Senescence represses the nuclear localization of the serum response factorand differentiation regulates its nuclear localization with lineage specificity

The differentiation of cultured 3T3T mesenchymal stem cells into adipocytes represses growth factor responsiveness by limiting the nuclear localizatio n of the serum response factor (SRF) that binds to and activates the promot ers of growth control genes that contain the serum response elements (SRE), such as junB and c-fos, The regulation of SRF nuclear localization by adip ocyte differentiation is specific, because we show that adipocyte different iation does not repress the nuclear localization of six other transacting f actors. To determine if repression of growth factor responsiveness that occ urs during senescence also represses the nuclear localization of SRF, we st udied normal human WI-38 fibroblasts at low versus high population doubling s, The results show that SRF localizes to the nucleus of proliferative cell s whereas in senescent cells SRF can not be detected in the nucleus, This r esult is apparent in both immunofluorescence assays and in western blot ana lysis. We next evaluated the cellular distribution of SRF in selected human tissues to determine whether the loss of proliferative potential in vivo c ould have a different effect on SRF nuclear localization. We found that in cells of the small bowel mucosa, differentiation modulates SRF nuclear loca lization in an opposite manner. Minimal SRF expression and nuclear localiza tion is evident in undifferentiated cells at the base of crypts whereas inc reased SRF expression and nuclear localization is evident in differentiated cells at the surface tip of the villus. These results together establish t hat regulation of SRF expression and nuclear localization is important in s enescence and differentiation in a lineage specific manner.