Abstract. Ng-CAM and N-CAM are cell adhesion molecules (CAMs), and each CAM can bind homophilically as demonstrated by the ability of CAM-coated beads (Covaspheres) to self-aggregate. We have found that the extent of aggregation of Covaspheres coated with either Ng-CAM or N-CAM was strongly inhibited by the intact 1D1 and 3F8 chondroitin sulfate proteoglycans of rat brain, and by the core glycoproteins resulting from chondroitinase treatment of the proteoglycans. Much higher concentrations of rat chondrosarcoma chondroitin sulfate proteoglycan (aggrecan) core proteins had no significant effect in these assays. The 1D1 and 3F8 proteoglycans also inhibited binding of neurons to Ng-CAM when mixtures of these proteins were adsorbed to polystyrene dishes.Direct binding of neurons to the proteoglycan core glycoproteins from brain but not from chondrosarcoma was demonstrated using an assay in which cellsubstrate contact was initiated by centrifugation, and neuronal binding to the 1D1 proteoglycans was specifically inhibited by the 1D1 monoclonal antibody. Different forms of the 1D1 proteoglycan have been identified in developing and adult brain. The early postnatal form (neurocan) was found to bind neurons more effectively than the adult proteoglycan, which represents the C-terminal half of the larger neurocan core protein. Our results therefore indicate that certain brain proteoglycans can bind to neurons, and that Ng-CAM and N-CAM may be heterophilic ligands for neurocan and the 3F8 proteoglycan. The ability of these brain proteoglycans to inhibit adhesion of cells to CAMs may be one mechanism to modulate cell adhesion and migration in the nervous system.

Abstract:Homologous receptor desensitization is an Important regulatory response to continuous activation by agonist that involves the uncoupling of a receptor from its G protein. When human retinoblastoma Y-79 cells expressing corticotropin-releasing factor (CRF) receptors were preincubated with CAF for 10 min-4 h, a time-dependent reduction in both the peak and sensitivity of OAF-stimulated intracellular cyclic AMP (cAMP) accumulation developed with a t 112 of 38 min and an EC50 of 6-7 nM OAF. OAF receptor desensitization was slowly reversible after a 4-h OAF preincubation with a t1,2 of 13 h and a full restoration of cAMP responsiveness to OAF at 24 h following the removal of 10 nM OAF. Because the ability of vasoactive intestinal peptide, forskolin, or (-)-isoproterenol to stimulate cAMP accumulation was not diminished in Y-79 cells desensitized with 10 nM ORF, the observed desensitization was considered to be a specific homologous action of OAF. OAF receptor desensitization was markedly attenuated by OAF receptor antagonists, which alone did not produce any appreciable reduction in OAF-stimulated cAMP accumulation. Although recent reports have demonstrated a rapid decline in steady-state levels of ORF receptor type 1 (OAF-Al) mRNA in anterior pituitary cells during several hours of exposure to OAF, there was no observed reduction in OAF-Al mANA levels when Y-79 cells were preincubated with 10 nM OAF for 10 min-24 h despite a rapid timeand concentration-dependent loss of OAF receptors from the retinoblastoma cell surface.

Abstract. Based on the observation that 5-aminolevulinic acid (ALA) induces the expression of heme oxygenase-1 (HO-1) in cultured melanoma cells, the role of HO-1 on the effectiveness of 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT) was examined. Transcriptional activation of the HO-1 gene is considered to be an adaptive response to oxidative and cellular stress and confers a protective capacity against cell and tissue injury, which could affect the responsiveness to ALA-PDT. A time-dependent accumulation (0-16 h) of protoporphyrin IX (PPIX) within melanoma cells was seen after incubation with ALA (0.5 mM ALA). Over the same time interval, a significant increase (up to 25-fold) in HO-1 protein expression was observed. Thus, the production and degradation of PPIX (via heme by HO-1) were simultaneously enhanced, leading to a reduced intracellular concentration of the photodynamically active substance PPIX. Diminishing HO-1 activity by the HO-1 inhibitor tin protoporphyrin IX (SnPPIX) significantly enhanced the formation of PPIX up to 1.8 fold. A further strong increase in HO-1 protein expression (up to 128-fold) was seen after ALA-PDT treatment. Induction of HO-1 is an essential step in the 'rescue response' of tumor cells. The pharmacological inhibition of HO-1 activity by SnPPIX leads to a considerable increase in the sensitivity of tumor cells to ALA-PDT treatment. At low radiation doses (0.42 J/cm 2 ), the percentages of death cells increased significantly from 7.3±1.3% to 43.7±6.4%. This effect could be further intensified by cellular depletion of HO-1 mRNA by siRNA. The combination of pharmacological inactivation of HO-1 with gene silencing led to an increase in the death rate of up to 54.1±8.6%. The results presented indicate that HO-1 can play a protective role against ALA-PDT mediated cytotoxicity so that a specific inhibition of HO-1 activity and/or expression might be used to increase the efficacy of ALA-based photodynamic therapy.

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