelium. In contrast, the Id1 transcriptional repressor has pro-proliferative and tumorigenic properties in this tissue. Here, we identify Id1 as a novel target of PKC alpha signaling. Using a highly specific antibody and a combined morphological/biochemical approach, we establish that Id1 is a nuclear protein restricted to proliferating intestinal crypt cells. A relationship between PKC alpha and Id1 was supported by the demonstration that (a) down-regulation of Id1 at the crypt/villus junction coincides with PKC alpha activation, and (b) loss ofPKC alpha in intestinal tumors is associated with increased levels of nuclear Id1. Manipulation of PKC alpha activity in IEC-18 nontransformed intestinal crypt cells determined that PKC alpha suppresses Id1 mRNA and protein via an Erk-dependent mechanism. PKC alpha, but not PKC delta, also inhibited Id1 expression in colon cancer cells. Id1 was found to regulate cyclin D1 levels in IEC-18 and colon cancer cells, pointing to a role for Id1 suppression in the antiproliferative/tumor suppressive activities of PKC alpha. Notably, Id1 expression was elevated in the intestinal epithelium of PKC alpha-knock-out mice, confirming that PKC alpha regulates Id1 in vivo. A wider role for PKC alpha in control of inhibitor of DNA binding factors is supported by its ability to down-regulate Id2 and Id3 in IEC-18 cells, although their suppression is more modest than that of Id1. This study provides the first demonstrated link between a specific PKC isozyme and inhibitor of DNA binding factors, and it points to a role for a PKC alpha -> 3 Erk (sic) Id1 -> cyclin D1 signaling axis in the maintenance of intestinal homeostasis.