Acinar Origin Research Articles

Morphologic and Ancillary Studies of Pediatric Acinic Cell Carcinoma: A Single Institute Experience.

Acinic cell carcinoma (AciCC) is the second most common pediatric malignant salivary gland tumor. However, there are limited pathology publications about this tumor in the pediatric population. We describe four pediatric AciCC cases diagnosed between 2000 and 2021 in our institute. Reticulin histochemistry plus immunohistochemistry for NR4A3 and DOG1 were performed on all cases. Histologically, all four cases featured a tumor-associated lymphoid proliferation and collagenous stroma, in which two formed central scars. The tumors were predominantly solid, with a lobular pattern and variably sized dilated spaces, including one case with focal microcysts. High-grade transformation was not observed in any of our cases. Reticulin stain and immunohistochemistry for NR4A3 showed distinct features between AciCC and non-neoplastic salivary gland parenchyma. DOG1 immunohistochemistry confirmed the acinar origin of AciCC. Our study reveals that pediatric AciCCs often present with tumor-associated lymphoid proliferation (TALP) and sclerosis. Special stains such as reticulin histochemistry and NR4A3 immunohistochemistry are helpful to separate tumor from adjacent benign parenchyma. The ancillary study is helpful for the diagnosis of small specimens. Our study is limited by its low case number, but we hope that our results will promote more studies on this rare salivary gland tumor in the pediatric population.

An In Vitro Barrier Model of the Human Submandibular Salivary Gland Epithelium Based on a Single Cell Clone of Cell Line HTB-41: Establishment and Application for Biomarker Transport Studies.

The blood–saliva barrier (BSB) consists of the sum of the epithelial cell layers of the oral mucosa and salivary glands. In vitro models of the BSB are inevitable to investigate and understand the transport of salivary biomarkers from blood to saliva. Up to now, standardized, cell line-based models of the epithelium of the submandibular salivary gland are still missing for this purpose. Therefore, we established epithelial barrier models of the submandibular gland derived from human cell line HTB-41 (A-253). Single clone isolation resulted in five different clones (B2, B4, B9, D3, and F11). Clones were compared to the parental cell line HTB-41 using measurements of the transepithelial electrical resistance (TEER), paracellular marker permeability assays and analysis of marker expression for acinar, ductal, and myoepithelial cells. Two clones (B9, D3) were characterized to be of acinar origin, one clone (F11) to be of myoepithelial origin and one isolation (B4) derived from two cells, to be presumably a mixture of acinar and ductal origin. Clone B2, presumably of ductal origin, showed a significantly higher paracellular barrier compared to other clones and parental HTB-41. The distinct molecular identity of clone B2 was confirmed by immunofluorescent staining, qPCR, and flow cytometry. Experiments with ferritin, a biomarker for iron storage, demonstrated the applicability of the selected model based on clone B2 for transport studies. In conclusion, five different clones originating from the submandibular gland cell line HTB-41 were successfully characterized and established as epithelial barrier models. Studies with the model based on the tightest clone B2 confirmed its suitability for transport studies in biomarker research.

Stress or injury induces cellular plasticity in salivary gland acinar cells.

Salivary gland function is severely disrupted by radiation therapy used to treat patients diagnosed with head and neck cancer and by Sjögren's syndrome. The resulting condition, which results in xerostomia or dry mouth, is due to irreversible loss of the secretory acinar cells within the major salivary glands. There are presently no treatments for the resolution of xerostomia. Cell-based approaches could be employed to repopulate acinar cells in the salivary gland but investigations into potential therapeutic strategies are limited by the challenges of maintaining and expanding acinar cells in vitro. We investigate the encapsulation of salivary gland cell aggregates within PEG hydrogels as a means of culturing secretory acinar cells. Lineage tracing was used to monitor the fate of acinar cells isolated from murine submandibular gland (SMG). Upon initial formation in vitro, SMG aggregates comprise both acinar and duct cells, with the majority cells of acinar origin. With longer culture times, acinar cells significantly decreased the expression of specific markers and activated the expression of keratins normally found in duct cells. A similar acinar-to-duct cell transition was also observed in vivo, following duct ligation injury. These results indicate that under conditions of stress (mechanical and enzymatic isolation from glands) or injury (duct ligation), salivary gland acinar cells exhibit plasticity to adopt a duct cell phenotype.

