α2-Macroglobulin (α2-M) is known as a wide-spectrum proteinase inhibitor and to bind covalently certain growth factors. We have previously characterized tumor-associated α2-M synthesized and secreted by human tumor cell lines. Of the cell lines studied, the melanoma cell line HMB-2 produced the largest amount of this glycoprotein. Immunofluorescence staining of cultured HMB-2 cells suggested that the cell population is heterogeneous with respect to α2-M production. We have now isolated clones from the parental HMB-2 cells and characterized eight representative ones in detail. They varied considerably in the quantity of α2-M secreted, from 4.2 to 46.5% of total protein. No relationship between the production of α2-M by these clones and their pigmentation or tumorigenicity in nude mice was found. However, statistically there was a strong correlation between the modal chromosome number and population doubling time (r2 = 0.88, P < 0.001) and also between the modal chromosome number and α2-M production (r2 = 0.73, P < 0.01). The growth rate of the clones correlated with the level of α2-M in culture medium (r2 = 0.69, P < 0.01). Clones with lower α2-M production had a proportionally shorter population doubling time than the clones with intermediate or high production. Northern hybridization indicated quantitative variation in the α2-M mRNA expressed by the parental cells and the clones, that was comparable but not identical with the quantity of α2-M in the respective culture media; both parental cells and clones expressed platelet-derived growth factor A-chain mRNAs with little difference in levels. Serum-free medium from low α2-M producer clones stimulated normal stationary fibroblasts significantly more than clones producing intermediate or high amounts of α2-M. α2-M decreased and anti-α2-M IgG increased the stimulation. These results suggest that production of tumor-associated α2-M is related to both autocrine and paracrine growth-stimulating activity of the tumor cells.
The project was supported by funds partly provided by the International Cancer Research Data Bank Program of the National Cancer Institute, NIH (US), under Contract N01-CO-65341 (International Cancer Research Technology Transfer-ICRETT), and partly by the International Union against Cancer; and funds from the Association pour la Recherche sur de Cancer, Villejuif, France.