Heterozygous inactivation of transforming growth factor-Β1 (TGF-Β1) and mutational activation of K-ras predisposes early lung tumor progression.

Abstract

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Growth inhibition of TGF-Β1 in lung epithelial cells is associated with direct, rapid stimulation of Ras. K-ras mutations are found in 30% of human lung adenocarcinomas, some of which show altered TGF-Β receptor expression, while high levels of TGF-Β1 are detected in the circulation of some of these patients. We hypothesized that heterozygous (HT) inactivation of TGF-Β1 and mutational activation of K-ras would predispose early lung tumor progression. We generated a mouse model system to examine the molecular effects of disruption of the TGF-Β signaling pathway in the presence of a spontaneously mutated activated K-ras mutation. HT TGF-Β1 mice, C57BL/6NCr TGF-Β1+/-, were mated with mu tated HT C57BL/6/129/sv K-ras+/- mice to generate four genotypes, including TGF-Β1+/+/K-ras+/+, TGF-Β1+/-/K-ras+/+, TGF-Β1+/+/K-ras+/-, and TGF-Β1+/-/K-ras+/-. Mice were maintained in a HT state because TGF-Β1 and K-ras null mice die in early stages of development. Mice with a single TGF-Β1 deletion or K-ras mutation were indistinguishable from wildtype littermates at birth. However, both TGF-Β1+/+/K-ras+/- and TGF-Β1+/-/K-ras+/- mice developed spontaneous lung lesions, ranging from hyperplasia, adenoma to adenocarcinoma within 1 month of birth. Heterozygous deletion of a TGF-Β1 allele in the presence of K-ras mutation (HT/K-ras) led to development of adenocarcinomas (invasive tumors) significantly earlier compared to animals with only a K-ras mutation with intact TGF-Β1 (WT/K-ras), The life span of HT/K-ras mice was significantly shorter than that of WT/K-ras mice. A pathway specific microarray analysis was performed on early spontaneous lung tumors (hyperplasia and adenomas) of HT/K-ras and WT/K-ras congenic mice. Expression profiles of genes involved in apoptosis, cell cycle regulation, angiogenesis, TGF-Β/BMP and MAPK pathways were examined. A total of 33 genes were differentially expressed in HT/K-ras mice compared to WT/K-ras mice. Brca2, p16, Gadd153, Pes1, BMP10 and Dad1 were among 16 genes that were down-regulated in HT/K-ras mouse lesions. CD29, Pmp22, Rad21, Adamts1, NfkB1, Akt1, Ptgs2 and TGF-Β RI were some of the genes that were overexpressed in lesions where K-ras was mutated and TGF-Β1 was heterozygous. Gene expression analysis data indicate that more profound early abrogation of apoptotic and cell cycle regulatory pathways in the presence of TGF-Β1 heterozygosity and activational mutation of K-ras are responsible for early progression to invasive lung lesions. Further examination of biological implications of these differentially expressed genes is necessary to understand the implications of abrogation of the TGF-Β pathway and activation of K-ras in the induction and maintenance of lung tumors.