Atypical flat lesions derive from pancreatic acinar cells

ObjectivesPancreatic ductal adenocarcinoma (PDAC) is thought to derive from different precursor lesions including the recently identified atypical flat lesions (AFL). While all precursor lesions and PDAC share ductal characteristics, there is an ongoing debate about the cellular origin of the different PDAC precursor lesions. In particular, pancreatic acinar cells have previously been shown to display a remarkable plasticity being able to undergo ductal dedifferentiation in the context of oncogenic stimuli. MethodsHistological analyses were performed in a murine PDAC model that specifically expresses oncogenic Kras in adult pancreatic acinar cells. Occurrence, characterization, and lineage tracing of AFLs were investigated. ResultsUpon expression of oncogenic Kras in adult pancreatic acinar cells, AFLs with typical morphology and expression profile arise. Lineage tracing confirmed that the AFLs were of acinar origin. ConclusionsUsing a murine PDAC model, this study identifies pancreatic acinar cells as a cellular source for AFLs.

A Proposal to Unify the Classification of Breast and Prostate Cancers Based on the Anatomic Site of Cancer Origin and on Long-term Patient Outcome

The similarity between the structure and function of the breast and prostate has been known for a long time, but there are serious discrepancies in the terminology describing breast and prostate cancers. The use of the large, thick-section (3D) histology technique for both organs exposes the irrationality of the breast cancer terminology. Pathologists with expertise in diagnosing prostate cancer take the anatomic site of cancer origin into account when using the terms AAP (acinar adenocarcinoma of the prostate) and DAP (ductal adenocarcinoma of the prostate) to distinguish between the prostate cancers originating primarily from the fluid-producing acinar portion of the organ (AAP) and the tumors originating either purely from the larger ducts (DAP) or from both the acini and the main ducts combined (DAP and AAP). Long-term patient outcome is closely correlated with the terminology, because patients with DAP have a significantly poorer prognosis than patients with AAP.The current breast cancer terminology could be improved by modeling it after the method of classifying prostate cancer to reflect the anatomic site of breast cancer origin and the patient outcome. The long-term survival curves of our consecutive breast cancer cases collected since 1977 clearly show that the non-palpable, screen-detected breast cancers originating from the milk-producing acini have excellent prognosis, irrespective of their histologic malignancy grade or biomarkers. Correspondingly, the breast cancer subtypes of truly ductal origin have a significantly poorer outcome, despite recent improvements in diagnosis and therapy. The mammographic appearance of breast cancers reflects the underlying tissue structure. Addition of these “mammographic tumor features” to the currently used histologic phenotypes makes it possible to distinguish the breast cancer cases of ductal origin with a poor outcome, termed DAB (ductal adenocarcinoma of the breast), from the more easily managed breast cancers of acinar origin, termed AAB (acinar adenocarcinoma of the breast), which have a significantly better outcome. This simple and easily communicable terminology could lead to better communication between the diagnostic and therapeutic team members and result in more rational treatment planning for the benefit of their patients.

Canine Pancreatic Clear Acinar Cell Carcinoma Showing an Unusual Mucinous Differentiation

A small solitary pancreatic nodule was identified in a 10-year-old mixed breed male dog. Microscopically, the lesion consisted of neoplastic clear acinar cells with diffuse, marked mucin production. Immunohistochemical and ultrastructural investigations confirmed the exocrine acinar origin of the tumour. The tumour was classified as a pancreatic clear acinar cell carcinoma (ACC) showing an unusual mucinous differentiation.

Abstract 2384: A novel mouse model for the study of c-MYC during initiation and progression of pancreatic ductal adenocarcinoma

