the early studies in the 1960s showed that the intestinal mucosa extract inhibited acid secretion in the dog stomach , which was designated  enterogastrone .  then ,  gastric inhibitory polypeptide ( gip )  was purified from porcine mucosal extracts and based on gastric acid inhibitory activity1 , 2 . dupr et  al.3 successfully showed that intravenous gip injection could enhance insulin secretion during intravenous glucose challenge in humans . several studies showed that gip was released by oral glucose loading or food absorption4 , 5 , and that gip also potentiated insulin secretion in a glucosedependent manner6 . thereafter , gip was spelled out as  glucosedependent insulinotropic polypeptide  reflecting its physiological insulinotropic activity , and was recognized as an  incretin . the sequence of purified gip peptide was determined and identified as consisting of 42 highly conserved amino acids among porcine , bovine and humans7 , 8 . gip142 was designated gip5000 ( natural gip ) , as its molecular weight is 4983.6 . whereas another lager molecule , gip8000 ( or 8kd gip ) , has been identified from porcine and human extracts from proximal jejunum , analyzed by ria after fractionation through gel filtration with different gip antisera9 , 10 . in addition to gut mucosa , several early studies showed that gip immunoreactivity was detectable in mammalian pancreatic extracts through ria . interestingly , a study showed that the pancreas was the major source of circulating gip in burmese pythons.11 in that study , both plasma gip and glucagon levels were sixfold increased by refeeding at 1  day after starvation . gip and glucagon were distributed more largely in the pancreas compared with the intestine or the stomach in the pythons . pancreatic distribution of gip remarkably decreased with refeeding , suggesting that gip was released from the pancreas as a part of digestive responses in the pythons , and concurrently increased the plasma concentration with glucagon . as some preliminary studies in the past showed that gip immunoreactivity through ria had been detectable in pancreatic extracts , several studies tried to elucidate whether the pancreatic tissue was actually releasing gip , and to find where gip was expressed in the pancreas . giplike material ,  but the authors could not exclude the possibility that a giplike peptide was absorbed from the circulation12 . another study examined various species including humans by immunohistochemistry , and showed that gip immunoreactivity could be expressed not only in the gut , but also in the islet cells13 . the gip antiserum used in that study was believed not to crossreact with either pancreatic glucagon or enteroglucagon . interestingly , many cells showing concomitant gip and glucagon immunoreactivity were observed in the surgical specimens from patients with pancreatic glucagonomas13 . however , a separate study concluded that gip was not present in the pancreas , stomach and large intestine , but was released from enteroendocrine cells of the duodenum and jejunum14 . furthermore , the authors indicated not gip but glicentinlike immunoreactivity was found in pancreatic cells . in these early studies , we need to consider limitations , as characterization or specificity of gip antisera or antibodies might be insufficient . these gip antisera or antibodies were polyclonal from rabbits or guinea pigs and raised against the whole gip molecule . in addition , the amino ( n ) terminus of gip and proglucagon gene products including pancreatic glucagon , glucagonlike peptide ( glp ) 1 , glp2 , glicentin and oxyntomodulin are well conserved in many species with higher homology . therefore , it was very difficult to exclude the possibility that gip antisera or antibodies might crossreact with some parts of the proglucagon gene products . meanwhile , buchan et  al.15 developed a mouse monoclonal antibody ( 3.65h ) specifically against the carboxyl ( c ) terminus of gip ( gip142 ) and compared with conventional rabbit and guineapig antisera to gip by immunochemical analysis . the analysis using this monoclonal antibody showed that cterminal immunoreactivity of gip142 was not observed in the mammalian pancreas , but abundantly found in the gut endocrine cells . as results from the study were definitely conclusive , few studies have been subsequently carried out , investigating gip distribution elsewhere except the gastrointestinal tract . the gip gene is translated to preprogip ( gip precursor ) messenger ribonucleic acid ( mrna ) with approximately 800 base pairs , and its transcript consists of 153 amino acids including a signal peptide of 21 residues16 . they showed that gip was coexpressed with prohormone convertase ( pc ) 1/3 in the intestinal sections from wildtype mice , but never with pc2 , when the