Control of mammary tumor differentiation by SKI-606 (bosutinib).

C-Src is infrequently mutated in human cancers but it mediates oncogenic signals of many activated growth factor receptors and thus remains a key target for cancer therapy. However, the broad function of Src in many cell types and processes requires evaluation of Src-targeted therapeutics within a normal developmental and immune-competent environment. In an effort to understand the appropriate clinical use of Src inhibitors, we tested an Src inhibitor, SKI-606 (bosutinib), in the MMTV-PyVmT transgenic mouse model of breast cancer. Tumor formation in this model is dependent on the presence of Src, but the necessity of Src kinase activity for tumor formation has not been determined. Furthermore, Src inhibitors have not been examined in an autochthonous tumor model that permits assessment of effects on different stages of tumor progression. Here we show that oral administration of SKI-606 inhibited the phosphorylation of Src in mammary tumors and caused a rapid decrease in the Ezh2 Polycomb group histone H3K27 methyltransferase and an increase in epithelial organization. SKI-606 prevented the appearance of palpable tumors in over 50% of the animals and stopped tumor growth in older animals with pre-existing tumors. These antitumor effects were accompanied by decreased cellular proliferation, altered tumor blood vessel organization and dramatically increased differentiation to lactational and epidermal cell fates. SKI-606 controls the development of mammary tumors by inducing differentiation.

The activin binding proteins follistatin and follistatin-related protein are differentially regulated in vitro and during cutaneous wound repair.

Follistatin is a secreted protein that binds activin in vitro and in vivo and thereby inhibits its biological functions. Recently, related human and murine genes, designated follistatin-related gene (FLRG), were identified, and their products were shown to bind activin with high affinity. In this study we further characterized the murine FLRG protein, and we analyzed its tissue-specific expression and regulation in comparison with those of follistatin. Transient expression of the mouse FLRG protein in COS-1 cells revealed that the FLRG cDNA encodes a secreted glycoprotein. FLRG mRNA was expressed at high levels in the lung, the testis, the uterus and, particularly, the skin. Immunohistochemistry revealed the presence of FLRG in the basement membrane between the dermis and the epidermis and around blood vessels. FLRG mRNA expression was induced in keratinocytes by keratinocyte growth factor, epidermal growth factor and transforming growth factor-beta 1, and in fibroblasts by platelet-derived growth factor and epidermal growth factor. The induction was more rapid, but weaker, than that of follistatin. Most interestingly, both follistatin and FLRG were expressed during the wound healing process, but their distribution within the wound was different. The different expression pattern of FLRG and follistatin and their differential regulation suggest different functions of these activin-binding proteins in vivo.

Impaired wound healing in transgenic mice overexpressing the activin antagonist follistatin in the epidermis.

Recently, we demonstrated a strong upregulation of activin expression after skin injury. Furthermore, overexpression of this transforming growth factor beta family member in the skin of transgenic mice caused dermal fibrosis, epidermal hyperthickening and enhanced wound repair. However, the role of endogenous activin in wound healing has not been determined. To address this question we overexpressed the soluble activin antagonist follistatin in the epidermis of transgenic mice. These animals were born with open eyes, and the adult mice had larger ears, longer tails and reduced body weight compared with non-transgenic littermates. Their skin was characterized by a mild dermal and epidermal atrophy. After injury, a severe delay in wound healing was observed. In particular, granulation tissue formation was significantly reduced, leading to a major reduction in wound breaking strength. The wounds, however, finally healed, and the resulting scar area was smaller than in control animals. These results implicate an important function of endogenous activin in the control of wound repair and scar formation.

Deletion of a novel protein kinase with PX and FYVE-related domains increases the rate of differentiation of Trypanosoma brucei.

Growth control of African trypanosomes in the mammalian host is coupled to differentiation of a non-dividing life cycle stage, the stumpy bloodstream form. We show that a protein kinase with novel domain architecture is important for growth regulation. Zinc finger kinase (ZFK) has a kinase domain related to RAC and S6 kinases flanked by a FYVE-related zinc finger and a phox (PX) homology domain. To investigate the function of the kinase during cyclical development, a stable transformation procedure for bloodstream forms of differentiation-competent (pleomorphic) Trypanosoma brucei strains was established. Deletion of both allelic copies of ZFK by homologous recombination resulted in reduced growth of bloodstream-form parasites in culture, which was correlated with an increased rate of differentiation to the non-dividing stumpy form. Growth and differentiation rates were returned to wild-type level by ectopic ZFK expression. The phenotype is stage-specific, as growth of procyclic (insect form) trypanosomes was unaffected, and Deltazfk/Deltazfk clones were able to undergo full cyclical development in the tsetse fly vector. Deletion of ZFK in a differentiation-defective (monomorphic) strain of T. brucei did not change its growth rate in the bloodstream stage. This suggests a function of ZFK associated with the trypanosomes' decision between either cell cycle progression, as slender bloodstream form, or differentiation to the non-dividing stumpy form.