Increased 78-kDa glucose-regulated protein (GRP78), C/EBP binding protein (CHOP), and X-box binding proteins (XBP1s) in human osteoarthritis (OA) chondrocytes. (A) Immunohistochemistry revealed expression of GRP78 and CHOP in human normal knee (grade I) and knee OA cartilage (mild and severe OA, grades II and IV, respectively). Data representative of a total five normal donors, and of six different donors each with mild and severe knee OA. (B) Reverse transcription (RT)-PCR was performed to assess XBP1 splicing at mRNA level in passage-1 human knee chondrocytes ranging from normal to severe OA, studying 1 × 106 cells, as described (Methods), in normal (grade I) and OA cells. Alternative splicing of XBP1 mRNA (248 bp), and increased XBP1 mRNA, were only seen in OA samples. The spliced XBP1 product XBP1s was 26 bp shorter (lower band), and was seen in chondrocytes of four of the five grade-II knee OA donors, and all five grade III-IV samples.