identical monoclonal antibody15 ( 3.65h ) against cterminal gip142 was used for detection of gip . pc1/3 null mice were largely impaired in processing to gip142 , whereas pc2 null mice presented normal processing of gip142 , analyzed by gel filtration of the gut extracts through ria . collectively , they concluded that pc1/3 was essential and sufficient for progip processing to gip142 , whereas pc2 did not play a role in gip production at least in the gut kcells . in contrast , the authors investigated progip processing in  vitro using pc2transfected gh4 cells and cell line tc1.9 cells . they interestingly pointed out the possibility that pc2 was able to cleave progip to  other gip fragments ,  which were not usually present in the gut extracts17 . it has been well recognized that glp1 and pyy are concomitantly expressed in identical colon lcells . nowadays , a lot of knowledge has been accumulated that many endocrine cells in the gastrointestinal tract can secrete two or more hormones , but anatomical distribution or combination of hormones are supposed to be destined by localization in the cephalo  caudal axis18 . proglucagon and progip mrna expression was identified in lasercaptured single duodenal endocrine cells by reverse transcription polymerase chain reaction in human biopsied samples19 . in addition , immunoreactivity of glp1 and gip was coexpressed with glucokinase in the single gut cells ( l / k or k / lcells ) . interestingly , the proportion of k / l cells in the duodenum was significantly increased in newly diagnosed type  2 diabetic patients19 . in our study20 , double incretinpositive cells expressed both transcription factors pdx1 and pax6 , whereas glp1 single positive cells expressed pax6 , but not pdx1 , in the rat ileum ( figure  1 ) . the reporter assay showed that pax6 could activate both gip and proglucagon promoters , but pdx1 was able to enhance only the gip promoter , when the 2.9kb human gip promoter or the 2.4kb rat proglucagon promoter was examined . reimann et  al.21 developed novel transgenic mice expressing venus fluorescent protein under the proglucagon promoter . they sorted isolated cells from the gut of transgenic mice by flow cytometry , and examined the expression of gip immunoreactivity by enzymelinked immunosorbent assay . it was shown that pyy was observed in the proglucagonpositive ( venuspositive ) cells only from the colon , but gip was occasionally expressed in venuspositive gut cells from the upper intestine . collectively , gip and glp1 might be in part housemates in a gut endocrine cell . glp1 ( red ) , gip ( blue ) and transcription factors , pax6 or pdx1 ( green ) . arrow shows a k / l cell expressing both glp1 and gip , while arrowhead shows an l  cell only expressing glp1 . similar to the doubleincretin positive  gut k / lcells  expressing both gip and proglucagon genes19 , 20 , it is postulated that gip might be coexpressed with glucagon in pancreatic cells . in fact , gip mrna expression was examined by reverse transcription polymerase chain reaction and in  situ hybridization , and present in the mouse islets22 . immunohistological analysis showed that gip was localized in islet cells of mice , humans and pythons , using an antibody recognizing the midportion of gip ( figure  2 ) . however , gip immunoreactivity was detected in islets not from wildtype mice , but from pc2 null mice , when the cterminal gip antibody15 ( 3.65h ) was used for immunostaining ( figure  3 ) , suggesting that gip in the pancreas might be processed by pc2 . though progip is typically cleaved by pc1/3 into gip142 in the gut as discussed17 , there is a putative site kgkk ( mouse rgkk ) , which can be cleaved by pc2 in the residues of human progip22 . therefore , progip could be processed not only to gip142 in the gut , but also to the shorter fragment gip by pc2 , then ultimately to gip130 amidated by the action of peptidylglycine amidating monooxygenase in pancreatic cells . gip antibody against the midportion of gip was used for detection22 , 23 ( magnification : 400 ) . monoclonal cterminal specific antibody against gip142 ( 3.65h ) was used for detection15 ( magnification : 400 ) . in the perfusion study of mouse pancreas gip130 also promoted somatostatin release from the perfused stomach at pharmacological concentrations similar to gip14223 . active gip was detectable from the culturing media of isolated islets from mice and humans by a gip receptormediated camp 3,5cyclic adenosine monophosphate assay with a creluciferase construct . in addition , gip neutralizing antibody or gip receptor antibody diminished glucosestimulated insulin release from the isolated islet , suggesting that gip induced insulin release in an  intraislet  paracrine manner22 . unexpectedly , coimmunoreactivity of gip and pc2 was also observed in the