Abstract Pancreatic cancer is one of the most deadly human malignant diseases. The median 5-year survival rate of patients is less than 5%. During the progression of precursor lesions into PDAC, somatic mutations within critical tumor susceptibility loci accumulate, including Kras, Cdkn2a, and Trp53. Also, overexpression of the c-Myc oncogene has been reported in a subset (i.e. between 30-40%) of primary PDACs and in more than half of pancreatic cancer cell lines. In addition to these published reports, we have analyzed the expression of the c-Myc oncoprotein within PanIN lesions as well as primary and metastatic PDACs and found this oncoprotein is already present in the earliest precursor lesions (PanINs). This finding indicates that upregulation of c-Myc is one of the earliest events in neoplastic transformation in the pancreas. Despite the significant role of c-Myc in the onset of human pancreatic cancer, this oncoprotein was found to be present only in a subset of primary PDACs and metastatic cases, implying that not all cases of human PDACs are “addicted” to this oncogene. To experimentally address whether c-Myc is required for the survival of cancer cells in primary and metastatic lesions that express this transcription factor, we have developed a novel mouse model that allows a spatially and temporally controlled expression of genes in a ligand (tetracycline)-controlled manner. Transgenic mice expressing c-Myc in the pancreatic ductal epithelium developed malignancies within 6 months. The majority of these tumors (about 75%) are ductal lesions and about 25% are poorly differentiated cancers that lack expression of CK19 and might be of acinar origin. Ablation of c-Myc expression in PanIN lesions as well as primary pancreatic cancers results in cell death and tumor regression, suggesting that the proliferation and survival of neoplastic pancreatic cells depend on the expression of the transforming oncogene. Our recent observations also show that the expression of c-Myc itself is sufficient to cause primary tumors that sporadically metastasize to the liver. These metastatic lesions in the mouse exhibit similar histopathological features and CK19 expression comparable to human PDAC. In summary, we have developed a novel mouse model in which c-Myc (and potentially other oncogenes) can be expressed in the pancreas of adult animals in a temporally and spatially controlled manner. Being able to regulate the expression of transforming oncogenes prior to and following neoplastic transformation, allows us to assess the continued significance of cancer initiating mutations at defined stages of the progressing disease. The in-depth characterization of this newly developed pancreatic cancer model will permit us to search for biomarkers that could be valuable to detect cancer shortly after disease initiation in vivo. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2384. doi:10.1158/1538-7445.AM2011-2384

This Month in Gastroenterology

This Month in Gastroenterology

Notch Signaling as Gatekeeper of Rat Acinar-to-β-Cell Conversion in Vitro

Exocrine acinar cells in the pancreas are highly differentiated cells that retain a remarkable degree of plasticity. After isolation and an initial phase of dedifferentiation in vitro, rodent acinar cells can convert to endocrine beta-cells when cultured in the presence of appropriate factors. The mechanisms regulating this phenotypic conversion are largely unknown. Using rat acinar cell cultures, we studied the role of Notch signaling in a model of acinar-to-beta-cell conversion. We report a novel lectin-based cell labeling method to demonstrate the acinar origin of newly formed insulin-expressing beta-cells. This method allows for specific tracing of the acinar cells. We demonstrate that growth factor-induced conversion of adult acinar cells to beta-cells is negatively regulated by Notch1 signaling. Activated Notch1 signaling prevents the reexpression of the proendocrine transcription factor Neurogenin-3, the key regulator of endocrine development in the embryonic pancreas. Interfering with Notch1 signaling allows modulating the acinar cell susceptibility to the differentiation-inducing factors. Its inhibition significantly improves beta-cell neoformation with approximately 30% of acinar cells that convert to beta-cells. The newly formed beta-cells mature when transplanted ectopically and are capable of restoring normal blood glycemia in diabetic recipients. We report for the first time an efficient way to reprogram one third of the acinar cells to beta-cells by adult cell type conversion. This could find application in cell replacement therapy of type 1 diabetes, provided that it can be translated from rodent to human models.

In Vivo Lineage Tracing Defines the Role of Acinar-to-Ductal Transdifferentiation in Inflammatory Ductal Metaplasia

Chronic injury results in regeneration of normal pancreatic tissue and formation of a metaplasia of ductal phenotype. Metaplastic ductal lesions are seen in pancreatitis as well as in specimens of pancreatic cancer and are thought to represent a condition with increased risk of neoplasia. Acinar-to-ductal transdifferentiation is thought to be the source of this metaplasia. This has been suggested for flat duct-like lesions called tubular complexes and for lesions exhibiting a mucinous metaplasia. However, available studies are based on interpretation of static data rather than on direct evidence. Transdifferentiation from acinar to ductal cells has never been confirmed in the adult pancreas. Here, we use Cre-loxP-based genetic lineage tracing in vivo to investigate whether transdifferentiation of acinar cells contributes to regeneration and metaplasia in pancreatitis. The results show that transdifferentiation does not play a role in regeneration of normal tissue. Acinar cells are regenerated by preexisting acinar cells and not from other cell types. Three different types of metaplastic ductal lesions are observed and analyzed. Whereas the majority of metaplastic lesions are not of acinar origin, acinar-to-ductal transdifferentiation is identified in a minority of mucinous metaplastic lesions. Here, we provide direct evidence that acinar-to-ductal transdifferentiation occurs in the adult pancreas in vivo. However, it accounts for only a minority of metaplastic lesions.

