The vascular endothelial growth factor (VEGF) receptor Flt-1 (VEGFR-1) modulates Flk-1 (VEGFR-2) signaling during blood vessel formation.

Mice lacking the vascular endothelial growth factor (VEGF) receptor flt-1 (VEGFR-1) die from vascular overgrowth, caused primarily by aberrant endothelial cell division (Kearney JB, Ambler CA, Monaco KA, Johnson N, Rapoport RG, Bautch VL: Vascular endothelial growth factor receptor Flt-1 negatively regulates developmental blood vessel formation by modulating endothelial cell division. Blood 2002, 99:2397-2407). Because a second high-affinity VEGF receptor, flk-1, produces a positive endothelial proliferation signal, it was logical to ask whether flt-1 affects developmental blood vessel formation by modulating signaling through flk-1. Differentiated embryonic stem cell cultures lacking flt-1 (flt-1-/-) had increased flk-1 tyrosine phosphorylation, indicating that flk-1 signaling is up-regulated in the mutant background. The selective flk-1 inhibitor SU5416 partially rescued the flt-1-/- mutant phenotype, and this rescue was accompanied by a decrease in the relative amount of flk-1 tyrosine phosphorylation. Thus reduced flk-1 signal transduction can partially compensate for the lack of flt-1. The flt-1-/- mutant phenotype was also partially rescued by Flt-1/Fc, a truncated flt-1 that binds and sequesters the VEGF ligand. Taken together, these data show that down-regulation of flk-1 signaling by two different strategies partially rescues the developmental vascular overgrowth seen in the absence of flt-1, and they support a model whereby flt-1 modulates the flk-1 signal at an early point in the pathway.

Targeted expression of c-Src in epidermal basal cells leads to enhanced skin tumor promotion, malignant progression, and metastasis.

In this study, we generated transgenic mice that overexpressed either a constitutively active human c-src mutant (src(530)) or a wild-type human c-src (src(wt)) in epidermal basal cells driven by human keratin 14 (HK14) or bovine keratin 5 (BK5) promoters, respectively. HK14.src(530) transgenic mice developed severe epidermal hyperplasia and hyperkeratosis, and did not survive beyond 3 weeks of age. Four transgenic founders were obtained after injection of a BK5.src(wt) construct with variable phenotypes, and three lines (lines A-C) were established. BK5.src(wt) founder D exhibited a severe skin phenotype similar to HK14.src(530) transgenic mice and died 5 days after birth. Line C transgenic mice also exhibited significant epidermal hyperplasia and hyperkeratosis, and developed spontaneous squamous cell carcinomas (SCCs) of the skin beginning at approximately 3 months of age (70% incidence at 1 year). Mice from lines A and B did not show a marked phenotype; however, elevated human src(wt) protein in the epidermis of line B mice was clearly evident. Additional analyses of line B transgenic mice showed an enhanced responsiveness to 12-O-tetradecanoylphorbol-13-acetate-induced epidermal hyperplasia and cell proliferation. Analysis of the susceptibility of line B mice to two-stage skin carcinogenesis revealed that papillomas and SCCs arose earlier and in greater numbers compared with nontransgenic littermates. In addition, malignant conversion occurred more rapidly, and the SCCs that developed in line B transgenic mice had a greater propensity to metastasize to peripheral lymph nodes and other organs. These observations support the hypothesis that c-src plays an important role in skin tumor promotion. In addition, the data show that elevated c-src activity enhances malignant progression and metastasis in this model system.

Switching of melanocyte pigmentation associated with pituitary pars intermedia tumors in Rb+/- and p27-/- female mice with yellow pelage.

As an incidental finding in a study of mammary tumorigenesis, two lines of genetically engineered mice were observed to develop pigmentation changes of the fur. Mice with targeted mutations of the Rb1 (Rb) and Cdkn1b (p27kip1) genes were crossed from C57BL/6 (black coat color; eumelanin) and 129Sv (wild-type agouti coat color) backgrounds, respectively, to one with a dominant yellow coat color (phaeomelanin) carrying a transgene for Agouti under a keratinocyte specific promoter. Both Rb+/- and p27-/- mice developed pituitary tumors of the pars intermedia that were associated with a switch to black (eumelanic) fur but were not observed in sibling Rb+/+ and p27+/+ mice. This phenomenon was observed first in the vibrissae and, subsequently one to two weeks later, as periorbital and dorsal patches, and was associated with pituitary lesions larger than four millimeters in the longest dimension. In Rb+/- mice, pigmentation change preceded a moribund state attributable to the tumors by two to four weeks, whereas in p27-/- mice, the pigmentation alteration was earlier, more gradual, and prolonged. The switch from phaeomelanin to eumelanin in the fur is most likely due to out-competition of the agouti gene product by alpha-melanocyte-stimulating hormone from the pituitary tumors, an effect masked in black or agouti mice.