Syndecans are transmembrane heparan sulfate proteoglycans controlling cell adhesion, migration, and proliferation. We previously showed that syndecan-2 is involved in the control of apoptosis in cultured osteosarcoma cells. These data led us to the hypothesis that syndecan-2 may play a role in the apoptotic signaling in bone tumors. We immunohistochemically analyzed tissue sections from biopsies from 21 patients with well-characterized osteosarcoma. These tissues expressed low levels of syndecan-2 compared with osteoblasts and osteocytes in normal bone. Cultured human osteosarcoma cells also produced lower mRNA levels of syndecan-2 than normal osteoblastic cells. Moreover, the presence of syndecan-2 correlated with spontaneous apoptosis in osteosarcoma tissues as assessed by detection of DNA fragmentation in situ. Overexpression of syndecan-2 resulted in decreased number of migrating and invading U2OS osteosarcoma cells in Matrigel. In addition, overexpression of syndecan-2 sensitized human osteosarcoma cells to chemotherapy-induced apoptosis, increasing the response to methotrexate, doxorubicin, and cisplatin. Consistently, knockdown of the proteoglycan using stable transfection with a plasmid coding small interfering RNA resulted in inhibition of chemotherapy-induced apoptosis. Analysis of syndecan-2 expression both in biopsies and in corresponding postchemotherapy-resected tumors, as well as in cells treated with methotrexate or doxorubicin, showed that the cytotoxic action of chemotherapy can be associated with an increase in syndecan-2. These results provide support for a tumor-suppressor function for syndecan-2 and suggest that dysregulation of apoptosis may be related to abnormal syndecan-2 expression or induction in osteosarcoma. Moreover, our data identify syndecan-2 as a new factor mediating the antioncogenic effect of chemotherapeutic drugs. [Cancer Res 2007;67(8):3708-15]

In spite of intensive studies, the intracellular mechanisms of regulation of mitochondrial function in heart and skeletal muscle are still obscure. One of the interesting observations in this area, made in numerous laboratories, is that in permeabilized oxidative muscle cells the apparent Km for exogenous ADP in the control of mitochondrial respiration is very high, in the range of 200-300·µmol·l -1 , in contrast to permeabilized fibers from fast-twitch skeletal muscles and isolated mitochondria in vitro: in both cases, the apparent Km for ADP is 15-20·µmol·l -1 (Anflous et al., 2001;Braun et al., 2001;Burelle and Hochachka, 2002;Dos Santos et al., 2002;Fontaine et al., 1995;Kay et al., 1997;Kummel, 1988;Kuznetsov et al., 1996;Liobikas et al., 2001;Milner et al., 2000;Saks et al., 1991Saks et al., , 1993Saks et al., , 1994Saks et al., , 1995Saks et al., , 1998aSaks et al., , 2001Seppet et al., 2001;Toleikis et al., 2001;Veksler et al., 1995). A trivial explanation of this phenomenon by formation of ADP concentration gradients between the medium and the core of the cells is excluded (Kay et al., 1997), since the Brownian movement of adenine nucleotides in water solution across the diffusion distance of <10·µm (permeabilized cardiomyocytes) is more rapid than the metabolic turnover of ADP and ATP . Besides, this trivial explanation is in conflict with the tissue specificity of the phenomenon mentioned above (Kuznetsov et al., 1996;Veksler et al., 1995). Another important recent observation is that the kinetics of regulation of mitochondrial respiration in ADP is approximately 20·µmol·l -1 ). An increase in the free Ca 2+ concentration (up to 3·µmol·l -1 , which is within physiological range), resulted in a very significant decrease of the apparent Km value to 20-30·µmol·l -1 , a decrease of Vmax of respiration in permeabilized intact fibers and a strong contraction of sarcomeres. In ghost cardiac fibers, from which myosin was extracted but mitochondria were intact, neither the high apparent Km for ADP (300-350·µmol·l -1 ) nor Vmax of respiration changed in the range of free Ca 2+ concentration studied, and no sarcomere contraction was observed. The exogenous-ADP-trapping system (pyruvate kinase + phosphoenolpyruvate) inhibited endogenous-ADP-supported respiration in permeabilized cells by no more than 40%, and this inhibition was reversed by creatine due to activation of mitochondrial creatine kinase. These results are taken to show strong structural associations (functional complexes) among mitochondria, sarcomeres and sarcoplasmic reticulum. Inside these complexes, mitochondrial functional state is controlled by channeling of ADP, mostly via energy-and phosphoryltransfer networks, and apparently depends on the state of sarcomere structures.

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