Physiologic levels of extracellular PPi must be produced and maintained by chondrocytes to suppress matrix deposition of hydroxyapatite (HA). But extracellular PPi rises markedly in cartilage in direct association with aging. Resultant matrix supersaturation with PPi rises markedly in cartilage in direct association with aging. Resultant matrix supersaturation with PPi stimulates calcium pyrophosphate (CPPD) and HA deposition. Cytokine and growth factor balance appears to normally keep extracellular PPi levels in check. In normal chondrocytes, TGFbeta increases and IGF-I decreases extracellular PPi. This effect of TGFbeta increases markedly with aging. Transport of PPi from the cell anterior also regulates extracellular PPi. Players in PPi metabolism include ANK, a multiple-pass chondrocyte membrane protein proposed to channel PPi and observed to be up-regulated in OA cartilage, the TGFbeta-inducible PPi-generating NTPPPH isoenzyme., PC-1, and the secreted TGFbeta- induced and IGF-I suppressed matrix constituent Cartilage Intermediate Layer Protein-1 (CILP-1), which had been reported to be an NTPPPH. CILP-1 expression becomes elevated in the middle zone in aging and OA cartilages, the principle region in which CPPD crystals deposit. We observe that CILP-1 lacks intrinsic NTPPPH activity. But CILP-1 is cleaved into two domains at a furin consensus site, and full length CILP- 1 and the N-terminal domain of CILP-1 block IGF-1 from suppressing chondrocyte extracellular PPi. We hypothesize that ANK and the NTPPPH PC-1, through coordinated expression regulated by TGFbeta and IGF1, produce synergistic effects on extracellular PPi and on cartilage matrix calcification. Second, we hypothesize that PPi is not simply a passive component of a calcium crystal salt, but that the effects of ANK and PC-1 through PPi also regulate chondrocyte gene expression and differentiation, in part via degradation of PPi to Pi. Third, we propose that CILP-1 functions in a CILP isoform-specific manner not shared by the 51% identical molecule CILP-2. Last, we hypothesize that excess CILP-1 expression promotes calcification in aging cartilage partly by antagonizing the effects on IGF- 1 on PPi metabolism. NTPPPH activity, extracellular PPi, CILP expression, and crystal deposition all increase in aging cartilages and in OA. Completion of the proposed studies to test these hypotheses will increase understanding of the molecular basis by which crystals deposit in cartilage and will help identify novel potential therapeutic targets.
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