Increased Unbound Retinol-binding Protein 4 Concentration Induces Apoptosis through Receptor-mediated Signaling

Chao-Hung Chen,Tusty-Jiuan Hsieh,Kun-Der Lin,Hsing-Yi Lin,Mei-Yueh Lee,Wei-Wen Hung,Pi-Jung Hsiao,Shyi-Jang Shin

doi:10.1074/jbc.m111.301721

Chao-Hung Chen, Tusty-Jiuan Hsieh + Show 6 more

Open Access

https://doi.org/10.1074/jbc.m111.301721

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Abstract

The increase of apo-/holo-retinol-binding protein 4 (RBP4) concentrations has been found in subjects with renal dysfunction and even in diabetic patients with microalbuminuria. Holo-RBP4 is recognized to possess cytoprotective function. Therefore, we supposed that the relative increase in apo-RBP4 might induce cell damage. In this study, we investigated the signal transduction that activated apoptosis in response to the increase of apo-/holo-RBP4 concentration. We found that increase of apo-/holo-RBP4 concentration ratio delayed the displacement of RBP4 with "stimulated by retinoic acid 6" (STRA6), enhanced Janus kinase 2 (JAK2)/STAT5 cascade, up-regulated adenylate cyclase 6 (AC6), increased cAMP, enhanced JNK1/p38 cascade, suppressed CRBP-I/RARα (cellular retinol-binding protein/retinoic acid receptor α) expression, and led to apoptosis in HK-2 and human umbilical vein endothelial cells. Furthermore, STRA6, JAK2, STAT5, JNK1, or p38 siRNA and cAMP-PKA inhibitor reversed the repression of CRBP-I/RARα and apoptosis in apo-RBP4 stimulation. In conclusion, this study indicates that the increase of apo-/holo-RBP4 concentration may influence STRA6 signaling, finally causing apoptosis.

Highlights

An increase of apo- to holo-retinol-binding protein 4 (RBP4) concentration in plasma is observed in subjects with renal dysfunction and is supposed to induce cell damage
We found that increase of apo-/holo-RBP4 concentration ratio delayed the displacement of RBP4 with “stimulated by retinoic acid 6” (STRA6), enhanced Janus kinase 2 (JAK2)/STAT5 cascade, up-regulated adenylate cyclase 6 (AC6), increased cAMP, enhanced JNK1/p38 cascade, suppressed cellular retinol-binding proteins (CRBPs)-I/retinoic acid receptors (RARs)␣ expression, and led to apoptosis in HK-2 and human umbilical vein endothelial cells
Increased Ratio of Apo- to Holo-RBP4 Concentration Enhanced RBP4 Binding with STRA6, Phosphorylation of STAT5/JAK2, Active Caspase 3, and Apoptosis in HK-2 and HUVEC Cells—The RBP4 binding assay demonstrated an increase in RBP4 binding activity after stimulation with the apo-/holo-RBP4 mixture at 0:50 and 5:45 (␮g/ml:␮g/ml) as compared with the stimulation with the apo-/holo-RBP4 mixture at 0:0, 15:35, 25:25, and 50:0 (␮g/ml:␮g/ml) in HK-2 (Fig. 1A) and HUVEC (Fig. 1B) at 3 h

Summary

Background

An increase of apo- to holo-RBP4 concentration in plasma is observed in subjects with renal dysfunction and is supposed to induce cell damage. Results: Increased apo-/holo-RBP4 ratio affects STRA6 signaling, which activates JAK2/STAT5 and induces apoptosis. The increase of apo-/holo-retinol-binding protein 4 (RBP4) concentrations has been found in subjects with renal dysfunction and even in diabetic patients with microalbuminuria. We found that increase of apo-/holo-RBP4 concentration ratio delayed the displacement of RBP4 with “stimulated by retinoic acid 6” (STRA6), enhanced Janus kinase 2 (JAK2)/STAT5 cascade, up-regulated adenylate cyclase 6 (AC6), increased cAMP, enhanced JNK1/p38 cascade, suppressed CRBP-I/RAR␣ (cellular retinol-binding protein/ retinoic acid receptor ␣) expression, and led to apoptosis in HK-2 and human umbilical vein endothelial cells. We sought to investigate whether the increase of apo-/holo-RBP4 concentration can repress CRBP-I/RAR␣ leading to apoptosis through JAK2/STAT5-activated cAMP/JNK1/p38 pathway in human renal proximal tubular cells (HK-2) and human umbilical vein endothelial cells

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