Endochondral bone growth is regulated by the rates of chondrocyte proliferation and differentiation. However, the intracellular mechanisms regulating these processes are poorly understood. Recently, interruption of the gene encoding the transcription factor activating transcription factor 2 (ATF-2) was shown to inhibit proliferation of chondrocytes in mice [Reimold, A. M., et al. Bone formation in vertebrates can occur in two different ways: intramembranous ossification, through which the bones of the skull are formed, and endochondral ossification, through which the axial and appendicular skeleton is formed. During endochondral bone formation, the precursor bones are first laid down as a cartilagenous model. In a second step, cartilage is then replaced by bone tissue (recently reviewed in ref.

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Exact coordination of growth plate chondrocyte proliferation is necessary for normal endochondral bone development and growth. Here we show that PTHrP and TGF␤ control chondrocyte cell cycle progression and proliferation by stimulating signaling pathways that activate transcription from the cyclin D1 promoter. The TGF␤ pathway activates the transcription factor ATF-2, whereas PTHrP uses the related transcription factor CREB, to stimulate cyclin D1 promoter activity via the CRE promoter element. Inhibition of cyclin D1 expression with antisense oligonucleotides causes a delay in progression of chondrocytes through the G1 phase of the cell cycle, reduced E2F activity, and decreased proliferation. Growth plates from cyclin D1-deficient mice display a smaller zone of proliferating chondrocytes, confirming the requirement for cyclin D1 in chondrocyte proliferation in vivo. These data identify the cyclin D1 gene as an essential component of chondrocyte proliferation as well as a fundamental target gene of TGF␤ and PTHrP during skeletal growth. INTRODUCTIONEndochondral bone growth is controlled by the coordinated proliferation and differentiation of growth plate chondrocytes (Cancedda et al., 1995). Numerous skeletal diseases (chondrodysplasias) are caused by genetic disturbances of these processes, resulting in skeletal deformities, dwarfism, and early onset osteoarthritis Olsen, 1997a, 1997b). Despite the recent identification of many genes that control chondrocyte proliferation and differentiation Olsen, 1997a, 1997b;Beier et al., 1999a), the intracellular signaling pathways and transcriptional mechanisms involved are not well defined. Transforming growth factor beta (TGF␤) and parathyroid hormone-related peptide (PTHrP) stimulate the proliferation of chondrocytes and chondrosarcoma cells in vitro (O'Keefe et al., 1988;Rosier et al., 1989;Guerne and Lotz, 1991;Loveys et al., 1993;Guerne et al., 1994). In addition, interruption of TGF␤ or PTHrP signaling in vivo in mice causes a reduction in the number of proliferative chondrocytes as well as premature differentiation of these cells (Amizuka et al., 1994;Karaplis et al., 1994;Serra et al., 1997;Yang et al., 2001). These data suggest that both factors are required for normal chondrocyte proliferation in vivo and in vitro. However, the intracellular signaling pathways activated by PTHrP and TGF␤ in chondrocytes as well as their target genes have yet to be identified.Cell cycle genes appear to play an important role in the control of chondrocyte proliferation and differentiation (Beier et al., 1999a;LuValle and Beier, 2000). Progression through the eukaryotic cell cycle is controlled by the activity of a family of kinases called the cyclin-dependent kinases or CDKs (Weinberg, 1995). CDK activity is strictly controlled by a number of mechanisms, including phosphorylation status and the presence of inhibitory proteins such as p16Ink4 or p21 Cip1/Waf1 . An absolute requirement for the activity of a given CDK is the presence of its specific cyclin partner. The D-type c...