The regulation of macrophage protein turnover by a colony stimulating factor (CSF‐1)

CSF‐1 is a hemopoietic growth factor that specifically regulates the survival, proliferation, and differentiation of mononuclear phagocytic cells. A homogeneous population of mononuclear phagocytes, bone marrow derived macrophages (BMM), were used to study the regulation of protein turnover by CSF‐1. Removal of CSF‐1 (∼0.4 nM) from exponentially growing BMM cultured in 15% fetal calf serum containing medium decreases the rate of DNA synthesis by more than 100‐fold. Addition of CSF‐1 to these cells causes them to resume DNA synthesis within 12 h. More immediate effects of CSF‐1 were observed on BMM protein metabolism. BMM cultured for 24 h in the absence of CSF‐1 reduce their protein synthetic rate by 50–60%. The protein synthetic rate commences to decrease at 2–3 h after CSF‐1 removal. Readdition of CSF‐1 to BMM previously incubated in its absence causes a return to the protein synthetic rate of exponentially growing cells within 2 h. In the presence of CSF‐1, BMM synthesize protein at a rate of ∼8.7%/h and degrade it at a rate of ∼0.9%/h. Removal of CSF‐1 results in a decrease in the protein synthetic rate to ∼3.4%/h and an increase in the rate of protein degradation to ∼3.4%/h. The rate of protein synthesis by BMM increases linearly with CSF‐1 concentration over the range of concentrations stimulating both survival and proliferation, while the rate of protein degradation decreases exponentially over the range of concentrations stimulating survival without proliferation. Therefore, it appears that the stimulation of the rate of protein synthesis and inhibition of the rate of protein degradation are two distinct effects of CSF‐1, both part of the pleiotropic response to this growth factor. The inhibition of the rate of protein degradation by CSF‐1 may be most significant for its survival inducing effect.