Liposarcomas - proliferation, senescence and the role of DDIT3

Sammanfattning: Lipomatous tumors comprise benign and malignant forms called lipomas and liposarcomas. Myxoid/round cell liposarcoma (MLS/RCLS) is the second most common liposarcoma and is characterized by the fusion oncogenes FUS-DDIT3 or EWSR1-DDIT3. To understand the morphology of MLS we investigated the role of the FUS-DDIT3 fusion in the development of MLS/RCLS in FUS-DDIT3- and DDIT3-transfected human HT1080 sarcoma cells. Cells expressing FUS-DDIT3 and DDIT3 grew as liposarcomas in immune-deficient mice. Microarray-based comparison of HT1080, the transfected cells, and an MLS/RCLS-derived cell line showed that the FUS-DDIT3- and DDIT3-transfected variants shifted toward an MLS/RCLS-like expression pattern. DDIT3-transfected cells responded in vitro to adipogenic factors by accumulation of fat and transformation to a lipoblast-like morphology. In conclusion, the fusion gene and normal DDIT3 induce a liposarcoma phenotype when expressed in a primitive sarcoma cell line. MLS/RCLS may develop from cell types other than preadipocytes. In addition, development of lipoblasts and the typical MLS/RCLS capillary network could be an effect of the DDIT3 transcription factor partner of the fusion oncogene. Further immunohistochemical investigation of the expression of the DDIT3 protein showed that major cell subpopulations of well differentiated tumors and MLS/RCLS tumors were found to express DDIT3 or the derived fusion protein. Our results suggest a dual, promoting and limiting, role for DDIT3 in formation of lipoblasts and liposarcoma morphology. Most liposarcoma types are characterized by genomic instability caused by impaired TP53 function. Further analysis of TP53 in MLS/RCLS with mass spectrometry, immunoblotting and immunohistochemistry show that a normal TP53 protein is produced in three of four MLS cell lines. This shows that the TP53 system is functional in the majority of MLS cases. MLS/RCLS tumors express proteins involved in cell senescence. In a study of 17 MLS/RCLS cases, large subpopulations of tumor cells expressed the RBL2 pocket protein together with senescence-associated heterochromatin binding protein 1γ and IL8 receptor β. The expression pattern suggests that MLS/RCLS tumors contain large subpopulations of senescent cells compatible with the slow growth of this tumor type.