Roles of TGF-beta family signaling in stem cell renewal and differentiation.

Transforming growth factor (TGF)-betas and their family members, including bone morphogenetic proteins (BMPs), Nodal and activins, have been implicated in the development and maintenance of various organs, in which stem cells play important roles. Stem cells are characterized by their ability to self-renew and to generate differentiated cells of a particular tissue, and are classified into embryonic and somatic stem cells. Embryonic stem (ES) cells self-renew indefinitely and contribute to derivatives of all three primary germ layers. In contrast, somatic stem cells, which can be identified in various adult organs, exhibit limited abilities for self-renewal and differentiation in most cases. The multi-lineage differentiation capacity of ES cells and somatic stem cells has opened possibilities for cell replacement therapies for genetic, malignant and degenerative diseases. In order to utilize stem cells for therapeutic applications, it is essential to understand the extrinsic and intrinsic factors regulating self-renewal and differentiation of stem cells. More recently, induced pluripotent stem (iPS) cells have been generated from mouse and human fibroblasts that resemble ES cells via ectopic expression of four transcription factors. iPS cells may have an advantage in regenerative medicine, since they overcome the immunogenicity and ethical controversy of ES cells. Moreover, recent studies have highlighted the involvement of cancer stem cells during the formation and progression of various types of cancers, including leukemia, glioma, and breast cancer. Here, we illustrate the roles of TGF-beta family members in the maintenance and differentiation of ES cells, somatic stem cells, and cancer stem cells.