This project was based on the hypothesis that understanding cellular plasticity could yield new insights into cancer and ageing. Our unifying hypothesis is that cellular plasticity lies at the basis of tissue regeneration (“adaptive cellular plasticity”), as well as at the origin of cancer (“maladaptive gain of cellular plasticity”) and ageing (“maladaptive loss of cellular plasticity”). We have taken advantage of our diverse background and integrate the above processes. A key experimental system will be our “reprogrammable mice” (with inducible expression of the four Yamanaka factors), which we regard as a tool to induce cellular plasticity in vivo. During the completion of this project, we have made relevant contributions to the fields of cellular reprogramming, cellular senescence, cancer, and ageing.
The project has been divided in three major objectives:
Objective #1 – Cellular plasticity and cancer: role of tumour suppressors in in vivo de-differentiation and reprogramming / impact of transient de-differentiation on tumour initiation / lineage tracing of Oct4 to determine whether a transient pluripotent-state occurs during cancer.
The main conclusion of this objective has been that the induction of cellular plasticity by the Yamanaka factors is not very tumorigenic. The expression of these plasticity factors together with oncogenes or mutagens did not have a dramatic impact on tumorigenesis. This indicates that cellular plasticity per se is not tumorigenic because it may drive cells into differentiated states that oppose tumorigenesis. This conclusion is of relevance for the use of transient expression of the Yamanaka factors in rejuvenation and regeneration.
Objective #2 – Cellular plasticity in tissue regeneration and ageing: impact of transient de- differentiation on tissue regeneration / contribution of the damage-induced microenvironment to tissue regeneration / impact of transient de-differentiation on ageing.
In contrast to cancer, the Yamanaka factors are able to induce a reversion of aging molecular markers in vivo. We have found that a single burst of transient and reversible expression of the Yamanaka factors is sufficient to reverse markers of cellular aging, including the transcriptome and the epigenome.
Another important conclusion of our work is the discovery that the Yamanaka factors need to operate in a tissue microenvironment modified by injury. In other words, the Yamanaka factors require cytokines associated to injury to trigger plasticity and rejuvenation. This is the case of interleukin-6, which we discovered as a critical factor for the induction of cellular plasticity by the Yamanaka factors.
Objective #3: New frontiers in cellular plasticity: chemical manipulation of cellular plasticity in vivo / new states of pluripotency / characterization of in vivo induced pluripotency and its unique properties. We anticipate that the completion of this project will yield new fundamental insights into cancer, regeneration and ageing.
We have identified markers of intermediate reprogramming in vivo that are applicable to several tissues. This will allow to unequivocally define for the first time those cells that have reached the state of partial reprogramming, which is the state when epigenomic rejuvenation reaches its maximum.
We have also found a novel mechanism to manipulate the strength of the enhancers that maintain cellular identity. This is based on small compound that inhibit a protein kinase known as MEDIATOR-kinase (or CDK8). The chemical inhibition of this kinase boosts the strength of transcriptional enhancers and therefore opposes cellular plasticity. Therefore, this tool may serve to oppose or to extinguish cellular plasticity. This may be important to prevent the emergence of cancer from plastic cells.
