Serine / threonine protein phosphatase 5 – a double-edged sword : in the progression towards diabetes

Abstract

Type 2 diabetes mellitus is increasing at an alarming rate worldwide. In the development and progression of type 2 diabetes mellitus, the insulin-producing pancreatic β-cells are commonly exposed to a hyperglycemic and hyperlipidemic environment, in which the levels of reactive oxygen species are elevated. In turn, reactive oxygen species can trigger an apoptotic response, by activating mitogen- activated protein kinase signaling networks, leading to β-cell death. When the functional β-cell mass is reduced to a level that, can no longer maintain euglycemia, type 2 diabetes mellitus is manifested. Therefore, there is a need to find new anti-diabetic drugs that are able to protect the loss of β-cell mass and in so doing prevent and treat diabetes. This thesis aimed at investigating the possible protective roles of serine/threonine protein phosphatase 5 (PP5) in this context.

We generated a PP5-deficient mouse line to evaluate the biological actions of PP5. Isolated mouse embryonic fibroblasts from mice lacking PP5 (Ppp5c-/-) and their wild-type littermates (Ppp5c+/+) were exposed to DNA damage-inducing agents to investigate the role of PP5 in response to genotoxic stress. Pancreatic islets isolated from both Ppp5c+/+ and Ppp5c-/- mice, and MIN6 cells treated with short-interfering RNA targeting PP5, were exposed to H2O2 or palmitate to test the hypothesis that PP5 acts to suppress apoptotic signaling in β-cells. Ppp5c+/+ and Ppp5c-/- mice were placed on either a standard diet or high-fat diet for ten weeks to examine the role of PP5 in a diabetic environment.

Our data revealed that the PP5-deficient mice were viable and fertile, demonstrating that PP5 is not essential for survival. However, the Ppp5c-/- mice were underrepresented in offspring from heterozygous mating, indicating that PP5 provides some advantage during embryonic development. Mouse embryonic fibroblasts lacking PP5 showed increased susceptibility to UV light-induced stress, suggesting that PP5 may promote cell survival. PP5 deficiency in mice was also associated with reduced weight gain, lower fasting glycemia and improved glucose tolerance. However, the genetic disruption of PP5 did not alter insulin sensitivity or islet volume. Comparison of mitogen-activated protein kinase signaling in islets from Ppp5c-/- mice and MIN6 cells with reduced PP5 levels revealed that the lack of PP5 was associated with enhanced H2O2- and palmitate-induced JNK phosphorylation and apoptosis. This indicates that PP5 can suppress stress-induced apoptosis in β-cells by a mechanism involving the regulation of JNK phosphorylation and, thereby, contributing to a protective effect. PP5 suppression in MIN6 cells correlated with hypersecretion of insulin in response to glucose.

When compared to wild-type littermate controls, Ppp5c-/- mice on a high-fat diet gained less weight. The mice lacking PP5 also had lower fasting glucose, which increased by high-fat diet feeding. A finding not observed in the Ppp5c+/+ mice. These observations may be explained, in part, by the enhanced serum insulin levels in the Ppp5c+/+ mice on the high-fat diet. Increased serum insulin was not observed in the Ppp5c-/- mice, which instead had lower levels of insulinemia after they were fed high-fat diet. High-fat diet feeding resulted in impaired glucose tolerance in both genotypes without an apparent difference in insulin sensitivity or β-cell function. These data indicate that the lack of PP5 protects mice against high-fat diet-induced weight gain but it cannot sustain glucose control during high-fat diet treatment.

Together, our data provide evidence that PP5 is involved in the regulation of β- cell apoptosis and glucose homeostasis. PP5 may represent a double-edged sword in the fight against diabetes, since a PP5 inhibitor useful to prevent the progression towards obesity and diabetes, could possibly also harm the β-cells exposed to a glucolipotoxic environment.