Protein dimerization between Lmo2 (Rbtn2) and Tal1 alters thymocyte development and potentiates T cell tumorigenesis in transgenic mice.

The LMO2 and TAL1 genes were first identified via chromosomal translocations and later found to encode proteins that interact during normal erythroid development. Some T cell leukaemia patients have chromosomal abnormalities involving both genes, implying that LMO2 and TAL1 act synergistically to promote tumorigenesis after their inappropriate co-expression. To test this hypothesis, transgenic mice were made which co-express Lmo2 and Tal1 genes in T cells. Dimers of Lmo2 and Tal1 proteins were formed in thymocytes of double but not single transgenic mice. Furthermore, thymuses of double transgenic mice were almost completely populated by immature T cells from birth, and these mice develop T cell tumours approximately 3 months earlier than those with only the Lmo2 transgene. Thus interaction between these two proteins can alter T cell development and potentiate tumorigenesis. The data also provide formal proof that TAL1 is an oncogene, apparently acting as a tumour promoter in this system.     

Activin A/erythroid differentiation factor induces thromboxane A2 synthetic activity in murine erythroleukemia cells

Activin A, a protein homologous to transforming growth factor beta, was shown to induce hemoglobin synthesis in murine erythroleukemia (MEL) cells and was also termed erythroid differentiation factor (EDF) (Eto, Y., Tsuji, T., Takezawa, M., Takano, S., Yokogawa, Y., and Shibai, H. (1987) Biochem. Biophys. Res. Commun. 142, 1095-1103). We found that activin A/EDF also induced thromboxane (TX) A2 synthetic activity in these cells. Synthesis of TXA2 from arachidonic acid is catalyzed by cyclooxygenase and TX synthase. Activin A/EDF induced the latter TX synthase activity, whereas the cyclooxygenase activity was constitutively expressed. The induction of this enzyme activity was inhibited by cycloheximide, suggesting that activin A/EDF induced de novo protein synthesis of TX synthase. Furthermore, we studied the relationship between the induction of TXA2 synthetic activity and erythroid differentiation in MEL cells, since the former is not an erythroid phenotype. We found 1) that the two responses to activin A/EDF were distinctly affected by the initial cell density; 2) that the dose-response curves for activin A/EDF were similar (ED50 = approximately 100 pM), whereas the time course of induction of TXA2 synthetic activity was much faster; and 3) that other erythroid differentiation inducers of MEL cells, namely dimethyl sulfoxide and hexamethylene bisacetamide, had little or no effect on TXA2 synthesis. These results indicate that activin A/EDF induces TXA2 synthetic activity independently of erythroid differentiation.