The objective of our research group is to decipher the molecular mechanisms that regulate the generation and maintenance of stem cells and their differentiation into functioning mature cells. Among these mechanisms, we are particularly interested in those that play a part in tumour processes in one way or another.
Over the last few years we have developed two clearly differentiated project lines that are focused on two different types of tissue, haematopoietic and intestinal, and which are linked by the study of the Notch signalling pathway. This pathway works as the primary regulation mechanism of cell differentiation and tissue homeostasis and is activated in response to interactions with its ligands through cell-to-cell contact. In general, activation of the Notch pathway inhibits cell differentiation, inducing the expression of different differentiation inhibitors, such as Hes and HRT.
In addition to patient samples, we use different mouse models and cell lines in the laboratory to study not only how the Notch pathway functions in general but also its specific functions in the context of its interaction with the Wnt and NFκB pathways in normal stem cells, progenitor cells and tumour cells.
Doctor Anna Bigas’ group is currently working in two main projects that tackle the different aspects of the Notch pathways function in normal haematopoietic stem cells and in leukaemic stem cells. The work led by Dr. Lluís Espinosa is primarily focused on studying the function of NFκB as a regulator of Notch activity and vice versa in the tumour transformation of the intestine.
We are using different strains of mutant mice for the Notch pathway with the aim of studying the role of this signalling in generating and maintaining haematopoietic stem cells in the embryo. This is a unique model that allows us to decipher the mechanisms that regulate haematopoietic stem cells during the embryonic stage, which is when they are generated. Using these models, we have demonstrated that Notch and its ligand Jagged-1 are essential for the differentiation of haematopoietic stem cells in the embryonic aorta, independently from the arterial programme (another programme of differentiation in which Notch is required) (Robert-Moreno et al., EMBO 2008). We are currently researching which are the transcriptional targets through which Notch regulates embryonic haematopoiesis. To do this, we are using expression analysis using biochips and ChIP-on-chip technology.
Another of this laboratory’s objectives is to determine if the signalling through the Notch pathway plays any role in the maintenance of leukaemic stem cells (LSCs) and colorectal cancer stem cells. To do this, we are developing different models both in vitro and in vivo. In fact, our intention is to use the information generated from the work with normal stem cells to understand the possible mechanisms that regulate the generation and maintenance of this population of tumour cells. Data obtained in our laboratory and others has revealed the existence of multiple connections between the Notch and Wnt pathways (Espinosa et al., JBC, 2003). In addition, through expression analysis using biochips, we have discovered that many of the genes that are expressed in colorectal cancer are common targets of Notch and b-catenin, and that b-catenin is responsible for activating Notch in these cells regulating the levels of Jagged-1. The elimination of a single allele of Jagged-1 is sufficient to reduce the size of the tumours in an APCMin/+ population (Rodilla et al., PNAS, in press), demonstrating how both pathways cooperate in tumour progression.
One of the primary objectives of the group was to study whether Notch cooperates with other signalling pathways in different contexts. As a result of this research, we observe that different elements of NFκB were capable of activating the transcription of genes dependent on Notch, such as Hes-1, in inducing the cytoplasmic export of nuclear corepressors (Espinosa et al., Journal of Cell Science 2002; Espinosa et al., MBC. 2003). In addition, we observe that IkBa was present in the chromatin of these genes under conditions of transcriptional repression, while the presence of IKKa was correlated with its activation (Aguilera et al., PNAS, 2004). In the model of colorectal cancer, we have demonstrated that IKKa attenuates the transcriptional repression of Notch target genes, such as Hes-1 and Hes-5, as well as the cIAP and c-Fos genes. Likewise, we have observed that the cytoplasmic export of SMRT and N-CoR constitutes a common characteristic of human colorectal tumours (Fernandez-Majada et al., PNAS, 2007; Fernandez-Majada et al., Cell Cycle, 2007). We are currently studying the mechanism that regulates the presence of active nuclear IKKa in colorectal cancer cells. For the moment, we have identified a truncated form of IKKa that is located in the nucleus of tumour cells and which we are characterising.
We have previously identified 14-3-3 as a decisive mediator in nuclear exportation of p65-IkBa, which is produced during the repression phase that follows the activation of NFκB (Aguilera et al., Journal of Cell Science 2006). We are currently studying whether this mechanism affects breast cancer cells, which lack the 14-3-3 protein isoform, and if this is the case, what is the physiological importance of this modification in human breast cancer.