Islet Β-cell Function Research Articles

A new topic concerning pancreatic islet α-cell

Brief discussion on exchange roles of islet α-cell and β-cell in the pathogenesis of type 2 diabetes,and the possibility of HbA1C as a new marker for assessing both pancreatic islet α-cell and β-cell function,so as to provide a new perspective for clinical diagnosis and treatment of type 2 diabetes. Key words: Islet α-cell; HbA1C; Diabetes mellitus, type 2

Characterization of an Apparently Novel β-Cell Line-enriched 80–88 kDa Transcriptional Activator of the MafA and Pdx1 Genes

MafA and Pdx1 represent critical transcriptional regulators required for the maintenance of pancreatic islet β-cell function. The in vivo β-cell-enriched expression pattern of these genes is principally directed by islet transcription factors binding within conserved Region 3 (base pairs (bp) -8118/-7750) of MafA and Area II (bp -2153/-1923) of the Pdx1 gene. Comprehensive mutational analysis of conserved MafA Region 3 revealed two new β-cell line-specific cis-activation elements, termed Site 4 (bp -7997 to -7988) and Site 12 (bp -7835 to -7826). Gel mobility and antibody super-shift analysis identified Pdx1 as the Site 4 binding factor, while an 80-88 kilodalton (kDa) β-cell line-enriched protein complex bound Site 12 and similar aligned nucleotides within Pdx1 Area II. The 80-88 kDa activator was also found in adult mouse islet extract. Strikingly, the molecular weight, DNA binding, and antibody recognition properties of this activator were unique when compared with all other key islet transcription factors tested, including Prox1 (83 kDa), Hnf1α (67 kDa), FoxA2 (48 kDa), MafA (46 kDa), Isl1 (44 kDa), Pdx1 (42 kDa), and Nkx2.2 (30 kDa). Collectively, these data define an apparently novel MafA Region 3 and Pdx1 Area II activator contributing to expression in β-cells.

Pre-culturing islets with mesenchymal stromal cells using a direct contact configuration is beneficial for transplantation outcome in diabetic mice.

We recently showed that co-transplantation of mesenchymal stromal cells (MSCs) improves islet function and revascularization in vivo. Pre-transplant islet culture is associated with the loss of islet cells. MSCs may enhance islet cell survival or function by direct cell contact mechanisms and soluble mediators. We investigated the capacity of MSCs to improve islet cell survival or β-cell function in vitro using direct and indirect contact islet-MSC configurations. We also investigated whether pre-culturing islets with MSCs improves islet transplantation outcome. The effect of pre-culturing islets with MSCs on islet function in vitro was investigated by measuring glucose-stimulated insulin secretion. The endothelial cell density of fresh islets and islets cultured with or without MSCs was determined by immunohistochemistry. The efficacy of transplanted islets was tested in vivo using a syngeneic streptozotocin-diabetic minimal islet mass model. Graft function was investigated by monitoring blood glucose concentrations. Indirect islet-MSC co-culture configurations did not improve islet function in vitro. Pre-culturing islets using a direct contact MSC monolayer configuration improved glucose-stimulated insulin secretion in vitro, which correlated with superior islet graft function in vivo. MSC pre-culture had no effect on islet endothelial cell number in vitro or in vivo. Pre-culturing islets with MSCs using a direct contact configuration maintains functional β-cell mass in vitro and the capacity of cultured islets to reverse hyperglycemia in diabetic mice.

The Effect of Simvastatin on Glucose Homeostasis in Streptozotocin Induced Type 2 Diabetic Rats

Objective. To investigate the effect of simvastatin on glucose homeostasis in streptozotocin induced type 2 diabetic rats. Methods. Forty male Wistar rats were randomly divided into four groups. Normal control rats were fed with standard diet, others were fed with high-fat diet. Diabetic rats were induced by a single intraperitoneal injection of STZ. The simvastatin intervention rats were fed with simvastatin during the experiment process, and the simvastatin treatment rats were fed with simvastatin after diabetes rats were induced. We measured body weight, fasting plasma glucose, cholesterol, high-density lipoprotein cholesterol, and triglyceride after an overnight fast. Results. The FPG was higher in diabetic rats when compared to normal control ones; the simvastatin intervention rats had a higher FPG compared to the diabetic rats and were more easily be induced to diabetes at the end of 4 weeks, FPG level of simvastatin treatment rats was increased compared with diabetic model rats after 12 weeks. Conclusion. These data indicate that simvastatin intervention rats may cause hyperglycemia by impairing the function of islet β cells and have an adverse effect on glucose homeostasis, especially on FPG level.

