Our current grant, NNX12AB88G, is a transfer grant from University of Colorado Denver to Duke University. This is a result of the PI’s moving from the University of Colorado to Duke. The major goals of this project are to understand the roles of free radicals and programmed cell death in space radiation induced mutagenesis and carcinogenesis. Towards these goals, we have made major progress in the funding period. Major achievements include:
A). The development of a non-invasive bioluminescence-based imaging method to monitor gamma H2AX foci, a major indicator of DNA double strand breaks (DSBs) (Li et al., Cancer Research, 2011, 71:4130-7). Using this reporter, we showed that space radiation induced DSBs came in two waves. The first wave occurs within minutes and lasted for hours while the second wave occurs after 24 hrs and lasted for more than a week. The second wave of DNA damage has strong implications for space radiation induced DNA damage and carcinogenesis. This type of damage could be responsible for persistent genetic instability often observed after radiation exposure.
B). The discovery of the “Phoenix Rising” pathway for wound healing and tissue regeneration. We discovered that dying cells in damaged tissues play a key role in mediating wound healing and tissue regeneration (Li et al., Science Signaling, 2010, 3:110: ra3) Surprisingly, caspase 3 and caspase 7, which are usually recognized as executioners of damaged or unwanted cells, play key facilitative roles regulating growth-promoting signals from dying cells. The elucidation of the counter-intuitive roles of apoptotic cells and apoptotic caspases significantly advanced our understanding of tissue homeostasis mechanisms in metazoan organisms.
C). We also found that the same “Phoenix Rising” pathway was hijacked by tumors during radiotherapy. Dying cells in tumors exposed to radiation release potent caspase 3-controlled growth signals that promote the growth of surviving tumor cells, which fuels the repopulation of damaged tumor. Most importantly, consistent with these findings, higher levels of activated caspase 3 in tumor samples from human patients correlated with worse prognosis. These results are counter-intuitive and significantly changed the way we view cell death during tumor radiotherapy.
D). Another discovery that we made concerns with surprising roles of caspases 3&8 in epigenetic reprogramming. We show that during induction of induced pluripotent stem (iPS) cells from human and mouse fibroblasts, caspases 3 and 8 are clearly activated. However, contrary to conventional wisdom, we found that blocking caspase activation did not increase the efficiency of iPSC induction. Instead, it significantly attenuated or completely blocked iPSC induction (Li et al., Cell Stem Cell, 2010, 7:508-20). These findings suggest that capases played a facilitative role for epigenetic reprogramming, a role that has not been suggested before. Our paper was the cover story for Cell Stem Cell. This finding may have significant implications for space radiation and other forms of carcinogenesis.
E). Still another significant finding in the funding period of this project involves the successful reprogramming of primary human fibroblasts into dopaminergic neurons (Liu et al,, Cell Research, 22:321-332). We showed that a combination of 5 transcription factors (Mash1, Ngn2, Sox2, Nurr1, and Pitx3) can directly reprogram human primary fibroblasts into doparminergic neurons. These cells should stimulate research in providing a promising autologous source for cell replacement therapy for Parkinson’s disease.
In addition to the above accomplishments, we have now focused our attention on the roles of caspases on space radiation induced carcinogenesis and mutagenesis. We have obtained preliminary evidence that caspase 3 activation plays a key role in mediating space radiation induced carcinogenesis and mutagenesis. In the remaining time of our grant (under request for no-cost extension), we will focus our attention on obtaining sufficient data to publish this very important new finding.