Regulation of Glucokinase As Lslets Adapt to Pregnancy
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Matschinsky FM, Magnuson MA (eds): Glucokinase and Glycemic Disease: From Basics to Novel Therapeutics. Front Diabetes. Basel, Karger, 2004, vol 16, pp 222-239 Regulationof Glucokinaseas lsletsAdapt to Pregnancy RobertL. Sorenson,Anthony J. Weinhaus,T. Clark Brelje Department of Genetics Cell Biology and Development, University of Minnesota Medical School, Minneapolis,Minn., USA Pregnancyis an occasionin the life history of the B-cell where there is an increasedneed for insulinthat'occurs over a relativelyshort period of time. This amountsto daysin rodentsand monthsin humans.The needfor enhanced islet function emergesas a consequenceof an increasein peripheralinsulin resistanceat the sametime as the placenta,a major targetorgan for insulin, develops.To accommodatethis increasein insulin demand,the islet must undergochanges that lead to increasedinsulin secretionunder normal glucose conditions. The primary short-termregulation of insulin secretionis achievedby ele- vating the glucoseconcentration. However, if this were the primary adaptive mechanismsduring pregnancy,there would be a need for persistenthyper- glycemia- a conditiondeleterious to the developingembryo, fetus and mother. Thus, in the face of this increaseddemand for insulin, islets must undergo structuraland functional changes.The outcomeof this long term upregulation of isletsmust be enhancedinsulin secretion at normalglucose levels. Failure of this long-termadaptive process can lead to gestationaldiabetes. Evidence for functional changes in islets during pregnancy first appearedin the 1960sshortly after the developmentof a sensitiveradioimm- unoassayfor insulin. Spellacyand coworkers[1 3] reportedthat there was a progressiveincrease in both fasting and glucose-stimulatedinsulin secretion throughoutthe courseof pregnancy.These and subsequentstudies led to the charucterizationof pregnancyas a condition of elevatedserum insulin levels, slightly lower blood glucose levels and peripheralinsulin resistance(see reviews[4, 5]). 300 E E' zso .a 'fr E zoo E G ; 300 .9 o X r<n'"" o 5, o E zoo c * roo f E q l (,50o 100 0 6 1012141820 0 6 101214171A20 0 610 12 14't820 Days pregnancy Days pregnancy Days pregnancy Fig. 1. F,ffect ofpregnancy on BrdU labeling ofislet nuclei (center panel) and extent ofglucose stimulated insulin secretionin rats (right panel) [9]. The increasein islet cell pro- liferation and insulin secretion correlateswith the onset of placental lactogen secretion (left panel). The two peaks of serum lactogen activity correspond to the secretion of PL-I and PLJI during rat pregnancy. lslet p-Cell Proliferation and Hypertrophy during Pregnancy Islets can respondto an increaseddemand for insulin by increasingislet' massand the functionalityof the B-cells.In fact, suggestionsthat isletsundergo changesduring pregnancywas suggestedas early as the 1930swhen it was first reportedthat there was an increasein total islet volume. Subsequently,a num- ber of studieshave shownthat the increasein islet massis due to both hyper- trophy and hyperplasia[see review, 6]. When we examinedDNA content/islet on day 20 andprotein contenton days l5 and 20 ofpregnancy,there wasa25o/o increasein DNA and a 50 and 100%increase in protein, respectively. Islet cell proliferationduring pregnancyin rats hasbeen examined by triti- atedthymidine incorporation and bromodeoxyuridine (BrdLf labeling.Tritiated thymidineincorporation was increased,2-to 3-fold on day 12-14 of pregnancy and then approachedcontrol levelsby day 19 [7, 8]. Similarly,there was a pro- gressiveincrease in BrdUlabeled nuclei from day l0 to 14 of pregnancyat which the labelingwas I 0-fold greaterthan in controls(fig. I ). Subsequently,the numberof labelednuclei/islet declined to control levelsby day l8 of pregnancy [9]. Theserates of BrdU labelingare consistent with a 30% increasein cell mass during pregnancy and correspond with the 25o/oincrease in DNA that was Glucokinase during Pregnancy 223 7 6 E 4 1 >U c 2 4 6 I 1012 2 4 6 8 1012 2 4 6 8 1012 =.E Oa CI o 5 '| 0 2 4 6 8 1012 2 4 6 8 1012 2 4 6 8 1012 Glucose (nM) 12 c €roo o o .EB J -c .E o6 o E o c p4 u 2.8 5.6 8.3 11.1 61012151920 Glucose (n^,f) Days pregnancy Sorenson/Weinhaus/Brelie observed.A study designedto determinethe numberof islets and islet volume during pregnancyindicated that the increasedislet volume resultsfrom growth of pre-existingislets and not from neo-formationof islets [10]. Thus, there is uniform agreementthat pregnancyresults in an increasein total islet mass.This growth is due to both B-cell hyperplasiaand hypertrophy. The increasein B-cell proliferation is first observedaround day 10 and corre- spondsto the observedincrease in placentallactogen [9]. The B-cell prolifera- tion peaksaround