Intra-islet glucagon secretion and action in the regulation of glucose homeostasis

Glucagon, a key hormone in the regulation of glucose homeostasis, acts as a counter-regulatory hormone to insulin by promoting hepatic glucose output. Under normal conditions, insulin and glucagon operate in concert to maintain the glucose level within a narrow physiological range. In diabetes, however, while insulin secretion or action is insufficient, the production and secretion of glucagon are excessive, contributing to the development of diabetic hyperglycemia. Within an islet, intra-islet insulin, in cooperation with intra-islet GABA, suppresses glucagon secretion via direct modulation of α-cell intracellular signaling pathways involving Akt activation, GABA receptor phosphorylation and the receptor plasma membrane translocation, while intra-islet glucagon plays an important role in modulating β-cell function and insulin secretion. Defects in the insulin-glucagon fine-tuning machinery may result in β-cell glucose incompetence, leading to unsuppressed glucagon secretion and subsequent hyperglycemia, which often occur under extreme conditions of glucose influx or efflux. Therefore, deciphering the precise molecular mechanisms underlying glucagon secretion and action will facilitate our understanding of glucagon physiology, in particular, its role in regulating islet β-cell function, and hence the mechanisms behind glucose homeostasis.

Novel therapy for type 1 diabetes: Autologous hematopoietic stem cell transplantation

Type 1 diabetes is characterized pathologically by autoimmune insulitis-related islet β-cell destruction. Although intensive insulin therapy for patients with type 1 diabetes can correct hyperglycemia, this therapy does not prevent all diabetes-related complications. Recent studies have shown that autologous hematopoietic stem cell transplantation (HSCT) is a promising new approach for the treatment of type 1 diabetes by reconstitution of immunotolerance and preservation of islet β-cell function. Herein we discuss the therapeutic efficacy and potential mechanisms underlying the action of HSCT and other perspectives in the clinical management of type 1 diabetes.

Effects of Chinese herbal medicine Danzhi Jiangtang Capsule and exercise on JNK signaling pathway in pancreatic tissues of diabetic rats

To explore the effects of exercise and Danzhi Jiangtang Capsule (DJC), a compound traditional herbal medicine, on the JNK signaling pathway in pancreatic tissues of diabetic rats and to investigate the possible mechanisms of exercise and DJC in treating diabetes. Seventy-eight male Wistar rats were injected with low dose of streptozotocin and fed a high-fat diet to establish a diabetic model in rats. Then 60 diabetic rats were divided into diabetes group, exercise group, DJC group and exercise combined with DJC group. Another twelve rats were used as normal control. After eight months of treatment, the expression levels of phosphor-c-Jun N-terminal kinase (p-JNK), pancreatic and duodenal homeobox-1 (PDX-1), and insulin protein in pancreatic tissues from rats were detected by immunohistochemical method and Western blotting. In pancreatic tissues of diabetes group, the expression level of p-JNK protein was significantly higher than that in the normal group (P<0.01), and the expression levels of PDX-1 and insulin protein were significantly decreased (P<0.01). After administration of exercise and DJC, the expression level of p-JNK protein in pancreatic tissues of the diabetes group was decreased significantly, while the expression levels of PDX-1 and insulin protein were increased significantly (P<0.05 or P<0.01). Exercise and DJC effectively protect isletβ-cell function in diabetic rats, which might be due to a decreased JNK signaling pathway.

Changes of insulin resistance and β‐cell function in women with gestational diabetes mellitus and normal pregnant women during mid‐ and late pregnant period: A case–control study

The aim of this study was to observe insulin resistance and β-cell function changes among women diagnosed with gestational impaired glucose tolerance or gestational diabetes mellitus (GDM) in mid-pregnancy. Sixty-four pregnant women receiving prenatal care underwent an oral glucose tolerance test at 20-24 weeks of gestation and an insulin release test. The GDM group included 34 pregnant women diagnosed with gestational impaired glucose tolerance or GDM, and the subjects with normal blood glucose were the control group. Insulin resistance and islet β-cell function changes were observed with the oral glucose tolerance test and insulin release test. The homeostatic model assessment-β levels in late pregnancy were higher than those in mid-pregnancy for both groups, and the primary time effect was statistically significant. The early insulin secretion index (ΔI(30)/ΔG(30)) values in mid- and late pregnancy were lower in the GDM group. The values of the area under the curve of blood glucose in mid- and late pregnancy were higher in the GDM group than those in the control group. Insulin resistance was higher in GDM patients than in normal pregnant women. Insulin resistance was aggravated, and β-cell's ability to compensate for the increased insulin resistance by modulating insulin secretion was aggravated, as gestational week increased in women with gestational diabetes and normal pregnant women. Insulin resistance in women with GDM is higher than in pregnant women with normal metabolism of glucose.

