A genetic screen to identify latent transforming growth factor beta activators

Modeling autosomal recessive cutis laxa type 1C in mice reveals distinct functions for Ltbp-4 isoforms

Recent studies have revealed an important role for LTBP-4 in elastogenesis. Its mutational inactivation in humans causes autosomal recessive cutis laxa type 1C (ARCL1C), which is a severe disorder caused by defects of the elastic fiber network. Although the human gene involved in ARCL1C has been discovered based on similar elastic fiber abnormalities exhibited by mice lacking the short Ltbp-4 isoform (Ltbp4S(-/-)), the murine phenotype does not replicate ARCL1C. We therefore inactivated both Ltbp-4 isoforms in the mouse germline to model ARCL1C. Comparative analysis of Ltbp4S(-/-) and Ltbp4-null (Ltbp4(-/-)) mice identified Ltbp-4L as an important factor for elastogenesis and postnatal survival, and showed that it has distinct tissue expression patterns and specific molecular functions. We identified fibulin-4 as a previously unknown interaction partner of both Ltbp-4 isoforms and demonstrated that at least Ltbp-4L expression is essential for incorporation of fibulin-4 into the extracellular matrix (ECM). Overall, our results contribute to the current understanding of elastogenesis and provide an animal model of ARCL1C.

Overexpression of transforming growth factor β1 in malignant prostate cells is partly caused by a runaway of TGF-β1 auto-induction mediated through a defective recruitment of protein phosphatase 2A by TGF-β type I receptor

OBJECTIVES

To elucidate the mechanism of transforming growth factor (TGF)-β1 overexpression in prostate cancer cells.

METHODS

Malignant (PC3, DU145) and benign (RWPE1, BPH1) prostate epithelial cells were used. Phosphatase activity was measured using a commercial kit. Recruitment of the regulatory subunit, Bα, of protein phosphatase 2A (PP2A-Bα) by TGF-β type I receptor (TβRI) was monitored by coimmunoprecipitation. Blockade of TGF-β1 signaling in cells was accomplished either by using TGF-β-neutralizing monoclonal antibody or by transduction of a dominant negative TGF-β type II receptor retroviral vector.

RESULTS

Basal levels of TGF-β1 in malignant cells were significantly higher than those in benign cells. Blockade of TGF-β signaling resulted in a significant decrease in TGF-β1 expression in malignant cells, but not in benign cells. Upon TGF-β1 treatment (10 ng/mL), TGF-β1 expression was increased in malignant cells, but not in benign cells. This differential TGF-β1 auto-induction between benign and malignant cells correlated with differential activation of extracellular signal-regulated kinase (ERK). Following TGF-β1 treatment, the activity of serine/threonine phosphatase and recruitment of PP2A-Bα by TβRI increased in benign cells, but not in malignant cells. Inhibition of PP2A in benign cells resulted in an increase in ERK activation and in TGF-β1 auto-induction after TGF-β1 (10 ng/mL) treatment.

CONCLUSIONS

These results suggest that TGF-β1 overexpression in malignant cells is caused, at least in part, by a runaway of TGF-β1 auto-induction through ERK activation because of a defective recruitment of PP2A-Bα by TβRI.

Stem cells, microenvironment mechanics, and growth factor activation

Physicochemical features of a cell's microenvironment can exert important effects on cell behavior and include the effects of matrix elasticity on cell differentiation processes, but molecular mechanisms are largely mysterious. Here we highlight recent reports of a mechanical dependence to growth factor activation, with a particular focus on release of TGFbeta (Transforming Growth Factor beta) from its large latent complex via forced unfolding. We discuss these processes and pathways in the contexts of matrix adhesion and fluid shearing as they might relate to stem cell differentiation and other mechanisms in development, disease, and repair.

Transforming growth factor-beta1 to the bone

TGF-beta1 is a ubiquitous growth factor that is implicated in the control of proliferation, migration, differentiation, and survival of many different cell types. It influences such diverse processes as embryogenesis, angiogenesis, inflammation, and wound healing. In skeletal tissue, TGF-beta1 plays a major role in development and maintenance, affecting both cartilage and bone metabolism, the latter being the subject of this review. Because it affects both cells of the osteoblast and osteoclast lineage, TGF-beta1 is one of the most important factors in the bone environment, helping to retain the balance between the dynamic processes of bone resorption and bone formation. Many seemingly contradictory reports have been published on the exact functioning of TGF-beta1 in the bone milieu. This review provides an overall picture of the bone-specific actions of TGF-beta1 and reconciles experimental discrepancies that have been reported for this multifunctional cytokine.