Recent studies from several laboratories have shown perturbations of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] metabolism in hypertension. While these perturbations may exert their effect on blood pressure via their actions on calcium metabolism, it is possible that this vitamin D metabolite may have direct effects on vascular smooth muscle cell (VSMC) physiology. To examine this, we studied the effect of 1,25(OH)2D3 on VSMC growth and found that this substance suppressed VSMC [3H]thymidine uptake; furthermore, this vitamin D metabolite also suppressed the stimulatory effect of epidermal growth factor (EGF) on VSMC proliferation. The concomitant presence of this substance appeared to be required for its action on VSMC growth since cells pretreated with the vitamin D metabolite for up to 72 hours and then washed of the substance grew normally and responded to EGF. Studies were also done to determine if 1,25(OH)2D3 had any effect on the function of EGF receptors on VSMC. Experiments using Iodine-125-labeled EGF showed no differences in the binding of this ligand to VSMC, either untreated or treated with 1,25(OH)2D3, which indicates the effect of the vitamin D metabolite on VSMC growth (when exposed to EGF) was not mediated by an alteration of EGF receptor function. The results of these studies have implications for the pathogenesis of vascular diseases such as hypertension and atherosclerosis.

Differentiation of smooth muscle cells (SMCs) is critical for proper vasculogenesis and angiogenesis. However, the molecular mechanisms controlling SMC differentiation are not completely understood. During embryogenesis, the transcription factor mesenchyme homeobox 1 (Meox1) is expressed in the early developing somite, which is one of the origins of SMCs. In the present study, we identified Meox1 as a positive regulator of SMC differentiation. We found that transforming growth factor-β (TGF-β) induces Meox1 expression in the initial phase of SMC differentiation of pluripotent murine C3H10T1/2 cells. shRNA-mediated Meox1 knockdown suppressed TGF-β-induced expression of SMC early markers, whereas Meox1 overexpression increased expression of these markers. Mechanistically, Meox1 promoted SMAD family member 3 (Smad3) nuclear retention during the early stage of TGF-β stimulation because Meox1 inhibited protein phosphatase Mg/Mn-dependent 1A (PPM1A) and thereby prevented PPM1A-mediated Smad3 dephosphorylation. Meox1 appears to promote PPM1A degradation, leading to sustained Smad3 phosphorylation, thus allowing Smad3 to stimulate SMC gene transcription. , Meox1 knockdown in mouse embryos impaired SMC marker expression in the descending aorta of neonatal mice, indicating that Meox1 is essential for SMC differentiation during embryonic development. In summary, the transcriptional regulator Meox1 controls TGF-β-induced SMC differentiation from mesenchymal progenitor cells by preventing PPM1A-mediated Smad3 dephosphorylation, thereby supporting SMC gene expression.

Mesenchyme homeobox 1 (MEOX1) is expressed in the early developing somite and derivatives during embryogenesis. Somite has been shown to be one of the origins or progenitors for SMC. Therefore, in this study, we studied the role of MEOX1 in SMC differentiation. Smooth muscle cell (SMC) differentiation is an important process during vasculogenesis and angiogenesis. It is well known that alterations of VSMC phenotype play a role in the progression of several cardiovascular disorders including atherosclerosis, hypertension, and restenosis. However, the molecular mechanisms controlling the differentiation and development of VSMC are largely unknown. We found that transforming growth factor‐β (TGF‐β) induces MEOX1 expression in the initial phase of SMC differentiation of pluripotent mesenchymal C3H10T1/2 cells. MEOX1 expression appears to be mediated by both PI3 kinase and Smad3 signaling pathways because blockade of these two pathways by specific inhibitors leads to a decreased expression of MEOX1 and SMC markers. Importantly, knockdown of MEOX1 by specific shRNA suppresses TGF‐β‐induced expression of SMC early markers including SM22α and calponin. MEOX1 overexpression, on the other hand, increases SMC marker expression. These results indicate that MEOX1 is a novel regulator for TGF‐β‐induced SMC differentiation.

Smooth muscle cell (SMC) differentiation is an important process during vasculogenesis and angiogenesis. It is well known that alterations of VSMC phenotype play a role in the progression of several cardiovascular disorders including atherosclerosis, hypertension, and restenosis. However, the molecular mechanisms controlling the differentiation and development of VSMC are largely unknown. Mesenchyme homeobox 1 (MEOX1) is expressed in the early developing somite and derivatives during embryogenesis. Somite has been shown to be one of the origins or progenitors for SMC. Therefore, in this study, we studied the role of MEOX1 in SMC differentiation. We found that transforming growth factor-β (TGF-β) induces MEOX1 expression in the initial phase of SMC differentiation of pluripotent mesenchymal C3H10T1/2 cells. MEOX1 expression appears to be mediated by both PI3 kinase and Smad3 signaling pathways because blockade of these two pathways by specific inhibitors leads to a decreased expression of MEOX1 and SMC markers. Importantly, knockdown of MEOX1 by specific shRNA suppresses TGF-β-induced expression of SMC early markers including SM22α and calponin. MEOX1 overexpression, on the other hand, increases SMC marker expression. These results indicate that MEOX1 is a novel regulator for TGF-β-induced SMC differentiation.