Characterization of benign and malignant prostate epithelial Hoechst 33342 side populations

The prostate epithelial stem cell has been proposed as the primary origin of neoplastic change in prostate cancer. However, the isolation and characterization of unexpanded prostate epithelial stem cells have proven problematic. A prostate epithelial side population (SP) has been isolated utilizing a modified Hoechst 33342 dye efflux assay from both benign and malignant prostate tissue. CD45(-ve), integrin alpha2(+ve) Hoechst 33342 SP and NSP cells were isolated by FACS, immunophenotyped and functionally characterized in 3D culture. FACS analysis revealed a verapamil sensitive SP accounting for 0.93 +/- 0.12% and 0.57 +/- 0.11% of the total epithelial population from both benign and malignant prostates. The benign SP phenotype revealed a heterogeneous cell population consisting predominantly of small basal cells containing minimal cytoplasm. Conversely, the malignant SP was of undetermined acinar origin and with a complete loss of expression of the CDK2 inhibitor p21(WAF1/Cip1). In vitro androgen-enhanced 3D culture of the benign and malignant SP cells led to the production of spheroids which had acinus like morphology and expressed primitive and basal cell markers. Incorporation of the CD133 marker isolated a further SP sub-fraction accounting for 0.037 +/- 0.01% of epithelial cells. Our observations are consistent with the Hoechst 33342 dye efflux assay isolating a stem cell enriched population which can be further sub-fractionated by CD133 selection. Moreover, the loss of the CDK inhibitor in malignancy is consistent with the hypothesis that neoplastic change originates in the stem cell compartment.

Endotoxin promotes preferential periportal upregulation of VLDL secretion in the rat liver

Zonation affects liver parenchymal cell function and metabolism as well as nonparenchymal cell activation, but whether VLDL production is zonated has yet to be elucidated. Infection induces enhanced VLDL secretion by the liver. Ex vivo studies were undertaken to examine the liver heterogeneity for VLDL formation and secretion and their in vivo response to endotoxin. Highly pure periportal (PP) and perivenous (PV) hepatocytes were isolated from fasted lipopolysaccharide-treated, fasted, and fed rats. They were used to assess their capacity to release VLDL-apolipoprotein B (apoB) and lipid classes in relation to de novo lipid synthesis and the expression of genes crucial to VLDL production. Despite the common superior ability of PP hepatocytes for lipid release and zonal differences in lipid synthesis, zonated secretion of VLDL particles was observed in septic but not in normal fed or fasted livers. The endotoxin-induced apoB secretion was more accentuated in PP hepatocytes; this was accompanied by a preferential PP increase in apoB and microsomal triglyceride transfer protein mRNA levels, whereas lipogenesis indicators were, if anything, similarly modified in hepatocytes of either acinar origin. We conclude that PP and PV hepatocytes exhibited similar capabilities for VLDL formation/secretion in normal conditions; however, the endotoxic pressure did zonate periportally.

Acinar cell cystadenocarcinoma of the pancreas: Report of rare case and review of the literature

Most exocrine pancreatic tumors are of ductal origin, whereas acinar cell adenocarcinomas are unusual (1% to 2% of all exocrine pancreatic neoplasms). We recently found a cystic adenocarcinoma of the pancreatic body whose cells had the characteristics of acinar cells, which we term acinar cell cystadenocarcinoma. Macroscopically, this tumor consists of a large multilocular cystic mass with a pseudocapsule and a spongy appearance on the cut surface. Microscopically, the cysts are lined by a single layer of cuboid/columnar cells. The cytoplasm has the characteristics of acinar cells, with eosinophilic granules in the apex and prominent nucleoli. Immunohistochemically, the cells express α1-antitrypsin, trypsin, and lipase in their cytoplasm, thus confirming the acinar origin of the tumor. A review of the literature revealed only 5 other cases of this tumor reported since its first description in 1981. Follow-up data are available for 4 of these; all of the affected patients had metastases at presentation or a few months later, and 2 died of the disease, at 13 and 37 months after diagnosis. Although this variant of adenocarcinoma of the pancreas is not prognostically different from the classic solid type (few patients survive more than 5 years), we believe that it is important because of its extreme rarity.

