Inhibition of colon tumor progression and angiogenesis by the Ink4a/Arf locus
The Ink4a/Arf locus encodes two uniquely arranged cell cycle inhibitors, p16 and Arf. Despite overlapping genomic sequences, p16 and Arf translate structurally dissimilar proteins that operate in largely discrete pathways. The Ink4a/Arf locus is frequently methylated in colorectal carcinoma and is the second most commonly inactivated tumor suppressor locus in all human cancers studied. These data suggest a broad role in the inhibition of tumor progression, although the specific steps and pathways countermanded by either p16 or Arf are poorly understood. We examined the influence of p16, Arf, and the complete Ink4a/Arf locus in the multiple intestinal neoplasia (Min) mouse model of colorectal carcinoma. Colon tumors in three month-old Min mice null for the complete Ink4a/Arf locus were moderately larger than those in wild type (wt) animals. Strikingly, one-half of the null colon tumors were visibly red in coloration, whereas most wt tumors were white, a significant difference even after normalizing for tumor area. Additionally, Ink4a/Arf-null colon tumors displayed more red blood cells (RBCs) near the tumor surface, twice the number of functional blood vessels, and features of carcinoma in situ absent in wt tumors. Red tumors had higher hemoglobin (Hgb) and vascular endothelial growth factor (VEGF) content than white tumors. In Min mice null specifically for p16, tumors trended toward greater area and more frequent red coloration than wt. Additionally, p16-null tumors had higher densities of necrosis although less frequent apoptosis, more numerous RBCs, and greater Hgb and VEGF content. Unexpectedly, p16-null neoplastic cells were less proliferative than wild type. Preliminary microarray analysis of tumor mRNA revealed elevated levels in p16-null tumors of several potential upregulators of vascularity, such as Arnt2, and reduced levels of clusterin, an angiogenesis inhibitor. Arf-null Min colon tumors were smaller and less frequently red than tumors from either Ink4a/Arf- or p16-null mice. These results provide direct evidence that the Ink4a/Arf locus inhibits colon tumor progression. Both p16 and Arf contribute to the tumor suppressive effects of the complete locus; p16 specifically inhibits intratumor angiogenesis. This inhibition is particularly significant, given the clinical relevance of vascularity to colon tumor detection, prevention, recurrence, and therapy. ^
Biology, Molecular|Biology, Cell
Steven L Gibson,
"Inhibition of colon tumor progression and angiogenesis by the Ink4a/Arf locus"
(January 1, 2007).
Dissertations available from ProQuest.