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Background and Purpose: Glycogen phosphorylase (GP) is the key enzyme for glycogen degradation. GP inhibitors (GPi-s) are glucose lowering agents that cause the accumulation of glucose in the liver as glycogen. Glycogen metabolism has implications in beta cell function. Glycogen degradation can maintain cellular glucose levels, which feeds into catabolism to maintain insulin secretion, and elevated glycogen degradation levels contribute to glucotoxicity. The purpose of this study was to assess whether influencing glycogen metabolism in beta cells by GPi-s affects the function of these cells.Experimental Approach: The effects of structurally different GPi-s were investigated on MIN6 insulinoma cells and in a mouse model of diabetes.Key Results: GPi treatment increased glycogen content and, consequently, the surface area of glycogen in MIN6 cells. Furthermore, GPi treatment induced insulin receptor β (InsRβ), Akt and p70S6K phosphorylation, as well as pancreatic and duodenal homeobox 1(PDX1) and insulin expression. In line with these findings, GPi-s enhanced non-stimulated and glucose-stimulated insulin secretion in MIN6 cells. The InsRβ was shown to co-localize with glycogen particles as confirmed by in silico screening, where components of InsR signalling were identified as glycogen-bound proteins. GPi-s also activated the pathway of insulin secretion, indicated by enhanced glycolysis, mitochondrial oxidation and calcium signalling. Finally, GPi-s increased the size of islets of Langerhans and improved glucose-induced insulin release in mice.Conclusion and Implications: These data suggest that GPi-s also target beta cells and can be repurposed as agents to preserve beta cell function or even ameliorate beta cell dysfunction in different forms of diabetes.Linked Articles: This article is part of a themed section on Inventing New Therapies Without Reinventing the Wheel: The Power of Drug Repurposing. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.2/issuetoc. [ABSTRACT FROM AUTHOR]

Both changes in intracellular calcium concentration ([Ca2+]i) and activation of certain protein kinase C (PKC) isoforms play a crucial role in keratinocyte functions. To better understand the interaction between these two signalling pathways we investigated the resting [Ca2+]i and the extracellular ATP-induced changes in [Ca2+]i on HaCaT cell clones overexpressing either the classical α or the β PKC isoform. These PKC isoenzymes were previously shown to decrease (α) or increase (β) cell proliferation and augment (α) or suppress (β) cell differentiation. Keratinocyte clones with decreased proliferation rate were found to have unaltered resting [Ca2+]i, but responded with greater calcium transients to the application of 180 μm of ATP. In contrast, clones with increased proliferation rate had elevated resting [Ca2+]i and suppressed calcium responses to ATP. Calcium transients on PKCβ clones displayed a faster falling phase. Each clone had a distinct purinergic receptor expression pattern, some of which paralleled the altered proliferation rate and calcium handling. Keratinocytes overexpressing PKCβ revealed decreased P2X1 and increased P2Y1 receptor expression as compared with the control or PKCα clones. The expression level of P2X7 was significantly increased in keratinocytes overexpressing PKCα. On the other hand neither the P2X2 nor the P2Y2 expression was altered significantly in the cell types investigated. These data indicate that a modified proliferation and differentiation pattern is associated with altered calcium handling in keratinocytes. The observations also suggest that different PKC isoenzymes have different effects on the phosphatidyl–inositol signalling pathway. [ABSTRACT FROM AUTHOR]

Keratinocyte proliferation and differentiation is strongly influenced by mechanical forces. We investigated the effect of osmotic changes in the development of HaCaT cells in culture using intracellular calcium measurements, electrophysiological recordings and molecular biology techniques. The application of hypotonic stress (174 mOsmol/l) caused a sustained hyperpolarization of HaCaT cells from a resting potential of −27 ± 4 to −51 ± 9 mV. This change was partially reversible. The surface membrane channels involved in the hyperpolarization were identified as chloride channels due to the lack of response in the absence of the anion. Cells responded with an elevation of intracellular calcium concentration to hypotonic stress, which critically depended on external calcium. The presence of phorbol-12-myristate-13-acetate in the culture medium for 12 h augmented the subsequent response to hypotonic stress. A sudden switch from iso- to hypotonic solution increased cell proliferation and suppressed the production of involucrin, filaggrin and transglutaminase, markers of keratinocyte differentiation. It is concluded that sudden mechanical forces increase the proliferation of keratinocytes through alterations in their membrane potential and intracellular calcium concentration. These changes together with additional modifications in channel expression and intracellular signalling mechanisms could underlie the increased proliferation of keratinocytes in hyperproliferative skin diseases. [ABSTRACT FROM AUTHOR]

