Knockdown of RNA binding protein musashi-1 leads to tumor regression in vivo.

Abstract

In the gut, tumorigenesis is thought to arise from the stem cell population located near the base of intestinal and colonic crypts. The RNA binding protein musashi-1 (Msi-1) is a putative intestinal and progenitor/stem cell marker. Msi-1 expression is increased during rat brain development and in APC(min/+) mice tumors. This study examined a potential role of Msi-1 in tumorigenesis. Msi-1 small interfering RNA (siRNA) was administered as a liposomal preparation to HCT116 colon adenocarcinoma xenografts in athymic nude mice and tumor volume was measured. Cell proliferation was assessed by hexosaminidase and 3-(4,5-dimethylthiazol 2-yl)-2,5-diphenyltetrazolium bromide MTT assays. siRNA-transfected cells were subjected to 12 Gy gamma-irradiation. Apoptosis was assessed by immunoreactive activated caspase-3 and mitosis was assessed by phosphorylated histone H3 staining. The tumor xenografts were stained similarly for phosphorylated histone H3, activated caspase-3, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling, Notch-1, and p21(WAF1). Furthermore, siRNA-transfected cells were subjected to cell-cycle analysis and Western blot analyses for Notch-1 and p21(WAF1). Knockdown of Msi-1 resulted in tumor growth arrest in xenografts, reduced cancer cell proliferation, and increased apoptosis alone and in combination with radiation injury. siRNA-mediated reduction of Msi-1 lead to mitotic catastrophe in tumor cells. Moreover, there was inhibition of Notch-1 and up-regulation of p21(WAF1) after knockdown of Msi-1. Our results show the involvement of Msi-1 in cancer cell proliferation, inhibition of apoptosis, and mitotic catastrophe, suggesting an important potential mechanism for its role in tumorigenesis.