Knockdown of Pdcd4 results in induction of proprotein convertase 1/3 and potent secretion of chromogranin A and secretogranin II in a neuroendocrine cell line.
BACKGROUND
Pdcd4 (programmed cell death 4) is up-regulated during apoptosis and seems to play an important role as a tumour suppressor. To gain further insights into its biological functions, we suppressed Pdcd4 expression in the neuroendocrine cell line Bon-1 via siRNA (small interfering RNA) technology.


RESULTS
Using this cell line, we found that suppression of Pdcd4 resulted in an increased release of CgA (chromogranin A) and Sg II (secretogranin II), and was accompanied by an up-regulation of intracellular PC1 (proprotein convertase 1/3). The enhanced secretion of CgA and Sg II seemed to be mediated by an activation of protein kinase Akt via PI3K (phosphoinositide 3-kinase). In accordance with this, inhibition of PI3K activity and, thereby, reduced phosphorylation of Akt was shown to enhance Pdcd4 expression. Neither the PKC (protein kinase C) signal transduction cascade nor the MAPK (mitogen-activated protein kinase) pathway seemed to play a role in the regulation of CgA and Sg II secretion by Pdcd4.


CONCLUSIONS
CgA is considered to be a marker for neuroendocrine tumours, and up-regulation of PC1 has been reported in various types of cancers. The repression of PC1 by Pdcd4 may represent a novel mechanism for the function of Pdcd4 as a tumour suppressor. Our results are of particular interest, as we observed that pioglitazone, an oral medication used in the treatment of Type 2 diabetes, decreased Pdcd4 levels, activated Akt, increased CgA and Sg II secretion and augmented PC1 protein in Bon-1 cells. Enhanced PC1 levels, leading to improved processing of proinsulin and proglucagon, may contribute to the benefits of pioglitazone therapy. The in vivo relevance of our findings was highlighted by data indicating elevated CgA amounts in the sera of patients treated with pioglitazone. This is the first study connecting Pdcd4 levels, secretion behaviour of neuroendocrine cells and regulation of PI3K activity.
Faculty of Medicine, Department of Gastroenterology, Endocrinology and Metabolism, University of Marburg, Baldingerstrasse, D-35033 Marburg, Germany. lankatbu@staff.uni-marburg.de
Brigitte
B
Lankat-Buttgereit
Sabine
S
Müller
Harald
H
Schmidt
Klaus G
KG
Parhofer
Thomas M
TM
Gress
Rüdiger
R
Göke
D051017:Apoptosis Regulatory Proteins / Q000235:genetics* / Q000378:metabolism; D045744:Cell Line, Tumor; D053379:Chromogranin A / Q000378:metabolism*; D003924:Diabetes Mellitus, Type 2 / Q000188:drug therapy / Q000378:metabolism; D004789:Enzyme Activation; D055785:Gene Knockdown Techniques; D006801:Humans; D008297:Male; D055099:Neuroendocrine Cells / Q000187:drug effects / Q000378:metabolism*; D000077205:Pioglitazone; D045664:Proprotein Convertase 1 / Q000235:genetics* / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics; D016601:RNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D053381:Secretogranin II / Q000378:metabolism*; D015398:Signal Transduction; D045162:Thiazolidinediones / Q000494:pharmacology / Q000627:therapeutic use; D015854:Up-Regulation
D051017:Apoptosis Regulatory Proteins; D053379:Chromogranin A; C114487:PDCD4 protein, human; D034741:RNA, Small Interfering; D016601:RNA-Binding Proteins; D053381:Secretogranin II; D045162:Thiazolidinediones; D045664:Proprotein Convertase 1; D000077205:Pioglitazone