Cyclic AMP is an intracellular messenger that regulates numerous processes in various types of cells. In pancreatic β-cells, cAMP potentiates the secretion of insulin by promoting Ca2+ signals and by amplifying Ca2+-triggered exocytosis. Whereas Ca2+ signals have been extensively characterized, little is known about the kinetics of cAMP signals. To enable measurements of the cAMP concentration beneath the plasma membrane ([cAMP]pm) of individual cells, a translocation biosensor was created based on fluorescent-protein-tagged subunits of protein kinase A (PKA). Evanescent wave microscopy imaging of biosensor-expressing clonal β-cells revealed that the insulinotropic hormones glucagon and GLP-1 triggered pronounced oscillations in [cAMP]pm. Simultaneous measurements of the intracellular Ca2+ concentration showed that cAMP and Ca2+ oscillations were synchronized and interdependent. [cAMP]pm oscillations were also triggered in clonal and primary mouse β-cells by an elevation of the glucose concentration from 3 to 11 mM. These oscillations were preceded and enhanced by elevations of Ca2+. However, conditions raising cytoplasmic ATP could trigger cAMP elevations also without accompanying Ca2+ changes, indicating that adenylyl cyclase activity may be directly controlled by the substrate concentration. Experiments with 3-isobutylmethylxanthine (IBMX) and various family-selective phosphodiesterase (PDE) inhibitors indicated that [cAMP]pm oscillations are generated by periodic formation of the messenger by adenylyl cyclases. PDE1 and PDE3 as well as IBMX-insensitive mechanisms shape [cAMP]pm, but no single PDE isoform was required for glucose generation of [cAMP]pm oscillations. Recordings of single-cell insulin secretion kinetics with a fluorescent biosensor that reports formation of the phospholipid PIP3 in the plasma membrane in response to autocrine insulin receptor activation showed that [cAMP]pm oscillations were paralleled by pulsatile insulin release. Whereas adenylyl cyclase inhibition suppressed both [cAMP]pm oscillations and pulsatile insulin release, elevation of [cAMP]pm enhanced secretion. Investigation of the effects of different temporal patterns of [cAMP]pm showed that brief [cAMP]pm elevation is sufficient to trigger cytoplasmic responses, whereas sustained elevation is required to induce translocation of the PKA catalytic subunit into the nucleus. In conclusion, these studies demonstrate for the first time in mammalian cells that [cAMP]pm oscillates in response to physiological stimuli. The glucose-induced [cAMP]pm oscillations are generated by periodic cAMP production mediated by interplay between ATP and Ca2+ in the sub-membrane space, and may contribute to both triggering and amplifying pathways of insulin secretion. Apart from regulating the precise kinetics of insulin exocytosis, temporal encoding of cAMP signals might constitute a basis for differential regulation of downstream cellular targets.

