Meta-analysis of gene expression in human pancreatic islets after in vitro expansion

Pancreatic islet transplantation as a potential cure for type 1 diabetes (T1D) cannot be scaled up due to a scarcity of human pancreas donors. In vitro expansion of beta-cells from mature human pancreatic islets provides an alternative source of insulin-producing cells. The exact nature of the expanded cells produced by diverse expansion protocols and their potential for differentiation into functional beta-cells remain elusive. We performed a large-scale meta-analysis of gene expression in human pancreatic islet cells, which were processed using three different previously described protocols for expansion and for which redifferentiation was attempted. All three expansion protocols induced dramatic changes in the expression profiles of pancreatic islets; many of these changes are shared among the three protocols. Attempts at redifferentiation of expanded cells induce a limited number of gene expression changes. Nevertheless, these fail to restore a pancreatic islet-like gene expression pattern. Comparison with a collection of public microarray datasets confirmed that expanded cells are highly comparable to mesenchymal stem cells. Genes induced in expanded cells are also enriched for targets of transcription factors important for pluripotency induction. The present data increase our understanding of the active pathways in expanded and redifferentiated islets. Knowledge of the mesenchymal stem cell potential may help development of drug therapeutics to restore beta-cell mass in T1D patients.

Growth factor-induced signaling of the pancreatic epithelium

Activated signaling proteins regulate diverse processes, including the differentiation of the pancreatic islet cells during ontogeny. Here we uncover the in vivo phosphorylation status of major growth factor-activated signaling proteins in normal adult mice and during pancreatic islet regeneration. We report elevated phospho-mitogen-activated protein kinase (phospho-MAPK), phospho-c-Jun-NH2-terminal kinase (phospho-JNK), and phospho-p38 MAPK expression during pancreatic regeneration. Immunoblotting experiments demonstrated elevated phosphorylation of p52 Src-homology/collagen (SHC) in the ductal network as well, substantiating the activation of this pathway. Furthermore, protein kinase B (PKB/Akt), a key signaling protein in the anti-apoptotic pathway, was phosphorylated to a greater extent in the ductal network from regenerating pancreas. We observed fibroblast growht factor (FGF)10 and platelet-derived growth factor (PDGF)AA expression in embryonic as well as regenerating adult pancreas. Epidermal growth factor (EGF) and PDGFAA stimulated MAPK and Akt phosphorylation, while FGF10 stimulated MAPK but not Akt phosphorylation in a time-dependent manner in freshly isolated cells from the adult ductal network. These data suggest that a heightened level of expression and stimulation of key signaling proteins underlie the expansion and differentiation processes that support pancreatic ontogeny and regeneration.

Insulin activates phospholipase C-gamma1 via a PI-3 kinase dependent mechanism in 3T3-L1 adipocytes

Previously we have shown that the insulin receptor and phospholipase C-gamma1 physically interact in the 3T3-L1 adipocyte cell line. In this study, we investigated the ability of insulin and PDGF to stimulate PLC-gamma1 enzyme activity as measured by PI-(4,5)P(2) hydrolysis. Both insulin and PDGF caused a rapid (<1 min) increase in PLC activity associated with the respective receptor. PDGF treatment resulted in a higher and more sustained stimulation of PLC-gamma1 activity compared to insulin (0.95 pmol/min/mg vs 0.68 pmol/min/mg). Furthermore, insulin and PDGF promoted increases in total cellular DAG, one of the products of PI-(4,5)P(2) hydrolysis. Insulin-stimulated PLC activity appears to be downstream of PI-3Kinase as the DAG increase was partially blocked by Wortmannin and addition of PI-(3,4,5)P(3) activated PLC-gamma1 in vitro. Inhibition of PLC using U73122 or an inhibitory peptide caused a decrease in insulin-stimulated 2-deoxyglucose transport and GLUT4 translocation that was rescued by the addition of OAG, a cell-permeable synthetic DAG.

