The RhoGDI-alpha/JNK signaling pathway plays a significant role in mycophenolic acid-induced apoptosis in an insulin-secreting cell line

Abstract

Mycophenolic acid (MPA)-induced beta-cell toxicity is an important factor for islet graft function. The signal transduction mechanisms underlying this process have not been fully explored. Using a proteomics approach, we examined protein expression patterns in MPA-treated RIN-5 cells and found that RhoGDI-alpha expression is altered by MPA-treatment. We examined the relationship between RhoGDI-alpha expression and activated JNK during MPA-induced apoptosis. Cells were treated with N-acetyl-cysteine (NAC), caspase inhibitor, JNK inhibitor, guanosine or GTP for 1 h before being treated with MPA. To investigate the regulatory effects of RhoGDI-alpha on JNK activity, we examined cells showing either elevated or reduced expression of RhoGDI-alpha as a result of transfection with cDNA or siRNA constructs, respectively. MPA significantly increased cell death, caspase-3 expression and JNK activation, but it decreased the expression of a protein spot 25 observed by two-dimensional electrophoresis. This protein 25 was identified as RhoGDI-alpha by mass spectrometry. MPA-induced cell death and down-regulation of RhoGDI-alpha were prevented by guanosine, GTP or a JNK inhibitor. However, MPA-induced cell death was partially restored by treatment with a caspase inhibitor, but not by NAC treatment. RhoGDI-alpha expression was not affected by treatment with NAC or caspase inhibitor. Over-expression of RhoGDI-alpha increased cell viability and decreased activated JNK expression following exposure to MPA, whereas knockdown of RhoGDI-alpha enhanced MPA-induced cell death and increased the activation of JNK. In conclusion, MPA induces significant apoptosis in insulin-secreting cells via down-regulation of RhoGDI-alpha linked with increased JNK expression. This RhoGDI-alpha/JNK pathway might be the focus of therapeutic target for the prevention of MPA-induced islet apoptosis.