Low-level phenolic estrogen pollutants impair islet morphology and β-cell function in isolated rat islets

Phenolic estrogen pollutants, a class of typical endocrine-disrupting chemicals, have attracted public attention due to their estrogenic activities of imitating steroid hormone 17β-estradiol (E(2)) effects. Exposure to these pollutants may disrupt insulin secretion and be a risk factor for type 2 diabetes. In this study, we investigated the direct effects of phenolic estrogen diethylstilbestrol (DES), octylphenol (OP), nonylphenol (NP), and bisphenol A (BPA) on rat pancreatic islets in vitro, whose estrogenic activities were DES>NP>OP>BPA. Isolated β-cells were exposed to E(2), DES, OP, NP, or BPA (0, 0.1, 0.5, 2.5, 25, and 250 μg/l) for 24 h. Parameters of insulin secretion, content, and morphology of β-cells were measured. In the glucose-stimulated insulin secretion test, E(2) and DES increased insulin secretion in a dose-dependent manner in a 16.7 mM glucose condition. However, for BPA, NP, or OP with lower estrogenic activity, the relationship between the doses and insulin secretion was an inverted U-shape. Moreover, OP, NP, or BPA (25 μg/l) impaired mitochondrial function in β-cells and induced remarkable swelling of mitochondria with loss of distinct cristae structure within the membrane, which was accompanied by disruption of mRNA expression of genes playing a key role in β-cell function (Glut2 (Slc2a2), Gck, Pdx1, Hnf1α, Rab27a, and Snap25), and mitochondrial function (Ucp2 and Ogdh). Therefore, these phenolic estrogens can disrupt islet morphology and β-cell function, and mitochondrial dysfunction is suggested to play an important role in the impairment of β-cell function.

Rab-geranylgeranyl transferase regulates glucose-stimulated insulin secretion from pancreatic β cells

A growing body of evidence implicates essential roles for small molecular weight G-proteins (e.g., Cdc42, Rac1, Arf6 and Rab3A and Rab27A) in islet β-cell function including glucose-stimulated insulin secretion (GSIS). One of the known mechanisms for optimal activation of small G-proteins involves post-translational prenylation, which is mediated by farnesyltransferase (FTase) and geranylgeranyl transferases (GGTases I and II). The FTase catalyzes incorporation of a 15-carbon farnesyl group while the GGTase mediates incorporation of a 20-carbon geranylgeranyl group into the C-terminal cysteines of G-proteins. The FTase, GGTase I and GGTase II prenylate Ras, Cdc42/Rac1, and Rab G-proteins, respectively. While considerable evidence exists on FTase/GGTase I-mediated regulation of GSIS, very little is known about GGTase II (also referred to as Rab GGTase; RGGT) and its regulatory proteins in the cascade of events leading to GSIS. Herein, we provide the first immunological evidence to suggest expression of α- and β-subunits of RGGT in clonal INS 832/13 β-cells, normal rat islets and human islets. Furthermore, Rab escort protein1 (REP1), which has been shown to be critical for prenylation of Rab G-proteins, is also expressed in these cells. Furthermore, evidence is presented to suggest that siRNA-mediated knockdown of α- or β-subunits of RGGT and REP1 markedly attenuates GSIS in INS 832/13 cells. These findings provide the first evidence in support of key roles for RGGT and its regulatory proteins in GSIS.

