Geniposide inhibits glucolipotoxicity and cooperates with Txnip knockdown to potentiate cell adaption to endoplasmic reticulum stress in pancreatic beta cells.

Thioredoxin-interacting protein (Txnip), a negative regulator of thioredoxin, has become an attractive therapeutic target to alleviate metabolic diseases. Our previous data demonstrated that geniposide improved glucose-stimulated insulin secretion by accelerating Txnip degradation and prevented the early-stage apoptosis of pancreatic β cells induced by palmitate, but the underlying mechanisms are still unclear. The objective of this study is to identify the role of Txnip in geniposide preventing the apoptosis of pancreatic β cells induced by high glucose and palmitate (HG/PA). The results revealed that geniposide attenuated HG/PA-induced cell apoptosis and the expression of Bax and caspase-3, while increasing mitochondrial membrane potential and the anti-apoptotic protein levels of heme-oxygenase-1 (HO-1) and Bcl-2 in INS-1 rat pancreatic β cells. Knockdown of the Txnip gene raised the levels of anti-apoptotic proteins HO-1 and Bcl-2 and geniposide potentiated the effect of Txnip when the INS-1 cells were challenged by HG/PA. Furthermore, geniposide enhanced the adoptive unfolded protein response by increasing the phosphorylation of PERK/eIF2α and IRE1α in HG/PA-treated INS-1 cells. The results together suggest that geniposide might be useful to antagonize glucolipotoxicity and Txnip might be a pleiotropic cellular factor in pancreatic β cells.

Liu M ，Liu C ，Shen S ，Liu J ，Yin F ... -  《-》

Unfolded protein response is involved in geniposide-regulating glucose-stimulated insulin secretion in INS-1 cells.

The growing evidence shows that in the early stage of type 2 diabetes mellitus (T2DM) development, when challenged by hyperglycemia and/or insulin resistance, pancreatic islets would produce more insulin to maintain the balance of blood sugar, but at the same time, endoplasmic reticulum (ER) stress will be initiated for the reason of over-compensation, which might be a crucial caused factor of dysfunction and death of pancreatic beta cell. In this study, we showed that high glucose induced a remarkably unfolded protein response (UPR) with the phosphorylation of PERK/eIF2α and IRE1α in INS-1 cells, but geniposide prevented the role of high glucose on the phosphorylation of PERK/eIF2α and IRE1α, respectively. Although inhibition of Txnip expression by siRNA had no significant effect on geniposide-regulating UPR, PERK and IRE1α were associated with geniposide-regulating Txnip degradation and glucose-stimulated insulin secretion (GSIS) in high glucose-cultured INS-1 cells. All these data suggest that geniposide might be an important regulator of ER stress and GSIS, and a promising compound for the treatment of T2DM. SIGNIFICANCE OF THE STUDY: Mounting evidence indicates that endoplasmic reticulum (ER) stress plays an essential role to maintain the normal cellular functions and dysfunction. In this study, we revealed that geniposide might be an important regulator of ER stress and glucose-stimulated insulin secretion in pancreatic beta cells.

Hao Y ，Shen S ，Yin F ，Zhang Y ，Liu J ... -  《-》

Geniposide accelerates proteasome degradation of Txnip to inhibit insulin secretion in pancreatic β-cells.

Liu CY ，Hao YN ，Yin F ，Zhang YL ，Liu JH ... -  《-》

Geniposide protects pancreatic β cells from high glucose-mediated injury by activation of AMP-activated protein kinase.

Our previous works indicated that geniposide could regulate glucose-stimulated insulin secretion (GSIS), and improved chronic high glucose-induced dysfunctions in pancreatic β cells, but the molecular mechanisms remain largely unknown. In the present study, we investigated the role of 5'-AMP-activated protein kinase (AMPK) in high glucose induced cell injury and explored the associated molecular mechanisms in rat INS-1 pancreatic β cells. Data suggested that geniposide obviously prevented the cell damage induced by high (25 mM) glucose in INS-1 cells, which increased the protein levels of cell apoptosis-associated enzymes, including heme oxygenase-1 (HO-1), and Bcl-2, but apparently attenuated the protein level of Bax, an apoptotic protein. In addition, Compound C, an AMPK inhibitor, remarkably inhibited the effects of geniposide on the protein levels of HO-1, Bcl-2, and Bax, but AICAR, an AMPK activator, potentiated the role of geniposide on the protein levels of HO-1, Bcl-2, and Bax. More importantly, geniposide directly prevented the cleavage of caspase-3 induced by high glucose, and this effect was also evidently prohibited by the pre-incubation of compound C in high glucose-treated INS-1 cells. Furthermore, using the method of RNA interfere, we further proved that treatment with AMPK siRNA attenuated the effects of geniposide on the apoptosis-associated proteins and cell viability. All these data suggest that AMPK plays a crucial role on geniposide antagonizing high glucose-induced pancreatic β cells injury.

Liu C ，Hao Y ，Yin F ，Zhang Y ，Liu J ... -  《-》

Glucagon-like peptide 1 receptor plays an essential role in geniposide attenuating lipotoxicity-induced β-cell apoptosis.

β-Cell apoptosis is considered to be a major cause of loss of β cells in diabetes. Geniposide could prevent oxidative stress-induced neuron apoptosis, and improved glucose stimulated insulin secretion by activating glucagon-like peptide 1 receptor (GLP-1R) in INS-1 cells. Here we have investigated whether geniposide can exert a direct effect against pancreatic β-cell lipoapoptosis. The results indicated that pretreatment pancreatic INS-1 cells with geniposide for 7h attenuated palmitate-induced β-cell apoptosis and active caspase-3 expression, but this effect was disappeared at 18 h. Long-term incubation with palmitate decreased GLP-1R expression in INS-1 cells, and exendin (9-39), an antagonist for GLP-1R, inhibited the effect of geniposide on palmitate-induced apoptosis in INS-1 cells. Moreover, geniposide also improved the impairment of GLP-1R signaling through enhancing the phosphorylation of Akt and Foxo1, and increased the expression of PDX-1 in palmitate-treated INS-1 cells. These results suggest that geniposide inhibits early stage of lipotoxicity-induced β-cell apoptosis, and GLP-1R plays a critical role in geniposide counteracting the action of lipotoxicity in INS-1 pancreatic β cells.

Liu J ，Yin F ，Xiao H ，Guo L ，Gao X ... -  《-》