Acyclic Retinoid Inhibits Neointima Formation Through Retinoic Acid Receptor Beta-Induced Apoptosis

Perlecan Heparan Sulfate Is Required for the Inhibition of Smooth Muscle Cell Proliferation by All-trans-Retinoic Acid

Smooth muscle cell (SMC) proliferation is a key process in stabilization of atherosclerotic plaques, and during restenosis after interventions. A clearer understanding of SMC growth regulation is therefore needed to design specific anti-proliferative therapies. Retinoic acid has been shown to inhibit proliferation of SMCs both in vitro and in vivo and to affect the expression of extracellular matrix molecules. To explore the mechanisms behind the growth inhibitory activity of retinoic acid, we hypothesized that retinoids may induce the expression of perlecan, a large heparan sulfate proteoglycan with anti-proliferative properties. Perlecan expression and accumulation was induced in murine SMC cultures by all-trans-retinoic acid (AtRA). Moreover, the growth inhibitory effect of AtRA on wild-type cells was greatly diminished in SMCs from transgenic mice expressing heparan sulfate-deficient perlecan, indicating that the inhibition is perlecan heparan sulfate-dependent. In addition, AtRA influenced activation and phosphorylation of PTEN and Akt differently in wild-type and mutant SMCs, consistent with previous studies of perlecan-dependent SMC growth inhibition. We demonstrate that AtRA regulates perlecan expression in SMCs and that the inhibition of SMC proliferation by AtRA is, at least in part, secondary to an increased expression of perlecan and dependent upon its heparan sulfate-chains.

Acyclic retinoid induces differentiation and apoptosis of murine hepatic stem cells

Introduction

The therapeutic potential of acyclic retinoid (ACR), a synthetic retinoid, has been confirmed in experimental and clinical studies. Therapeutic targets include precancerous and cancer stem cells. As ACR is also involved in developmental processes, its effect on normal hepatic stem cells (HpSCs) should be investigated for understanding the underlying mechanisms. Here, we examined effects of the acyclic retinoid peretinoin on fresh isolated murine HpSCs.

Methods

We isolated c-kit⁻CD29⁺CD49f^+/lowCD45⁻Ter119⁻ cells from murine fetal livers using flow cytometry. To evaluate the effect of ACR, we traced clonal expansion and analyzed cell differentiation as well as apoptosis during the induction process by immunofluorescent staining and marker gene expression.

Results

ACR dose-dependently inhibited HpSCs expansion. Stem cell clonal expansion was markedly inhibited during the culture period. Moreover, ACR showed a significant promotion of HpSC differentiation and induction of cellular apoptosis. The expression of stem cell marker genes, Afp, Cd44, and Dlk, was downregulated, while that of mature hepatocyte genes, Alb and Tat, and apoptosis-related genes, Annexin V and Caspase-3, were upregulated. Flow cytometry showed that the proportion of Annexin V-positive cells increased after ACR incubation compared with the control. Data obtained by immunofluorescent staining for albumin and Caspase-3 corroborated the data on gene expression. Finally, we found that ACR directly regulates the expression of retinoic acid receptors and retinoid X receptors.

Conclusions

These findings indicate that ACR inhibits the clonal expansion of normal HpSCs in vitro and promotes the differentiation of immature cells by regulating receptors of retinoic acid.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0046-9) contains supplementary material, which is available to authorized users.

Acyclic retinoid inhibits angiogenesis by suppressing the MAPK pathway

Acyclic retinoid (ACR) is currently under clinical trial as an agent to suppress the recurrence of hepatocellular carcinoma (HCC) through its ability to induce apoptosis in premature HCC cells. ACR has an anticancer effect in vivo as well, although it shows weak apoptosis-inducing activity against mature HCC cells, suggesting the existence of an additional action mechanism. In this study, we investigated the antiangiogenic activity of ACR. ACR inhibited angiogenesis within chicken chorioallantoic membrane (CAM) in as similar a manner as all-trans retinoic acid (atRA). Although suppression of angiogenesis by atRA was partially rescued by the simultaneous addition of angiopoietin-1, suppression of angiogenesis by ACR was not rescued under the same condition at all. Conversely, although suppression of angiogenesis by ACR was partially inverted by the simultaneous addition of vascular endothelial growth factor (VEGF), suppression of angiogenesis by atRA was not affected under the same condition. These results suggested that mechanisms underlying the suppression of angiogenesis by ACR and atRA were different. ACR selectively inhibited the phosphorylation of VEGF receptor 2 (VEGFR2) and of extracellular signal-regulated kinase (ERK) without changing their protein expression levels, and inhibited endothelial cell growth, migration, and tube formation. The inhibition of the phosphorylation of ERK, endothelial growth, migration, tube formation, and angiogenesis by ACR was rescued by the overexpression of constitutively active mitogen-activated protein kinase (MAPK). Finally, ACR, but not atRA, inhibited HCC-induced angiogenesis in a xenografted CAM model. These results delineate the novel activity of ACR as an antiangiogenic through a strong inhibition of the VEGFR2 MAPK pathway.

Krüppel-like factor 4 inhibits proliferation by platelet-derived growth factor receptor beta-mediated, not by retinoic acid receptor alpha-mediated, phosphatidylinositol 3-kinase and ERK signaling in vascular smooth muscle cells

Proliferation inhibition of vascular smooth muscle cells (VSMCs) is governed by the activity of a transcription factor network. Krüppel-like factor 4 (Klf4), retinoic acid receptor (RAR alpha), and platelet-derived growth factor receptor (PDGFR) are expressed in VSMCs and are components of such a network. However, the relationship among them in the regulation of VSMC proliferation remains unknown. Here, we investigated the mechanisms whereby Klf4 mediates the growth inhibitory effects in VSMCs through RAR alpha and PDGFR beta. We demonstrated that Klf4 directly binds to the 5' regulatory region of RAR alpha, down-regulates RAR alpha expression, and specifically inhibits RAR alpha-mediated phosphatidylinositol 3-kinase (PI3K) and ERK signaling in cultured VSMCs induced by the synthetic retinoid Am80. Of particular interest, Klf4 inhibits RAR alpha and PDGFR beta expression while blocking PI3K and ERK signaling induced by Am80 and PDGF-BB, respectively. The anti-proliferative effects of Klf4 on neointimal formation depend largely on PDGFR-mediated PI3K signaling without involvement of the RAR alpha-activated signaling pathways. These findings provide a novel mechanism for signal suppression and growth inhibitory effects of Klf4 in VSMCs. Moreover, the results of this study suggest that Klf4 is one of the key mediators of retinoid actions in VSMCs.

Acyclic retinoid inhibits functional interaction of transcription factors Krüppel-like factor 5 and retinoic acid receptor-alpha

We show that transcription factor Krüppel-like factor 5 (KLF5), which is important in cardiovascular remodeling, interacts with retinoic acid receptor-alpha (RARalpha) to regulate downstream gene expression. Here, we investigated whether acyclic retinoid (ACR) regulates KLF5 and inhibits vascular remodeling. Co-immunoprecipitation and pull-down binding assay showed that ACR attenuates functional interaction of KLF5 and RARalpha. ACR affects KLF5 functions by regulating transactivation of platelet-derived growth factor A (PDGF-A) chain. ACR may be a new vascular therapy to target KLF5 in cardiovascular pathology.