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The 21-kDa protein is an extracellular matrix (ECM) component whose synthesis is stimulated transiently during oncogenic transformation ofchicken embryo fibroblasts (CEF) or after treatment of normal cells with the tumor promoter phorbol 12-myristate 13-acetate. Biochemical characterization indicates that the protein is related, but not identical, to two members of the family of tissue inhibitors of metalloproteinases, TIMP-1 and TIMP-2. The cDNA of the 21-kDa protein was recently cloned, and based upon its deduced amino acid sequence and other supporting data we propose that it is another member of this family, a TIMP-3. We now report electrophoretic purification of sufficient quantities of this protein to determine its function. The protein promotes the detachment of transforming cells from the ECM. Although its presence in the matrix may be necessary for cell release it is not the only factor involved because it does not influence the adhesive properties of nontransformed cells. It also appears to accelerate the morphological changes associated with cell transformation and stimulates the proliferation of growth-retarded, nontransformed cells maintained under low serum conditions. Based on these data we hypothesize that the 21-kDa protein promotes the development of the transformed phenotype in cultured cells.The extracellular matrix (ECM) is a dynamic tissue compartment that functions in the regulation of morphogenesis, differentiation (1), and cell proliferation (2). One of its primary functions is proposed to be the sequestration of growth factors (3) that may be released locally in response to physiological stimuli (4). Cell-ECM interactions also play an important role in diseases involving abnormal growth and development, such as cancer, which is characterized by alterations in the synthesis and degradation of matrix components (5).We have been studying the potential roles of various ECM components in oncogenic transformation and have reported characterization of the 21-kDa protein found concentrated in the ECM of transforming chicken embryo fibroblasts (CEF) (6, 7). Synthesis of this protein (Mr 21,000) is stimulated early in the transformation of cells infected with temperaturesensitive mutants of Rous sarcoma virus (RSV) or by treatment of normal, uninfected cells with the tumor promoter phorbol 12-myristate 13-acetate. These observations implicated the 21-kDa protein in the development of the transformed phenotype but did not reveal its precise function.The NH2-terminal amino acid sequence of purified 21-kDa protein is >60% identical to a consensus sequence of mammalian tissue inhibitors of metalloproteinases (TIMPs) and the protein displays metalloproteinase inhibitor activity (8). It is the major inhibitor in the ECM and its solubility properties appear unique among inhibitors with a TIMP-like sequence. Its cDNA was recently cloned and sequenced (9), and its deduced amino acid sequence indicates that it is related to, but distinct from, TIMP-1 and TIMP-2. Based on these and other supporting ...

The cDNA encoding secreted alkaline phosphatase (SEAP) is a useful tool for investigating the function of known or putative enhancer/promoter elements. SEAP has the unusual properties of extreme heat stability and resistance to the phosphatase inhibitor L-homoarginine. Therefore, endogenous alkaline phosphatase activity in transfected cells can be minimized by pretreatment of samples at 65 degrees C and incubation with the inhibitor. With the use of the chemiluminescent substrate CSPD, 10(-13) g of enzyme can be detected in culture medium, and the enzyme activity can be detected as early as 24 h after transfection. The chemiluminescence-based SEAP assay is about 10-fold more sensitive than similar assays using firefly luciferase as the reporter enzyme. The SEAP activity can also be assayed with a fluorescent substrate MUP, which provides sensitivity comparable to luciferase. Since the enzyme is secreted to culture medium, the enzyme assay can be performed on small samples of the culture supernatant. Because preparation of cell lysates is not required, assaying for SEAP activity is faster and more convenient than assaying for intracellular reporters. Furthermore, because the transfected cells are not disturbed by the sampling procedure, the same cultures can be repeatedly sampled for time-course studies or used for further investigations.

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