The Role of PTEN in β-Cell Growth

Abstract

This paper describes the biological functions of PTEN and the PTEN regulated signaling pathway in pancreatic β-cells. PTEN has been shown to regulate the regeneration of β-cells. We review the pathways that are controlled by PTEN signaling and their functions in β-cell regeneration. In particular, we describe the unique effect of Pten deletion in β-cells. Unlike its effect in other tissues, Pten deletion does not lead to tumor formation but does enhance β-cell proliferation and function. In addition to the literature review, we also report new results exploring PTEN loss in adult β-cells. We demonstrate that inducing PTEN loss in adult cells has the same regenerative effects previously found for prenatal deletion.

Keywords: AKT; PTEN; pancreatic β-cells.

Fig. (1). The biological function of PTEN

PTEN is a lipid phosphatase. Its function is to remove the phosphate from the 3′ position of PI-3,4,5-P₃ (PIP₃) and form PI-4,5-P₂ (PIP₂). The accumulation of PI-3,4,5-P₃ is responsible for the activation of a number of downstream kinases that contain the Pleckstrin Homology (PH) domain. PI-3,4,5-P₃ binds to the PH domains of these downstream molecules and initiates their activation events. Since PI-3,4,5-P₃ accumulation is caused by the activation of PI3K, a kinase that adds the 3′ phosphate to PI-4,5-P₂, the functions of PTEN and PI3K directly oppose each other. PI3K is activated when growth factors bind to their receptor tyrosine kinases (RTKs) to induce mitogenic signals. It is also activated when insulin binds to its receptor. Thus, PTEN, by antagonizing the function of PI3K inhibits mitogenic signals and blocks the signal transduction of insulin receptor activity.

Fig. (2). Substrates of AKT

A well characterized target of PI3K and PTEN signaling is AKT. Accumulation of PI-3,4,5-P₃ leads to the activation of AKT. AKT is a serine/threonine kinase and has a number of downstream targets. Through these targets, AKT regulates cell survival (through caspase, BAD and FOXO); cell growth (through p21, p27, MDM2, cyclin D (Cyn D) as well as FOXO); AKT also regulates mTOR signaling indirectly by inhibiting TSC2 (tuberous sclerosis complex 2 ). Through mTOR and glycogen synthase kinase (GSK3 β), AKT signaling pathway also controls protein translation. In addition, activation of AKT also leads to upregulation of lipogenic and glycolytic genes such as steroyl-CoA response element binding protein (srebp), fatty acid synthase (FAS) and glucokinase (GK). Together with mobilization of glucose transporters (GLUT), induction of these genes by AKT activation leads to enhanced cellular metabolic processes.

Fig. (3). The function of PTEN in the nucleus

PTEN can shuttle between nucleus and cytosol. In the nucleus, PTEN directly interacts with CENP-C to control Centromere stability. PTEN also interacts with p53 and controls its stability in the nucleus as well. Furthermore, nuclear PTEN control DNA repair and cyclin D through Rad 51 and MAPK, respectively.

Fig. (4). Deletion of Akt2 does not affect islet mass in Pten null islets

We deleted Akt2 in the Pten^loxP/loxP; Rip-Cre⁺ mice to test whether the PI3K/AKT signaling mediated β-cell regeneration. We found that deletion of Akt2 has little effect on the regenerative islet phenotype we observed in the Pten deletion models. Pm, Pten^loxP/loxP; Rip-Cre⁺; Dm, Pten^loxP/loxP; Rip-Cre⁺;Akt2^−/−. Repre-sentative image from 10 animals.

Fig. (5). Reduced p16 expression in Pten mutant β-cells

Deletion of Pten in the β-cells (Pten^loxP/loxP; Rip-Cre⁺, Mut) led to reduction of cell cycle inhibitor p16 compared to control mice (Pten^loxP/loxP; Rip-Cre⁻, Con). Immunohistochemistry staining of p16 (green) in β-cells (red) show that p16 levels are reduced in the Pten mutant islets vs. control islets. Images are from pancreatic sections of 8 months old mice. Representative of 4 animals.

Fig. (6). Induced Pten loss in adult β-cells leads to increased islet mass

PTEN loss is induced in 3 month old Pten^loxP/loxP; Rip-CreER⁺ mice with treatment of tamoxifen. Pancreata are sectioned 3 months later. Mice treated with tamoxifen (Pten deleted) displayed larger and more islets. Representative image from 6 animals.

Fig. (7). Pten loss does not lead to development of insulinomas

We analyzed the Pten mutant (Pten^loxP/loxP; Rip-Cre⁺) mice 14.5 months of age (right panels) and found that the islets in these older mice are even larger compared with the controls (left panels). They maintain the normal cellular structures and still produce insulin (red, bottom panels) and other endocrine hormones (green, stained with a cocktail of somatostatin, pancreatic polypeptide and glucagon). n=3.