Acinar cell cystadenocarcinoma of the pancreas: Report of rare case and review of the literature

Most exocrine pancreatic tumors are of ductal origin, whereas acinar cell adenocarcinomas are unusual (1% to 2% of all exocrine pancreatic neoplasms). We recently found a cystic adenocarcinoma of the pancreatic body whose cells had the characteristics of acinar cells, which we term acinar cell cystadenocarcinoma. Macroscopically, this tumor consists of a large multilocular cystic mass with a pseudocapsule and a spongy appearance on the cut surface. Microscopically, the cysts are lined by a single layer of cuboid/columnar cells. The cytoplasm has the characteristics of acinar cells, with eosinophilic granules in the apex and prominent nucleoli. Immunohistochemically, the cells express α1-antitrypsin, trypsin, and lipase in their cytoplasm, thus confirming the acinar origin of the tumor. A review of the literature revealed only 5 other cases of this tumor reported since its first description in 1981. Follow-up data are available for 4 of these; all of the affected patients had metastases at presentation or a few months later, and 2 died of the disease, at 13 and 37 months after diagnosis. Although this variant of adenocarcinoma of the pancreas is not prognostically different from the classic solid type (few patients survive more than 5 years), we believe that it is important because of its extreme rarity.

Nitric oxide-dependent in vitro secretion of amylase from innervated or chronically denervated parotid glands of the rat in response to isoprenaline and vasoactive intestinal peptide.

The basal in vitro release of amylase was similar from rat parotid lobules of innervated and chronically denervated glands and was unaffected by the inhibitors used in this study. The secretion of amylase induced by isoprenaline or vasoactive intestinal peptide (VIP) was reduced by one-third to one-half from the lobules of the innervated glands and even more so from the lobules of the denervated glands by ODQ, an inhibitor of soluble guanyl cyclase which is activated by nitric oxide (NO) and catalyses the cGMP production. The use of N (omega)-propyl-L-arginine (N-PLA) revealed that the evoked secretion of amylase in the denervated glands depended on the activity of neuronal type NO synthase to synthesize NO. Since the denervated gland is virtually devoid of NO synthase-containing nerve fibres, the neuronal type NO synthase was most probably of a non-neuronal source. NO-dependent amylase secretion was agonist related, since amylase secretion evoked by bethanechol and neuropeptide Y was not reduced by ODQ or N-PLA. Hence, under physiological conditions, activation of beta-adrenoceptors (sympathetic activity) and VIP receptors (parasympathetic activity) is likely to cause secretion of parotid amylase partly through a NO/cGMP-dependent intracellular pathway involving the activity of neuronal type NO synthase, possibly of acinar origin.

Transgenic mice expressing cholecystokinin 2 receptors in the pancreas.

Several studies argue for the presence of CCK2 receptors in the human pancreas but their physiological role in normal exocrine pancreas and their contribution to pancreatic pathologies is unknown. In order to allow an easy investigation of their pancreatic function, we created the ElasCCK2 transgenic mice expressing the human receptor in pancreatic exocrine cells. In this model, the CCK2 receptor is specifically expressed in the exocrine pancreas and has typical molecular and binding features. It is functional and mediates enzyme release but stimulating concentrations of agonists are not physiological. Results of phenotypic and long-term studies show that activation of CCK2 receptors stimulates growth of the pancreas in correlation with an increase of acinar tissue. This finding is also consistent with the demonstration of an efficient coupling of the transgenic receptor to protein synthesis. Alterations in pancreatic histology and development of preneoplastic lesions are apparent from postnatal day 50. Moreover, expression of this G-protein-coupled receptor leads to the development of tumours in older animals with an incidence of 15%. Although tumours have distinct phenotypes they all exhibit ductular structures. Immunohistochemical analysis of these structures shows their acinar origin. These data, linking for the first time the development of pancreatic carcinogenesis in vivo to the expression of the CCK2 receptor, support a key role of the CCK2 receptor in the initiation of pancreatic cancer. Moreover, ElasCCK2 mice provide a model for carcinogenesis by transformation and dedifferentiation of acinar cells.