Stimulation of MAPK cascades by insulin and osmotic shock: lack of an involvement of p38 mitogen-activated protein kinase in glucose transport in 3T3-L1 adipocytes

Osmotic shock and insulin stimulate GLUT4 translocation and glucose transport via mechanisms that are for the most part distinct yet convergent. In this article, we investigated the effect of osmotic shock and insulin on the activation of the mitogen-activated protein kinase (MAPK) cascades in differentiated 3T3-L1 adipocytes. The MAPKs are activated by phosphorylation on conserved tyrosine and threonine residues. Both sorbitol and insulin strongly stimulated extracellular regulated kinase (ERK) 1 and 2 phosphorylation (8- and 18-fold, respectively). In contrast, c-jun-NH2-terminal kinase (JNK)/stress-activated protein kinase (SAPK) phosphorylation was stimulated only by sorbitol (sevenfold) and not by insulin. Phosphorylation of p38 MAPK was stimulated strongly by sorbitol (22-fold) but weakly by insulin (2.7-fold). Measurement of intrinsic JNK and p38 MAPK activity confirmed the phosphorylation studies. JNK and p38 MAPK were activated only significantly by sorbitol. The MAPKs are phosphorylated by dual-specificity kinases (mitogen-activated ERK-activating kinase [MEK] or MAPK kinase [MKK]). As expected, sorbitol and insulin both stimulated MEK phosphorylation. MKK4 was phosphorylated only in response to sorbitol, and neither of the stimuli caused phosphorylation of MKK3 or 6. To determine the functional significance of the observed activation of p38 MAPK in response to insulin and osmotic shock, we used three pyridinyl imidazole p38 MAPK inhibitors, SB203580, SB202190, and PD169316. Insulin and osmotic shock-stimulated glucose transport was not inhibited by any inhibitor at concentrations that were shown to block p38 MAPK activity. Furthermore, activation of the p38 MAPK pathway by treatment of cells with anisomycin did not stimulate glucose transport. These results suggest that activation of the p38 MAPK pathway is not involved in the stimulation of glucose transport.

Association of the insulin receptor with phospholipase C-gamma (PLCgamma) in 3T3-L1 adipocytes suggests a role for PLCgamma in metabolic signaling by insulin

Phospholipase C-gamma (PLCgamma) is the isozyme of PLC phosphorylated by multiple tyrosine kinases including epidermal growth factor, platelet-derived growth factor, nerve growth factor receptors, and nonreceptor tyrosine kinases. In this paper, we present evidence for the association of the insulin receptor (IR) with PLCgamma. Precipitation of the IR with glutathione S-transferase fusion proteins derived from PLCgamma and coimmunoprecipitation of the IR and PLCgamma were observed in 3T3-L1 adipocytes. To determine the functional significance of the interaction of PLCgamma and the IR, we used a specific inhibitor of PLC, U73122, or microinjection of SH2 domain glutathione S-transferase fusion proteins derived from PLCgamma to block insulin-stimulated GLUT4 translocation. We demonstrate inhibition of 2-deoxyglucose uptake in isolated primary rat adipocytes and 3T3-L1 adipocytes pretreated with U73122. Antilipolytic effect of insulin in 3T3-L1 adipocytes is unaffected by U73122. U73122 selectively inhibits mitogen-activated protein kinase, leaving the Akt and p70 S6 kinase pathways unperturbed. We conclude that PLCgamma is an active participant in metabolic and perhaps mitogenic signaling by the insulin receptor in 3T3-L1 adipocytes.

Effects of dietary protein restriction on the fractional rates of protein synthesis in perfused rat hindlimb

The effects of dietary protein restriction on protein synthesis were investigated in perfused rat hindlimb. The fractional rate of protein synthesis was measured with [3H]phenylalanine in young adult (7-week-old) rats fed a low protein (5% casein) diet and a protein-free diet for 3 weeks. The low protein diet (LPD) allowed a moderate gain in body weight. The fractional rate of protein synthesis fell to 70% of the control value in LPD group and further fell to less than a half in the protein-free diet (PFD) group. Thus, the protein synthesis rate decreased as the dietary protein content was reduced. The fall of protein synthesis was mainly accompanied by the reduction of RNA activity (mg protein/mg RNA/day) rather than RNA concentration (RNA/protein). The rate of protein breakdown was calculated by subtracting growth rate from protein synthesis rate. The breakdown rate was decreased in LPD group and increased slightly in PFD group. From the low rates of protein synthesis and breakdown, it appears that dietary protein restriction, at least allowing a gain in body weight, makes the turnover rate slow down. The overall changes in protein synthesis obtained in the perfused hindlimb are consistent with the reported results in vivo.

