Branched and linear PEI/DNA complexes differ in their ability to transfect cells. The greater efficiency of linear PEI might be due to an inherent kinetic instability under salt conditions. Understanding how to employ this kinetic instability of linear PEI could help in designing future vectors with greater flexibility and transfection efficiency in vivo.

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Systemic application of positively charged polycation/DNA complexes has been shown to result in predominant gene expression in the lungs. Targeting gene expression to other sites, eg distant tumors, is hampered by nonspecific interactions largely due to the positive surface charge of transfection complexes. In the present study we show that the positive surface charge of PEI (25 kDa branched or 22 kDa linear)/DNA complexes can be efficiently shielded by covalently incorporating transferrin at sufficiently high densities in the complex, resulting in a dramatic decrease in nonspecific interactions, eg with erythrocytes, and decreased gene expression in the lung. Systemic application of transferrinshielded PEI/DNA complexes into A/J mice bearing subcut-

The role of protein kinase C (PKC) in ischemic preconditioning remains controversial because of difficulties with both its measurement and pharmacological manipulation. We investigated preconditioning in isolated neonatal rat cardiocytes by expressing constitutively active isotypes of PKC. Observations at differing durations of simulated ischemia suggested ␤-galactosidase (␤-gal) activity reflected viability within transfected myocytes. Preconditioning with 90 min of ischemia significantly increased ␤-gal activity and myocyte survival after 6 h of ischemia; an effect abolished by PKC inhibitors. After co-transfection with plasmids encoding ␤-gal and either constitutively active mutants of PKC-␦, PKC-␣, wild type PKC-␦, or empty vector, cardiocytes were subjected to 6 h of ischemia. Only PKC-␦, rendered constitutively active by a limited deletion within the pseudosubstrate domain, consistently increased resistance to simulated ischemia (␤-gal activity was 85.6 ؎ 11.9% versus 53.7 ؎ 6.5% (p < 0.01) and dead myocytes 46.8 ؎ 3.4% versus 68.7 ؎ 2.8% (p < 0.01)). Since transfection was apparent in only 5-12% of cells, the results suggested a protective bystander effect that was confirmed by co-culture of transfected myocytes with untransfected myocytes. In neonatal cardiocytes expression of active PKC-␦ increases resistance to simulated ischemia. This observation may provide further insight into the mechanism and possible avenues for therapeutic exploitation of preconditioning.

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