Diabetogenic effects of glucocorticoids : role of protein phosphatase 5 and GLP-1 receptor activation

Abstract

Obesity, the metabolic syndrome and type 2 diabetes are inextricably linked together, and all increase with rapid pace globally. Glucocorticoid (GC) excess is associated with diabetogenic effects, including insulin resistance and glucose intolerance, and a role for GCs has also been proposed in the development and pathology of the metabolic syndrome. Studies have indicated that patients with glucose intolerance or the metabolic syndrome have elevated levels of cortisol and in addition, the metabolic syndrome shares many phenotypical characteristics and pathologies with cortisol excess.

This thesis aimed at studying diabetogenic effects of GCs and the role of protein phosphatase 5 (PP5) and GLP-1 receptor activation in a setting of GC excess in mice in vivo. Apoptosis of insulin producing pancreatic β-cells is believed to play an important role in the pathologic development from insulin resistance and disturbed glucose tolerance to overt diabetes. Mechanisms underlying GC-mediated direct cytotoxic effects on β-cells were studied in vitro with a special emphasis on the involvement of PP5.

For studies in vivo, mice were exposed to GCs for five weeks via their drinking water and characterized in terms of glucose and lipid handling. The results revealed several features mimicking the metabolic syndrome in humans. Mice became obese with ectopic fat deposition and dyslipidemia, they became hyperglycemic and hyperinsulinemic with insulin resistance and they developed hypertension. Treatment with the GLP-1 receptor agonist liraglutide slowed progression towards obesity and ectopic fat deposition and improved glucose control. Mice with a global knock-out of PP5 (Ppp5c-/-) were in part protected against GC-induced hyperglycemia and hyperinsulinemia. Ppp5c-/- mice receiving vehicle exhibited better glucose handling than Ppp5c+/+ cognates during glucose tolerance test. The observed effects of the model were reversed after GC removal. Furthermore, the studies revealed insights into β-cell adaptation to the increased insulin demand, in this setting of GC-induced insulin resistance. Increased islet volume due to cell proliferation, increased β-cell and α-cell mass, increased insulin secretory capacity and islet chaperone expression were found in GC-treated mice.

The in vitro study revealed that insulinoma cells and isolated pancreatic islets succumbed to detrimental direct effects of the synthetic GC dexamethasone via increased apoptosis. Dexamethasone activated mitogen-activated protein kinase (MAPK) signaling as evident by enhanced phosphorylation of p38 MAPK and c-Jun N-terminal kinase (JNK). Inhibition of p38 MAPK attenuated dexamethasone-induced apoptosis and decreased phosphorylation of the GC receptor. In contrast, inhibition of JNK augmented the cytotoxic effect of dexamethasone. Downregulation or lack of PP5 made cells and islets more susceptible to the apoptotic GC-effects with concomitant activation of p38 MAPK, suggesting that MAPKs and PP5 work in concert to regulate the cytotoxic effects of GCs in these cells.

In conclusion, the model mimicking the metabolic syndrome in humans could be valuable for studying mechanisms behind development of the metabolic syndrome and diabetes, as well as the multifaceted relations between GC excess and disease. Furthermore, liraglutide can be beneficial for patients at risk of developing metabolic complications in the setting of GC surplus. Lack of PP5 was protective against the deleterious effects of GCs in vivo, however, in the in vitro situation with direct effects of GCs on β-cells, a lack of PP5 was instead detrimental, leading to the conclusion that the actions of PP5 might be double edged and contradictive in different tissues.