Impaired autophagic function in rat islets with aging

Type 2 diabetes is characterized by a deficit in β-cell function and mass, and its incidence increases with age. Autophagy is a highly regulated intracellular process for degrading cytoplasmic components, particularly protein aggregates and damaged organelles. Impaired or deficient autophagy is believed to cause or contribute to aging and age-related disease. Autophagy may be necessary to maintain structure, mass, and function of pancreatic β-cells. In this study, we investigated the effects of age on β-cell function and autophagy in pancreatic islets of 4-month-old (young), 14-month-old (adult), and 24-month-old (old) male Wistar rats. We found that islet β-cell function decreased gradually with age. Protein expression of the autophagy markers LC3/Atg8 and Atg7 exhibited a marked decline in aged islets. The expression of Lamp-2, a good indicator of autophagic degradation rate, was significantly reduced in the islets of old rats, suggesting that autophagic degradation is decreased in the islets of aged rats. However, protein expression of beclin-1/Atg6, which plays an important role in the induction and formation of the pre-autophagosome structure by associating with a multimeric complex of autophagy regulatory proteins (Atg14, Vps34/class 3 PI3 kinase, and Vps15), was most prominent in the islets of adult rats, and was higher in 24-month-old islets than in 4-month-old islets. The levels of p62/SQSTM1 and polyubiquitin aggregates, representing the functions of autophagy and proteasomal degradation, were increased in aging islets. 8-Hydroxydeoxyguanosine, a marker of mitochondrial and nuclear DNA oxidative damage, exhibited strong immunostaining in old islets. Analysis by electron microscopy demonstrated swelling and disintegration of cristae in the mitochondria of aged islets. These results suggest that β-cell and autophagic function in islets decline simultaneously with increasing age in Wistar rats, and that impaired autophagy in the islets of older rats may cause accumulation of misfolded and aggregated proteins and reduce the removal of abnormal mitochondria in β-cells, leading to reduced β-cell function. Dysfunctional autophagy in islets during the aging process may be an important mechanism leading to the development of type 2 diabetes.

A VGF-Derived Peptide Attenuates Development of Type 2 Diabetes via Enhancement of Islet β-Cell Survival and Function

Deterioration of functional islet β-cell mass is the final step in progression to Type 2 diabetes. We previously reported that overexpression of Nkx6.1 in rat islets has the dual effects of enhancing glucose-stimulated insulin secretion (GSIS) and increasing β-cell replication. Here we show that Nkx6.1 strongly upregulates the prohormone VGF in rat islets and that VGF is both necessary and sufficient for Nkx6.1-mediated enhancement of GSIS. Moreover, the VGF-derived peptide TLQP-21 potentiates GSIS in rat and human islets and improves glucose tolerance in vivo. Chronic injection of TLQP-21 in prediabetic ZDF rats preserves islet mass and slows diabetes onset. TLQP-21 prevents islet cell apoptosis by a pathway similar to that used by GLP-1, but independent of the GLP-1, GIP, or VIP receptors. Unlike GLP-1, TLQP-21 does not inhibit gastric emptying or increase heart rate. We conclude that TLQP-21 is a targeted agent for enhancing islet β-cell survival and function.

MiR-33a Modulates ABCA1 Expression, Cholesterol Accumulation, and Insulin Secretion in Pancreatic Islets

Changes in cellular cholesterol affect insulin secretion, and β-cell–specific deletion or loss-of-function mutations in the cholesterol efflux transporter ATP-binding cassette transporter A1 (ABCA1) result in impaired glucose tolerance and β-cell dysfunction. Upregulation of ABCA1 expression may therefore be beneficial for the maintenance of normal islet function in diabetes. Studies suggest that microRNA-33a (miR-33a) expression inversely correlates with ABCA1 expression in hepatocytes and macrophages. We examined whether miR-33a regulates ABCA1 expression in pancreatic islets, thereby affecting cholesterol accumulation and insulin secretion. Adenoviral miR-33a overexpression in human or mouse islets reduced ABCA1 expression, decreased glucose-stimulated insulin secretion, and increased cholesterol levels. The miR-33a–induced reduction in insulin secretion was rescued by cholesterol depletion by methyl-β-cyclodextrin or mevastatin. Inhibition of miR-33a expression in apolipoprotein E knockout islets and ABCA1 overexpression in β-cell–specific ABCA1 knockout islets rescued normal insulin secretion and reduced islet cholesterol. These findings confirm the critical role of β-cell ABCA1 in islet cholesterol homeostasis and β-cell function and highlight modulation of β-cell miR-33a expression as a means to influence insulin secretion.

