Coping mechanism limits efficiency
Senescence, the ‘coping mechanism’ that can kick in when a cell is subjected to a biochemical insult, may also act as a barrier to the efficient reprogramming of normal adult cells to become stem cells, new research from the CSC’s Cell Proliferation group and five other independent teams indicates.
Induced pluripotent stem (iPS) cells have far-reaching implications for research and regenerative medicine, but the rate at which normal cells can be successfully induced to become pluripotent is very low – between 0.1%–1% – suggesting the existence of barriers limiting induction efficiency.
In recent years, a growing body of work carried out by teams worldwide has shown that iPS cells can be made by inducing adult somatic cells, including fibroblasts and keratinocytes, with four transcription factors, termed Oct4, Sox2, Klf4 and c-Myc. Work carried out by the Cell Proliferation group and by teams at other institutes has indicated that the same transcription factors can activate genes and pathways that trigger senescence. This state is an irreversible growth arrest that normally limits the ability of cells to proliferate indefinitely, but it can also be triggered by ‘insults’, such as those sent by cancerous cells (oncogene-induced senescence – ‘OIS’). OIS has been shown to impede the growth of tumours, especially in the early stages.
This new research shows that senescence may also be one of the barriers that limit the efficient production of iPS cells. First Author Ana Banito and coworkers in the group, led by Jesus Gil, found that each transcription factor individually diminished cell growth, which suggested a complex scenario when all four factors act in concert. As identified for OIS, remodelling of a gene region called INK4a/ARF is a crucial event in the onset of senescence induced during reprogramming (termed reprogramming-induced senescence – RIS). The study also shows that the p53/p21CIP1 pathway, which is also critical in OIS, is engaged at different levels in response to the expression of the four reprogramming transcription factors. A major cause for concern is that pathways and mechanisms controlling tumour progression overlap with those controlling stem cell pluripotency. Indeed, the ability of the iPS cells to generate a particular tumour called a teratoma is currently used as an indication of pluripotency.
Improving the rate at which stem cells can be produced safely is key to the advancement of stem cell technology. While the inhibition of senescence has been shown to improve the efficiency of iPS cell production, the derivation of cells lacking p53 or p16INK4a – which are also important tumour suppressors – would introduce an unacceptable risk.
In order to circumvent this problem, the paper suggests, reversible compounds could be introduced into cells during reprogramming in order to transiently inhibit senescence. Defining the precise timing that would be required is key to this approach being successful. A number of groups worldwide are now engaged in screening for potential compounds to improve the efficiency of iPS generation.
Banito, A., Rashid, S. T., Acosta, J. C., Li, S., Pereira, C. F., Geti, I., Pinho, S., Silva, J. C., Azuara, V., Walsh, M., Vallier, L., Gil, J., August 2009. Senescence impairs successful reprogramming to pluripotent stem cells. Genes & Development. In press. doi:10.1101/gad.1811609