Age-dependent FOXO regulation of p27Kip1 expression via a conserved binding motif in rat muscle precursor cells.

Previously, we have demonstrated that forkhead box O3a (FOXO3a) overexpression increased p27(Kip1) promoter activity and protein expression, whereas it decreased proliferation in muscle precursor cells (MPCs). The objectives of the present study were to 1) locate and identify FOXO regulatory elements in the rat p27(Kip1) promoter using deletion analysis of a promoter/reporter construct and 2) determine if age-related differences exist in FOXO-induced p27(Kip1) expression. The full-length (-4.0/+0.4 kb) rat p27(Kip1) promoter construct revealed that both FOXO1 and FOXO3a induced an increase in transcriptional activity. Interestingly, MPCs isolated from old animals exhibited an increased FOXO3a-induced p27(Kip1) promoter activity compared with MPCs isolated from young animals. Deletion of a 253-bp portion of the 5'-untranslated region (UTR) resulted in a significant decrease in FOXO-induced p27(Kip1) promoter expression. Site-specific mutation of a daf-16 family protein-binding element (DBE) within this 253-bp portion of the 5'-UTR also demonstrated a decrease in FOXO-induced p27(Kip1) promoter expression. These data suggest that a putative FOXO regulatory element located in the 5'-UTR of the rat p27(Kip1) gene plays a role in the age-dependent differences in FOXO3a-dependent p27(Kip1) promoter expression. These findings have implications for developing treatment strategies aimed at increasing the proliferation of MPCs and regenerative capacity of aged skeletal muscle.