Mouse C3H teratoma-derived adipogenic cell line 1246 which grows and undergoes adipose differentiation in defined medium strictly requires insulin for both processes. Several variant insulin-independent cell lines have been isolated. One of them called 1246-3A was particularly studied. 1246-3A cells have lost their ability to differentiate and have become tumorigenic. They produce growth promoting activities which can stimulate the growth of the parent cell line 1246 and which were characterized. It was found that 1246-3A cells produce an insulin related factor (IRF) similar to pancreatic insulin and acting as an autostimulating growth factor; TGF-alpha like polypeptides (55 kDa and 15 kDa); TGF-beta like polypeptide and a 13 kDa polypeptide growth factor different from TGF-alpha and TGF-beta. It was shown that the TGF-alpha and beta polypeptides and the 13 kDa fractions also inhibited adipose differentiation of 1246 cells. Based on these results, it can be suggested that the adipose differentiation inhibitors inhibit the differentiation of the producer cells in an autocrine fashion and can account for the change of phenotype observed in the insulin-independent variant 1246-3A cells. Moreover, it is possible to assume that growth factor production can be responsible for the lesser growth factor requirement of tumorigenic insulin-independent cell line. In order to examine these possibilities, several approaches are proposed: 1) to isolate highly tumorigenic insulin-independent cell lines from the 1246-3A cells. In vitro growth properties and the ability of the cells to synthesize of endogenous growth factors will be examined and compared to the ones of 1246 and 1246-3A cells both at the cellular and molecular levels; 2) to characterize the growth factor produced by the highly tumorigenic cell line; 3) transfect the parent cell lines with expression vectors for the various polypeptide growth factors produced by the insulin-independent cell line 1246-3A (insulin, TGF-alpha, TGF-beta) and examine the growth and differentiation properties in vitro, and tumorigenic properties in vivo of transfected cells, and 4) to obtain revertant cells from the insulin-independent cell lines and investigate their growth and differentiation properties, in order to determine if nontumorigenic properties and ability to differentiate can be recovered. This study will be part of the long-term objective of our investigations related to characterization of the nature of factors controlling growth and differentiation of mesenchymal derived cell lines and understanding their mechanism of action.
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