Donald E. Fleenor

Differential and Complementary Effects of Glucose and Prolactin on Islet DNA Synthesis and Gene Expression

The mechanisms by which lactogenic hormones promote β-cell expansion remain poorly understood. Because prolactin (PRL) up-regulates β-cell glucose transporter 2, glucokinase, and pyruvate dehydrogenase activities, we reasoned that glucose availability might mediate or modulate the effects of PRL on β-cell mass. Here, we used male rat islets to show that PRL and glucose have differential but complementary effects on the expression of cell cyclins, cell cycle inhibitors, and various other genes known to regulate β-cell replication, including insulin receptor substrate 2, IGF-II, menin, forkhead box protein M1, tryptophan hydroxylase 1, and the PRL receptor. Differential effects on gene expression are associated with synergistic effects of glucose and PRL on islet DNA synthesis. The effects of PRL on gene expression are mirrored by β-cell overexpression of signal transducer and activator of transcription 5b and are opposed by dexamethasone. An ad-small interfering RNA specific for cyclin D2 attenuates markedly the effects of PRL on islet DNA synthesis. Our studies suggest a new paradigm for the control of β-cell mass and insulin production by hormones and nutrients. PRL up-regulates β-cell glucose uptake and utilization, whereas glucose increases islet PRL receptor expression and potentiates the effects of PRL on cell cycle gene expression and DNA synthesis. These findings suggest novel targets for prevention of neonatal glucose intolerance and gestational diabetes and may provide new insight into the pathogenesis of β-cell hyperplasia in obese subjects with insulin resistance.

Regulation of Islet β-Cell Pyruvate Metabolism: Interactions of Prolactin, Glucose, and Dexamethasone

Prolactin (PRL) induces beta-cell proliferation and glucose-stimulated insulin secretion (GSIS) and counteracts the effects of glucocorticoids on insulin production. The mechanisms by which PRL up-regulates GSIS are unknown. We used rat islets and insulinoma (INS-1) cells to explore the interactions of PRL, glucose, and dexamethasone (DEX) in the regulation of beta-cell pyruvate carboxylase (PC), pyruvate dehydrogenase (PDH), and the pyruvate dehydrogenase kinases (PDKs), which catalyze the phosphorylation and inactivation of PDH. PRL increased GSIS by 37% (P < 0.001) in rat islets. Glucose at supraphysiological concentrations (11 mm) increased PC mRNA in islets; in contrast, PRL suppressed PC mRNA levels in islets and INS-1 cells, whereas DEX was without effect. Neither PRL nor DEX altered PC protein or activity levels. In INS-1 cells, PRL increased PDH activity 1.4- to 2-fold (P < 0.05-0.001) at glucose concentrations ranging from 2.5-11 mm. DEX reduced PDH activity; this effect was reversed by PRL. PDK1, -2, -3, and -4 mRNAs were detected in both islets and insulinoma cells, but the latter expressed trivial amounts of PDK4. PRL reduced PDK2 mRNA and protein levels in rat islets and INS-1 cells and PDK4 mRNA in islets; DEX increased PDK2 mRNA in islets and INS-1 cells; this effect was reversed by PRL. Our findings suggest that PRL induction of GSIS is mediated by increases in beta-cell PDH activity; this is facilitated by suppression of PDKs. PRL counteracts the effects of DEX on PDH and PDK expression, suggesting novel roles for the lactogens in the defense against diabetes.

The mouse CD7 gene: identification of a new element common to the human CD7 and mouse Thy-1 promoters

Abstract Human CD7 (hCD7) is a 40 000 Mr member of the immunoglobulin gene superfamily that is expressed early in natural killer (NK) and T-lymphocyte development. CD7 is involved in lymphocyte activation, as ligation of CD7 activates NK and TCRγδ T lymphocytes, and ligation of CD7 on TCRαβ T lymphocytes induces a non-mitogenic calcium flux. We have previously cloned and characterized the gene for human CD7 (hCD7) and have described its expression in transgenic mice. Recently a mouse cDNA homologous to hCD7 was reported, which we mapped to the corresponding mouse chromosomal location as hCD7. We now report the identification and characterization of a mouse CD7 (mCD7) genomic clone. We demonstrated that the mCD7 gene was similar both in size and structural organization to hCD7. Comparison of the 5′ flanking sequences of the mCD7 and hCD7 genes revealed two regions of sequence similarity. Electrophoretic mobility shift assay confirmed both of these regions to be sites of tissue-restricted protein binding in vitro. The more 3′ similarity region also shared sequence with a region in the mouse Thy-1 gene 5′ flanking region, suggesting that this sequence may be a cis-acting regulatory element common to all three genes. Thus, the promoter regions and exonic organization were similar in the human CD7, mouse CD7, and mouse Thy-1 genes.