Dynamic contrast-enhanced magnetic resonance imaging as a tool to monitor the blood supply to an artificial cavity used as a site for islet transplantation in rats.

BACKGROUND: The transplantation of islets of Langerhans isolated from one donor pancreas can rarely release a diabetic recipient from insulin injections. The major reason is the destruction of 50%-60% of the transplanted tissue, which proceeds typically within a few hours after the insertion of the islets into the portal vein. Therefore, several groups have focused on development of an artificial site for islet transplantation. The main aim of the present study was to test the efficacy of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) to evaluate the blood supply feeding the artificially created cavities for islet transplantation. METHODS: Two rounded devices were implanted: one device subcutaneously and the second one into the greater omentum of each animal. On the day of implantation as well as 1, 3, and 4 weeks later, we quickly injected the vascular specific MR contrast agent Vasovist (0.05 mL/100 g) intravenously. Penetration of the contrast agent was monitored by DCE-MRI. The influence of the contrast agent on the signal intensity observed within selected target areas was calculated with the use of ImageJ software. RESULTS: The penetration of the contrast agent was detected by the increase in signal intensity within implanted devices. The signal increase caused by the contrast compound was normalized to kidney tissue. On day of implantation of the device, no signal due to the contrast agent was detected in all devices. However, over the following weeks, there was an increase in signal detection within the omental device to 34%, 21%, and 14% of that of the kidney. Within the subcutaneously implanted devices there was an increase in signal detection up to 11%, 10%, and 7% of that detected in the kidney. CONCLUSIONS: The optimal time for transplantation of pancreatic islets into our omental device was 1 week after implantation of the scaffold. Also, the blood supply feeding the subcutaneous devices was regarded to be inadequate.

Vascularization of artificial beds for pancreatic islet transplantation in a rat model.

An alternative prevascularized bed with a subcutaneously located entrance would substantially improve islet engraftment, requiring much less invasive surgery. Studies have described times necessary for the creation of an artificial pouch suitable for subsequent islet transplantation. Polymeric mesh shaped in rounded scaffolds were implanted both subcutaneously and into the major omentum of Brown Norway female rats (n = 7). The connective tissue together with vessels were embedded into scaffolds at 1 week without regard to site. In contrast to the major omentum, vessels within the subcutaneous connective tissue surrounding the devices started to decline in 2 weeks and almost disappeared 1 week later. Magnetic resonance imaging (MRI) detected changes in fibrous tissue surrounding the wall, but only large veins located beside the devices were visible using basic MRI. The blood supply to the internal surface of the created beds was important for islet engraftment, but information could be obtained only by using dynamic contrast-enhanced MRI.

Inducible expression of the regulatory protein kinase CK2beta subunit: incorporation into complexes with catalytic CK2 subunits and re-examination of the effects of CK2beta on cell proliferation.

The regulatory subunit of protein kinase CK2, designated CK2beta, exists both free in cells and in complexes with the CK2 catalytic subunits. Growing evidence suggests that CK2beta has functions dependent and independent of the CK2 catalytic subunits. There have been indications that CK2beta has functions associated with DNA damage responses and in the control of cell proliferation. For example, transient and stable constitutive overexpression of CK2beta in mammalian cells was previously shown to perturb cell cycle progression and to attenuate proliferation. To systematically investigate the molecular mechanisms responsible for these effects of CK2beta on cell proliferation, we generated human osteosarcoma U2OS cell lines with tetracycline-regulated expression of CK2beta. Increased expression of CK2beta results in increases in total cellular CK2 activity, but no changes in cell cycle profiles or proliferation. Furthermore, following exposure to ultraviolet radiation, p53 induction was identical regardless of the levels of CK2beta in cells. Mouse 3T3-L1 cells stably transfected with CK2beta also showed no alterations in cell proliferation. The differences between these results and those previously reported emphasize the complex nature of CK2beta and its cellular functions. Furthermore, these results indicate that increased expression of CK2beta is not by itself sufficient to effect alterations in cell proliferation.

Functional specialization of CK2 isoforms and characterization of isoform-specific binding partners.

In mammals, protein kinase CK2 has two isozymic forms of its catalytic subunit, designated CK2alpha and CK2alpha'. CK2alpha and CK2alpha' exhibit extensive similarity within their catalytic domains but have completely unrelated C-terminal sequences. To systematically examine the cellular functions of each CK2 isoform in mammalian cells, we have generated human osteosarcoma U2-OS cell lines with the expression of active or inactive versions of each CK2 isoform under the control of an inducible promoter. Examination of these cell lines provides evidence for functional specialization of CK2 isoforms at the cellular level in mammals with indications that CK2alpha' is involved in the control of proliferation and/or cell survival. To understand the molecular basis for functional differences between CK2alpha and CK2alpha', we have undertaken studies to identify proteins that interact specifically with each isoform of CK2 and could contribute to the regulation of their independent functions. A novel pleckstrin-homology domain containing protein, designated CK2-interacting protein 1 (i.e. CKIP-1) was isolated using the yeast two hybrid system as a protein that interacts with CK2alpha but not CK2alpha'. When expressed in cells as a fusion with green fluorescent protein, CKIP-1 localizes to the cell membrane and to the nucleus. In this study, we present evidence from deletion analysis of CKIP-1 suggesting that a C-terminal region containing a putative leucine zipper has a role in regulating its nuclear localization. Collectively, our data supports a model whereby CKIP-1 is a non-enzymatic regulator of CK2alpha that regulates the cellular functions of CK2alpha by targeting or anchoring CK2alpha to specific cellular localization or by functioning as an adapter to integrate CK2alpha-mediated signaling events with components of other signal transduction pathways.

Inducible expression of protein kinase CK2 in mammalian cells. Evidence for functional specialization of CK2 isoforms.

Protein kinase CK2 (formerly casein kinase II) exhibits elevated expression in a variety of cancers, induces lymphocyte transformation in transgenic mice, and collaborates with Ha-Ras in fibroblast transformation. To systematically examine the cellular functions of CK2, human osteosarcoma U2-OS cells constitutively expressing a tetracycline-regulated transactivator were stably transfected with a bidirectional plasmid encoding either catalytic isoform of CK2 (i.e. CK2alpha or CK2alpha') together with the regulatory CK2beta subunit in order to increase the cellular levels of either CK2 isoform. To interfere with either CK2 isoform, cells were also transfected with kinase-inactive CK2alpha or CK2alpha' (i. e. GK2alpha (K68M) or CK2alpha'(K69M)) together with CK2beta. In these cells, removal of tetracycline from the growth medium stimulated coordinate expression of catalytic and regulatory CK2 subunits. Increased expression of active forms of CK2alpha or CK2alpha' resulted in modest decreases in cell proliferation, suggesting that optimal levels of CK2 are required for optimal proliferation. By comparison, the effects of induced expression of kinase-inactive CK2alpha differed significantly from the effects of induced expression of kinase-inactive CK2alpha'. Of particular interest is the dramatic attenuation of proliferation that is observed following induction of CK2alpha'(K69M), but not following induction of CK2alpha(K68M). These results provide evidence for functional specialization of CK2 isoforms in mammalian cells. Moreover, cell lines exhibiting regulatable expression of CK2 will facilitate efforts to systematically elucidate its cellular functions.