Abstract A75: Retinoblastoma protein promotes Wnt signaling pathway activation in MC3T3 osteoblasts

Abstract

The retinoblastoma protein (pRb) is a transcriptional regulator of osteoblast differentiation. Wnt signaling also regulates this process. While Wnt signaling hinders degradation of β‐catenin leading to its nuclear accumulation and transcription of Wnt target genes, lack of Wnt activity promotes β‐catenin degradation. Since inactivation of both the pRb and Wnt pathways is commonly observed in osteosarcomas, we intend to determine if these pathways are functionally linked. Using MC3T3 pRb+/+ and pRb−/− cells, we performed immunoblots of Dishevelled (Dvl), which promotes Wnt activity when phosphorylated, and qRT‐PCR analyses of c‐myc, a known Wnt target gene, and GSK3‐β, an inducer of β‐catenin degradation. We found a 2‐fold increase in the phosphorylated/total Dvl ratio in the pRb−/− relative to pRb+/+ cells. Our results also showed 50% and 80% decreases in GSK3‐β and c‐myc mRNA levels, respectively, in the pRb−/− relative to pRb+/+ cells. Our results suggest that pRb may have a dual effect on Wnt: it inhibits Wnt by inhibiting Dvl phosphorylation and increasing GSK3‐β mRNA levels, while it promotes Wnt by promoting c‐myc mRNA levels. Research supported by RCMI Grant #5G12RR003050.