Primary cilia attenuate hedgehog signalling in neoplastic chondrocytes

Abstract

Primary cilia can act as either a negative or positive regulator of the hedgehog (Hh) signaling pathway. Many cartilage tumors are characterized by abnormal activation of the Hh pathway. Here, we report that the presence of primary cilia occurs at a low frequency (12.4%) in neoplastic chondrocytes from malignant human chondrosarcomas, compared with chondrocytes from normal articular cartilage (67.7%). To determine the function of primary cilia in cartilaginous neoplasia, we studied benign cartilage tumors that are formed in mice by chondrocyte-specific overexpression of Gli2, a downstream transcriptional activator of the Hh pathway. Col2A1-Gli2 mice were crossed with Ift88+/− mice, which display a partial loss of ciliogenesis. Surprisingly, cartilage tumors developed in Ift88+/− mice that were phenotypically similar to those that arise in Col2A1-Gli2 mice. Further activation of the Hh pathway was observed in Col2A1-Gli2; Ift88+/− mice compared with either Col2A1-Gli2 or Ift88+/− mice, which was associated with an increased incidence of cartilage tumors. Chondrosarcomas were established in explant cultures, and treated with choral hydrate, which disrupts the functional primary cilia. Thus, treatment resulted in hyperactivity of the Hh signaling pathway, as well as cellular changes that could promote tumor growth. Primary cilia functions to inhibit Hh signaling in neoplastic chondrocytes. The activation of Hh signaling is sufficient to induce benign cartilage tumors without another oncogenic initiating event. Moreover, as primary cilia suppress Hh pathway activation in chondrosarcoma, cellular mechanisms inhibiting proper cilia function may be important in maintaining the neoplastic phenotype.

Hedgehog Pathway Inhibition in Chondrosarcoma Using the Smoothened Inhibitor IPI-926 Directly Inhibits Sarcoma Cell Growth

“…In tumors such as chondrosarcoma, which rely on ligand-dependent Hh activation and exhibit high levels of GLI1 and PTCH1 expression, downregulation of GLI1 and PTCH1 could serve as biomarkers for response to Hh pathway inhibition. In addition, as most chondrosarcomas have an identifiable primary cilium, the presence of cilia may serve as a biomarker for response to Hh blockade, as recently demonstrated in a transgenic mouse model of cartilage neoplasia (46). As a potential treatment for patients, IPI-926, a potent and selective SMO antagonist, may have advantages as targeted therapy for Hh-dependent tumors compared with other pathway inhibitors.…”

Primary Cilia and Intraflagellar Transport Proteins in Bone and Cartilage

“…In human chondrosarcomas, the percentage of ciliated neoplastic chondrocytes is only 12.4%, which is a very low frequency as compared with 67.7% in normal articular cartilage (Ho et al 2013). In the mouse peripheral chondrosarcoma, primary cilia are absent on the majority of chondrocytes, which might contribute to the loss of polarity of chondrocytes in peripheral chondrosarcoma (de Andrea et al 2015).…”

“…Similar to Col2α1-Gli2 mice, IFT88 +/-mice (partial loss IFT88 leads a partial loss of ciliogenesis) also develop cartilage tumors. More evidence demonstrates that the disruption of cilia (by decreasing IFT88 expression or treating with choral hydrate) in Gli2-overexpressed chondrosarcomas results in further activation of the Hh signaling pathway and promotes tumor growth, suggesting that cilia act as an inhibitor of Hh signaling in neoplastic chondrocytes (Ho et al 2013). A recent study suggested that loss of primary cilia in chondrocytes creates a proliferative block and might help to select those cells that have lost cell-cycle regulatory genes, which promotes tumor growth (de Andrea et al 2015).…”

“…Cilia are randomly located on the cell surface (de Andrea et al 2010;de Andrea and Hogendoorn 2012). In human malignant chondrosarcomas, cilium incidence in neoplastic chondrocytes is only 12.4% (Ho et al 2013). In the mouse peripheral chondrosarcoma, primary cilia are absent in the majority of chondrocytes (de Andrea et al 2015).…”

“…In osteochondroma, Hh gradient is disrupted, showing homogeneous pattern (Benoist-Lasselin et al 2006). Chondrocyte-specific overexpression of Gli2 (Col2α1-Gli2, activated Hh signaling in growth plate) leads to cartilage tumors in mice (Ho et al 2013). Similar to Col2α1-Gli2 mice, IFT88 +/-mice (partial loss IFT88 leads a partial loss of ciliogenesis) also develop cartilage tumors.…”