day 14 ofpregnancyand then returnsto control levelsby the end of pregnancy.From this dataone can surmisethat the capacityfor insulin secretion,based on islet mass,is increasedabout 2-fold during pregnancy. Insulin Secretion during Pregnancy During pregnancyin rodents,fasting serum insulin levels are increased aboutT5o/oand glucose levels are decreased by about25%ll l-131. Glucosetol- erancetests are normal, but showenhanced insulin secretion[11]. A similar sit- uationis observedin humansduring pregnancy [-5]. Therehave been relatively few studiesexamining insulin secretionfrom islets isolatedduring pregnancy. These studies showedthat pregnancyresults in enhancedglucose-stimulated insulin secretionfrom theseislets [8, ll, l4]. Greenet al. |4, 15] alsonoted that there was a leftward shift in the glucoseresponse curves for insulin secretion and insulin synthesis.This was the first report indicatingthat the thresholdfor glucose-stimulatedinsulin secretioncould be loweredbelow that observedunder normal, non-pregnantconditions. That is, an increasein the islet's sensitivityto glucosestimulation. Although these studies indicated that there was an increasein insulin secretionduring pregnancy,there were no studiesexamining the correlationof secretionparameters with changesin placentallactogen (PL) secretionthrough- out pregnancy.To investigatethe temporalprofile of changesin islets during pregnancy,we examinedinsulin secretionduring gestationin rats (fig. 2) I9l. Differencesin both the thresholdof glucose-stimulatedinsulin secretionand the amountof insulin releasedabove this thresholdcould be detectedbv dav l0 Fig. 2. The top six panels show glucose-dependentinsulin secretion from perfused pancreasesduring the course ofpregnancy in comparison to the controls [9]. The shift in the glucose-stimulationthreshold and abovethreshold insulin secretionis first detectedon day I 0. The differences increase until day 15 and then return to control levels by day 20 ofpreg- nancy.The bottom left panel showsthe ratio ofinsulin secretionduring pregnancy compared to controls for each glucose concentration.The bottom right panel showsthe fold increasein insulin secretioncompared to control at a normal blood glucose concentration of 5.6mM. Glucokinaseduring Pregnancy 225 (fig.2). By day l2,the thresholdwaslowered from5.7mM glucoseIo3.3mM, remainedat this level throughday 15, and returnedtowards normal by day 20. concomitant with the increasedsensitivity of B-cellsto glucose,the amountof insulin releasedabove the thresholdwas increased by day 12,peakedonday 15, and returned to control levels by day 20. This lowering of the thresholdfor glucose-stimulatedinsulin secretionis an important feature of islets as they adaptto pregnancy.It is only by this maneuverthat a large increasein insulin secretioncan be achievedat normal blood glucoselevels. The magnitudeof this effect canbe seenby comparingthe ratesof insulin secretionat 5.6mMglucose from pregnantanimals versus controls (fig. 2). The onsetofthe changesin islet cell proliferationand insulin secretioncor- relate with the appearanceof circulating PL [9]. However,pL remains elevated until the end of pregnancywhen islet cell proliferation and insulin secretionhave returnedto control levels.Since islets culturedin the presenceof pRL or pL show persistentincreases in islet cell dMsion and insulin secretion,it is likely that the increasein steroidsor other effectorsduring the later stagesofpregrrancy are coun- teracting the effects of PL. This hypothesisis supportedby experimentsin vitro whereislets are cultured with bothPRL andprogesterone [16]. In this case,insulin secretionand Brdu labeling increasedduring the first 4 days, but subsequently returnedto control levelsby day 8. This temporalpattern of changesin islet func- tion closely mimics that observed in islets during pregnancy [9]. Similarly, increasedplasma glucocorticoid levels during the later part of pregnancycould effectively reversethe lactogen-inducedupregulation of islet function by inhibit- ing insulin secretionand cell proliferationwhile increasingapoptosis [17]. The summaryof the evidencethat the adaptationof islets to pregnancyis mediatedby way of lactogenicactivity (i.e. pRL or pL) is shownin table l. In addition,the magnitudeof the effectsobserved with the homologouspRL or pL in vitro is comparableto thoseobserved during pregnancy. It should also be mentioned that the physiological relevance of pL secretion has been questionedbecause placentas of many speciesdo not producea distinct PL (e.g. rabbits,pigs, dogs, and cats) [18]. In addition, the occurrenceof normal pregnanciesin womenwith undetectableserum concen- trations of human PL, becauseof a gene deletion,has beenreported Ug-211. unfortunately, insulin secretionduring pregnancyhas not been examinedin either of these conditions.An