Role of pancreatic-derived factor in type 2 diabetes: evidence from pancreatic β cells and liver

Pancreatic-derived factor (PANDER) is a cytokine-like protein that is highly expressed in pancreatic islets. In vitro, PANDER pretreatment or viral-mediated overexpression promotes apoptosis of islet β cells. Under conditions of insulin resistance, chronic hyperglycemia potently activates PANDER expression and stimulates the cosecretion of insulin and PANDER in β cells. PANDER binds to the liver cell membrane and induces insulin resistance, resulting in increased gluconeogenesis. Recently, PANDER was found to be expressed in rodent and human liver, and its expression is increased in the liver of diabetic mice and rats. Hepatic overexpression of PANDER promotes lipogenesis in the liver and induces insulin resistance in C57BL/6 mice, whereas the inactivation of hepatic PANDER markedly reduces steatosis, insulin resistance, and hyperglycemia in db/db mice. PANDER deficiency protects mice from high-fat-diet-induced hyperglycemia by decreasing gluconeogenesis in the liver. In summary, PANDER plays an important role in the progression of type 2 diabetes by negatively regulating islet β-cell function and insulin sensitivity in the liver.

Relationship of a single-sample urinary C-peptide/creatinine ratio and 24 h urinary C-peptide with islet β-cell function in type 2 diabetic patients

Blood C-peptide,first-void fasting urinary C-peptide/creatinine ratio ( UCPCR ),second-void fasting UCPCR,and 24 h urinary C-peptide (UCP) were determined in 90 type 2 diabetics and 30 health volunteers.The results showed that first-void fasting UCPCR and second-void fasting UCPCR were positively related to 24 h UCP and the index of islet β-cell function( all P＜0.01 ). Key words: Urinary C-peptide/creatinine ratio; Diabetes mellitus, type 2 ; Void fasting urine; 24 h urine; Islet β-cell function

Novel Role for SGK3 in Glucose Homeostasis Revealed in SGK3/Akt2 Double-Null Mice

The phosphatidylinositol-3-kinase-dependent kinase, Akt2, plays a central role in mediating insulin effects in glucose-metabolizing tissues. Akt2 knockout mice display insulin resistance with a reactive increase in pancreatic islet mass and hyperinsulinemia. The related phosphatidylinositol-3-kinase-dependent kinase, serum- and glucocorticoid-regulated kinase 3 (SGK3), is essential for normal postnatal hair follicle development but plays no apparent role in glucose homeostasis. We report here an unexpected role of SGK3 in islet β-cell function, which is revealed in Akt2/SGK3 double-knockout (DKO) mice. DKO mice have markedly worse glucose homeostasis than Akt2 single-null animals, including greater baseline glucose, and greater rise in blood glucose after glucose challenge. However, surprisingly, our data strongly support the idea that this exacerbation of the glucose-handling defect is due to impaired β-cell function, rather than increased insulin resistance in peripheral tissues. DKO mice had lower plasma insulin and C-peptide levels, lower β-cell mass, reduced glucose-stimulated insulin secretion, and greater sensitivity to exogenous insulin than Akt2 single nulls. We further demonstrated that SGK3 is strongly expressed in normal mouse islets and, interestingly, that β-catenin expression is dramatically lower in the islets of DKO mice than in those of Akt2(-/-)/SGK3(+/+) or Akt2(-/-)/SGK3(+/-) mice. Taken together, these data strongly suggest that SGK3 plays a previously unappreciated role in glucose homeostasis, likely through direct effects within β-cells, to stimulate proliferation and insulin release, at least in part by controlling the expression and activity of β-catenin.

Endocrine Pancreatic Function Changes After Acute Pancreatitis

This study aimed to investigate the impairment of pancreatic endocrine function and the associated risk factors after acute pancreatitis (AP). Fifty-nine patients were subjected to tests of pancreatic function after an attack of pancreatitis. The mean time after the event was 3.5 years. Pancreatic endocrine function was evaluated by fasting blood glucose (FBG), glycosylated hemoglobin, fasting blood insulin, and C-peptide. Homeostasis model assessment was used to evaluate insulin resistance and islet β-cell function. Pancreatic exocrine function was evaluated by fecal elastase 1. Factors that could influence endocrine function were also investigated. Nineteen patients (32%) were found to have elevated FBG, whereas 5 (8%) had abnormal glycosylated hemoglobin levels. The levels of FBG, fasting blood insulin, and C-peptide were higher in patients than in controls (P < 0.01). The islet β-cell function of patients was lower than that of controls (P < 0.01), whereas insulin resistance index was higher among patients (P < 0.01). Obesity, hyperlipidemia, and diabetes-related symptoms were found to be associated with endocrine insufficiency. Pancreatic exocrine functional impairment was found at the same time. Endocrine functional impairment with insulin resistance was found in patients after AP. Obesity, hyperlipidemia, and diabetes-related symptoms increased the likelihood of developing functional impairment after AP.

