We identified a novel inhibitor of growth family member 2 (ING2) isoform, ING2b, which shares exon 2 with ING2a, but lacks the N-terminal p53 binding region. Contrary to ING2a, ING2b's promoter has no p53 binding sites. Consistently, activation of p53 led to suppression of ING2a, leaving ING2b unaffected. Through isoform-specific targeting, we showd that ING2a knockdown suppressed cell growth only in the presence of p53, ING2b knockdown had no effect on cell growth, and knockdown of both induced cell cycle arrest and apoptosis independently of p53. ING2a and ING2b have compensatory roles that protect cells from cell cycle arrest and apoptosis and may be involved in development of chemotherapeutic resistance. ING2 is associated with chromatin remodeling and regulation of gene expression by binding to a methylated histone H3K4 residue and recruiting HDAC complexes to the region.
The aim of our study is to investigate the regulation of ING2 expression and the clinical significance of upregulated ING2 in colon cancer. We have shown that the ING2 mRNA level in colon cancer tissue increased to more than twice that in normal mucosa in 45% of colorectal cancer cases that we examined. A putative NK-kappaB binding site was found in the ING2 promoter region. We confirmed that NF-kappaB could bind to the ING2 promoter by EMSA and luciferase assays. Subsequent microarray analyses revealed that ING2 upregulates expression of matrix metalloproteinase 13 (MMP13), which enhances cancer invasion and metastasis. ING2 regulation of MMP13 expression was confirmed in both ING2 overexpression and knockdown experiments. MMP13 expression was further induced by coexpression of ING2 with HDAC1 or with mSin3A, suggesting that the ING2-HDAC1-m-Sin3A complex members regulate expression of MMP13. An in vitro invasion assy was performed to determine functional significance of ING2 upregulation. ING2 overexpressed cells exhibitied greater invasive potential. Taken together, upregulation of ING2 was associated with colon cancer and MMP13-dependent cellular invasion, indicating that ING2 expression might be involved with cancer invasion and metastasis.
