Expression of the vav oncogene in somatic cell hybrids

Human Vav1 expression in hematopoietic and cancer cell lines is regulated by c-Myb and by CpG methylation

Vav1 is a signal transducer protein that functions as a guanine nucleotide exchange factor for the Rho/Rac GTPases in the hematopoietic system where it is exclusively expressed. Recently, Vav1 was shown to be involved in several human malignancies including neuroblastoma, lung cancer, and pancreatic ductal adenocarcinoma (PDA). Although some factors that affect vav1 expression are known, neither the physiological nor pathological regulation of vav1 expression is completely understood. We demonstrate herein that mutations in putative transcription factor binding sites at the vav1 promoter affect its transcription in cells of different histological origin. Among these sites is a consensus site for c-Myb, a hematopoietic-specific transcription factor that is also found in Vav1-expressing lung cancer cell lines. Depletion of c-Myb using siRNA led to a dramatic reduction in vav1 expression in these cells. Consistent with this, co-transfection of c-Myb activated transcription of a vav1 promoter-luciferase reporter gene construct in lung cancer cells devoid of Vav1 expression. Together, these results indicate that c-Myb is involved in vav1 expression in lung cancer cells. We also explored the methylation status of the vav1 promoter. Bisulfite sequencing revealed that the vav1 promoter was completely unmethylated in human lymphocytes, but methylated to various degrees in tissues that do not normally express vav1. The vav1 promoter does not contain CpG islands in proximity to the transcription start site; however, we demonstrated that methylation of a CpG dinucleotide at a consensus Sp1 binding site in the vav1 promoter interferes with protein binding in vitro. Our data identify two regulatory mechanisms for vav1 expression: binding of c-Myb and CpG methylation of 5′ regulatory sequences. Mutation of other putative transcription factor binding sites suggests that additional factors regulate vav1 expression as well.

Defective Vav expression and impaired F-actin reorganization in a subset of patients with common variable immunodeficiency characterized by T-cell defects

Common variable immunodeficiency (CVID) is a primary immune disorder characterized by impaired antibody production, which is in many instances secondary to defective T-cell function (T-CVID). We have previously identified a subset of patients with T-CVID characterized by defective T-cell receptor (TCR)-dependent protein tyrosine phosphorylation. In these patients, ZAP-70 fails to be recruited to the TCR as the result of impaired CD3zeta phosphorylation, which is, however, not dependent on defective Lck expression or activity. Here we show that neither Fyn nor CD45 is affected in these patients. On the other hand, T-CVID T cells show dramatic defects in the Vav/Rac pathway controlling F-actin dynamics. A significant deficiency in Vav protein was indeed observed; in 3 of 4 patients with T-CVID, it was associated with reduced VAV1 mRNA levels. The impairment in Vav expression correlated with defective F-actin reorganization in response to TCR/CD28 co-engagement. Furthermore, TCR/CD28-dependent up-regulation of lipid rafts at the cell surface, which requires F-actin dynamics, was impaired in these patients. The actin cytoskeleton defect could be reversed by reconstitution of Vav1 expression in the patients' T cells. Results demonstrate an essential role of Vav in human T cells and strongly suggest Vav insufficiency in T-CVID.

PU.1 regulates the expression of the vav proto-oncogene

Vav is a guanine nucleotide exchange factor for the rho/rac GTPases that is upregulated in the embryo during the transition from primitive to definitive hematopoiesis. It is one of several genetic markers that correlates with the differentiation of the intraembryonic definitive hematopoietic stem cell. Subsequently, in the adult, vav is expressed predominantly in cells of the hematopoietic system. A heat-resistant protein complex that bound to a 23-bp segment, which is essential for vav promoter activity, was found to be present in myeloid cells but not T-cells. The complex was absent in non-hematopoietic cells which normally do not express vav. Using a saturation mutagenesis method, Mutex, a "footprint" of the protein binding site (AGAGGAAGT) was obtained that was consistent with the consensus binding site for PU.1. A specific antibody to PU.1 supershifted the complex and identified the presence of PU.1 within the complex. A GST fusion protein of the human PU.1 bound to the same consensus sequence as the heat-resistant complex from myeloid lineages. Specific mutation of the GGAA PU.1 core binding site silenced vav promoter activity and a dominant negative PU.1 inhibited the transactivation of PU.1 at the vav promoter as measured by the expression of the EGFP reporter gene. In addition, PCR analysis of immunoprecipitated chromatin using specific antibodies for PU.1 detected the co-immunoprecipitation of DNA containing the vav promoter. These results suggest that PU.1 is essential for transcriptional activity of the vav promoter in myeloid cells.

Regulation of the vav proto-oncogene by LKLF

The transcriptional induction of the vav proto-oncogene coincides with the first appearance of the definitive hematopoietic stem cell in the aorta-gonad-mesonephros region. Vav promoter activity was dependent on a previously identified 23 bp DNA segment containing PU.1 and Runx1/AML-1 binding sites and on a newly identified, highly conserved, 12 bp DNA segment (Box B). The sequence of Box B was identical in the human, mouse and rat species. Mutation of the CACCC core sequence led to diminished vav promoter activity. A protein complex which bound to Box B was found in hematopoietic cells but not in cells which did not express vav. A double-stranded oligonucleotide containing a mutation of the CACCC core was less effective in electro-mobility shift assay competitions than the wild-type sequence. UV crosslinking studies identified a 37 kDa DNA binding protein which interacted with Box B in U937 cells. Antibody supershift assays identified this protein as lung Krüppel-like factor (LKLF). LKLF, expressed as a glutathione S-transferase fusion protein, was capable of binding to Box B. A dominant-negative LKLF was able to inhibit the expression of enhanced green fluorescent protein by the vav promoter and chromatin immunoprecipitations detected LKLF bound to the vav promoter in U937 but not HeLa cells. These in vitro results suggest future in vivo experiments to examine the role of LKLF, a gene required for vasculogenesis, in the induction of vav during the genesis of the definitive hematopoietic stem cell from the vascular endothelium.

Genomic organization and regulation of the vav proto-oncogene

Vav and vav2 are members of the dbl family of guanine nucleotide exchange factors (GEF) for the rho/rac family of GTP binding proteins. Vav is expressed primarily in hematopoietic cells, while vav2 has a wider tissue distribution. The genomic structure of the human vav proto-oncogene was studied by identifying and sequencing all 27 exons of the gene from overlapping P1 and cosmid clones. The gene spans a 77-kb region on chromosome 19. In contrast, the coding region of vav2 is distributed over 30 exons spanning 227-kb. The overall organization of the exons which encode both proteins was found to be similar. In humans, alternative splicing of exons 6, 16 and 28 generated at least two distinct vav2 mRNA species. Several differences from the original vav cDNA sequence were noted. The most important difference was the identification of amino acid 718 as isoleucine, rather than threonine. This change warrants the reclassification of the vav SH2 domain as a type 3 SH2, instead of a type 2 SH2 as originally proposed by Songyang et al. (Mol. Cell. Biol. 14 (1994) 2777-2785). A series of vav promoter deletions were constructed using the enhanced green fluorescent protein (EGFP) as a reporter gene. A 23-bp segment that included a potential CBF/AML-1 binding site was found to be essential for EGFP expression in U937 cells. The same constructs were not active in HeLa cells, which do not express vav. A potential c-myb DNA binding site within the vav promoter was not required for EGFP expression.