Signals Controlling Cell Proliferation

The major interest of my laboratory is the control of proliferation in normal and cancer ells and the genes and gene-products whose interplay regulates proliferation and differentiation.

To understand how growth factors mediate cell proliferation and differentiation, we are studying the mechanism of action and the regulation of expression of fibroblast growth factors (FGF). FGF represents a large family of growth factors which signal through their interaction with tyrosine kinase receptors (FGFR) which also make-up a gene family. FGF signaling plays a major role in a variety of developmental processes, ranging from gastrulation to bone morphogenesis. Ectopic or excessive FGF expression can lead to oncogenesis. The main projects currently being carried out include:

1. The physiological role of FGFs, using transgenic mice or gene-ablation techniques. We have generated FGF1, FGF2 and FGF1/FGF2 knockout mice and show that these factors play no essential role in development, but affect the production of specific neurons, the development of hematopoietic precursor cells, and wound-healing. Expression of FGF4 during embryonic development is important for limb formation. We have produced mice in which specific DNA elements regulating FGF4 expression in vitro have been deleted, and demonstrated that these elements control FGF4 expression in myotomes and limb buds also in vivo. In collaboration with Dr. M. Mohamamdi (Dept. of Pharmacology) we are also conducting structural studies of the mechanism by which FGFs interacts with FGF receptors and with Heparin.
   
   
2. The regulation of skeletal development by FGF signaling. Skeletal development is controlled by a small number of signaling molecules that regulate the commitment, proliferation and differentiation of osteogenic cells. Unregulated FGF signaling, due to FGFR activating mutations, causes a variety of dominant bone morphogenetic disorders in humans, including several forms of dwarfism and craniosynostosis syndromes, showing that FGF signaling plays an important role in bone development. Excessive FGF signaling alters bone development by affecting the dynamics of growth and differentiation of chondrocytes and osteoblasts, the two major cell types involved in bone formation. We are studying the biological response of these two cell types to FGF and the key pathways involved in this process.
   
   
   1. In chondrocytes, we found that FGF signaling inhibits proliferation and increases apoptosis both in vitro and in vivo. These effects are cell type-specific and require the activation of STAT1, a signal transducing transcription factor which is not normally activated by FGF in other cell types. STAT1 deficiency corrects the chondrodysplastic phenotype of mice overexpressing FGF2. In addition, we have shown that FGF-induced growth arrest requires the activity of two Retinoblastoma (Rb) family members, p107 and p130, but not Rb itself. FGFs induce in chondrocytes a complex network of signaling and transcriptional events which ultimately result in growth arrest and induction of several aspects of chondrocytes hypertrophic differentiation. One of the earliest and critical events following FGF treatment is the very rapid dephosphorylation of p107, that is likely due to activation of a phosphatase. Furthermore, sustained activation of ERK 1/2 kinases and downregulation of AKT function also appear to contribute to the growth arrest induced by FGF. To identify the mechanisms which direct signal-transduction to growth-inhibitory pathways, we are investigating the precise role that these downstream effectors play in the response of chondrocytes to FGF. In particular, we are focusing on the mechanism of p107 dephosphorylation and the role of STAT1 and STAT3 in chondrocyte proliferation and differentiation.
      
      
   2. Immature osteoblasts respond to FGF with increased proliferation, while differentiating cells undergo apoptosis. Sustained FGF signaling inhibits differentiation. We have examined the program of gene expression in osteoblasts expressing activated FGFR mutants and detected a significant down-regulation of WNT target gene expression. Concomitantly, we have observed a dramatic induction of expression of Sox2, a transcription factor of the HMG domain family whose expression was generally thought to be confined to early embryonic development and the CNS. Sox2 is also induced by FGF treatment of normal osteoblasts and is clearly detectable in cranial osteoblasts in vivo. Wnt signaling is important to promote osteoblast function and high bone mass in humans and mice. We have found that Sox2 can interact with b-catenin to inhibit Wnt signaling, and that constitutive expression of Sox2 in osteoblasts inhibits differentiation and causes downregulation of several Wnt target genes. Thus the induction of Sox2 by FGF, and consequent inhibition of Wnt signaling, could be a major mechanism by which FGFs inhibit osteoblast differentiation and, when aberrantly expressed, cause craniofacial malformations. To better define the mechanisms by which FGF controls the proliferation and differentiation of osteoblasts, we are studying a mouse model of FGF induced craniosynostosis. We also plan to further define the role of SOX2 and the cross-talk between FGF and WNT signaling in regulating osteoblast differentiation.