Serine/threonine protein kinase SGK1 in glucocorticoid-dependent transdifferentiation of pancreatic acinar cells to hepatocytes.
ABSTRACT Elevated glucocorticoid levels result in the transdifferentiation of pancreatic acinar cells into hepatocytes through a process that requires a transient repression of WNT signalling upstream of the induction of C/EBP-β. However, the mechanism by which glucocorticoid interacts with WNT signalling is unknown. A screen of microarray data showed that the serine/threonine protein kinase SGK1 (serum- and glucocorticoid-regulated kinase 1) was markedly induced in the model B-13 pancreatic rat acinar cell line after glucocorticoid treatment (which converts them into hepatocyte-like 'B-13/H' cells) and this was confirmed at the level of mRNA (notably an alternatively transcribed SGK1C form) and protein. Knockdown of SGK1 using an siRNA designed to target all variant transcripts inhibited glucocorticoid-dependent transdifferentiation, whereas overexpression of the human C isoform (and also the human SGK1F isoform, for which no orthologue in the rat has been identified) alone - but not the wild-type A form - inhibited distal WNT signalling Tcf/Lef transcription factor activity, and converted B-13 cells into B-13/H cells. These effects were lost when the kinase functions of SGK1C and SGK1F were mutated. Inhibition of SGK1 kinase activity also inhibited glucocorticoid-dependent transdifferentiation. Expression of SGK1C and SGK1F resulted in the appearance of phosphorylated β-catenin, and recombinant SGK1 was shown to directly phosphorylate purified β-catenin in vitro in an ATP-dependent reaction. These data therefore demonstrate a crucial role for SGK1 induction in B-13 cell transdifferentiation to B-13/H hepatocytes and suggest that direct phosphorylation of β-catenin by SGK1C represents the mechanism of crosstalk between glucocorticoid and WNT signalling pathways.
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ABSTRACT: Introduction: Expression of serum-and-glucocorticoid-inducible kinase-1 (SGK1) is low in most cells, but dramatically increases under certain pathophysiological conditons, such as glucocorticoid or mineralocorticoid excess, inflammation with TGFβ release, hyperglycemia, cell shrinkage and ischemia. SGK1 is activated by insulin and growth factors via phosphatidylinositide-3-kinase, 3-phosphoinositide-dependent kinase and mammalian target of rapamycin. SGK1 sensitive functions include activation of ion channels (including epithelial Na(+) channel ENaC, voltage gated Na(+) channel SCN5A transient receptor potential channels TRPV4 - 6, Ca(2+) release activated Ca(2+) channel Orai1/STIM1, renal outer medullary K(+) channel ROMK, voltage gated K(+) channels KCNE1/KCNQ1, kainate receptor GluR6, cystic fibrosis transmembrane regulator CFTR), carriers (including Na(+),Cl(-) symport NCC, Na(+),K(+),2Cl(-) symport NKCC, Na(+)/H(+) exchangers NHE1 and NHE3, Na(+), glucose symport SGLT1, several amino acid transporters), and Na(+)/K(+)-ATPase. SGK1 regulates several enzymes (e.g., glycogen synthase kinase-3, ubiquitin-ligase Nedd4-2) and transcription factors (e.g., forkhead transcription factor 3a, β-catenin, nuclear factor kappa B). Areas covered: The phenotype of SGK1 knockout mice is mild and SGK1 is apparently dispensible for basic functions. Excessive SGK1 expression and activity, however, contributes to the pathophysiology of several disorders, including hypertension, obesity, diabetes, thrombosis, stroke, fibrosing disease, infertility and tumor growth. A SGK1 gene variant (prevalence ∼ 3 - 5% in Caucasians and ∼ 10% in Africans) is associated with hypertension, stroke, obesity and type 2 diabetes. SGK1 inhibitors have been developed and shown to reduce blood pressure of hyperinsulinemic mice and to counteract tumor cell survival. Expert opinion: Targeting SGK1 may be a therapeutic option in several clinical conditions, including metabolic syndrome and tumor growth.Expert Opinion on Investigational Drugs 03/2013; · 4.74 Impact Factor
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ABSTRACT: The rat pancreatic “B-13” acinar cell is a stable progenitor cell line that differentiates into hepatocyte-like cells (B-13/H cells) in 2D un-coated plastic culture with simple culture media in response to glucocorticoid exposure. Examination of cytochrome P450 indicated that the expression of a range of genes were similar to freshly isolated hepatocytes and that these gene products were functional on the basis of spectrophotometrically-detectable reduced carbon-monoxide haemoprotein and metabolism of several drugs. Since normal hepatocytes readily de-differentiate under similar conditions, we hypothesized that B-13 cells have undergone a variety of alterations that stabilise a progenitor phenotype and restrict differentiation to hepatocytes only (which if capitulated in human cells, could generate a readily accessible supply of functional human hepatocytes in vitro). To examine this hypothesis, the B-13 karyotype; pluripotency-inducing transcription factor expression and forced over-expression of these factors in B-13 cells were examined. B-13 cells were also injected into NOD/SCID mice and engraftment and differentiation assessed by RT-PCR, Western blotting, immunohistochemistry and fluorescent in situ hybridization (FISH). B-13 cells expressed four pluripotency-inducing transcription factors c-Myc, Klf4, Oct4 and Sox2 with only c-Myc expression maintained after glucocorticoid treatment. Over-expression of the pluripotency-inducing transcription factors blocked B-13/H formation in response to glucocorticoid. Injection of B-13 cells into NOD/SCID mice resulted in their engraftment to the pancreas and liver, with restricted differentiation to hepatocytes in the liver. The cells did not engraft to any other tissues examined. The ability of B-13 cells to specifically generate functional hepatocytes in vitro in response to glucocorticoid is therefore associated with genetic rearrangements that may facilitate expression of genes associated with plasticity (without leading to pluripotency), which are repressed by glucocorticoid treatment.Toxicol Res. 01/2013; 2(6):308 - 320.