Structure of the murine Pit1 phosphate transporter/retrovirus receptor gene and functional characterization of its promoter region

The emerging role of phosphate in vascular calcification

Vascular calcification is recognized as a major contributor to cardiovascular disease (CVD) in end stage renal disease (ESRD) patients. Susceptibility to vascular calcification is genetically determined and actively regulated by diverse inducers and inhibitors. One of these inducers, hyperphosphatemia, promotes vascular calcification and is a nontraditional risk factor for CVD mortality in ESRD patients. Vascular smooth muscle cells (SMCs) respond to elevated phosphate levels by undergoing an osteochondrogenic phenotype change and mineralizing their extracellular matrix through a mechanism requiring sodium-dependent phosphate cotransporters. Disease states and cytokines can increase expression of sodium-dependent phosphate cotransporters in SMCs, thereby increasing susceptibility to calcification even at phosphate concentrations that are in the normal range.

Role of interleukin-8 in PiT-1 expression and CXCR1-mediated inorganic phosphate uptake in chondrocytes

OBJECTIVE

The proinflammatory chemokine interleukin-8 (IL-8) induces chondrocyte hypertrophy. Moreover, chondrocyte hypertrophy develops in situ in osteoarthritic (OA) articular cartilage and promotes dysregulated matrix repair and calcification. Growth plate chondrocyte hypertrophy is associated with expression of the type III sodium-dependent inorganic phosphate (Pi) cotransporter phosphate transporter/retrovirus receptor 1 (PiT-1). This study was undertaken to test the hypothesis that IL-8 promotes chondrocyte hypertrophy by modulating chondrocyte PiT-1 expression and sodium-dependent Pi uptake, and to assess differential roles in this activity.

METHODS

The selective IL-8 receptor CXCR1 and the promiscuous chemokine receptor CXCR2 were used. Human knee OA cartilage, cultured normal bovine knee chondrocytes, and immortalized human articular chondrocytic CH-8 cells were transfected with CXCR1/CXCR2 chimeric receptors in which the 40-amino acid C-terminal cytosolic tail domains were swapped and site mutants of a CXCR1-specific region were generated.

RESULTS

Up-regulated PiT-1 expression was detected in OA cartilage. IL-8, but not IL-1 or the CXCR2 ligand growth-related oncogene alpha, induced PiT-1 expression and increased sodium-dependent Pi uptake by >40% in chondrocytes. The sodium/phosphate cotransport inhibitor phosphonoformic acid blocked IL-8-induced chondrocyte hypertrophic differentiation. Signaling mediated by kinase Pyk-2 was essential for IL-8 induction of PitT-1 expression and Pi uptake. Signaling through the TSYT(346-349) region of the CXCR1 cytosolic tail, a region divergent from the CXCR2 cytosolic tail, was essential for IL-8 to induce Pi uptake.

CONCLUSION

Our results link low-grade IL-8-mediated cartilaginous inflammation in OA to altered chondrocyte differentiation and disease progression through PiT-1 expression and sodium-dependent Pi uptake mediated by CXCR1 signaling.

Species-specific mechanisms control the activity of the Pit1/PIT1 phosphate transporter gene promoter in mouse and human

The Pit1 phosphate transporter is involved in regulated phosphate handling in bone forming cells. In this study, we compared the structure of the murine and human Pit1/PIT1 promoters and characterized cis-acting elements controlling Pit1/PIT1 expression. The Pit1/PIT1 promoter sequence and its location relative to the first transcribed exon are conserved and similar transcription factor binding sites are found at identical positions in mouse and human. Luciferase reporter gene assays in transiently transfected mouse ATDC5 chondrocytes and human SaOS-2 osteoblasts indicated that the activity of the mouse Pit1 promoter depends on several cis-acting elements, including ATF/CREB, Sp1 and AP-1 sites, an E-box and a TATA box. In contrast, the activity of the human promoter essentially requires a TATA-like sequence and one single Sp1 site. This Sp1 site binds Sp1, Sp3, as well as unidentified proteins present in SaOS-2 nuclear extracts and co-transfection experiments in SL2 cells indicate that Sp1 and Sp3 activate transcription from the human PIT1 promoter. These data suggest that, despite similarities in promoter structure, changes in the relative importance of conserved transcription factor binding sites cause species-dependent differences in Pit1 promoter function, which allow Sp1-related proteins to play a particularly important role in human.

Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes

During endochondral bone formation and fracture healing, cells committed to chondrogenesis undergo a temporally restricted program of differentiation that is characterized by sequential changes in their phenotype and gene expression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes undergo a similar but restricted differentiation program that does not proceed to mineralization, except in pathologic conditions such as osteoarthritis. The pathogenesis of disorders of cartilage development and metabolism, including osteochondrodysplasia, fracture non-union, and osteoarthritis remain poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrogenesis and cartilage maturation. Differentiation was monitored over a 4-week period using a combination of morphological, biochemical, and molecular markers that have been characterized in vivo and in vitro. CFK2 cells expressed the type III sodium-dependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity of Glvr-1 by extracellular phosphate and parathyroid hormone-related protein was restricted to an early stage of CFK2 differentiation, as evidenced by expression of type II collagen, proteoglycan, and Ihh. On the other hand, regulated expression and activity of a calcium-sensing receptor by extracellular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activity and parathyroid hormone/parathyroid hormone-related protein receptor expression. On the basis of these temporally restricted changes in the sensing and transport of phosphate and calcium, we predict that extracellular phosphate plays a role in the commitment of chondrogenic cells to differentiation, whereas extracellular calcium plays a role at a later stage in their differentiation program.