p16(INK4a) expression begins early in human colon neoplasia and correlates inversely with markers of cell proliferation.
OBJECTIVE
p16(INK4a) is a cell cycle inhibitor and a major tumor-suppressor protein, but the regulation of p16(INK4a) is poorly understood and the physiologic settings in which it exerts its antiproliferative effects are unknown. A role for p16(INK4a) in intestinal neoplasia is suggested by the observation that the promoter region is methylated in a subset of human colon tumors. We examined the expression of the protein in specimens representing the full spectrum of neoplastic progression in the human colon and determined whether expressing cells showed evidence of cell cycle inhibition.


METHODS
We studied p16(INK4a) expression by immunoprecipitation, immunoblotting, reverse-transcription polymerase chain reaction (RT-PCR), immunohistochemistry, and immunofluorescence in matched normal and neoplastic colonic tissue from 70 patients.


RESULTS
p16(INK4a) expression was very low in normal mucosa, with staining observed in rare epithelial cells at the base of crypts. A distinctly higher expression was found in 4 of 7 aberrant crypt foci, 32 of 36 adenomas, 18 of 28 primary carcinomas, and 5 of 5 metastatic carcinomas. Within each neoplasm p16(INK4a) staining was heterogeneous, with higher expression commonly seen in areas bordering normal tissue. p16(INK4a) staining correlated inversely with that of Ki67, cyclin A, and the retinoblastoma protein, suggesting that cell cycle progression was inhibited.


CONCLUSIONS
These results suggest that p16(INK4a) expression begins in the earliest detectable stages of neoplastic progression in the human colon and exerts a continuous, piecemeal constraint on tumor growth.
Dai
C Y
CY
Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA.
Furth
E E
EE
Mick
R
R
Koh
J
J
Takayama
T
T
Niitsu
Y
Y
Enders
G H
GH
D015153:Blotting, Western; D002352:Carrier Proteins / Q000032:analysis / Q000235:genetics*; D002455:Cell Division; D002462:Cell Membrane / Q000737:chemistry; D003110:Colonic Neoplasms / Q000737:chemistry / Q000235:genetics* / Q000473:pathology*; D051358:Cyclin-Dependent Kinase 4; D019941:Cyclin-Dependent Kinase Inhibitor p16; D018844:Cyclin-Dependent Kinases / Q000032:analysis; D006801:Humans; D007150:Immunohistochemistry; D007413:Intestinal Mucosa / Q000737:chemistry / Q000378:metabolism / Q000473:pathology; D011518:Proto-Oncogene Proteins; D012004:Rectal Neoplasms / Q000737:chemistry / Q000235:genetics / Q000473:pathology; D016160:Retinoblastoma Protein / Q000032:analysis; D020133:Reverse Transcriptase Polymerase Chain Reaction
D002352:Carrier Proteins; D019941:Cyclin-Dependent Kinase Inhibitor p16; D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein; C495900:CDK4 protein, human; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases