beta-cell-specific deletion of the Igf1 receptor leads to hyperinsulinemia and glucose intolerance but does not alter beta-cell mass.

Regulation of glucose homeostasis by insulin depends on the maintenance of normal beta-cell mass and function. Insulin-like growth factor 1 (Igf1) has been implicated in islet development and differentiated function, but the factors controlling this process are poorly understood. Pancreatic islets produce Igf1 and Igf2, which bind to specific receptors on beta-cells. Igf1 has been shown to influence beta-cell apoptosis, and both Igf1 and Igf2 increase islet growth; Igf2 does so in a manner additive with fibroblast growth factor 2 (ref. 10). When mice deficient for the Igf1 receptor (Igf1r(+/-)) are bred with mice lacking insulin receptor substrate 2 (Irs2(-/-)), the resulting compound knockout mice show a reduction in mass of beta-cells similar to that observed in pancreas of Igf1r(-/-) mice (ref. 11), suggesting a role for Igf1r in growth of beta-cells. It is possible, however, that the effects in these mice occur secondary to changes in vascular endothelium or in the pancreatic ductal cells, or because of a decrease in the effects of other hormones implicated in islet growth. To directly define the role of Igf1, we have created a mouse with a beta-cell-specific knockout of Igf1r (betaIgf1r(-/-)). These mice show normal growth and development of beta-cells, but have reduced expression of Slc2a2 (also known as Glut2) and Gck (encoding glucokinase) in beta-cells, which results in defective glucose-stimulated insulin secretion and impaired glucose tolerance. Thus, Igf1r is not crucial for islet beta-cell development, but participates in control of differentiated function.

Impaired insulin secretion and beta-cell loss in tissue-specific knockout mice with mitochondrial diabetes.

Mitochondrial dysfunction is an important contributor to human pathology and it is estimated that mutations of mitochondrial DNA (mtDNA) cause approximately 0.5-1% of all types of diabetes mellitus. We have generated a mouse model for mitochondrial diabetes by tissue-specific disruption of the nuclear gene encoding mitochondrial transcription factor A (Tfam, previously mtTFA; ref. 7) in pancreatic beta-cells. This transcriptional activator is imported to mitochondria, where it is essential for mtDNA expression and maintenance. The Tfam-mutant mice developed diabetes from the age of approximately 5 weeks and displayed severe mtDNA depletion, deficient oxidative phosphorylation and abnormal appearing mitochondria in islets at the ages of 7-9 weeks. We performed physiological studies of beta-cell stimulus-secretion coupling in islets isolated from 7-9-week-old mutant mice and found reduced hyperpolarization of the mitochondrial membrane potential, impaired Ca(2+)-signalling and lowered insulin release in response to glucose stimulation. We observed reduced beta-cell mass in older mutants. Our findings identify two phases in the pathogenesis of mitochondrial diabetes; mutant beta-cells initially display reduced stimulus-secretion coupling, later followed by beta-cell loss. This animal model reproduces the beta-cell pathology of human mitochondrial diabetes and provides genetic evidence for a critical role of the respiratory chain in insulin secretion.

Salt-sensitive hypertension and reduced fertility in mice lacking the prostaglandin EP2 receptor.

Prostaglandins (PGs) are ubiquitous lipid mediators derived from cyclooxygenase metabolism of arachidonic acid that exert a broad range of physiologic activities, including modulation of inflammation, ovulation and arterial blood pressure. PGE2, a chief cyclooxygenase product, modulates blood pressure and fertility, although the specific G protein-coupled receptors mediating these effects remain poorly defined. To evaluate the physiologic role of the PGE2 EP2 receptor subtype, we created mice with targeted disruption of this gene (EP2-/-). EP2-/- mice develop normally but produce small litters and have slightly elevated baseline systolic blood pressure. In EP2-/- mice, the characteristic hypotensive effect of intravenous PGE2 infusion was absent; PGE2 infusion instead produced hypertension. When fed a diet high in salt, the EP2-/- mice developed profound systolic hypertension, whereas wild-type mice showed no change in systolic blood pressure. Analysis of wild-type and EP2-/- mice on day 5 of pregnancy indicated that the reduced litter size of EP2-/- mice is due to a pre-implantation defect. This reduction of implanted embryos could be accounted for by impaired ovulation and dramatic reductions in fertilization observed on day 2 of pregnancy. These data demonstrate that the EP2 receptor mediates arterial dilatation, salt-sensitive hypertension, and also plays an essential part in female fertility.

Plasma fibronectin supports neuronal survival and reduces brain injury following transient focal cerebral ischemia but is not essential for skin-wound healing and hemostasis.

Fibronectin performs essential roles in embryonic development and is prominently expressed during tissue repair. Two forms of fibronectin have been identified: plasma fibronectin (pFn), which is expressed by hepatocytes and secreted in soluble form into plasma; and cellular fibronectin (cFn), an insoluble form expressed locally by fibroblasts and other cell types and deposited and assembled into the extracellular matrix. To investigate the role of pFn in vivo, we generated pFn-deficient adult mice using Cre-loxP conditional gene-knockout technology. Here we show that pFn-deficient mice show increased neuronal apoptosis and larger infarction areas following transient focal cerebral ischemia. However, pFn is dispensable for skin-wound healing and hemostasis.

Identification of a sequence in the PEPCK gene that mediates a negative effect of insulin on transcription.

Phosphoenolpyruvate carboxykinase (PEPCK) governs the rate-limiting step in gluconeogenesis. Glucocorticoids and adenosine 3',5'-monophosphate (cAMP) increase PEPCK gene transcription and gluconeogenesis, whereas insulin has the opposite effect. Insulin is dominant, since it prevents cAMP and glucocorticoid-stimulated transcription. Glucocorticoid and cAMP response elements have been located in the PEPCK gene and now a 15-base pair insulin-responsive sequence (IRS) is described. Evidence for a binding activity that recognizes this sequence is presented.

Coexpression of glucose transporters and glucokinase in Xenopus oocytes indicates that both glucose transport and phosphorylation determine glucose utilization.

A Xenopus oocyte expression system was used to examine how glucose transporters (GLUT 2 and GLUT 3) and glucokinase (GK) activity affect glucose utilization. Uninjected oocytes and low rates of both glucose transport and phosphorylation; expression of GLUT 2 or GLUT 3 increased glucose phosphorylation approximately 20-fold by a low Km, endogenous hexokinase at glucose concentrations < or = 1 mM, but not at higher glucose concentrations. Coexpression of functional GK isoforms with GLUT 2 or 3 increased glucose utilization approximately an additional two- to threefold primarily at the physiologic glucose concentrations of 5-20 mM. The Km for glucose of both the hepatic and beta cell isoforms of GK, determined in situ, was approximately 5-10 mM when coexpressed with either GLUT 2 or GLUT 3. The increase in glucose utilization by coexpression of GLUT 3 and GK was dependent upon glucose phosphorylation since two missense GK mutations linked with maturity-onset diabetes, 182: Val-->Met and 228:Thr-->Met, did not increase glucose utilization despite accumulation of both a similar amount of immunoreactive GK protein and glucose inside the cell. Coexpression of a mutant GK and a normal GK isoform did not interfere with the function of the normal GK enzyme. Since the coexpression of GK and a glucose transporter in oocytes resembles conditions in the hepatocyte and pancreatic beta cell, these results indicate that increases in glucose utilization at glucose concentrations > 1 mM depend upon both a functional glucose transporter and GK.

Location and characterization of two widely separated glucocorticoid response elements in the phosphoenolpyruvate carboxykinase gene.

Chimeric genes were constructed by fusion of various regions of the 5'-flanking sequence from the phosphoenolpyruvate carboxykinase (GTP) (PEPCK) gene to the chloramphenicol acetyltransferase-coding sequence and to simian virus 40 splice and polyadenylation sequences. These were used to demonstrate that two glucocorticoid regulatory elements (GREs) combine to confer glucocorticoid responsiveness upon the PEPCK gene in H4IIE hepatoma cells. Both elements, a distal one whose 5' boundary is located between -1264 and -1111 base pairs and a proximal one located between -468 and -420 base pairs relative to the transcription initiation site, act independently, in various positions and orientations, and upon the heterologous thymidine kinase promoter. Each element accounts for half of the maximal response of the chimeric genes. Therefore, two widely separated enhancerlike elements contribute equally to the response of the PEPCK gene to glucocorticoid hormones. Neither of the PEPCK GREs contains the TGTTCT consensus sequence associated with most other GREs.

BZP, a novel serum-responsive zinc finger protein that inhibits gene transcription.

