TREATMENT OF RESTING ZONE CHONDROCYTES WITH TRANSFORMING GROWTH-FACTOR-BETA-1 INDUCES DIFFERENTIATION INTO A PHENOTYPE CHARACTERISTIC OF GROWTH ZONE CHONDROCYTES BY DOWN-REGULATING RESPONSIVENESS TO 24,25-(OH)(2)D-3 AND UP-REGULATING RESPONSIVENESS TO 1,25-(OH)(2)D-3

To determine if transforming growth factor-beta 1 (TGF-beta 1) can ind uce the differentiation of resting zone (RC) chondrocytes, confluent, fourth passage cultures of these cells were pretreated for 24, 36, 48, 72, and 120 h with TGF-beta 1. At the end of pretreatment, the media were replaced with new media containing 10(-10)-10(-8) mol/L 1,25-(OH) (2)D-3 and the cells incubated for an additional 24 h, This second tre atment was chosen because prior studies had shown that only the more m ature growth zone (GC) chondrocytes respond to this vitamin D-3 metabo lite. The effect of TGF-beta pretreatment on cell maturation was asses sed by measuring alkaline phosphatase (ALPase)-specific activity. In a ddition, changes in matrix protein synthesis were assessed by measurin g collagen synthesis, as well as (SO4)-S-35 incorporation into proteog lycans. When RC cells were pretreated for 120 h with TGF-beta 1, treat ment with 1,25-(OH)(2)D-3 caused a dose-dependent increase in ALPase-s pecific activity and collagen synthesis, with no effect on proteoglyca n production. RC cells pretreated with 1,25-(OH)(2)D-3 responded like RC cells that had not received any pretreatment. RC cells normally res pond to 24,25-(OH)(2)D-3; however, RC cultures pretreated for 120 h wi th TGP-beta 1 lost their responsiveness to 24,25-(OH)(2)D-3. These res ults indicate that TGF-beta 1 directly regulates the maturation of RC chondrocytes into GC chondrocytes and support the hypothesis that this growth factor may play a significant role in regulating chondrocyte m aturation during endochondral ossification. (C) 1998 by Elsevier Scien ce Inc. All rights reserved.