Characterization of CD44 intracellular domain interaction with RUNX2 in PC3 human prostate cancer cells.

Background: Expression of CD44 receptor is associated with the onset of several tumors. The intracellular domain of CD44 (CD44-ICD) has been implicated as a co-transcription factor for RUNX2 in the regulation of expression of MMP-9 in breast carcinoma cells. Previous studies from our laboratory demonstrated the role of CD44 in migration and invasion of PC3 prostate cells through activation of MMP-9. CD44 signaling regulates the phosphorylation and hence the localization of RUNX2 in the nucleus. The role of CD44-ICD has not been studied in prostate cancer cells. This study aimed to explore the role of CD44-ICD and RUNX2 in the regulation of expression of metastasis-related genes.

Methods: PC3 and PC3 cells overexpressing RUNX2 protein were analyzed for RUNX2/CD44-ICD interaction by immunoprecipitation, immunoblotting, and Immunofluorescence analyses. Wound healing and tumorsphere formation analyses were also done in these cells. The real-time PCR analysis was used to detect the expression levels of different genes.

Results: Expression of CD44 and RUNX2 was observed only in PC3 cells (androgen receptor positive) and not in LNCaP or PCa2b cells (androgen receptor negative). Therefore, CD44-ICD fragment (~ 15-16 kDa) was observed in PC3 cells. Moreover, localization of CD44-ICD was more in the nucleus than in the cytoplasm of PC3 cells. Inhibition of cleavage of CD44 with a γ-secretase inhibitor, DAPT reduced the formation of CD44-ICD; however, accumulation of CD44-external truncation fragments (~ 20 and ~ 25 kDa) was detected. RUNX2 and CD44-ICD interact in the nucleus of PC3 cells, and this interaction was more in PC3 cells transfected with RUNX2 cDNA. Overexpression of RUNX2 augments the expression of metastasis-related genes (e.g., MMP-9 and osteopontin) which resulted in increased migration and tumorsphere formation.

Conclusions: We have shown here a strong functional relationship between CD44-ICD and RUNX2 in PC3 cells. RUNX2 forms a complex with CD44-ICD as a co-transcriptional factor, and this complex formation not only activates the expression of metastasis-related genes but also contributes to migration and tumorsphere formation. Therefore, RUNX2 and CD44-ICD are potential targets for anti-cancer therapy, and attenuation of their interaction may validate the regulatory effects of these proteins on cancer migration and progression.