We report the fortuitous isolation of cDNA clones encoding a novel zinc finger DNA-binding protein termed BZP. The protein encoded is 114 kDa and contains eight zinc finger motifs, seven of which are present in two clusters at opposite ends of the molecule. Both finger clusters bound to the 9-bp sequence AAAGGTGCA with apparent Kds of approximately 2.5 nM. Two of the finger motifs within the amino- and carboxy-terminal finger clusters share 63% amino acid identity. BZP inhibited transcription of the herpes simplex virus thymidine kinase promoter when copies of the 9-bp target motif were linked in cis, suggesting that it functions as a transcriptional repressor. BZP mRNA and immunoreactivity were detected in several established cell lines but were most abundant in hamster insulinoma (HIT) cells, the parental source of the cDNAs. In mouse tissues, BZP mRNA and immunoreactivity were identified in cells of the endocrine pancreas, anterior pituitary, and central nervous system. Interestingly, in HIT cells proliferating in culture, BZP immunoreactivity was predominately nuclear in location, whereas it was usually located in the cytoplasm in most neural and neuroendocrine tissues. Serum deprivation of HIT cells caused BZP immunoreactivity to become predominantly cytoplasmic in location and attenuated its inhibitory effect on transcription, thereby suggesting that the both the subcellular location and the function of this protein are modulated by factors in serum.

Multiple elements in the upstream glucokinase promoter contribute to transcription in insulinoma cells.

beta-cell type-specific expression of the upstream glucokinase promoter was studied by transfection of fusion genes and analysis of DNA-protein interactions. A construct containing 1,000 bp of 5'-flanking DNA was efficiently expressed in HIT M2.2.2 cells, a beta-cell-derived line that makes both insulin and glucokinase, but not in NIH 3T3 cells, a heterologous cell line. In a series of 5' deletion mutations between bases -1000 and -100 (relative to a base previously designated +1), efficient expression in HIT cells was maintained until -280 bp, after which transcription decreased in a stepwise manner. The sequences between -180 and -1 bp contributing to transcriptional activity in HIT cells were identified by studying 28 block transversion mutants that spanned this region in 10-bp steps. Two mutations reduced transcription 10-fold or more, while six reduced transcription between 3- and 10-fold. Three mutationally sensitive regions of this promoter were found to bind to a factor that was expressed preferentially in pancreatic islet beta cells. The binding sites, designated upstream promoter elements (UPEs), shared a consensus sequence of CAT(T/C)A(C/G). Methylation of adenine and guanine residues within this sequence prevented binding of the beta-cell factor, as did mutations at positions 2, 3, and 5. Analysis of nuclear extracts from different cell lines identified UPE-binding activity in HIT M2.2.2 and beta-TC-3 cells but not in AtT-20, NIH 3T3, or HeLa cells; the possibility of a greatly reduced amount in alpha-TC-6 cells could not be excluded. UV laser cross-linking experiments supported the beta-cell type expression of this factor and showed it to be approximately 50 kDa in size. Gel mobility shift competition experiments showed that this beta-cell factor is the same that binds to similar elements, termed CT boxes, in the insulin promoter. Thus, a role for these elements (UPEs or CT boxes), and the beta-cell factor that binds to them, in determining the expression of genes in the beta cells of pancreatic islets is suggested.

Identification of basal and cyclic AMP regulatory elements in the promoter of the phosphoenolpyruvate carboxykinase gene.

Promoter elements important for basal and cyclic AMP (cAMP)-regulated expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene have been identified by analysis of a series of PEPCK promoter mutations in transfection experiments. Fusion genes containing wild-type and mutated PEPCK promoter sequences from -600 to +69 base pairs (bp) fused to the coding sequence for chloramphenicol acetyltransferase were studied. Internal deletion mutations that replaced specific bases with a 10-bp linker within the region from -129 bp to -18 bp of the PEPCK promoter were examined. In addition, wild-type and mutated DNA templates were used as probes in DNase I protection experiments to determine sites of protein-DNA interaction. The PEPCK promoter contains a binding site for nuclear factor 1-CAAT. Deletion of the 5' end of this binding site reduced the size of the DNase I footprint in this region but had no effect on promoter activity. In contrast, deletion or disruption of the 3' end of this binding site completely eliminated protein binding and reduced promoter activity by 50%. Deletion of core sequences of the cAMP regulatory element (CRE) resulted in loss of cAMP responsiveness and an 85% decrease in basal promoter activity, indicating that the CRE also functions as a basal stimulatory element. Mutation of the core sequence of the CRE resulted in loss of the DNase I footprint over the CRE. Internal deletions flanking the CRE showed no loss of induction by cAMP but did have reduced promoter activity. This delimits the CRE to an 18-bp region between nucleotides -100 and -82. Analysis of mutations that disrupted bases between the CRE and the initiation site identified a basal inhibitory element adjacent to a basal stimulatory element, both located just 3' of the CRE, as well as a basal stimulatory element coincident with the TATA consensus sequence centered at -27. These data demonstrate that several cis-acting elements are located within 130 nucleotides of the initiation site of the PEPCK gene and that the CRE is essential for both basal promoter activity and cAMP-regulated expression of this gene.

