Mechanistic Studies On Tissue Transglutaminase And Its Roles In Cell Growth And Survival

Tissue transglutaminase (tTG) is a dual functional acyl transferase/GTPase that has important roles in many cellular processes, as well as in the development of various disease states. Here we show that tTG expression is frequently up-regulated in high-grade brain tumors or Glioblastoma (GBM), and that tTG is essential for the growth and survival of these highly aggressive cancer cells. We discovered a novel mechanism through which tTG mediates the transformed phenotypes of GBMs, which involves its ability to enhance the signaling activity of the cell surface receptor tyrosine kinase and proto-oncogene, the epidermal growth factor receptor (EGFR). In particular, tTG can associate with and functionally inactivate the E3 ubiquitin ligase c-Cbl, a negative regulator of EGFR, thus delaying its downregulation and extending its signaling half-life. Interestingly, the ability of tTG to influence c-Cbl function is independent of its crosslinking activity, but dependent on a specific conformation that tTG adopts. X-ray crystallographic studies showed that tTG can exist in a nucleotide-bound, crosslinking inactive closed conformation, or a nucleotide free, crosslinking competent, open conformation. Previous findings from my first study and by others showed that ectopically expressed forms of tTG that adopt a closed conformation in cells, promote cell growth and survival, whereas tTG mutants that adopt an open conformation trigger cell death. Thus, we set out to better understand the molecular mechanisms that regulate the transition of tTG between these two different conformational states. Two pairs of intramolecular hydrogen bonds formed between the C-terminal end of tTG and its catalytic core domain were identified and shown to play an important role in maintaining tTG in its closed conformation. Disrupting these interactions causes tTG to adopt a constitutively open configuration and trigger cell death. Collectively, these new insights into the regulation and function of tTG have not only led to its identification as an important player in brain tumor progression, but also raise the possibility that developing strategies that cause tTG to adopt an open conformation could be beneficial for the treatment of human cancers