The Expression of Apolipoprotein E in Osteoblastic Cells on Titanium

Abstract

Apolipoprotein E (ApoE) is a plasma protein that plays a major role in lipoprotein metabolism. Recent findings indicate that it is also involved in bone metabolism. ApoE mRNA is highly expressed during bone formation around titanium implants and adding ApoE to cultured osteoblasts enhances bone formation. However, the mechanism of ApoE expression in osteoblasts on titanium is not well understood. This study investigated the mechanism regulating ApoE mRNA expression in osteoblasts on titanium.
Rat or mouse primary osteoblasts or MC3T3-E1 cells were cultured on titanium, cover glasses, or culture dishes. The bone nodule formation on each substrate was measured, and the expression of ApoE and/or osteocalcin (OCN) was analyzed using reverse-transcription (RT) and real-time polymerase chain reaction (PCR). Bone nodules formed on titanium and culture dishes, but not on cover glasses. RT-PCR showed strong expression of ApoE in the cells on titanium and culture dishes, but not those on the cover glasses. Real-time PCR analysis indicated that on day 14 the percentage expression of ApoE and OCN by cells on the cover glasses relative to those on titanium was 18% and 29%, respectively. Although MC3T3-E1 cells did not express ApoE mRNA, adding ApoE or conditioned medium of cultured primary osteoblasts induced the expression of ApoE in MC3T3-E1 cells.
These findings suggest that primary osteoblasts express ApoE during bone formation, which indicates that ApoE is involved not only in lipoprotein metabolism but also in bone formation on biomaterials such as titanium.