The functional role of the EGFR/ErbB system for bone development and homeostasis

EGFR activity is primarily determined by the availability of seven peptide growth factors. These ligands have both overlapping and specific functions, conferring the system a high degree of robustness. ErbB2, in contrast, has no known ligand. Nonetheless, ErbB2 is a potent initiator of intracellular signaling pathways by forming heterodimers with the EGFR. Although the EGFR, its ligands, and the structurally related receptor ErbB2/neu have been reported to be expressed in skeletal cells and to affect postnatal somatic growth since many years, their functions in bone cells remain poorly defined. The purpose of the present project is to characterize further the role the EGFR and ERBB2 in bone biology and pathology by employing the following approaches: 1) Unpublished work from our laboratories revealed that osteoblast-specific overexpression of the EGFR ligand amphiregulin profoundly increased trabecular and cortical bone mass in transgenic mice. We propose to fully characterize the bone phenotype of this mouse line. 2) Knockout mice lacking EGFR or ERBB2 are not viable. To evaluate the relevance of these receptors for bone, we propose to specifically inactivate their genes in osteoblasts by employing already available EGFR and ErbB2 "floxed" mouse lines. 3) In a recently finished project, we showed that EGFR signaling is not required for the anabolic response to intermittent PTH on cancellous and endocortical bone surfaces, but that the EGFR pathway is essential for the PTH-driven periosteal expansion at the femoral shaft. We propose experiments to elucidate the mechanisms behind this sitespecific effect of the EGFR in long bones.

In this project, we have characterized further the role of the epidermal growth factor receptors EGFR and ErbB2 in bone biology. The epidermal growth factor receptor (EGFR) is essential for the growth of many tissues under physiological circumstances, but also for the growth of many tumor cells. The EGFR and the structurally related receptor ErbB2 are ex- pressed in the cell membrane of target cells, and mediate signaling of seven growth factors, the EGFR ligands. The receptor ErbB2 has no known ligand, but forms complexes with the EGFR in the cell membrane. It is known that the EGFR, its ligands, and ErbB2 are expressed in skeletal cells. However, their precise functions in bone cells remain poorly defined. Because the EGFR and its ligands are expressed in almost every tissue in the body, it is necessary to target overexpression or deficiency to a specific tissue, in this case bone. Here, we generated transgenic mice characterized by gain or loss of function of the EGFR system in bone-building cells, so-called osteoblasts.We found that overexpression of the EGFR ligand amphiregulin in bone-building cells leads to an increase in bone mass in fast-growing bone sites by slightly downregulating bone re- sorption, and, hence, altering bone balance in favor of bone formation. However, this amphiregulin-induced increase in bone mass was observed only in young mice, and did not persist until older ages. Conversely, deficiency of the EGFR, but not of ErbB2, in bone-building osteoblasts led to a mild decrease in bone mass in the spine. The EGFR system has also been implicated to play a role in mediating the effects of one of the few bone anabolic anti- osteoporosis therapies, namely the therapy with intermittent parathyroid hormone (PTH). Therefore, we tested the hypothesis that the known PTH-induced upregulation of the expression of amphiregulin in bone plays a role in the increase in bone formation caused by intermittent PTH therapy. However, we found that amphiregulin is dispensable for the bone anabolic effects of intermittent PTH.This project has enhanced our knowledge about the function of the EGFR system in bone. An improved understanding of the role of growth factors such as the EGFR system in bone biology may eventually lead to better prevention and therapy of bone diseases such as osteoporosis.