Positive and negative regulators of transforming growth factor beta 1/bone morphogenetic protein pathways are constitutively expressed in mesangial cells

Background

Phenotypic alterations of mesangial cells (MC) may compromise intraglomerular hemodynamics, particularly during hypoperfusional states. Transforming growth factor beta 1 (TGF-β1) is a key player in glomerular normal and pathological functioning. It has been recently demonstrated that Gremlin is a downstream target in the TGF-β1 pathway in MC, and facilitates cell proliferation by directly antagonizing bone morphogenetic proteins (BMPs). Conversely, BMPs, which also belong to the TGF-β1 superfamily, present an inhibitory effect on MC multiplication. During early development, the antiproliferative activity of BMPs depends on the upregulation of the inhibitor of DNA binding genes (ID), which works as the endogenous dominant negative of basic HLH, transcription factors involved in cell differentiation, growth and death. Thus, BMP antagonists and ID appear to have opposite effects in pathways activated by members of the TGF-β1 superfamily.

Aim

This study begins to characterize the expression patterns of the aforementioned genes in cultured MC.

Methods

Primary MC were obtained from adult male Wistar rats. Cells were grown in 20% fetal bovine serum until confluence and then kept in serum-free medium for 24 hours. Total RNA was extracted from quiescent cells and cDNA synthesized using oligo-(d)T primers. Analysis of mRNA gene expression was performed through quantitative realtime polymerase chain reaction.

Results

Initially we confirmed the constitutive expression of Gremlin in rat MC. In addition, two other BMP antagonists, the headinducing factor Cerberus and the tumor suppressor Dan, were also found at very similar expression levels. Quantitative realtime polymerase chain reaction analysis also demonstrated for the first time the presence of ID in quiescent rat MC. In fact, all four members of this family of genes (ID1-ID4) were expressed at relatively high levels in quiescent MC.

Conclusions

Our study demonstrates the expression of proproliferative and antiproliferative genes that play a significant role in TGF-β1/BMP-activated pathways in MC. The simultaneous, constitutive expression of these genes in adult, nontransformed rat MC suggests the presence of a novel autocrine loop, which may modulate MC proliferation. Additional studies are underway to characterize the functional role of BMP antagonists and IDcultured rat MC. The elucidation of functional interaction between BMP antagonists and ID may enhance our knowledge about the molecular circuitry required for MC proliferation.