Effects of DP and CRTH2 activation on osteoblast function

Abstract

Modulation of PGs by inhibition or stimulation is a promising approach for the management of pain and inflammation in patients with rheumatic disease. Based on recent results from our laboratories as well as on the literature, we hypothesise that Prostaglandin D[subscript 2] (PGD[subscript 2]) is an important anabolic agent for osteoblasts. Our results show that the PGD[subscript 2] decreases the osteoblasts proliferation acting probably through the CRTH2 receptor. Surprisingly, when DK-PGD[subscript 2] was used alone or with Naproxen, although the proliferation decreased with the dose, it seemed to be restored to the control level at higher concentrations of DK-PGD[subscript 2] . Thus, we hypothesise the existence of other compensatory mechanisms. The PGD[subscript 2] had no relevant effect alone or when used with Naproxen, but seemed to decrease the osteoblast differentiation when used with Diclofenac at a higher concentration only. When vitamin D was added to all conditions, PGD[subscript 2] had an inhibitory effect on the differentiation (dose-response), but this could not be replicated when Naproxen was used. In a test with Diclofenac, we can assume a decreasing trend-line for differentiation when augmenting the PGD[subscript 2] dose, but the effect is not statistically relevant. In a competition test with PGD[subscript 2] and DP/CRTH2 antagonists, blocking DP receptor yielded no effect on differentiation, and blocking the CRTH2 receptor showed a relevant decrease at high concentration of PGD[subscript 2]. The effect was similar in a test with PGD[subscript 2] and PPAR[gamma] antagonist suggesting that it might have a compensatory, positive effect that reversed DP activation. The PGD[subscript 2] has a slight positive effect on the osteoblast matrix mineralisation (with Naproxen), but not through its receptors since use of DP/CRTH2 antagonists did not abrogate this. In a competition test with PGD[subscript 2] and DP/CRTH2 antagonists we had no response. When we used the PGD[subscript 2] in the presence of PPAR[gamma] antagonist, the calcification decreased significantly, indicating that the positive effect of PGD[subscript 2] on calcification works rather through this receptor.