Growth factor

Overview
The term growth factor refers to a naturally occurring protein capable of stimulating cellular proliferation and cellular differentiation. Growth factors are important for regulating a variety of cellular processes.

Growth factors typically act as signaling molecules between cells. Examples are cytokines and hormones that bind to specific receptors on the surface of their target cells.

They often promote cell differentiation and maturation, which varies between growth factors. For example, bone morphogenic proteins stimulate bone cell differentiation, while vascular endothelial growth factors stimulate blood vessel differentiation.

Growth factors versus cytokines
Growth factor is sometimes used interchangeably among scientists with the term cytokine. Historically, cytokines were associated with hematopoietic (blood forming) cells and immune system cells (e.g., lymphocytes and tissue cells from spleen, thymus, and lymph nodes). For the circulatory system and bone marrow in which cells can occur in a liquid suspension and not bound up in solid tissue, it makes sense for them to communicate by soluble, circulating protein molecules. However, as different lines of research converged, it became clear that some of the same signaling proteins the hematopoietic and immune systems used were also being used by all sorts of other cells and tissues, during development and in the mature organism.

While growth factor implies a positive effect on cell division, cytokine is a neutral term with respect to whether a molecule affects proliferation. In this sense, some cytokines can be growth factors, such as G-CSF and GM-CSF. However, some cytokines have an inhibitory effect on cell growth or proliferation. Yet others, such as Fas ligand are used as "death" signals; they cause target cells to undergo programmed cell death or apoptosis.

Example of growth factors
Individual growth factor proteins tend to occur as members of larger families of structurally and evolutionarily related proteins. There are dozens and dozens of growth factor families such as TGF-beta (transforming growth factor-beta), BMP (bone morphogenic protein), neurotrophins (NGF, BDNF, and NT3), fibroblast growth factor (FGF), and so on.

Several well known growth factors are:
 * Transforming growth factor beta (TGF-β)
 * Granulocyte-colony stimulating factor (G-CSF)
 * Granulocyte-macrophage colony stimulating factor (GM-CSF)
 * Nerve growth factor (NGF)
 * Neurotrophins
 * Platelet-derived growth factor (PDGF)
 * Erythropoietin (EPO)
 * Thrombopoietin (TPO)
 * Myostatin (GDF-8)
 * Growth differentiation factor-9 (GDF9)
 * Basic fibroblast growth factor (bFGF or FGF2)
 * Epidermal growth factor (EGF)
 * Hepatocyte growth factor (HGF]

Uses in medicine
For the last two decades, growth factors have been increasingly used in the treatment of hematologic and oncologic diseases like:


 * neutropenia
 * myelodysplastic syndrome (MDS)
 * leukemias
 * aplastic anaemia
 * bone marrow transplantation