Background and aims: Type 2 diabetes mellitus results from complex interplay between genetic predisposition and environmental factors that together promote impairment of insulin-producing β-cells. Elevated levels of glucose, fatty acid palmitate and apolipoprotein CIII (apoCIII) are implicated in this process. To delineate effects of these factors, the role of enhanced carnitine palmitoyltransferase 1 (CPT1) expression in glucolipotoxic cells, glucose-dependency of the unfolded protein response (UPR) in palmitate-induced apoptosis and activation of mitogen activated protein kinases (MAPKs) in apoCIII-induced apoptosis were evaluated. In addition, protein profiles of β-cell exposed to elevated levels of glucose or palmitate were generated to identify proteins regulated by these nutrients. Methodology: INS-1E cells were cultured at different glucose concentrations in the absence or presence of palmitate or apoCIII for up to 48 hours. CPT1 was over-expressed with a Tet-ON regulated adenovirus. In cells exposed to apoCIII, inhibitors of MAPKs p38 or ERK1/2 were included during culture. After culture, apoptosis, insulin secretion, expression of UPR-markers and MAPKs and protein profiles were determined. Results: INS-1E cells exposed to elevated levels of glucose and palmitate showed deranged insulin secretion with increased insulin secretion at non-stimulatory glucose level, enhanced apoptosis and induced expression of UPR-markers. Over-expression of CPT1 reduced basal insulin secretion and attenuated apoptosis. Palmitate-induced apoptosis was accentuated by increasing the culture glucose concentration. Markers of UPR were not modulated by the glucose concentration in INS-1E cell exposed to palmitate, however. ApoCIII-induced apoptosis in INS-1E cells was accompanied by activation of p38 and ERK1/2. Protein profiling of INS-1E cells exposed to elevated levels of glucose or palmitate revealed changes in expression of multiple β-cell proteins implicated in glucose metabolism, defence against reactive oxygen species, protein translation/folding/degradation and insulin granular trafficking. Conclusions: Over-expression of CPT1 counteracts β-cell glucolipotoxicity. Activation of UPR is not a major determinant for palmitate-induced β-cell apoptosis. ApoCIII-induced β-cell apoptosis involves activation of MAPKs. The identified differentially expressed proteins indicate a central role of altered glucose metabolism and protein synthesis in gluco- and lipotoxic β-cells and may provide specific molecular mechanisms offering new ways of treating the disease.

The adaptor protein Shb and the non-receptor tyrosine kinase Frk have been implicated in intracellular signalling in insulin-producing beta cells. In this thesis, knockout mice are used to further elucidate the role of Shb and Frk for beta cell number, cytokine-induced cell death, and glucose homeostasis. In addition, the effect of Shb deficiency upon tumour growth is studied in a mouse model of endogenous tumourigenesis. Previously, overexpression of Frk has been associated with increased beta cell replication, and increased susceptibility to cytokine induced beta cell destruction. To test whether Frk has a non-redundant role in regulating beta cell mass, beta cell number in Frk-/- mice was assessed at different stages of life. The results showed that Frk is involved in regulating beta cell number during embryonal and early postnatal life, but is probably redundant in the adult. An earlier study had suggested that Shb participates in cytokine-induced beta cell death, a model of autoimmune diabetes. To test this further, Shb-/- islets were exposed to cytokines, or to an ER-stress inducing agent. Shb knockout islets exhibited decreased cell death, and this effect appeared to be independent of NO, JNK, p38 MAP kinase, FAK and c-Abl, but may involve an augmented induction of Hsp70. Furthermore, glucose homeostasis in Shb-/- mice was impaired, with elevated basal blood sugar concentration and reduced glucose-induced insulin secretion. Previously Shb deficient mice had showed an impaired ability to sustain growth of implanted tumour cells, due to reduced angiogenesis. Tumour growth and angiogenesis were here assessed in an inheritable tumour model. Shb deficient mice exhibited fewer tumours, and reduced vessel density in small tumours, indicating impaired angiogenesis. However, a few large tumours developed in Shb-/- mice, suggesting that tumours can escape the angiogenic restriction caused by the absence of Shb.

Type 1 diabetes mellitus is a severe life-long disease with a pronounced risk of developing secondary complications. One way to avoid the latter is to restore the fine tuning of blood glucose homeostasis by transplantation of pancreatic islets. However, isolated islets need to be properly engrafted and to re-establish a vascular network in order to regain function. Earlier studies have shown that pancreatic islets experimentally transplanted to e.g. the liver or the kidney become poorly revascularized. In the present thesis, mice deficient of the angiostatic factor thrombospondin-1 (TSP-1) were found to have an impaired beta-cell function. Development of this beta-cell dysfunction was prevented by treatment of TSP-1 deficient mice from birth with the TGFbeta-1 activating sequence of TSP-1. TSP-1 in islets was predominantly expressed in the endothelial cells. Isolated islet endothelial cells was observed to have a low proliferatory and migratory capacity towards angiogenic stimuli, but this could be reversed by neutralizing antibodies to the angiostatic factors alpha1-antitrypsin, endostatin or TSP-1. Transient downregulation of TSP-1 expression in mouse islet cells prior to transplantation improved graft revascularization, blood perfusion, oxygenation and function when evaluated one-month post-transplantation. The same result was achieved when islets or recipients of islets were pre-treated with the hormone prolactin one-month post-transplantation. The present study illustrates the importance of the angiostatic factor TSP-1 for islet beta-cell function and engraftment of islets following transplantation. Interference with TSP-1 can possibly be used to improve the results of clinical islet transplantation.

Although the reason for insufficient release of insulin in diabetes mellitus may vary depending on the type and stage of the disease, it is of vital importance that an amplified insulin biosynthesis can meet the increased need during periods of hyperglycemia. The insulin mRNA is highly abundant in beta cells and changes in insulin mRNA levels are, at least in part, controlled by altered rates of mRNA degradation. Since the mechanisms behind the control of insulin messenger stability and translation are still largely obscure, the work presented in this thesis therefore aimed to further investigate the role of insulin mRNA binding proteins in the control of insulin mRNA break-down and utilization for insulin biosynthesis. To clarify how glucose regulates insulin mRNA stability and translation we studied the correlation between polypyrimidine tract binding protein (PTB) gene expression and insulin mRNA levels. It was found that an increase in PTB mRNA and protein levels is paralleled by an increase in insulin mRNA levels. It was also found that PTB binds to the 5’-untranslated region of the insulin mRNA and that insulin mRNA can be translated through a cap-independent mechanism in human islets of Langerhans, possibly due to the interaction with PTB. Further it was discovered that the suppressed insulin biosynthesis in human islets during glucotoxicity is partly due to an induction of the microRNA miR-133a. This induction leads to decreased levels of PTB and insulin biosynthesis rates in human islets. Finally, we were able to identify two proteins, hnRNP U and TIAR, that bind specifically to the insulin mRNA in vitro, and show that the stability and translation of insulin mRNA is oppositely affected by these proteins. In conclusion, insulin producing cells seem to be able to regulate insulin messenger stability and translation by a control mechanism in which the binding of specific proteins to the insulin messenger dictates the outcome. A better understanding of the events leading to insulin production may in the future aid in optimal diagnosis and treatment of type 2 diabetes.

The prevalence of type II diabetes is increasing at an alarming rate due to the western world lifestyle. Type II diabetes is characterized by an insulin resistance distinguished by impaired glucose uptake in adipose and muscle tissues. The molecular mechanisms behind the insulin recistance and also the knowledge considering normal insulin signalling in fat cells, especially in humans, are still unclear. Insulin receptor substrate (IRS) is known to be important for medating the insulin-induced signal from the insulin receptor into the cell. We developed and optimized a method for transfection of primary human adipocytes by electroporation. By recombinant expression of proteins, we found a proper IRS to be crucial for both mitogenic and metabolic signalling in human adipocytes. In human, but not rat, primary adipocytes we found IRS1 to be located at the plasma membrane in non-insulin stimulated cells. Insulin stimulation resulted in a two-fold increase of the amount of IRS1 at the plasma membrane in human cells, compared with a 12-fold increase in rat cells. By recombinant expression of IRS1 we found the species difference between human and rat IRS1 to depend on the IRS proteins and not on properties of the host cell. The adipocytes function as an energy store, critical for maintaining the energy balance, and obesity strongly correlates with insulin resistance. The insulin sensitivity of the adipocytes with regard to the size of the cells was examined by separating small and large cells from the same subject. We found no increase of the GLUT4 translocation to the plasma membrane following insulin stimulation in the large cells, whereas there was a two-fold increase in the small cells. This finding supports the idea of a causal relationship between the enlarged fat cells and reduced insulin sensitivity found in obese subjects. The insulin receptor is located and functional in a specific membrane structure, the caveola. The morphology of the caveola and the localization of the caveolar marker proteins caveolin-1 and -2 were examined. Caveolae were shown to be connected to the exterior by a narrow neck. Caveolin was found to be located at the neck region of caveolae, which imply importance of caveolin for maintaining and sequestering caveolae to the plasma membrane. In conclusion, the transfection technique proved to be highly useful for molecular biological studies of insulin signal transduction and morphology in primary adipocytes.

<p>L1 syndrome is an X-linked recessive disorder, characterised by congenital hydrocephalus, adducted thumbs, spastic paraplegia, agenesis of the corpus callosum and mental retardation. The disease is caused by mutations in the L1CAM gene, encoding a protein predominantly expressed in the nervous system. This protein has been implicated in a variety of processes including neuronal migration, neurite outgrowth and fasciculation, myelination, and long-term memory formation.</p><p>L1 syndrome was suspected at genetic counselling of a family with a boy suffering from congenital hydrocephalus and mental retardation. Complete sequencing of L1CAM, performed by an external laboratory, revealed a novel mutation in the family, including a boy, affected with L1 syndrome, his sister, his mother and his maternal grandmother.</p><p>To verify this mutation and to be able to detect mutations in the L1CAM gene locally in the future, a method for mutational analysis of this gene was set up using PCR optimisation and cycle sequencing.</p><p>Sequencing of L1CAM was then performed on DNA samples from the four family members and the mutation was verified. A point mutation (c.3458-1G>C) in the 5’ splice site of exon 26 was detected in all of them. This new, not previously described, mutation is supposed to cause a deletion of the 26th exon and a frameshift in the part of the protein encoded by exons 27 and 28. This means that almost the entire cytoplasmic domain of the protein would have a loss of function, explaining the symptoms affecting the boy.</p>

<p>The Notch signalling pathway is an evolutionary conserved pathway, named after the Notch receptors, Notch1-4 in mammals, which upon cell-cell contact and ligand binding releases the intracellular domain (NICD). NICD translocates into the nucleus where it binds the transcriptional repressor RBP-Jk, which together with co-activators belonging to the Mastermind-like family of proteins form a transcriptional activation complex. This complex activates genes controlling cell fate decision, embryonic development, proliferation, differentiation, adult homeostasis and stem cell maintenance. On the other hand, disrupted Notch signalling may result in pathological conditions like cancer, although the mechanisms behind the disruption are often complex and in many cases largely unknown.</p><p>Notch1 drives the lymphocyte differentiation towards a T-cell fate and activating mutations in the gene have been suggested to be involved in T-cell lymphoma. In <em>paper I, </em>genetic alterations in <em>Notch1 </em>and the Notch1 regulating gene <em>CDC4 </em>were investigated in tumours from murine T-cell lymphoma induced with phenolphthalein, 1,3-butadiene or 2’,3’-dideoxycytidine. We identified activating <em>Notch1</em> mutations in 39% of the lymphomas, suggesting that <em>Notch1 </em>is<em> </em>an important target gene for mutations in chemically induced lymphomas.<em></em></p><p>While it is known that constitutively activated Notch signalling has a clear oncogenic function in several solid malignancies as well, the molecular mechanisms are less known in this context. Unpublished data of our lab, together with other recent studies, suggest that mutations of Notch and Notch-related genes <em>per se</em> are uncommon in solid malignancies including colorectal cancer, while a growing body of evidence indicates that aberrant Wnt/b-catenin signalling may result in pro-tumoural Notch activation in these contexts. In <em>paper II</em>, we therefore investigated potential transcriptional interactions between the Notch and Wnt signalling pathways in colorectal cancer cell lines. The proximal Notch and Wnt pathway gene promoters were bioinformatically identified and screened for putative TCF/LEF1 and RBP-Jk sites. In canonical Wnt signalling, Apc negatively regulates b-catenin leading to repression of TCF/LEF1 target genes. Upon repression of the Wnt pathway we observed that several genes in the Notch pathway, including <em>Notch2</em>, were transcriptionally downregulated. We also confirmed binding of Lef1 to <em>Notch2</em> as well as other Notch pathway gene promoters and luciferase assays showed an increased activity for at least one LEF1/TCF-site in the <em>Notch2</em> promoter upon co-transfection of HT29 or HCT116 cells with mutated b-catenin. HT29 cell lines were also treated with the g-secretase inhibitor DAPT, leading to inactivation of the Notch pathway by preventing release of NICD. However, results showed no effects on Apc, b-catenin or their target <em>cyclin D1</em>. Taken together, these results indicate that the Wnt pathway may function as a regulator of the Notch pathway through the TCF/LEF1 target gene program in colon cancer cell lines.</p><p>In summary, Notch pathway deregulation is of importance in both murine T-cell lymphoma and human colorectal cancer, although the mechanisms differ. The current results give new insights in Notch pathway alterations as well as the signalling networks in which the Notch pathway interacts, and thus increase the understanding of Notch’s involvement in malignant diseases.</p> / Studies on molecular genetic alterations in colorectal cancer

<p>Prostaglandin (PG) E₂ is an eicosanoid derived from the polyunsaturated twenty carbon fatty acid arachidonic acid (AA). PGE₂ has physiological as well as pathophysiological functions and is known to be a key mediator of inflammatory responses. Formation of PGE₂ is dependent upon the activities of three specific enzymes involved in the AA cascade; phospholipase A₂ (PLA₂), cyclooxygenase (COX) and PGE synthase (PGEs). Although the research within this field has been intense for decades, the regulatory mechanisms concerning the PGE₂ synthesising enzymes are not completely established.</p><p>PGE₂ was investigated in human monocytes with or without lipopolysaccharide (LPS) pre-treatment followed by stimulation with calcium ionophore, opsonised zymosan or phorbol myristate acetate (PMA). Cytosolic PLA₂a (cPLA₂a) was shown to be pivotal for the mobilization of AA and subsequent formation of PGE₂. Although COX-1 was constitutively expressed, monocytes required expression of COX-2 protein in order to convert the mobilized AA into PGH₂. The conversion of PGH₂ to the final product PGE₂ was to a large extent due to the action of microsomal PGEs-1 (mPGEs-1). In addition, experiments with inhibitors of extracellular signal regulated kinase and p38 activation, indicated that phosphorylation of cPLA₂α was markedly advantageous for the formation of PGE₂.</p><p>Ellagic acid, a natural polyphenolic compound found in fruits and nuts, was shown to inhibit stimuli induced release of PGE₂ in human monocytes. The effect of ellagic acid was not due to a direct effect on the activities of the enzymes but rather to inhibition of the LPS-induced protein expression of COX-2, mPGEs-1 and cPLA₂a.</p>

A prerequisite for a multicellular organism to survive is the ability to correctly replicate and repair DNA while minimizing the number of heritable mutations. To achieve this, cells need a balanced supply of deoxyribonucleoside triphosphates (dNTPs), the precursors for DNA synthesis. The rate-limiting step in de novo biosynthesis of dNTPs is catalyzed by the enzyme ribonucleotide reductase (RNR). The classic eukaryotic RNR enzyme consists of a large and a small subunit. Together, these subunits form a heterotetrameric RNR complex. The larger subunit harbours active sites whereas the smaller subunit contains a stable tyrosyl free radical. Both subunits are required for RNR activity. Since failure to correctly regulate de novo dNTP biosynthesis can lead to misincorporation of nucleotides into DNA, genetic abnormalities and cell death, RNR activity is tightly regulated. The regulation of RNR activity involves cell cycle-specific expression and degradation of the RNR proteins, as well as binding of allosteric effectors to the large RNR subunit. In this thesis, in vitro assays based on purified recombinant RNR proteins, in combination with in vivo assays, have been used successfully to study the regulation of RNR activity in response to DNA damage. I present new findings regarding the function of an alternative mammalian RNR small subunit, and on the role of a small RNR inhibitor protein of fission yeast, during normal growth and after DNA damage. I also show conclusively that there are fundamental differences in the regulation of dNTP biosynthesis between the cells of higher and lower eukaryotes after DNA damage.