Insulin-like growth factor-1 signaling in engineered articular cartilage

Articular cartilage can easily become damaged or diseased and it does not have the ability to heal itself. A tissue engineering approach to regenerate cartilage is to integrate chondrocytes, the primary cell in cartilage, with biomaterials and biomolecules. Currently, there is limited knowledge on how all these factors influence the expression of upstream insulin-like growth factor-1 (IGF-1) signaling molecules. In an effort to better understand how IGF-1 and phenotypic, type II collagen, expression can be modified by altering construct properties, chondrocytes were embedded in alginate hydrogels. Increasing alginate concentration resulted in an upregulation of IGF-1 expression and by increasing cell density further enhanced IGF-1 expression. Additional changes in chondrocyte signaling were observed when exogenously delivering IGF-1 to the constructs. IGF-1 expression decreased while the receptor for IGF-1 (IGF-1R) expression as well as type II collagen increased in the presence of excess IGF-1 indicating that it has a key role in IGF-1 and chondrocyte interaction. An extracellular matrix molecule, such as hyaluronic acid (HA), provides anchorage sites for chondrocytes and therefore the influence of HA on IGF-1 signaling was also investigated. The incorporation of HA created a dual affect by entrapping exogenously delivered IGF-1 as well as directly interacting with chondrocytes. As a result, the IGF-1 expression levels varied depending on HA concentration and there was also a lack of correlation with IGF-1R expression. The upregulated expression of type I collagen, a fibroblastic marker, by chondrocytes indicated that HA can overcome the beneficial affects of IGF-1. These in vitro works were also compared to an in vivo study. Alginate/HA constructs with embedded chondrocytes were pre-cultured with IGF-1 and then subcutaneously implanted into mice. Similar levels of type II collagen were observed for the constructs. However, by increasing the HA content a decrease in IGF-1 synthesis occurred with an increase in type I collagen staining. Pre-incubating the samples with IGF-1 led to a further decrease in IGF-1 production but was able to reverse the affects of HA on type I collagen expression. This research demonstrates that construct properties can alter endogenous IGF-1 signaling and overall shows the importance of understanding these details when engineering an articular cartilage construct.