The Smad5 Gene Is Involved in the Intracellular Signaling Pathways That Mediate the Inhibitory Effects of Transforming Growth Factor-β on Human Hematopoiesis

Autocrine transforming growth factor-beta regulation of hematopoiesis: many outcomes that depend on the context

Transforming growth factor-beta (TGF-beta) is a pleiotropic regulator of all stages of hematopoieis. The three mammalian isoforms (TGF-beta1, 2 and 3) have distinct but overlapping effects on hematopoiesis. Depending on the differentiation stage of the target cell, the local environment and the concentration and isoform of TGF-beta, in vivo or in vitro, TGF-beta can be pro- or antiproliferative, pro- or antiapoptotic, pro- or antidifferentiative and can inhibit or increase terminally differentiated cell function. TGF-beta is a major regulator of stem cell quiescence, at least in vitro. TGF-beta can act directly or indirectly through effects on the bone marrow microenvironment. In addition, paracrine and autocrine actions of TGF-beta have overlapping but distinct regulatory effects on hematopoietic stem/progenitor cells. Since TGF-beta can act in numerous steps in the hematopoietic cascade, loss of function mutations in hematopoeitic stem cells (HSC) have different effects on hematopoiesis than transient blockade of autocrine TGF-beta1. Transient neutralization of autocrine TGF-beta in HSC has therapeutic potential. In myeloid and erythroid leukemic cells, autocrine TGF-beta1 and/or its Smad signals controls the ability of these cells to respond to various differentiation inducers, suggesting that this pathway plays a role in determining the cell fate of leukemic cells.

The role of Smad signaling in hematopoiesis

The TGF-beta family of ligands, including TGF-beta, bone morphogenetic protein (BMP) and activin, signal through Smad pathways to regulate the fate of hematopoietic progenitor and stem cells during development and postnatally. BMP regulates hematopoietic stem cell (HSC) specification during development, while TGF-beta1, 2 and 3 are not essential for the generation of HSCs. BMP4 can increase proliferation of human hematopoietic progenitors, while TGF-beta acts as a negative regulator of hematopoietic progenitor and stem cells in vitro. In contrast, TGF-beta signaling deficiency in vivo does not affect proliferation of HSCs and does not affect lineage choice either. Therefore, the outcome of Smad signaling is very context dependent in hematopoiesis and regulation of hematopoietic stem and progenitor cells is more complicated in the bone marrow microenvironment in vivo than is seen in liquid cultures ex vivo. Smad signaling regulates hematopoiesis by crosstalk with other regulatory signals and future research will define in more detail how the various pathways interact and how the knowledge obtained can be used to develop advanced cell therapies.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.