We used the Cre-loxP system to generate an osteoblast-specific null mutation of Pten in mice [Col1CrePtenflox/flox mice] using α 1(I)-collagen Cre transgenic mice and Ptenflox mice to examine the effects of Pten in osteoblasts in vivo. Semi-quantitative PCR experiments revealed that Pten might be deleted from most osteoblasts in mutant mice. Mutant mice were born alive and appeared healthy. They developed no spontaneous bone tumors during the 24-month observation period. However, mutant mice showed increased bone density ; histomorphometric measurements revealed increased bone volume that resulted from increased bone formation caused by increased osteoblast functions in mutant mice. Furthermore, mutant osteoblasts were larger than WT cells both in vitro and in vivo, and bone marrow cells were more numerous in the mutant mice. Therefore, we conclude that Pten inhibits bone formation primarily by inhibiting osteoblast activity. Inhibition of Pten or activation of PI3K in osteoblasts might be useful for future treatment of bone diseases such as osteoporosis or pathological bone fracture
