Background: LIM mineralization protein-1 (LMP-1), an intracellular protein, is thought to induce secretion of soluble factors that convey its osteoinductive activity. Although evidence suggests that LMP-1 may be a critical regulator of osteoblast differentiation in vitro and in vivo, little is known about its mechanism of action. The purpose of the present study was to identify candidates for the induced secreted factors and to describe the time sequence of histological changes during bone formation induced by LMP-1.
Methods: Human lung carcinoma (A549) cells were used to determine if LMP-1 overexpression would induce expression of bone morphogenetic proteins (BMPs) in vitro. Cultured A549 cells were infected with recombinant replication-deficient human type-5 adenovirus containing the LMP-1 or LacZ cDNA. Cells were subjected to immunohistochemical analysis after forty-eight hours. Finally, sixteen athymic rats received subcutaneous implants consisting of collagen disks loaded with human buffy-coat cells that were infected with one of the above two viruses. Rats were killed at intervals, and explants were studied with histological and immunohistochemical analyses.
Results: In vitro experiments with A549 cells showed that AdLMP-1-infected cells express elevated levels of BMP-2, BMP-4, BMP-6, BMP-7, and TGF-ß1 (transforming growth factor-beta 1) protein. Human buffy-coat cells infected with AdLMP-1 also demonstrated increased levels of BMP-4 and BMP-7 protein seventy-two hours after ectopic implantation in athymic rats, confirming the in vitro hypothesis.
Conclusions: The osteoinductive properties of LMP-1 involve synthesis of several BMPs and the recruitment of host cells that differentiate and participate in direct membranous bone formation.
Clinical Relevance: Ex vivo gene therapy with the LMP-1 cDNA-induced secretion of multiple BMPs may provide an alternative to implantation of large doses of a single BMP to induce new bone formation.