Beta-catenin plays an important role in embryogenesis and carcinogenesis by controlling either cadherin-mediated cell adhesion or transcriptional activation of target gene expression. In many types of cancers nuclear translocation of beta-catenin has been observed. Our data indicate that during melanoma progression an increased dependency on the transcriptional function of beta-catenin takes place. Blockade of beta-catenin in metastatic melanoma cell lines efficiently induces apoptosis, inhibits proliferation, migration and invasion in monolayer and 3-dimensional skin reconstructs and decreases chemoresistance. In addition, subcutaneous melanoma growth in SCID mice was almost completely inhibited by an inducible beta-catenin knockdown. In contrast, the survival of benign melanocytes and primary melanoma cell lines was less affected by beta-catenin depletion. However, enhanced expression of beta-catenin in primary melanoma cell lines increased invasive capacity in vitro and tumor growth in the SCID mouse model. These data suggest that beta-catenin is an essential survival factor for metastatic melanoma cells, whereas it is dispensable for the survival of benign melanocytes and primary, non-invasive melanoma cells. Furthermore, beta-catenin increases tumorigenicity of primary melanoma cell lines. The differential requirements for beta-catenin signaling in aggressive melanoma versus benign melanocytic cells make beta-catenin a possible new target in melanoma therapy.

All models discussed here have their defined strengths, but also limitations with respect to their predictive features. Understanding the preclinical models in a more profound way should lead to optimized clinical trials, thereby expanding the therapeutic arsenal and improving patient outcome further.

The E3 ubiquitin ligase and tumor suppressor APC/CCdh1 is crucial for cell cycle progression, development and differentiation in many cell types. However, little is known about the role of Cdh1 in hematopoiesis. Here we analyzed Cdh1 expression and function in malignant hematopoiesis. We found a significant decrease of Cdh1 in primary acute myeloid leukemia (AML) blasts compared to normal CD34+ cells. Thus, according to its important role in connecting cell cycle exit and differentiation, decreased expression of Cdh1 may be a mechanism contributing to the differentiation block in leukemogenesis. Indeed, knockdown (kd) of Cdh1 in HL-60 cell line (AML with maturation, FAB M2) led to less differentiated cells and a delay in PMA-induced differentiation. Acute promyelocytic leukemia (APL, FAB M3) is an AML subtype which is highly vulnerable to differentiation therapy with all-trans retinoic acid (ATRA). Accordingly, we found that APL is resistant to a Cdh1-kd mediated differentiation block. However, further depletion of Cdh1 in APL significantly reduced viability of leukemia cells upon ATRA-induced differentiation. Thus, low Cdh1 expression may be important in AML biology by contributing to the differentiation block and response to therapy depending on differences in the microenvironment and the additional genetic background.

The E2F1 transcription factor enhances apoptosis by DNA damage in tumors lacking p53. To elucidate the mechanism of a potential cooperation between E2F1 and chemotherapy, whole-genome microarrays of chemoresistant tumor cell lines were performed focusing on the identification of cooperation response genes (CRG). This gene class is defined by a synergistic expression response upon endogenous E2F1 activation and drug treatment. Cluster analysis revealed an expression pattern of CRGs similar to E2F1 mono-therapy, suggesting that chemotherapeutics enhance E2F1-dependent gene expression at the transcriptional level. Using this approach as a tool to explore E2F1-driven gene expression in response to anticancer drugs, we identified novel apoptosis genes such as the tumor suppressor TIEG1/KLF10 as direct E2F1 targets. We show that TIEG1/KLF10 is transcriptionally activated by E2F1 and crucial for E2F1-mediated chemosensitization of cancer cells. Our results provide a broader picture of E2F1-regulated genes in conjunction with cytotoxic treatment that allows the design of more rational therapeutics.