Inhibition of calcium phosphate-DNA coprecipitates induced cell death by phosphocitrates

Phosphocitrate [PC] is a powerful inhibitor of biological crystallization and a potential disease modifying drug for crystal associated diseases such as crystals associated osteoarthritis [OA]. Recently, it has been reported that a new PC complex salt, calcium sodium PC [CaNaPC], is much more powerful than its precursor, sodium PC [NaPC], in reducing the size of chemically-induced calcified plaques in rat when examined using a calcergy assay (1). The molecular mechanisms underlying such a superior activity as a calcification inhibitor over its precursor NaPC are currently unknown. In order to evaluate the potential of CaNaPC as a disease modifying drug for crystals associated OA, we examined and compared CaNaPC and its precusor NaPC using several cell- based assays. CaNaPC was found to have an inhibitory potency similar to that of NaPC toward preventing the stimulating effects of basic calcium phosphate [BCP] crystals on the induction of MMP1, thymidine uptake and endocytosis. However, CaNaPC proved much more powerful than NaPC in the inhibition of amorphous calcium phosphate-DNA coprecipitates-induced cell death. These results suggest that the superior anti-calcification activity of NaCaPC over NaPC observed in rat is probably due to its superior activity in the inhibition of the effects associated with amorphous calcium phosphate clusters/aggregates/precipitates but not the effects associated with BCP crystals. Since amorphous calcium phosphate clusters/aggregates/precipitates are precursors of BCP crystals and coexist with calcium-containing crystals in calcified tissues (2-6), these amorphous clusters/aggregates/precipitates, similar to BCP crystals, may have played a significant role in pathological calcifications and in the development of crystals associated diseases such as crystals associated OA. The superior activity of CaNaPC over its precursor NaPC in the inhibition of amorphous calcium phosphate-DNA coprecipitates-induced cell death may, at least in part, explain its powerful anti-calcification activity in vivo. The findings suggest that CaNaPC through a dual action of inhibiting both the detrimental biological effects of formed BCP crystals and preforming amorphous calcium phosphate clusters/aggregates/precipitates, could present as a better disease-modifying drug for crystals associated OA than its parent NaPC.