In the present study, we investigated the ability of RNA interference technology to suppress TASK-2 potassium channel expression in human embryonic kidney (HEK293) cells stably transfected with TASK-2 cDNA and in rat isolated intact pulmonary arteries.Lipofectamine-induced transfection of a specific siRNA sequence targeted against TASK-2 resulted in a dose- and time-dependent decrease in TASK-2 channel protein expression. In siRNA-transfected cells the TASK-2 peak currents were significantly smaller than in control cells at every investigated pH, while the pH sensitivity was not altered. Using scrambled siRNA as a negative control, there were no significant changes in TASK-2 protein expression or current compared to mock-transfected cells.In TASK-2 siRNA-transfected small pulmonary arteries, but not in scrambled siRNA-treated vessels, myocyte resting membrane potential at pH 7.4 was significantly less negative and the hyperpolarisations in response to increasing pH from 6.4 to 8.4 were significantly smaller compared with control.The application of levcromakalim (10 μM), NS1619 (33 μM) and a potassium channel inhibitor cocktail (5 mM 4-aminopyridine, 10 mM tetraethylammonium chloride, 30 μM Ba2+ and 10 μM glibenclamide) had similar effects in control and in siRNA-transfected vessels. The TASK-1 (anandamide-sensitive) contribution to resting membrane potential was comparable in each group. Clofilium (100 μM) generated significantly smaller responses in transfected artery segments.These results suggest that RNA interference techniques are effective at inhibiting TASK-2 channel expression in cultured cells and in intact vessels and that TASK-2 channels have a functional role in setting the membrane potential of pulmonary artery myocytes.British Journal of Pharmacology (2006) 147, 496–505. doi:10.1038/sj.bjp.0706649; published online 23 January 2006 [ABSTRACT FROM AUTHOR]

Papp H, Czifra G, Lázár J, Gönczi M, Csernoch L, Kovács L, Bíró T. Protein kinase C isozymes regulate proliferation and high cell density-mediated differentiation in HaCaT keratinocytes. Protein kinase C (PKC) isoforms play pivotal roles in the regulation of differentiation of normal human epidermal keratinocytes (NHEK). In this study, we investigated the participation of the PKC system in the proliferation and high cell density-induced differentiation of the human immortalized keratinocyte line HaCaT. HaCaT keratinocytes possessed a characteristic PKC isoform pattern (PKCα, β, γ,δ,ε,η,θ,ζ), which altered during proliferation and differentiation. The GF109203X compound, a selective PKC inhibitor, suppressed the expressions of the late (granular cell) differentiation markers involucrin (INV) and filaggrin (FIL), and the terminal marker keratinocytes-specific transglutaminase-1 (TG), but did not affect the level of the early (spinous cell) marker keratin 10 (K10) and cellular proliferation. Phorbol 12-myristate 13-acetate (PMA), an activator of PKC, inhibited proliferation, elevated intracellular calcium concentration, decreased the expression of K10, and increased the expressions of INV, FIL, and TG. These data indicate that the endogenous activation of PKC regulates the expressions of the late differentiation markers, and that the exogenous activation of PKC by PMA results in the induction of terminal differentiation. Because the cellular effects of PMA were accompanied by differential down-regulations of the sensitive PKC isoforms in proliferating and differentiating cultures, our findings argue for the differential roles of the existing PKC isoforms in the regulation of cellular proliferation and high cell density-induced differentiation of HaCaT cells. [ABSTRACT FROM AUTHOR]

Abstract: Capacitive calcium influx is associated with the release of calcium from internal stores and participates in intracellular calcium homeostasis. In keratinocytes, its activation is linked to the stimulation of the phospho-inositide (PI) pathway and seems to be altered in psoriasis. An overnight treatment of isolated HaCaT keratinocytes with phorbol 12-myristate 13-acetate (PMA) selectively downregulated the classical, calcium-dependent protein kinase C (PKC) isoenzyme PKCα in preconfluent cells. This was parallelled by an increased capacitative calcium influx with no effects on the PI pathway. These observations were strengthened in measurements using cyclopiazonic acid which revealed a 47% increase in PMA pretreated as compared with control cells in the calcium influx rate through store-operated calcium channels (SOC-s) following the emptying of the intracellular calcium stores. In confluent as compared with preconfluent cultures PKCε was markedly increased, while other isoenzymes were not affected. In parallel, the kinetics of capacitative calcium influx were altered, showing clear inactivation. PMA pretreatment in these cells had little effect on PKCα but downregulated both PKCβ and PKCε, and did not increase the influx through SOC-s. These observations support the differential regulation of SOC-s by PKC and suggest the involvement of several PKC isoenzymes in human keratinocytes. [ABSTRACT FROM AUTHOR]