Squamous Cell Carcinoma of the Pancreas With Cystic Degeneration

Most nonendocrine pancreatic neoplasms are adenocarcinomas of ductal cell or acinar origin. Primary carcinomas of the pancreas with squamous differentiation are rare enough to warrant a search for other primary tumors. In the past few decades, well-documented individual reports and large series reviews support the view that these squamous neoplasms are indeed of pancreatic origin and not uncommonly exhibit cystic degeneration. Late manifestation and unfavorable prognosis seem to be uniform features. We report a case with many of these features.

Rat serum induces a differentiated phenotype in a rat parotid acinar cell line.

To establish a continuous cell line, freshly prepared rat parotid acinar cells were stably transfected with a plasmid vector containing the SV40 large T antigen. The acinar origin of these cells was confirmed by Western blotting, enzyme analysis, and morphological analysis. Transformed cells grown in 10% rat serum showed a modest reduction in cell number after 7 days and a concentration- and time-dependent increase in amylase levels approximately 16 times greater than those observed in fetal bovine serum-treated cells. Ultrastructural analysis revealed that cells grown in rat serum harbored protein-filled secretory granules localized adjacent to the endoplasmic reticulum, and punctate amylase-specific immunofluorescence distributed throughout the cytoplasm was consistent with the presence of amylase in secretory organelles. Clonal cells express tissue-specific proline-rich proteins and the four protein kinase C isozymes present in primary culture. Carbachol and isoproterenol stimulated [3H]protein secretion and isoproterenol enhanced amylase secretion from cells grown in rat serum. Moreover, norepinephrine, carbachol, and substance P produced a time- and concentration-dependent rise in cytoplasmic Ca2+. This continuous cell line of parotid acinar cells, which after treatment with rat serum retains the basic structural and functional properties of primary culture cells, will be utilized as a model system for studying long-term biological processes that regulate parotid cell function.

Squamous Cell Carcinoma Arising in Hormonally Treated Adenocarcinoma of the Prostate

Adenocarcinoma of acinar origin comprises almost all of the primary prostatic carcinomas. In contrast, prostatic squamous cell carcinoma is extraordinarily rare. We report a case of primary squamous cell carcinoma of the prostate that developed in a 64-year-old man, 3 months after treatment with diethylstilbestrol (DES) and flutamide for stage D2 adenocarcinoma of the prostate. This case report describes a transformation from adenocarcinoma to pure squamous cell carcinoma following the use of DES. The etiology of the squamous cell carcinoma in the present case and those previously reported in the literature are discussed.

Measurement of Metabolic Fluxes through Pyruvate Kinase, Phosphoenolpyruvate Carboxykinase, Pyruvate Dehydrogenase, and Pyruvate Carboxylase in Hepatocytes of Different Acinar Origin

Isolated hepatocytes were prepared from the periportal and perivenous regions of the liver of 18-h-starved rats. These showed characteristic enzyme patterns and an enhanced rate of ureagenesis in the periportal cells; however, total cellular ATP content was unchanged in the two cell types. Measurements of py- ruvate kinase flux showed no significant difference in the overall rate in the two cell types; however, the flux through phosphoenolpyruvate (PEP) carboxykinase was significantly higher in the periportal cells, such that the percentage of PEP being metabolized by py- ruvate kinase was enhanced in the perivenous cells. The increase in partitioning of PEP through pyruvate kinase could account for only a small percentage of the difference in gluconeogenic flux in the two cell types, suggesting that the rate of provision of PEP was the principal limiting factor for glucose synthesis. The flux through pyruvate dehydrogenase showed no significant metabolic zonation, whereas pyruvate carboxylase flux was enhanced in the periportal zone. The partitioning of pyruvate between pyruvate carboxylase and pyruvate dehydrogenase was increased 2.8-fold in the periportal cells compared to that in the perivenous cells and it is suggested that this, together with possible alterations in phosphoenolpyruvate carboxykinase, is primarily responsible for the different gluconeogenic rates in the two zones of the liver.