Signal Transduction in Malignant Cells – Transformation, Activation and Differentiation

Abstract: All aspects of cell life are regulated by signal transduction mechanisms. This thesis describes the regulatory roles of a few key signal transduction molecules involved in three major biological responses. The studied pathways include platelet derived growth factor (PDGF)-BB induced transformation of murine fibroblasts, interferon (IFN)-? stimulated monocyte activation and all-trans retinoic acid (ATRA) induced myeloid differentiation.We found that intact phosphoinositide 3OH-kinase (PI3K) activity is essential in the signaling pathway that leads to the morphological alterations and migration pattern characteristic of PDGF-BB transformed NIH/sis and NIH/COL1A1 fibroblasts. Furthermore, our data indicated that the small Rho-GTPase, Rac1 is the predominant mediator of these signals downstream of PI3K.The study of the IFN-? induced activation of monocytic U-937 cells showed that upregulation of the high affinity receptor for IgG (Fc?RI) is dependent on the coordination of several regulatory events: the PKR-mediated serine 727 phosphorylation of Stat1, the expression of the hematopoietic lineage specific transcription factor PU.I, and the activation of the NF?B pathway.ATRA-induced differentiation and cell cycle arrest are impaired in U-937 sublines expressing phosphorylation deficient Stat1 (Stat1Y701F and Stat1S727A). The findings in paper III indicated that the expression pattern of the myeloid specific transcription factors Stat2, ICSBP and c/EBP? was altered in the sublines and that intact Stat1 activation is critical for maintaining the balance of the transcriptional network during ATRA induced terminal differentiation.Finally, ATRA-induced differentiation and growth arrest were blocked by treatment with the IKK?/? inhibitor BMS345541 or by ectopic expression of the NF?B super repressor I?B? (S32A/S36A). The fact that I?B(AA) sublines differentiated normally in response to vitamin D3, showed that NF?B inhibition specifically affected ATRA induced responses. Notably we suggest that the activity of the NF?B pathway may interfere with the differentiation process via a direct effect on the RAR/RXR mediated transcription.