Tissue-specific, developmental, hormonal, and dietary regulation of rat phosphoenolpyruvate carboxykinase-human growth hormone fusion genes in transgenic mice.

The cytosolic phosphoenolpyruvate carboxykinase (PEPCK) gene is expressed in multiple tissues and is regulated in a complex tissue-specific manner. To map the cis-acting DNA elements that direct this tissue-specific expression, we made transgenic mice containing truncated PEPCK-human growth hormone (hGH) fusion genes. The transgenes contained PEPCK promoter fragments with 5' endpoints at -2088, -888, -600, -402, and -207 bp, while the 3' endpoint was at +69 bp. Immunohistochemical analysis showed that the -2088 transgene was expressed in the correct cell types (hepatocytes, proximal tubular epithelium of the kidney, villar epithelium of the small intestine, epithelium of the colon, smooth muscle of the vagina and lungs, ductal epithelium of the sublingual gland, and white and brown adipocytes). Solution hybridization of hGH mRNA expressed from the transgenes indicated that white and brown fat-specific elements are located distally (-2088 to -888 bp) and that liver-, gut-, and kidney-specific elements are located proximally (-600 to +69 bp). However, elements outside of the region tested are necessary for the correct developmental pattern and level of PEPCK expression in kidney. Both the -2088 and -402 transgenes responded in a tissue-specific manner to dietary stimuli, and the -2088 transgene responded to glucocorticoid stimuli. Thus, different tissues utilize distinct cell-specific cis-acting elements to direct and regulate the PEPCK gene.

Phosphoenolpyruvate carboxykinase is necessary for the integration of hepatic energy metabolism.

We used an allelogenic Cre/loxP gene targeting strategy in mice to determine the role of cytosolic phosphoenolpyruvate carboxykinase (PEPCK) in hepatic energy metabolism. Mice that lack this enzyme die within 3 days of birth, while mice with at least a 90% global reduction of PEPCK, or a liver-specific knockout of PEPCK, are viable. Surprisingly, in both cases these animals remain euglycemic after a 24-h fast. However, mice without hepatic PEPCK develop hepatic steatosis after fasting despite up-regulation of a variety of genes encoding free fatty acid-oxidizing enzymes. Also, marked alterations in the expression of hepatic genes involved in energy metabolism occur in the absence of any changes in plasma hormone concentrations. Given that a ninefold elevation of the hepatic malate concentration occurs in the liver-specific PEPCK knockout mice, we suggest that one or more intermediary metabolites may directly regulate expression of the affected genes. Thus, hepatic PEPCK may function more as an integrator of hepatic energy metabolism than as a determinant of gluconeogenesis.

Hepatocyte nuclear factor 3beta (Foxa2) is dispensable for maintaining the differentiated state of the adult hepatocyte.

Liver-specific gene expression is controlled by a heterogeneous group of hepatocyte-enriched transcription factors. One of these, the winged helix transcription factor hepatocyte nuclear factor 3beta (HNF3beta or Foxa2) is essential for multiple stages of embryonic development. Recently, HNF3beta has been shown to be an important regulator of other hepatocyte-enriched transcription factors as well as the expression of liver-specific structural genes. We have addressed the role of HNF3beta in maintenance of the hepatocyte phenotype by inactivation of HNF3beta in the liver. Remarkably, adult mice lacking HNF3beta expression specifically in hepatocytes are viable, with histologically normal livers and normal liver function. Moreover, analysis of >8,000 mRNAs by array hybridization revealed that lack of HNF3beta affects the expression of only very few genes. Based on earlier work it appears that HNF3beta plays a critical role in early liver development; however, our studies demonstrate that HNF3beta is not required for maintenance of the adult hepatocyte or for normal liver function. This is the first example of such functional dichotomy for a tissue specification transcription factor.

