High serum levels of IL-6 correlate with poor outcome in breast cancer patients. However, no data are available on the relationship between IL-6 and mammary stem/progenitor cells, which may fuel the genesis of breast cancer in vivo. Herein, we address this issue in the MCF-7 breast cancer cell line and in primary human mammospheres (MS), multicellular structures enriched in stem/progenitor cells of the mammary gland. MS from node invasive breast carcinoma tissues expressed IL-6 mRNA at higher levels than did MS from matched nonneoplastic mammary glands. In addition, IL-6 mRNA was detected only in basal-like breast carcinoma tissues, an aggressive breast carcinoma variant showing stem cell features. IL-6 treatment triggered Notch-3-dependent upregulation of the Notch ligand Jagged-1 and promotion of MS and MCF-7-derived spheroid growth. Moreover, IL-6 induced Notch-3-dependent upregulation of the carbonic anhydrase IX gene and promoted a hypoxia-resistant/invasive phenotype in MCF-7 cells and MS. Finally, autocrine IL-6 signaling relied upon Notch-3 activity to sustain the aggressive features of MCF-7-derived hypoxia-selected cells. In conclusion, these data support the hypothesis that IL-6 induces malignant features in Notch-3-expressing stem/progenitor cells from human ductal breast carcinoma and normal mammary gland.

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As the cellular component of articular cartilage, chondrocytes are responsible for maintaining in a low-turnover state the unique composition and organization of the matrix that was determined during embryonic and postnatal development. In joint diseases, cartilage homeostasis is disrupted by mechanisms that are driven by combinations of biological mediators that vary according to the disease process, including contributions from other joint tissues. In osteoarthritis (OA), biomechanical stimuli predominate with upregulation of both catabolic and anabolic cytokines and recapitulation of developmental phenotypes, whereas in rheumatoid arthritis (RA), inflammation and catabolism drive cartilage loss. In vitro studies in chondrocytes have elucidated signaling pathways and transcription factors that orchestrate specific functions that promote cartilage damage in both OA and RA. Thus, understanding how the adult articular chondrocyte functions within its unique environment will aid in the development of rational strategies to protect cartilage from damage resulting from joint disease. This review will cover current knowledge about the specific cellular and biochemical mechanisms that regulate cartilage homeostasis and pathology. IntroductionAdult articular cartilage is an avascular tissue composed of a specialized matrix of collagens, proteoglycans, and noncollagen proteins, in which chondrocytes constitute the unique cellular component. Although chondrocytes in this context do not normally divide, they are assumed to maintain the extracellular matrix (ECM) by low-turnover replacement of certain matrix proteins. During aging and joint disease, this equilibrium is disrupted and the rate of loss of collagens and proteoglycans from the matrix may exceed the rate of deposition of newly synthesized molecules. Originally considered an inert tissue, cartilage is now considered to respond to extrinsic factors that regulate gene expression and protein synthesis in chondrocytes. Numerous studies in vitro and in vivo during the last two decades have confirmed that articular chondrocytes are able to respond to mechanical injury, joint instability due to genetic factors, and biological stimuli such as cytokines and growth and differentiation factors that contribute to structural changes in the surrounding cartilage matrix [1]. Mechanical influences on chondrocyte function are considered to be important in the pathogenesis of osteoarthritis (OA), but chondrocyte responses to molecular signals may vary in different regions, including the calcified cartilage, and also occur at different stages over a long time course (Figure 1). In rheumatoid arthritis (RA), the inflamed synovium is the major source of cytokines and proteinases that mediate cartilage destruction in areas adjacent to the proliferating synovial pannus (Figure 2) [2]. However, the basic cellular mechanisms regulating chondrocyte responses are very different in OA and RA. Moreover, mechanistic insights from in vitro studies ideally should be interpreted in light...

In the context of OA disease, NF-κB transcription factors can be triggered by a host of stress-related stimuli including pro-inflammatory cytokines, excessive mechanical stress and ECM degradation products. Activated NF-κB regulates the expression of many cytokines and chemokines, adhesion molecules, inflammatory mediators, and several matrix degrading enzymes. NF-κB also influences the regulated accumulation and remodeling of ECM proteins and has indirect positive effects on downstream regulators of terminal chondrocyte differentiation (including β-catenin and Runx2). Although driven partly by pro-inflammatory and stress-related factors, OA pathogenesis also involves a “loss of maturational arrest” that inappropriately pushes chondrocytes towards a more differentiated, hypertrophic-like state. Growing evidence points to NF-κB signaling as not only playing a central role in the pro-inflammatory stress-related responses of chondrocytes to extra- and intra-cellular insults, but also in the control of their differentiation program. Thus unlike other signaling pathways the NF-κB activating kinases are potential therapeutic OA targets for multiple reasons. Targeted strategies to prevent unwanted NF-κB activation in this context, which do not cause side effects on other proteins or signaling pathways, need to be focused on the use of highly specific drug modalities, siRNAs or other biological inhibitors that are targeted to the activating NF-κB kinases IKKα or IKKβ or specific activating canonical NF-κB subunits. However, work remains in its infancy to evaluate the effects of efficacious, targeted NF-κB inhibitors in animal models of OA disease in vivo and to also target these strategies only to affected cartilage and joints to avoid other undesirable systemic effects.

Human cartilage is a complex tissue of matrix proteins that vary in amount and orientation from superficial to deep layers and from loaded to unloaded zones. A major challenge to efforts to repair cartilage by stem cell-based and other tissue engineering strategies is the inability of the resident chondrocytes to lay down new matrix with the same structural and resilient properties that it had upon its original formation. This is particularly true for the collagen network, which is susceptible to cleavage once proteoglycans are depleted. Thus, a thorough understanding of the similarities and particularly the marked differences in mechanisms of cartilage remodeling during development, osteoarthritis, and aging may lead to more effective strategies for preventing cartilage damage and promoting repair. To identify and characterize effectors or regulators of cartilage remodeling in these processes, we are using culture models of primary human and mouse chondrocytes and cell lines and mouse genetic models to manipulate gene expression programs leading to matrix remodeling and subsequent chondrocyte hypertrophic differentiation, pivotal processes which both go astray in OA disease. Matrix metalloproteinase (MMP)-13, the major type II collagen-degrading collagenase, is regulated by stress-, inflammation-, and differentiation-induced signals that not only contribute to irreversible joint damage (progression) in OA, but importantly, also to the initiation/onset phase, wherein chondrocytes in articular cartilage leave their natural growth-and differentiation-arrested state. Our work points to common mediators of these processes in human OA cartilage and in early through late stages of OA in surgical and genetic mouse models.

MElal, a 16-base-pair nuclear factor binding site residing between the c-MYC P1 and P2 transcription initiation sites, is required for P2 activity.

Comparative nucleotide sequence analysis of a rearranged c-myc gene in a murine plasmacytoma and c-myc cDNA from normal spleen reveals that chromosomal translocation in the plasmacytoma breaks the c-myc gene within the first exon or intron. In the plasmacytoma truncated c-myc RNAs initiate from newly exposed promoter sites. Nevertheless, the myc polypeptide produced in the plasmacytoma is probably the same as that from the intact c-myc gene because the exon lost by breakage and translocation is non-coding. The second and third exons of the mouse c-myc gene are substantially conserved in the v-myc gene of the avian retrovirus, MC29.

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