Serum stimulation of parathyroid hormone-related peptide gene expression in ROS 17/2.8 osteosarcoma cells through transcriptional and posttranscriptional mechanisms

The gene encoding PTH-related peptide (PTHrP), a protein that plays a primary role in the development of humoral hypercalcemia of malignancy, is widely expressed in normal and neoplastic tissues. This study demonstrates that expression of the PTHrP gene has features of early response genes, including up-regulation after serum repletion of serum-starved ROS 17/2.8 (rat osteosarcoma) cells. The PTHrP messenger RNA (mRNA) levels were induced within 30 min and peaked at 4 h. Elevated mRNA levels were accompanied by an increase in secreted PTHrP. The serum effects on PTHrP mRNA levels were blocked by actinomycin D, suggesting a requirement for gene transcription. Nuclear run-on assays revealed a 3-fold increase in PTHrP gene transcription 4 h after exposure to serum. Deletions of the 5' flanking sequence of the rat PTHrP gene fused to the chloramphenicol acetyltransferase gene and transfected into ROS 17/2.8 cells showed that the serum-responsive region is located between -1.05 kb and -0.3 kb upstream of the transcription start site. PTHrP mRNA levels were also induced by cycloheximide, another feature common to early response genes. The PTHrP mRNA half-life in serum-starved cells was 56 min. Serum treatment prolonged the half-life 2.7-fold, suggesting serum-induced stabilization of the mRNA. Insulin and epidermal growth factor also induced PTHrP mRNA expression in a time-dependent manner analogous to serum, indicating that the effects of serum may be mediated, at least partially, through these agents. In summary, serum up-regulated PTHrP mRNA expression through both transcriptional and posttranscriptional mechanisms. This rapid stimulation by growth factors suggests that PTHrP may contribute to the early cellular response after growth factor stimulation.