Retinoids regulate TGFbeta signaling at the level of Smad2 phosphorylation and nuclear accumulation.

Indirect regulation of transforming growth factor (TGF)-beta signaling by retinoids occurs on a long-term timescale, secondary to transcriptional events. Studies by our group show loss of retinoid X receptor (RXR) alpha results in increased TGFbeta2 in the midgestational heart, which may play a role in the cardiac defects seen in this model [S.W. Kubalak, D.R. Hutson, K.K. Scott and R.A. Shannon, Elevated transforming growth factor beta2 enhances apoptosis and contributes to abnormal outflow tract and aortic sac development in retinoic X receptor alpha knockout embryos, Development 129 (2002) 733-746.]. Acute and direct interactions between retinoid and TGFbeta signaling, however, are not clearly understood. Treatment of dispersed hearts and NIH3T3 cells for 1 h with TGFbeta and retinoids (dual treatment) resulted in increased phosphorylated Smad2 and Smad3 when compared to treatment with TGFbeta alone. Of all dual treatments, those with the RXR agonist Bexarotene, resulted in the highest level of phosphorylated Smad2, a 7-fold increase over TGFbeta2 alone. Additionally, during dual treatment phosphorylation of Smad2 occurs via the TGFbeta type I receptor but not by increased activation of the receptor. As loss of RXRalpha results in increased levels of Smad2 phosphorylation in response to TGFbeta treatment and since nuclear accumulation of phosphorylated Smad2 is decreased during dual treatment, we propose that RXRalpha directly regulates the activities of Smad2. These data show retinoid signaling influences the TGFbeta pathway in an acute and direct manner that has been unappreciated until now.

The expanding role for retinoid signaling in heart development.

The importance of retinoid signaling during cardiac development has long been appreciated, but recently has become a rapidly expanding field of research. Experiments performed over 50 years ago showed that too much or too little maternal intake of vitamin A proved detrimental for embryos, resulting in a cadre of predictable cardiac developmental defects. Germline and conditional knockout mice have revealed which molecular players in the vitamin A signaling cascade are potentially responsible for regulating specific developmental events, and many of these molecules have been temporally and spatially characterized. It is evident that intact and controlled retinoid signaling is necessary for each stage of cardiac development to proceed normally, including cardiac lineage determination, heart tube formation, looping, epicardium formation, ventricular maturation, chamber and outflow tract septation, and coronary arteriogenesis. This review summarizes many of the significant milestones in this field and particular attention is given to recently uncovered cross-talk between retinoid signaling and other developmentally significant pathways. It is our hope that this review of the role of retinoid signaling during formation, remodeling, and maturation of the developing heart will serve as a tool for future discoveries.

Holding their own: the noncanonical roles of Smad proteins.

The identification of Smads as protein transcription factors in 1995 led to elucidation of the canonical transforming growth factor-beta (TGF-beta) signaling pathway. In the years that have followed, nuances of the pathway have been realized, and the once-simple scheme of ligand to receptor to activated transcription factor is now understood to be highly regulated at each step and riddled with crosstalk from other pathways. The Smads are also recognized as important players outside of canonical TGF-beta-dependent signaling and are responsible for regulating diverse cellular processes. New evidence suggests that Smad7 plays an integral role in maintaining cell-cell adhesion through direct regulation of beta-catenin. Receptor-activated Smads regulate the processing of a subset of microRNAs, particularly miR-21. The number of reports demonstrating the interactions of Smads with proteins outside of canonical TGF-beta signaling is increasing, although the functional relevance of these interactions is not known. Investigating these interactions will likely yield more evidence that Smads serve important and diverse purposes beyond their original reported function as signal transducers in the TGF-beta pathway.