Formation and Function of the Rbl2p-beta-Tubulin Complex

The yeast protein Rbl2p suppresses the deleterious effects of excess beta-tubulin as efficiently as does alpha-tubulin. Both in vivo and in vitro, Rbl2p forms a complex with beta-tubulin that does not contain alpha-tubulin, thus defining a second pool of beta-tubulin in the cell. Formation of the complex depends upon the conformation of beta-tubulin. Newly synthesized beta-tubulin can bind to Rbl2p before it binds to alpha-tubulin. Rbl2p can also bind beta-tubulin from the alpha/beta-tubulin heterodimer, apparently by competing with alpha-tubulin. The Rbl2p-beta-tubulin complex has a half-life of ~2.5 h and is less stable than the alpha/beta-tubulin heterodimer. The results of our experiments explain both how excess Rbl2p can rescue cells overexpressing beta-tubulin and how it can be deleterious in a wild-type background. They also suggest that the Rbl2p-beta-tubulin complex is part of a cellular mechanism for regulating the levels and dimerization of tubulin chains

Sprouty-2 regulates oncogenic K-ras in lung development and tumorigenesis

Somatic activation of Ras occurs frequently in human cancers, including one-third of lung cancers. Activating Ras mutations also occur in the germline, leading to complex developmental syndromes. The precise mechanism by which Ras activation results in human disease is uncertain. Here we describe the phenotype of a mouse engineered to harbor a germline oncogenic K-ras(G12D) mutation. This mouse exhibits early embryonic lethality due to a placental trophoblast defect. Reconstitution with a wild-type placenta rescues the early lethality, but mutant embryos still succumb to cardiovascular and hematopoietic defects. In addition, mutant embryos demonstrate a profound defect in lung branching morphogenesis associated with striking up-regulation of the Ras/mitogen-activated protein kinase (MAPK) antagonist Sprouty-2 and abnormal localization of MAPK activity within the lung epithelium. This defect can be significantly suppressed by lentiviral short hairpin RNA (shRNA)-mediated knockdown of Sprouty-2 in vivo. Furthermore, in the context of K-ras(G12D)-mediated lung tumorigenesis, Sprouty-2 is also up-regulated and functions as a tumor suppressor to limit tumor number and overall tumor burden. These findings indicate that in the lung, Sprouty-2 plays a critical role in the regulation of oncogenic K-ras, and implicate counter-regulatory mechanisms in the pathogenesis of Ras-based disease

Nkx2-1 Represses a Latent Gastric Differentiation Program in Lung Adenocarcinoma

Tissue-specific differentiation programs become dysregulated during cancer evolution. The transcription factor Nkx2-1 is a master regulator of pulmonary differentiation that is downregulated in poorly differentiated lung adenocarcinoma. Here we use conditional murine genetics to determine how the identity of lung epithelial cells changes upon loss of their master cell-fate regulator. Nkx2-1 deletion in normal and neoplastic lungs causes not only loss of pulmonary identity but also conversion to a gastric lineage. Nkx2-1 is likely to maintain pulmonary identity by recruiting transcription factors Foxa1 and Foxa2 to lung-specific loci, thus preventing them from binding gastrointestinal targets. Nkx2-1-negative murine lung tumors mimic mucinous human lung adenocarcinomas, which express gastric markers. Loss of the gastrointestinal transcription factor Hnf4α leads to derepression of the embryonal proto-oncogene Hmga2 in Nkx2-1-negative tumors. These observations suggest that loss of both active and latent differentiation programs is required for tumors to reach a primitive, poorly differentiated state.Burroughs Wellcome Fund (Career Award for Medical Scientists)National Institutes of Health (U.S.) (Grant U01-CA84306)National Institutes of Health (U.S.) (Grant K08-CA154784-01)Howard Hughes Medical InstituteLudwig Center for Molecular OncologyNational Cancer Institute (U.S.) (Cancer Center Support (Core) Grant P30-CA14051