Insulin-dependent diabetes mellitus (IDDM) is
known to be associated with an increased risk of osteopenia.
However, the cellular and molecular mechanisms for IDDMinduced
alterations of the bone are not well understood. The
effects of IDDM on bone metabolism were investigated using
rats rendered diabetic by an injection of streptozotocin (STZ).
After 4 weeks, the diabetic rats exhibited bone loss, low levels
of osteocalcin, insulin-like growth factor-I (IGF-I) and bone
alkaline phosphatase (ALP) activity with normal levels of bone
tartrate-resistant acid phosphatase (TRAP) and cathepsin K
activity, and urinary excretion of deoxypyridinoline (Dpd).
Histological analysis showed a decrease in the number of
osteoblasts with a normal number of osteoclasts in the
metaphysis of the proximal tibia. The decreased expression of
ALP, osteoclacin and collagen mRNA was associated with a
decrease in the expression of runt-related transcription factor 2
(Runx2), Osterix and distal-less homeobox 5 (Dlx5) and an
unaltered expression of bone morphogenic protein-2 (BMP2).
The protein levels of Runx2, phosphorylated glycogen
synthase kinase 3β (GSK3β), active β-catenin and β-catenin
decreased. The activation of Akt was inhibited. The mRNA
and protein levels of sclerosteosis (Sost) and Dickkopf 1
(Dkk1), inhibitors of Wnt signaling, increased. The mRNA
expression of IGF-I and the IGF-I receptor (IGF-IR) was
suppressed. These changes observed in the bone of diabetic
rats were reversed by treatment with insulin, but not by normalization
of the circulating IGF-I levels by treatment with IGF-I.
These results suggest that insulin-deficiency in IDDM decreases
osteoblastogenesis associated with inhibition of Wnt signaling
through the increased expression of Sost and Dkk1 and the
inhibition of Akt activation.