Oncogenic Ki-ras but not oncogenic Ha-ras blocks integrin beta1-chain maturation in colon epithelial cells.
Human colorectal tumors commonly contain mutations in Ki-ras but rarely, if ever, in Ha-ras. The selectivity for Ki-ras mutations in this tumor was explored using the HD6-4 colon epithelial cell line which contains no ras mutations. After adhesion to an extracellular matrix, HD6-4 cells polarize into columnar goblet cells with distinct apical and basal regions. Stable HD6-4 transfectants were made with mini-gene constructs of the oncogenic cellular Ki-ras4BG12V gene, the oncogenic Ha-rasG12V gene, or mini-gene constructs of wild-type Ki-ras4B as a control. Ki-ras mutations, but not Ha-ras mutations, disrupted colon epithelial cell apicobasal polarity and adhesion to collagen I and laminin. Three Ha-ras transfectants and three Ki-ras transfectants exhibited Ras proteins expressing the Val-12 mutation by Western blotting with pan-rasG12V antibody. Only wild-type Ki-ras transfectant cells and oncogenic Ha-ras transfectant cells synthesized the mature, fully glycosylated forms of beta1 integrin. Instead of the mature integrin beta1-chain, a faster migrating beta1-chain intermediate was detected on the cell surface and in the cytoplasm of the oncogenic Ki-ras transfectants. Expression of the oncogenic Ki-ras gene caused the altered beta1 integrin maturation because phosphorothiolated antisense oligonucleotides to Ki-ras reduced expression of both the mutant Ki-Ras protein and the aberrant integrin beta1-chain and increased expression of the mature integrin beta1-chain. Altered glycosylation generated the new beta1 integrin form since integrin core beta1-chain proteins of the same molecular weight were yielded in Ki-ras, Ha-ras, and control transfectants after removal of sugar residues with endoglycosidase F or following tunicamycin treatment to inhibit glycosylation. The selective effect of oncogenic Ki-ras on beta1 integrin glycosylation was not due to selective activation of mitogen-activated protein kinases because both mutated Ki- and Ha-ras genes activated this pathway and increased cell proliferation. Since blocking the glycosylation of integrin beta1-chain inhibited the adherence, polarization, and subsequent differentiation of colon epithelial cells, the selective effects of the oncogenic cellular Ki-ras gene on integrin beta1-chain glycosylation may account, at least in part, for the selection of Ki-ras mutations in human colon tumors.
Yan
Z
Z
State University of New York Health Science Center, Department of Pathology, Syracuse, New York 13210, USA.
Chen
M
M
Perucho
M
M
Friedman
E
E
D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002460:Cell Line; D016764:Cell Polarity; D002471:Cell Transformation, Neoplastic / Q000235:genetics; D003106:Colon / Q000166:cytology / Q000378:metabolism*; D005109:Extracellular Matrix / Q000378:metabolism; D006031:Glycosylation; D006801:Humans; D019012:Integrin beta1 / Q000378:metabolism*; D007413:Intestinal Mucosa / Q000166:cytology / Q000378:metabolism*; D009154:Mutation; D016376:Oligonucleotides, Antisense / Q000235:genetics / Q000378:metabolism; D015689:Oncogene Protein p21(ras) / Q000235:genetics / Q000378:metabolism; D014162:Transfection; D018631:ras Proteins / Q000235:genetics / Q000378:metabolism*
D019012:Integrin beta1; D016376:Oligonucleotides, Antisense; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D015689:Oncogene Protein p21(ras); D018631:ras Proteins