Insulin- and thyroid hormone-independent adaptation of myofibrillar proteolysis to glucocorticoids

Myofibrillar protein breakdown in skeletal muscle progresses through two distinct phases in response to chronic glucocorticoid administration in the rat, i.e., an early phase lasting 4-5 days, during which proteolysis increases followed by a later phase during which proteolysis decreases. The possible involvement of insulin and the iodothyronines in this phenomenon has now been examined. Diabetic, thyroidectomized, and normal rats were treated with corticosteroid for 10-11 days, and at timed intervals muscle proteolysis was evaluated by measuring the release of 3-methyl-L-histidine (3-MH) and tyrosine from the perfused hindquarter as well as the excretion of 3-MH in the urine. Corticosterone (CTC) administration to normal rats increased plasma insulin, whereas plasma 3,5,3'-triiodothyronine responded with an early rise followed by a fall after 4-5 days. However, the biphasic response of myofibrillar proteolysis to chronic glucocorticoid treatment was not abolished in CTC-treated diabetic or thyroidectomized rats. CTC treatment increased release of tyrosine by perfused muscle of diabetic rats but, unlike 3-MH release, did not diminish later. Thus the adaptation of myofibrillar proteolysis to chronic glucocorticoid treatment appears to be independent of insulin and thyroid hormones. However, insulin may play a role in curtailing glucocorticoid-induced breakdown of nonmyofibrillar proteins.

Sensitivity of myofibrillar proteins to glucocorticoid-induced muscle proteolysis

Previous studies have reported that prolonged administration of pharmacological doses of glucocorticoids in young rats results in a rise in urinary 3-methyl-L-histidine (3-MH) excretion followed by a fall to initial levels by 8 days. To determine whether this response reflects events in skeletal muscle, protein breakdown in this tissue was evaluated using the perfused hindquarter preparation with rats treated with corticosterone (10 mg X 100 g-1 X day-1) for 2, 4, or 8 days. Myofibrillar and total cell proteolysis were evaluated by measuring the release of 3-MH and tyrosine, respectively, after inhibition of protein synthesis with cycloheximide. Corticosterone treatment resulted in an early increase (1-4 days) followed by a fall (4-8 days) in 3-MH excretion. 3-MH release by the perfused hindquarter of treated rats responded in a similar manner, in that its release increased at days 2 and 4 and decreased to control levels by day 8. On the other hand, corticosterone treatment did not affect the release of tyrosine by the perfused hindquarter. Corticosterone treatment diminished protein synthesis in muscle by 30-50% (P less than 0.01), which unlike 3-MH release by perfused muscle persisted throughout the treatment period. The data indicate that glucocorticoids specifically augment the breakdown of myofibrillar proteins in skeletal muscle. This response is attenuated with prolonged treatment and is unrelated to a loss of metabolic effectiveness of the steroid. Also our findings suggest that the breakdown of myofibrillar and nonmyofibrillar proteins might be regulated independently.

Synergism of triiodothyronine and corticosterone on muscle protein breakdown

The concerted effect of triiodothyronine (T3) and corticosterone on muscle protein synthesis and breakdown was studied. Thyroidectomized young male rats were treated with T3 (1.5 microgram/100 g body weight per day), corticosterone (10 mg/100 g body weight per day) and both T3 and corticosterone for 4 days. On the 3rd day of the experiment urine was collected to measure N tau-methylhistidine excretion as an index of muscle protein breakdown. On the last day of the experiment, the rates of protein synthesis in skeletal muscles were measured by the large-dose [3H]phenylalanine method. N tau-Methylhistidine excretion was slightly increased by T3 treatment and it was increased about 3-times by corticosterone treatment. When both T3 and corticosterone were administered, it was increased about 6-fold. The rate of muscle protein breakdown calculated from the difference between the rate of protein synthesis and the growth rate was consistent with these findings. The rate of muscle protein synthesis was increased by T3, and it was decreased by corticosterone. The rate was the same as that of the thyroidectomized control group when the animals were given T3 and corticosterone, showing that T3 restrained the inhibiting effect of corticosterone on muscle protein synthesis. The results indicate that a physiological level of T3 enhances the catabolic action of pharmacological doses of glucocorticoids on muscle protein breakdown.