Senescent WI-38 fibroblasts overexpress LPC-1, a putative transmembrane shock protein

We have recently reported the isolation of cDNAs for a number of genes that are differentially expressed between nonproliferating early (young) and late (senescent) population doubling level (PDL) WI-38 human, fetal lung- derived, fibroblast-like cells. We now demonstrate that one of these isolates, LPC-1 (Late PDL cDNA-1), derives from an approximately 2.9-kb mRNA species that is expressed at a two- to fivefold higher level in serum- starved, confluent, senescent versus similarly treated young WI-38 cells. Nucleotide sequence analysis of this eDNA confirms its identity with that of a cDNA encoding a marker (p63) for the rough endoplasmic recticulum and a related swine hepatic cardiogenic shock protein. We show that LPC-1 expression in early PDL WI-38 cells is strictly cell cycle-regulated and its expression peaks 9-12 h after serum stimulation of G₀ cultures. The steady state levels of LPC-1 transcript in early PDL cells preceeding and following its peak expression are low, reflecting basal levels seen in G₀ upon removal of serum. Late PDL cells, however, seem to have lost this tight cell cycle regulation seen in early PDL cells and inappropriately express high levels of the transcript after serum stimulation. Specific antiserum detects a protein of approximately 63 kDa by Western analysis and elicits intense cytoplasmic staining of senescent fibroblasts by immunohistochemistry. Related genomic sequences are found in all mammalian species examined as well as in the chicken. These findings are consistent with the hypothesis that senescent WI- 38 cells exhibit a state of growth arrest fundamentally distinct from that of quiescent (G₀) young cells.