Hepatocyte-specific mutation establishes retinoid X receptor alpha as a heterodimeric integrator of multiple physiological processes in the liver.

A large number of physiological processes in the adult liver are regulated by nuclear receptors that require heterodimerization with retinoid X receptors (RXRs). In this study, we have used cre-mediated recombination to disrupt the mouse RXRalpha gene specifically in hepatocytes. Although such mice are viable, molecular and biochemical parameters indicate that every one of the examined metabolic pathways in the liver (mediated by RXR heterodimerization with PPARalpha, CARbeta, PXR, LXR, and FXR) is compromised in the absence of RXRalpha. These data demonstrate the presence of a complex circuitry in which RXRalpha is integrated into a number of diverse physiological pathways as a common regulatory component of cholesterol, fatty acid, bile acid, steroid, and xenobiotic metabolism and homeostasis.

Glucokinase gene structure. Functional implications of molecular genetic studies.

Glucokinase is expressed in both the liver and the pancreatic beta-cell and plays a key role in the metabolism of glucose by both tissues. Expression of this enzyme is differentially regulated; hepatic glucokinase is stimulated by insulin and repressed by cAMP, whereas beta-cell glucokinase activity is increased by glucose. Recently, the glucokinase gene has been characterized and was found to contain two different transcription control regions. One region regulates transcription of the gene in the liver, whereas the other region, which lies at least 12 kilobases further upstream, controls transcription in the pancreatic beta-cell. The finding of two different transcription control regions in a single glucokinase gene provides a genetic basis for the tissue-specific differential regulation of glucokinase and will serve as the basis for further studies to identify and characterize the different regulatory elements and factors in the liver and beta-cell, which are presumably involved. Comparison of different glucokinase cDNAs isolated from hepatic, insulinoma, and islet cDNA libraries indicates that at least three glucokinase isoforms are generated by differential RNA processing of the glucokinase gene transcripts. Whether any of these glucokinase isoforms are functionally unique remains to be determined.

Pancreatic beta-cell glucokinase: closing the gap between theoretical concepts and experimental realities.

There remains a wide gap between theoretical concepts and experimental realities in the enzyme kinetics and biochemical genetics of the pancreatic beta-cell glucokinase-glucose sensor. It is the goal of present efforts in many laboratories to bridge this gap. This perspective intends to provide a timely review of this crucial aspect of research in glucose homeostasis. It deals briefly with some fundamentals of glucokinase enzyme kinetics, offers some pertinent biochemical genetic considerations, takes stock of the current experimental database of the field by emphasizing human studies and referring to recent mouse studies, and ventures a few extrapolations into the future of this endeavor.

Effects of glucose on insulin secretion, glucokinase activity, and transgene expression in transgenic mouse islets containing an upstream glucokinase promoter-human growth hormone fusion gene.

We have analyzed in organ culture the effects of glucose on glucose-induced insulin secretion, glucokinase (GK) activity, and human growth hormone (hGH) expression in pancreatic islets from transgenic mice containing an upstream GK promoter-hGH fusion gene. Freshly isolated islets from these mice had a normal insulin secretory response to glucose but showed subtle defects after culture in low or high glucose for 4 days that may have been due to the accumulation of hGH in the culture media. Islets cultured from both normal and transgenic mice had approximately a fourfold induction of GK activity in response to an increased concentration of glucose in the culture media, whereas no such change in total islet hGH production was observed. Immunocytochemical localization of hGH in islets cultured in 3 mM glucose showed a pattern similar to that in freshly isolated islets. However, after culture in 30 mM glucose, hGH immunostaining became strikingly more heterogeneous. We conclude 1) that GK-hGH transgene expression does not appear to adversely affect glucose-stimulated insulin secretion in vivo or in freshly isolated islets, 2) that glucose does not induce transgene expression, thus providing additional evidence against an effect of glucose on GK gene transcription in the islet, and 3) that glucose stimulates the co-release of hGH with insulin, thereby enhancing the heterogeneous staining pattern seen among pancreatic beta-cells.

Glucokinase gene locus transgenic mice are resistant to the development of obesity-induced type 2 diabetes.

