Growth hormone (GH)-activated signal transducer and activator of transcription 5 (STAT5) and the glucocorticoid (GC)-responsive glucocorticoid receptor (GR) are important signal integrators in the liver during metabolic and physiologic stress. Their deregulation has been implicated in the development of metabolic liver diseases, such as steatosis and progression to fibrosis. Using liver-specific STAT5 and GR knockout mice, we addressed their role in metabolism and liver cancer onset. STAT5 single and STAT5/GR double mutants developed steatosis, but only double-mutant mice progressed to liver cancer. Mechanistically, STAT5 deficiency led to the up-regulation of prolipogenic sterol regulatory element binding protein 1 (SREBP-1) and peroxisome proliferator activated receptor gamma (PPAR-γ) signaling. Combined loss of STAT5/GR resulted in GH resistance and hypercortisolism. The combination of both induced expression of adipose tissue lipases, adipose tissue lipid mobilization, and lipid flux to the liver, thereby aggravating STAT5-dependent steatosis. The metabolic dysfunctions in STAT5/GR compound knockout animals led to the development of hepatic dysplasia at 9 months of age. At 12 months, 35% of STAT5/GR-deficient livers harbored dysplastic nodules and ∼60% hepatocellular carcinomas (HCCs). HCC development was associated with GH and insulin resistance, enhanced tumor necrosis factor alpha (TNF-α) expression, high reactive oxygen species levels, and augmented liver and DNA damage parameters. Moreover, activation of the c-Jun N-terminal kinase 1 (JNK1) and STAT3 was prominent.ConclusionHepatic STAT5/GR signaling is crucial for the maintenance of systemic lipid homeostasis. Impairment of both signaling cascades causes severe metabolic liver disease and promotes spontaneous hepatic tumorigenesis. (hepatology 2011;54:1398–1409)

The D816V-mutated variant of Kit triggers multiple signaling pathways and is considered essential for malignant transformation in mast cell (MC) neoplasms. We here describe that constitutive activation of the Stat5-PI3K-Akt-cascade controls neoplastic MC development. Retrovirally transduced active Stat5 (cS5 F ) was found to trigger PI3K and Akt activation, and to transform murine bone marrow progenitors into tissue-infiltrating MCs. Primary neoplastic Kit D816V ؉ MCs in patients with mastocytosis also displayed activated Stat5, which was found to localize to the cytoplasm and to form a signaling complex with PI3K, with consecutive Akt IntroductionMast cells (MCs) are critical effector cells in innate and acquired immunity. 1,2 Under various circumstances and pathologic conditions, MCs increase in number and accumulate in various tissues and organs. In many cases, reactive MC hyperplasia is found. 1 However, MCs (MC progenitors) may also undergo neoplastic transformation. 3,4 Disorders that lead to enhanced proliferation and/or accumulation of neoplastic MCs are well defined by WHO criteria. [3][4][5][6] MCs are derived from pluripotent hematopoietic cells in the bone marrow and undergo terminal maturation in their ultimate tissue destinations under the influence of stem cell factor, also known as Kit ligand. [7][8][9] Studies in MC-deficient mouse strains displaying mutations in the stem cell factor (SCF) gene or the gene encoding the SCF receptor, c-Kit, as well as activating c-Kit mutations that are considered to represent major transforming hits in mastocytosis, underline the importance of SCF and Kit for MC development. [10][11][12][13][14][15][16] Binding of SCF to Kit induces activation of various signaling molecules including phospholipase C, the Src family tyrosine kinase, the scaffolding molecule Gab2, the MAP Kinases Erk1/2, the JAK tyrosine kinase, the Phosphatidyl-inositol 3-kinase (PI3K), and the Stat transcription factors. [17][18][19] Lessons from gene deletion studies in mice have indicated that PI3K, Gab2, and Stat5 play a critical role in MC development and function, suggesting that these molecules may represent important downstream effectors of c-Kit signaling. [20][21][22] Moreover, recent data have shown that Stat5 and Gab2 are also required for signaling via the high affinity IgE receptor Fc⑀RI that plays a critical role in MC function and allergic response. 23,24 Besides their physiologic role in MCs, accumulating evidence suggests that persistent Stat5 and PI3K activation is frequently found in hematopoietic neoplasms and solid tumors. 25,26 It has also been described that disease-related oncogenic tyrosine kinases like Tel-Jak2, Bcr-Abl, Tel-PDGFR␤, mutated Kit or Flt3 receptors, and the Jak2 (V617F) mutant, detectable in most myeloproliferative disorders (MPDs), induce constitutive activation of Stat5, PI3K and its downstream effector, the serine threonine kinase Akt. [27][28][29][30][31][32][33][34][35] Moreover, Stat5 proteins were found to be required for Tel-Jak2-and Bcr-Abl-induc...