Effect of aging on islet beta-cell function and its mechanisms in Wistar rats

Type 2 diabetes mellitus is characterized by islet β-cell dysfunction and its incidence increases with age. However, the mechanisms underlying the effect of aging on islet β-cell function are not fully understood. We characterized β-cell function in 4-month-old (young), 14-month-old (adult), and 24-month-old (old) male Wistar rats, and found that islet β-cell function decreased gradually with age. Old rats displayed oral glucose intolerance and exhibited a decrease in glucose-stimulated insulin release (GSIR) and palmitic acid-stimulated insulin release (PSIR). Furthermore, total superoxide dismutase (T-SOD), CuZn superoxide dismutase (CuZn-SOD), and glutathione peroxidase (GSH-Px) activity decreased, whereas serum malondialdehyde (MDA) levels increased in the older rats. Moreover, we detected a significant reduction in β-cell proliferation and an increase in the frequency of apoptotic β-cells in the islets of rats in the old group. Finally, Anxa1 expression in the islets of old rats was significantly upregulated. These data provide new insights into the development of age-related β-cell dysfunction in rats.

Immunological characteristics and pancreatic β-cell function in first-degree relatives of type 1 diabetes mellitus

Oral glucose tolerance test(OGTT)was performed in 419 first-degree relatives(FDRs)of type 1 diabetes mellitus. GADA, IA-2A, and IAA were determined by radioligand assay, and the positive rates were 7.16%, 1.43%, and 1.26%, respectively. Intravenous glucose tolerance test(IVGTT)and nateglinide-OGTT were performed in 39 controls, 11 first-degree relatives with positive autoantibody(Ab+group), 14 ones with negative autoantibody(Ab-group)during 5-7 days.The first-phase insulin release(FPIR), area under insulin release during 0-10 min [AUC0-10] of IVGTT and the value of(ΔI30/ΔG30)of nateglinide-OGTT in Ab+group were lower than those of control and(2.75±0.37 vs 3.61±1.05)mU/mmol, all P＜0.05]. The 1st min insulin release in Ab+group was lower than that of Ab-group [(3.80±0.30 vs 4.52±0.70)mU/L, P＜0.05]. The HOMA-IR was higher in Ab-group than that in control group(2.92±1.04 vs 1.96±1.22, P＜0.05). The results suggest that the positivity rates of autoantibodies in FDRs of type 1 diabetes mellitus are very close to those of Caucasian. There exist insulin secretion defects in FDRs with positive autoantibody while insulin resistance in FDRs with negative autoantibody. Key words: Dabetes mellitus, type 1; First-degree relatives; Islet autoantibodies; Islet β-cell function; Nateglinide

Proteasome activator PA28γ stimulates degradation of GSK3-phosphorylated insulin transcription activator MAFA

MAFA is a member of the MAF family of basic leucine zipper transcription factors and is a critical regulator of insulin gene expression and islet β-cell function. To be degraded by the proteasome, MAFA must be phosphorylated by GSK3 and MAP kinases at multiple serine and threonine residues (Ser49, Thr53, Thr57, Ser61, and Ser65) within its amino-terminal domain. In this study, we report that MAFA degradation is stimulated by PA28γ (REGγ and PSME3), a member of a family of proteasome activators that bind and activate the 20S proteasome. To date, only a few PA28γ-proteasome pathway substrates have been identified, including steroid receptor coactivator 3 (SRC3) and the cell cycle inhibitor p21(CIP1). PA28γ binds to MAFA, induces its proteasomal degradation, and thereby attenuates MAFA-driven transcriptional activation of the insulin promoter. Co-expression of GSK3 enhanced the PA28γ-mediated degradation of MAFA, but mutants that contained alanine substitutions at the MAFA phosphorylation sites did not bind PA28γ and were resistant to degradation. We also found that a PA28γ mutant (N151Y) that did not stimulate p21 degradation enhanced MAFA degradation, and another mutant (K188D) that promoted greater p21 degradation did not enhance MAFA degradation.These results suggest that PA28γ stimulates MAFA degradation through a novel molecular mechanism that is distinct from that for the degradation of p21.