No abstract

In this study, we investigated the in vitro and in vivo biologic activity of bone morphogenetic protein 2 (BMP-2) released from four sustained delivery vehicles for bone regeneration. BMP-2 was incorporated in 1) a gelatin hydrogel, 2) poly(lactic-co-glycolic acid) (PLGA) microspheres embedded in a gelatin hydrogel, 3) microspheres embedded in a poly(propylene fumarate) (PPF) scaffold and 4) microspheres embedded in a PPF scaffold surrounded by a gelatin hydrogel. A fraction of the incorporated BMP-2 was radiolabeled with 125 I to determine its in vitro and in vivo release profiles. The release and bioactivity of BMP-2 were tested weekly over a period of 12 weeks in preosteoblast W20-17 cell line culture and in a rat subcutaneous implantation model. Outcome parameters for in vitro and in vivo bioactivity of the released BMP-2 were alkaline phosphatase (AP) induction and bone formation, respectively. The four implant types showed different in vitro release profiles over the 12-week period, which changed significantly upon implantation. The AP induction by BMP-2 released from gelatin implants showed a loss in bioactivity after 6 weeks in culture, while the BMP-2 released from the other implants continued to show bioactivity over the full 12-week period. Micro-CT and histological analysis of the delivery vehicles after 6 weeks of implantation showed significantly more bone in the microsphere/PPF scaffold composites (implant 3, p < 0.02). After 12 weeks, the amount of newly formed bone in the microsphere/PPF scaffolds remained significantly higher than in the gelatin and microsphere/gelatin hydrogels (p < 0.001), however there was no statistical difference compared to the microsphere/PPF/gelatin composite. Overall, the results from this study show that BMP-2 could be incorporated into various bone tissue engineering composites for sustained release over a prolonged period of time with retention of bioactivity.

Activation of 2-5A-dependent ribonuclease by 5'-phosphorylated, 2',5'-linked oligoadenylates, known as 2-5A, is one pathway of interferon action. Unaided uptake into HeLa cells of 2-5A linked to an antisense oligonucleotide resulted in the selective ablation of messenger RNA for the double-stranded RNA (dsRNA)-dependent protein kinase PKR. Similarly, purified, recombinant human 2-5A-dependent ribonuclease was induced to selectively cleave PKR messenger RNA. Cells depleted of PKR activity were unresponsive to activation of nuclear factor-kappa B (NF-kappa B) by the dsRNA poly(I):poly(C), which provides direct evidence that PKR is a transducer for the dsRNA signaling of NF-kappa B.

2‐5A‐dependent RNase is the terminal factor in the interferon‐regulated 2‐5A system thought to function in both the molecular mechanism of interferon action and in the general control of RNA stability. However, direct evidence for specific functions of 2‐5A‐dependent RNase has been generally lacking. Therefore, we developed a strategy to block the 2‐5A system using a truncated form of 2‐5A‐dependent RNase which retains 2‐5A binding activity while lacking RNase activity. When the truncated RNase was stably expressed to high levels in murine cells, it prevented specific rRNA cleavage in response to 2‐5A transfection and the cells were unresponsive to the antiviral activity of interferon alpha/beta for encephalomyocarditis virus. Remarkably, cells expressing the truncated RNase were also resistant to the antiproliferative activity of interferon. The truncated RNase is a dominant negative mutant that binds 2‐5A and that may interfere with normal protein‐protein interactions through nine ankyrin‐like repeats.

Apoptosis of viral infected cells appears to be one defense strategy to limit viral infection. Interferon can also confer viral resistance by the induction of the 2-5A system comprised of 2'-5' oligoadenylate synthetase (OAS), and RNase L. Since rRNA is degraded upon activation of RNase L and during apoptosis and since both of these processes serve antiviral functions, we examined the role RNase L may play in cell death. Inhibition of RNase L activity, by transfection with a dominant negative mutant, blocked staurosporine-induced apoptosis of NIH3T3 cells and SV40-transformed BALB/c cells. In addition, K562 cell lines expressing inactive RNase L were more resistant to apoptosis induced by decreased glutathione levels. Hydrogen peroxide-induced death of NIH3T3 cells did not occur by apoptosis and was not dependent upon active RNAse L. Apoptosis regulatory proteins of the Bcl-2 family did not exhibit altered expression levels in the absence of RNase L activity. RNase L is required for certain pathways of cell death and may help mediate viral-induced apoptosis.