Insulin-like growth factor (IGF) signaling is thought to play a role in the development and progression of multiple cancer types. To date, therapeutic strategies aimed at disrupting IGF signaling have largely focused on antibodies that target the IGF-I receptor (IGF-IR). Here, we describe the pharmacologic profile of BI 836845, a fully human monoclonal antibody that utilizes an alternative approach to IGF signaling inhibition by selectively neutralizing the bioactivity of IGF ligands. Biochemical analyses of BI 836845 demonstrated high affinity to human IGF-I and IGF-II, resulting in effective inhibition of IGF-induced activation of both IGF-IR and IR-A in vitro. Cross-reactivity to rodent IGFs has enabled rigorous assessment of the pharmacologic activity of BI 836845 in preclinical models. Pharmacodynamic studies in rats showed potent reduction of serum IGF bioactivity in the absence of metabolic adverse effects, leading to growth inhibition as evidenced by reduced body weight gain and tail length. Moreover, BI 836845 reduced the proliferation of human cell lines derived from different cancer types and enhanced the antitumor efficacy of rapamycin by blocking a rapamycininduced increase in upstream signaling in vitro as well as in human tumor xenograft models in nude mice. Our data suggest that BI 836845 represents a potentially more effective and tolerable approach to the inhibition of IGF signaling compared with agents that target the IGF-I receptor directly, with potential for rational combinations with other targeted agents in clinical studies. Mol Cancer Ther; 13(2); 399-409. Ó2013 AACR.

IntroductionHematopoietic development is regulated by cytokine-or growth factor-activated signaling pathways, among which the Janus kinase (Jak)/signal transducer and activator of transcription (Stat) pathway plays a major role. [1][2][3] Four Jak kinases and 7 Stat proteins regulate a wide spectrum of cellular functions such as proliferation, survival, and differentiation. 4 Stats are latent transcription factors that constantly shuttle between the nucleus and the cytoplasm. Their activity is tightly regulated by protein tyrosine kinases. Phosphorylation of a positionally conserved tyrosine residue triggers dimerization, efficient nuclear translocation, and subsequent binding of Stats to specific promoter sequences. 5 Target genes regulated by Stat5 proteins and their cofactors are essential for hematopoietic stem cell maintenance, lineage commitment, self-renewal, and survival of committed hematopoietic progenitors as well as for mature cells of both myeloerythroid and lymphoid lineage. [6][7][8] Although there is considerable functional overlap, Stat5a and Stat5b proteins also have distinct functions due to isoformspecific differences in mRNA levels, 3,5 nucleocytoplasmic shuttling, 9,10 and activation by tyrosine and/or serine phosphorylation. 11 Hyperactivated Stat5a and Stat5b proteins have been implicated in several hematopoietic malignancies and many solid tumors. [1][2][3] Yet to date, there are no reports of activating mutations in Stat5 proteins. Persistent tyrosine phosphorylation of Stat5 was found to be caused by deregulated cytokine signaling 12 or perturbation of upstream molecules, which mostly results from receptor mutations or chromosomal rearrangement. Well-known examples of this are the point mutation Jak2-V617F 13,14 and chromosomal translocations such as Bcr-Abl. 15,16 Furthermore, it has been shown that the presence of Stat5 proteins is required for Bcr-Abl-induced transformation and development of leukemia. 17 This is why Stat5 proteins and their activation status are thought to have a key role in leukemogenesis. Moreover, the transforming capacity of oncogenic Stat5 is not limited to its role as a transcription factor in the nucleus. Recent studies have demonstrated that oncogenic Stat5a also has a cytoplasmic role that links Jak/Stat signaling to the activation of the PI3K-Akt-mTOR signaling pathway via Gab2 [18][19][20] and that Stat5 acts as a repressor of Bcl-2 member-regulating microRNAs (miRNAs) 15/16. 21 In addition to tyrosine phosphorylation, the activity of several Stat proteins was shown to be modulated by serine phosphorylation (reviewed in Decker and Kovarik 22 ). In the highly homologous Stat5a and Stat5b proteins, distinct serine residues in the carboxyterminal transactivation domain, the least conserved region of Stat proteins, 2,3 were found to be phosphorylated. These serine moieties, however, are located in perfectly conserved Pro-Ser-Pro (PSP) motifs at positions 725 (Stat5a) and 730 (Stat5b 11,[23][24][25][26][27] Using a mouse transplantation model, 28,29 the pr...