Transformation and tumor promoter sensitive phosphoproteins in JB-6 mouse epidermal cells: one is also sensitive to heat stress.

Abstract

JB-6 mouse epidermal cells undergo irreversible transformation when exposed to tumor-promoting agents such as 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Phosphoprotein changes related to transformation were sought in four tumor cell lines related to JB-6 cells. Two dimensional polyacrylamide gel electrophoresis showed altered abundances of five phosphoproteins in the tumor cell lines compared with five untransformed clones. The mol. wt. in Kilodaltons and isoelectric points in pH units were: 120/6.0, 80/4.5, 55/6.5, 37/5.0 and 23-25/4.5. In all four transformants pp80 was markedly decreased and the pp23-25 doublet increased. In two of the four transformants pp120 and pp55 were increased and pp37 decreased. Treatment of untransformed clones with TPA affected only one of the phosphoproteins altered in the transformants. Treatment of untransformed clones with TPA produced a 2-fold increase in pp80 after 5 h. pp80 returned to baseline levels by 24 h and changed little in the continuous presence of TPA for up to 96 h. The increase in pp80 with short term TPA treatment occurred in all of the untransformed clones but none of four transformants. Late preneoplastic (P+) JB-6 cells only require treatment with a tumor promoter to transform. Early preneoplastic (P-) JB-6 cells require prior transfection of DNA from late preneoplastic JB-6 cells to transform in response to tumor promoter treatment. Quantitative analysis of pp80 in early preneoplastic, late preneoplastic, and tumor cell lines showed an inverse relationship between the level of pp80 and degree of preneoplastic progression in these cells. pp80 represents approximately 2% of total cellular phosphoprotein in JB-6 cells, shows microheterogeneity of both mol. wt. and isoelectric point, occurs in the particulate fraction of cells and is readily solubilized by 1% Triton. pp80 is increased by heat stress and shares other properties with the recently described mammalian heat stress protein, hsp 80. pp80's decrease in four out of four tumor cell lines, inverse correlation with stage of preneoplastic progression and responsiveness to TPA in preneoplastic but not in tumor cell line suggest that pp80 may be closely linked to biochemical mechanisms underlying transformation in this cell system.