Bone anabolic effects of subcutaneous treatment with basic fibroblast growth factor alone and in combination with estrogen in osteopenic ovariectomized rats

Abstract

Although basic fibroblast growth factor (bFGF) is a potent stimulator of bone formation when administered intravenously, less is known regarding the effects of this peptide on bone following subcutaneous (sc) administration. In addition, it is unknown whether coadministration of estrogen enhances the bone response to treatment with bFGF. Therefore, the purpose of this study was (1) to characterize the skeletal response to sc injection of a high dose of bFGF, and (2) to determine whether concurrent administration of estrogen affects the skeletal response to bFGF treatment. Female Sprague-Dawley rats were ovariectomized (ovx) or sham-operated (sham) at 3 months of age and left untreated for 2 months to establish cancellous osteopenia in the ovx group. The sham rats ( n = 10) and one group of ovx rats ( n = 9) were then injected sc with vehicle alone for 3 weeks. Two additional groups of ovx rats were injected sc with bFGF ( n = 10) or with bFGF + estrogen ( n = 10) for 3 weeks. bFGF was administered sc at a daily dose of 1 mg/kg/day and estrogen was administered sc 4 days per week at a dose of 10 μg/kg for the 3-week duration of treatment. Lumbar vertebrae were collected and processed undecalcified for quantitative bone histomorphometry. Cancellous bone volume was lower and cancellous bone turnover was higher in vehicle-treated ovx rats than in vehicle-treated sham rats. Subcutaneous treatment of ovx rats with bFGF for 3 weeks resulted in a 4-fold increase in osteoblast surface and an 8-fold increase in osteoid surface in comparison to vehicle treatment of ovx rats. Osteoid volume was also markedly increased in the bFGF-treated ovx rats (7 ± 4%) in comparison to vehicle-treated ovx rats (<0.1%). Osteoblast surface, osteoid surface, and osteoid volume were nearly identical in ovx rats treated with bFGF alone and with bFGF + estrogen. Although the majority of the osteoid in bFGF- and bFGF + estrogen-treated animals was deposited along mineralized bone surfaces, osteoid spicules without any connections to preexisting bone surfaces were also detected, providing definitive proof for bone formation within bone marrow in response to bFGF administration. Osteoclast surface, an index of bone resorption, was not affected by bFGF treatment. However, cotreatment of ovx rats with bFGF + estrogen resulted in lower osteoclast surface in comparison to treatment of ovx rats with either vehicle or bFGF alone. In summary, these findings indicate that administration of a high dose of bFGF via sc injection markedly increases bone formation and may be a useful treatment for cancellous osteopenia in the estrogen-deplete skeleton. The anabolic effects of bFGF on bone are not enhanced by concurrent treatment with estrogen at the replacement dose used in this study.