Jeanette Bennett

High glucose up-regulates ENaC and SGK1 expression in HCD-cells

Background/Aim: Diabetic nephropathy is associated with progressive renal damage, leading to impaired function and end-stage renal failure. Secondary hypertension stems from a deranged ability of cells within the kidney to resolve and appropriately regulate sodium resorption in response to hyperglycaemia. However, the mechanisms by which glucose alters sodium re-uptake have not been fully characterised. Methods: Here we present RT-PCR, western blot and immunocytochemistry data confirming mRNA and protein expression of the serum and glucocorticoid inducible kinase (SGK1) and the α conducting subunit of the epithelial sodium channel (ENaC) in a model in vitro system of the human cortical collecting duct (HCD). We examined changes in expression of these elements in response to glucose challenge, designed to mimic hyperglycaemia associated with type 2 diabetes mellitus. Changes in Na+ concentration were assessed using single-cell microfluorimetry. Results: Incubation with glucose, the Ca2+-ionophore ionomycin and the cytokine TGF-β1 were all found to evoke significant and time-dependent increases in both SGK1 and αENaC protein expression. These molecular changes were correlated to an increase in Na+-uptake at the single-cell level. Conclusion: Together these data offer a potential explanation for glucose-evoked Na+-resorption and a potential contributory role of SGK1 and ENaCs in development of secondary hypertension, commonly linked to diabetic nephropathy.

Calcium-sensing receptor activation increases cell-cell adhesion and ß-cell function

Background/Aims: The extracellular calcium-sensing receptor (CaR) is expressed in pancreatic β-cells where it is thought to facilitate cell-to-cell communication and augment insulin secretion. However, it is unknown how CaR activation improves β-cell function. Methods: Immunocytochemistry and western blotting confirmed the expression of CaR in MIN6 β-cell line. The calcimimetic R568 (1µM) was used to increase the affinity of the CaR and specifically activate the receptor at a physiologically appropriate extracellular calcium concentration. Incorporation of 5-bromo-2’-deoxyuridine (BrdU) was used to measure cell proliferation, whilst changes in non-nutrient-evoked cytosolic calcium were assessed using fura-2-microfluorimetry. AFM-single-cell-force spectroscopy related CaR-evoked changes in epithelial (E)-cadherin expression to improved functional tethering between coupled cells. Results: Activation of the CaR over 48hr doubled the expression of E-cadherin (206±41%) and increased L-type voltage-dependent calcium channel expression by 70% compared to control. These changes produced a 30% increase in cell-cell tethering and elevated the basal-to-peak amplitude of ATP (50µM) and tolbutamide (100µM)-evoked changes in cytosolic calcium. Activation of the receptor also increased PD98059 (1-100µM) and SU1498 (1-100µM)-dependent β-cell proliferation. Conclusion: Our data suggest that activation of the CaR increases E-cadherin mediated functional tethering between β-cells and increases expression of L-type VDCC and secretagogue-evoked changes in [Ca2+]i. These findings could explain how local changes in calcium, co-released with insulin, activate the CaR on neighbouring cells to help ensure efficient and appropriate secretory function.

TGF-β1 Mediates Glucose-evoked Up-regulation of Connexin-43 Cell-to-cell Communication in HCD-cells

Background/Aims: In the current study we examined if the multifunctional cytokine TGF-β1 mediated glucose-evoked increases in connexin-43(Cx43)-mediated intercellular communication in cells of the human collecting duct (HCD). Methods: RT-PCR and western blot analysis were used to confirm mRNA and protein expression of TGF-β1 and Cx43 in HCD-cells. The effect of TGF-β1 and high glucose (25mM) on Cx43 protein expression, cytoskeletal organisation and cell-cell communication was determined in the presence/absence of TGF-β1 specific immuno-neutralising antibodies. Functional cell-cell communication was determined using Ca2+-microfluorimetry. Results: At 24hrs, high glucose (25mM) significantly increased Cx43 mRNA and protein expression. Changes were mimicked by TGF-β1 (2ng/ml) at low glucose (5mM). Both high glucose and TGF-β1 mediated changes were completely reversed by a pan-specific immuno-neutralising antibody to TGF-β. Furthermore, high glucose-evoked changes were inhibited by a TGF-β1-specific monoclonal antibody. Mannitol (25mM), an osmotic control for high glucose, failed to alter Cx43 expression. TGF-β1 evoked changes in Cx43 expression were biphasic. An early (4-8hr) transient decrease in expression was followed by an increase in protein expression (12-24hr). The decrease in Cx43 expression was paralleled by a transient reorganisation of the actin cytoskeleton, whilst increased Cx43 expression at 24hrs coincided with a TGF-β1 specific increase in touch-evoked transmission of Ca2+-signals between coupled cells. Conclusions: High glucose evoked a TGF-β1 mediated increase in Cx43 expression and gap-junction mediated cell-cell communication in HCD-cells. These changes may maintain epithelial integrity of the collecting duct following hyperglycaemic assault as observed in diabetes.