K-ras exon 4A has a tumour suppressor effect on carcinogen-induced murine colonic adenoma formation.
K-ras encodes two isoforms, K-ras 4A and 4B, that are jointly affected by K-ras activating mutations, which are prevalent in colorectal cancer (CRC). CRC shows alterations in the expressed K-ras 4A : 4B isoform ratio in favour of K-ras 4B, in tumours both with and without K-ras mutations. The present study evaluated whether K-ras 4A expression can suppress colonic adenoma development in the absence of its oncogenic allele. Mice with homozygous targeted deletions of K-ras exon 4A (K-ras(tmDelta4A/tmDelta4A)) that can express the K-ras 4B isoform only, along with heterozygous K-ras(tmDelta4A/+) and wild-type mice, were given ten weekly 1,2-dimethylhydrazine (DMH) treatments to induce colonic adenomas. There was a significant increase in both the number and the size of colonic adenomas in DMH-treated K-ras(tmDelta4A/tmDelta4A) mice, with reduced survival, compared with heterozygous and wild-type mice. No K-ras mutations were found in any of the 30 tumours tested from the three groups. Lack of expression of K-ras 4A transcripts was confirmed, whereas the relative expression levels of K-ras 4B transcripts were significantly increased in the adenomas of K-ras(tmDelta4A/tmDelta4A) mice compared with K-ras(tmDelta4A/+) and wild-type mice. Immunohistochemical studies showed that adenomas of K-ras(tmDelta4A/tmDelta4A) mice had significantly increased cell proliferation and significantly decreased apoptosis with evidence of activation of MapKinase and Akt pathways, with increased phospho-Erk1/2 and both phospho-Akt-Thr308 and phospho-Akt-Ser473 immunostaining, compared with adenomas from K-ras(tmDelta4A/+) and wild-type mice. In conclusion, following DMH treatment, K-ras exon 4A deletion promoted increased number and size of colonic adenomas showing increased K-ras 4B expression, increased proliferation, decreased apoptosis, and activation of MapKinase and Akt pathways, in the absence of K-ras mutations. Therefore, K-ras 4A expression had a tumour suppressor effect on carcinogen-induced murine colonic adenoma formation, explaining the selective advantage of the altered K-ras 4A : 4B isoform ratio found in human colorectal cancer.
Luo
Feijun
F
Department of Pathology, University of Cambridge, Addenbrooke's Hospital, Hills Road, Cambridge, CB2 2QQ, UK.
Ye
Hongtao
H
Hamoudi
Rifat
R
Dong
Gehong
G
Zhang
Wenyan
W
Patek
Charles E
CE
Poulogiannis
George
G
Arends
Mark J
MJ
D019813:1,2-Dimethylhydrazine; D000236:Adenoma / Q000139:chemically induced / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D000818:Animals; D017209:Apoptosis; D002273:Carcinogens; D049109:Cell Proliferation; D002471:Cell Transformation, Neoplastic / Q000235:genetics / Q000378:metabolism; D003110:Colonic Neoplasms / Q000139:chemically induced / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D004789:Enzyme Activation / Q000502:physiology; D005091:Exons / Q000235:genetics; D016147:Genes, Tumor Suppressor; D011905:Genes, ras; D020935:MAP Kinase Signaling System / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009154:Mutation; D051057:Proto-Oncogene Proteins c-akt / Q000502:physiology; D016283:Proto-Oncogene Proteins p21(ras) / Q000172:deficiency / Q000235:genetics*
D002273:Carcinogens; D051057:Proto-Oncogene Proteins c-akt; C495971:Hras protein, mouse; D016283:Proto-Oncogene Proteins p21(ras); D019813:1,2-Dimethylhydrazine