Detection of PDGF-2 homodimers in human tumor cells

The v-sis oncogene encodes a protein structurally and functionally related to human platelet-derived growth factor (PDGF). In the present studies, we show that the primary translational product of the human sis proto-oncogene is a 26-kd protein, p26(c-sis). This product is processed to yield a disulfide-linked homodimer, p56(c-sis), which is further processed to a 35,000-dalton dimer, p35(c-sis). Like the v-sis gene product, the PDGF-2 precursor undergoes N-linked glycosylation, implying its processing through the endoplasmic reticulum. The PDGF-2 product was shown to possess functional properties of PDGF. Whereas lysates of COS-1 cells lacked mitogenic activity lysates of COS-1 cells transfected with a c-sis/PDGF-2 expression vector specifically stimulated DNA synthesis of quiescent fibroblasts. Moreover, this activity was completely inhibitable by PDGF antibody. Identical forms of the sis/PDGF-2 product were identified in human tumor cells that expressed c-sis/PDGF-2 transcripts. These proteins were shown to be specifically associated with the membrane component of the tumor cells and were not detectable secreted into the culture medium. These findings support the concept that expression of the sis/PDGF-2 product in human cells responsive to its proliferative action can be an important step in the processes leading to malignancy.