Negative regulation of pancreatic and duodenal homeobox-1 by somatostatin receptor subtype 5.
Somatostatin receptor subtype 5 (SSTR5) mediates the inhibitory effect of somatostatin and its analogs on insulin expression/secretion and islet cell proliferation. We provide biochemical and genetic evidence that SSTR5 exerted its physiological actions via down-regulating pancreatic and duodenal homeobox-1 (PDX-1), a β-cell-specific homeodomain-containing transcription factor. Cotransfection of SSTR5 with PDX-1 resulted in dose-dependent inhibition of PDX-1 expression in human embryonic kidney 293 cells. SSTR5 agonist RPL-1980 inhibited PDX-1 expression and abolished glucagon-like peptide 1-stimulated PDX-1 expression in mouse insulinoma β-TC-6 cells. SSTR5 knockdown by short hairpin RNA led to increased PDX-1 expression that was accompanied by enhanced insulin secretion stimulated by high glucose in β-TC6 cells and alternated expressions of cell cycle proteins that favor cell proliferation in mouse insulinoma MIN6 cells. Quantitative RT-PCR analysis showed that cotransfected SSTR5 inhibited PDX-1 mRNA expression, whereas knockdown of SSTR5 increased PDX-1 mRNA expression. In addition, we found that cotransfected wild-type SSTR5 increased PDX-1 ubiquitination in human embryonic kidney 293 cells, whereas SSTR5 P335L, a hypofunctional single nucleotide polymorphism of SSTR5, inhibited PDX-1 ubiquitination. SSTR5 knockout resulted in increased expression of PDX-1, insulin, and proliferating cell nuclear antigen in the islets of sstr(-/-) mice. Immunohistochemistry analysis showed that SSTR5 P335L was associated with elevated expression of PDX-1 in human pancreatic neuroendocrine tumor. Taken together, our studies demonstrated that SSTR5 is a negative regulator for PDX-1 expression and that SSTR5 may mediate the inhibitory effects of somatostatin and its analogs on insulin expression/secretion and cell proliferation via down-regulating PDX-1 at both transcriptional and posttranslational levels.
Department of Surgery, David Geffen School of Medicine at University of California, Los Angeles, California 90095, USA.
Guisheng
G
Zhou
Shi-He
SH
Liu
Kelly M
KM
Shahi
Hua
H
Wang
Xueyan
X
Duan
Xia
X
Lin
Xin-Hua
XH
Feng
Min
M
Li
William E
WE
Fisher
Francesco J
FJ
Demayo
David
D
Dawson
F Charles
FC
Brunicardi
D000818:Animals; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D015536:Down-Regulation; D052216:Glucagon-Like Peptide 1 / Q000378:metabolism; D005947:Glucose; D057809:HEK293 Cells; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000096:biosynthesis* / Q000378:metabolism; D000078790:Insulin Secretion; D007340:Insulinoma; D051379:Mice; D018345:Mice, Knockout; D020641:Polymorphism, Single Nucleotide; D018809:Proliferating Cell Nuclear Antigen / Q000096:biosynthesis; D034622:RNA Interference; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering; D017481:Receptors, Somatostatin / Q000235:genetics / Q000378:metabolism*; D013004:Somatostatin / Q000378:metabolism*; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*; D054875:Ubiquitination
D018398:Homeodomain Proteins; D007328:Insulin; D018809:Proliferating Cell Nuclear Antigen; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D017481:Receptors, Somatostatin; D015534:Trans-Activators; C084010:pancreatic and duodenal homeobox 1 protein; D013004:Somatostatin; D052216:Glucagon-Like Peptide 1; C094573:somatostatin receptor 5; D005947:Glucose