The Role of Plasmacytoma Growth Factor in the in Vitro Responses of Murine Plasmacytoma Cells

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Abstract
Many plasmacytomas arising in BALB/c mice are dependent upon a specific, macrophage-derived plasmacytoma growth factor for survival and proliferation in vitro. Adherent cells taken from the peritoneal oil granuloma in which the early plasmacytomas arise and proliferate produce 50 times more PCT-GF activity in vitro than do normal peritoneal cells, thus suggesting a possible in vivo role for PCT-GF. Purification and amino acid sequencing of PCT-GF derived from the murine macrophage cell line, P388D1, have identified a 23 kDa protein with a unique NH2-terminal sequence. This molecule is now known as murine IL6. As part of the characterization of murine Il-6, genomic sequences have been localized to the proximal end of mouse chromosome 5 via Southern analysis of restriction fragment length polymorphisms. The removal of IL6 from IL6-dependent PCT cell lines results in a growth arrest in early G1. This is accompanied by a rapid and specific loss of transferrin receptor expression and results in eventual cell death. It appears that the response to IL6 is at least partially dependent on Ca++ because functional Ca++ channels are necessary for the PCT cells to pass through G1 and to maintain transferrin receptor expression.