The Rac1 splice form Rac1b promotes K-ras-induced lung tumorigenesis.

Rac1b, an alternative splice form of Rac1, has been previously shown to be upregulated in colon and breast cancer cells, suggesting an oncogenic role for Rac1b in these cancers. Our analysis of NSCLC tumor and matched normal tissue samples indicates Rac1b is upregulated in a significant fraction of lung tumors in correlation with mutational status of K-ras. To directly assess the oncogenic potential of Rac1b in vivo, we employed a mouse model of lung adenocarcinoma, in which the expression of Rac1b can be conditionally activated specifically in the lung. Although expression of Rac1b alone is insufficient to drive tumor initiation, the expression of Rac1b synergizes with an oncogenic allele of K-ras resulting in increased cellular proliferation and accelerated tumor growth. Finally, we show that in contrast to our previous findings demonstrating a requirement for Rac1 in K-ras-driven cell proliferation, Rac1b is not required in this context. Given the partially overlapping spectrum of downstream effectors regulated by Rac1 and Rac1b, our findings further delineate the signaling pathways downstream of Rac1 that are required for K-ras driven tumorigenesis.

Pirin downregulation is a feature of AML and leads to impairment of terminal myeloid differentiation.

Terminal differentiation of blood cells requires the concerted action of a series of transcription factors that are expressed at specific stages of maturation and function in a cell-type and dosage-dependent manner. Leukemogenic oncoproteins block differentiation by subverting the normal transcriptional status of hematopoietic precursor cells. Pirin (PIR) is a putative transcriptional regulator whose expression is silenced in cells bearing the acute myeloid leukemia-1 eight-twenty-one (AML1/ETO) and promyelocytic leukemia/retinoic acid receptor (PML/RAR) leukemogenic fusion proteins. A role for PIR in myeloid differentiation has not to date been reported. In this study we show that PIR expression is significantly repressed in a large proportion of acute myeloid leukemias (AMLs), regardless of subtype or underlying karyotypic abnormalities. We show that PIR expression increases during in vitro myeloid differentiation of primary hematopoietic precursor cells, and that ablation of PIR in the U937 myelomonocytic cell line or in murine primary hematopoietic precursor cells results in impairment of terminal myeloid differentiation. Gene expression profiling of U937 cells after knockdown of PIR revealed increased expression of genes associated with the early phases of hematopoiesis, in particular, homeobox A (HOXA) genes. Our results suggest that PIR is required for terminal myeloid maturation, and its downregulation may contribute to the differentiation arrest associated with AML.