Induction of functional differentiation and growth inhibition in vitro of canine osteosarcoma by 22-oxacalcitriol, calcitriol and all-trans retinoic acid

The effects of 22-oxacalcitriol (OCT), calcitriol and all-trans retinoic acid (ATRA) on the induction of functional differentiation and growth inhibition of the canine osteosarcoma cell line POS were investigated in vitro via bone differentiation markers and proliferation assays, respectively. The intracellular alkaline phosphatase (ALP) activity and the gamma-carboxyglutamic acid osteocalcin (GLA-OC) and procollagen type I C peptide (PIP) production were used as markers of differentiation. Treatment with 10(-8) M concentrations of all drugs for 72 h significantly inhibited growth (P < 0.0001) and increased ALP activity and GLA-OC and PIP production in POS. OCT, calcitriol and ATRA significantly increased the: ALP activity from 1.58 +/- 0.14 mumol/min/mg protein (mean +/- SD; control) to 2.50 +/- 0.09 (P < 0.0001), 2.30 +/- 0.14 (P < 0.0001) and 2.00 +/- 0.14 (P = 0.0008), respectively; GLA-OC production from 0.71 +/- 0.01 ng/ml (control) to 2.87 +/- 0.01 (P < 0.0001), 2.87 +/- 0.11 (P < 0.0001) and 1.36 +/- 0.06 (P < 0.0001), respectively; and PIP production from 433.91 +/- 23.29 ng/ml (control) to 536.54 +/- 15.46 (P = 0.0002), 497.06 +/- 1.99 (P = 0.0028) and 481.66 +/- 0.01 (P = 0.0104), respectively. This study demonstrated that treatment with these drugs induced a phenotypic maturation of POS cells into cells that exhibit the properties of functionally mature bone cells with parallel growth inhibition. The effects of these drugs on functional differentiation may be useful to complement the progression of a normal osteogenic differentiation process in the sarcoma.