gut when the antibody against the midportion of gip , but not the cterminal gip antibody15 ( 3.65h ) , was used for detection , suggesting that a subset of gut endocrine cells are secreting gip130 , but not gip142 , through progip processing by pc223 . in the developing pancreas of the human fetus , gip expression was also found in pancreatic cells22 . intense gip immunostaining was sparse , but largely coincident with insulin immunoreactivity at 10  weeks of fetal age . gip immunoreactivity was more prevalent and largely restricted to cell areas at 15  weeks . another study showed that gip mrna and immunoreactivity were detected in the islets of mouse embryos by reverse transcription polymerase chain reaction or by immunohistochemistry . both antisense and small interfering rna against gip decreased the number of pdx1positive and sox9positive cells , and inhibited the differentiation of cells in the cultured embryonic pancreas , suggesting that pancreatic gip might play a key role in the development of pancreatic islets24 . incretin 2015  that gip130 expression was enhanced with cell expansion concomitantly with decreased cell area in diabetic rodent models including nod ( non obese diabetes ) , akita and lowdose streptozotocin ( ldstz ) mice ( yf , ty , yt , jh , yt , aa , ym , hm ) . it is crucial to investigate whether gip130 has the therapeutic potential to treat diabetes and to protect cells against chronic hyperglycemia . it is also interesting whether gip130 has different actions to extrapancreatic tissues compared with gip142 . widenmaier et  al.25 synthesized a dipeptidylpeptidase  4resistant gip130 ( dgip130 ) , and evaluated its effects on glucose homeostasis and preservation of cell mass in several rodent diabetic models . dgip130 improved acute glucose tolerance and insulin secretion in both lean and obese rats during the intraperitoneal glucose tolerance test . in addition , chronic treatment with dgip130 inhibited cell apoptosis , preserved cell mass , and improved postprandial glycemia and insulin secretion in stz rats and zucker diabetic fatty rats . interestingly , exogenous dgip130 did not induce bodyweight gain . in in  vitro studies , dgip130 greatly reduced action on lipoprotein lipase activity in cultured 3t3l1 adipocytes compared with gip142 , although gip130 equivalently promoted insulin secretion and inhibited cell death against staurosporine . incretin 2015  that the longacting gip130 with pegylation could suppress the progression to hyperglycemia and improve glucose tolerance in ldstz mice . the gip130 analog did not affect glycemia in nondiabetic mice , but reduced glycated hemoglobin levels and improved glucose excursions after an intraperitoneal glucose tolerance test in ldstz mice , compared with nontreated ldstz mice . the gip130 analog also reduced cell expansion in the islets and suppressed plasma glucagon levels compared with nontreated ldstz mice ( unpubl . , we might be able to expect therapeutic potentials of gip130 for some diabetic conditions . collectively , gip130 might play important roles in glucose metabolism including insulin secretion and islet protection . however , further studies should be carried out to elucidate the mechanism of secretion and actions , pathophysiology , and the therapeutic potentials of gip130 .
abstractglucosedependent insulinotropic polypepide ( gip ) was first extracted from porcine gut mucosa and identified as  incretin  decades ago . 
 though early studies have shown the possible gip isoforms by gel filtration profiles from porcine or human intestinal extracts analyzed by radioimmunoassay ( ria ) , gip is currently believed to consist of 42 amino acids ( gip142 ) , which are released from gut kcells and promote postprandial insulin release . 
 in fact , gip142 is usually processed from progip by the action of prohormone convertase ( pc ) 1/3 in the gut . 
 gip expression is occasionally found in the intestinal glucagonlike peptide1secreting cells , suggesting gene expression of both gip and proglucagon can coexist in identical cells . 
 however , gip142 immunoreactivity is rarely found in cells or other pancreatic endocrine cells of wildtype mammals . 
 interestingly , we found that shortform gip130 is expressed in and released from pancreatic cells and a subset of enteroendocrine cells through progip processing by pc2 . 
 gip130 is also insulinotropic and modulates glucosestimulated insulin secretion in a paracrine manner . 
 it is also suggested that shortform gip130 possibly plays a crucial role for the islet development . 
 it has not been well elucidated whether expression of gip130 is modulated in the diabetic status , and whether gip130 might have therapeutic potentials . 
 our preliminary data suggest that shortform gip130 might play important roles in glucose metabolism .