Transgenic mice that overexpress the entire glucokinase (GK) gene locus have been previously shown to be mildly hypoglycemic and to have improved tolerance to glucose. To determine whether increased GK might also prevent or diminish diabetes in diet-induced obese animals, we examined the effect of feeding these mice a high-fat high-simple carbohydrate low-fiber diet (HF diet) for 30 weeks. In response to this diet, both normal and transgenic mice became obese and had similar BMIs (5.3 +/- 0.1 and 5.0 +/- 0.1 kg/m2 in transgenic and non-transgenic mice, respectively). The blood glucose concentration of the control mice increased linearly with time and reached 17.0 +/- 1.3 mmol/l at the 30th week. In contrast, the blood glucose of GK transgenic mice rose to only 9.7 +/- 1.2 mmol/l at the 15th week, after which it returned to 7.6 +/- 1.0 mmol/l by the 30th week. The plasma insulin concentration was also lower in the GK transgenic animals (232 +/- 79 pmol/l) than in the controls (595 +/- 77 pmol/l), but there was no difference in plasma glucagon concentrations. Together, these data indicate that increased GK levels dramatically lessen the development of both hyperglycemia and hyperinsulinemia associated with the feeding of an HF diet.

Concordant glucose induction of glucokinase, glucose usage, and glucose-stimulated insulin release in pancreatic islets maintained in organ culture.

Using cultured islets as the experimental system, this study established dosage-response and time-dependency curves of the inductive glucose effect on glucose-stimulated insulin release, glucose usage, and glucokinase activity. Glucose-stimulated insulin release in islets cultured for 1, 2, or 7 days was increased as a function of glucose concentration in the culture medium and as a function of time. Glucose usage in the cultured islets showed a close relationship with glucose concentration in the culture medium at both 2 and 7 days of culture. Glucokinase activity increased in islets cultured for 1, 2, or 7 days as a function of increasing glucose concentrations in the culture medium and as a function of time. The V(max) of glucokinase in islets cultured for 7 days in medium containing 30 mM glucose was twice the value of freshly isolated islets and was almost fivefold higher than that in islets cultured for 7 days in 3 mM glucose. The glucose induction of glucose-stimulated insulin release, of glucose usage, and of glucokinase activity were tightly correlated. The biochemical mechanisms of glucose induction of islet glucokinase were further studied. Immunoblotting with an antibody against C-terminal peptide of glucokinase showed that densities of a 52,000-kD protein band from tissue extracts of islets cultured for 7 days in 3, 12, and 30 mM glucose were 25, 44, and 270% compared with that of extract from freshly isolated islets (100%). RNA blot analysis of glucokinase mRNA demonstrated virtually the same levels in fresh islets and islets after 7 days of culture in 3 or 30 mM glucose. The adaptive response of glucokinase to glucose appears therefore to be occurring at a translational or posttranslational site in cultured islets. These data greatly strengthen the concept that glucose is the regulator that induces the activity of glucokinase, which in turn determines the rate change of glucose usage as well as glucose-stimulated insulin release from beta-cells. Thus, the hypothesis that glucokinase is the glucose sensor of beta-cells is strengthened further.

Characterization of the Pal motifs in the upstream glucokinase promoter: binding of a cell type-specific protein complex correlates with transcriptional activation.

The upstream glucokinase (GK) promoter is expressed specifically in several different neural/neuroendocrine (NE) cell types, including the pancreatic beta-cell and pituitary corticotrope. Previously, a mutational and evolutionary analysis of this promoter identified two identical 9-bp motifs (TGGTCACCA) termed Pal-1 and Pal-2 that are essential for high level expression in HIT M2.2.2 cells, an insulinoma cell line. Here we show that these motifs are also necessary for efficient expression in AtT-20 cells, a corticotrope-derived cell line, and that proteins from both NE and non-NE cells bind to the Pal motifs, although the DNA-protein complexes differ by cell type. Complexes formed using nuclear extracts from NE cells contained an extra NE cell-specific band and differed in the relative abundance of two other bands when compared with non-NE cells, UV laser cross-linking experiments further supported the cell-specific binding of two proteins, 110 and 150 kDa in size, to these motifs. The presence or absence of the NE-specific band correlates with transcription of GK promoter fusion gene constructs, suggesting a key role for this protein in determining the cell-specific expression of GK. The Pal motifs themselves do not function as enhancers but seem to be essential components of a larger transcriptional regulatory domain that is active only in certain NE cells. Together, these studies suggest that the NE cell-specific expression of the upstream GK promoter involves the formation of a distinct protein complex on the two Pal motifs.