Abstract
We have previously reported the presence of a 28-kDa protein in human mammary adenocarcinoma MCF-7 cells, whose phosphorylation by phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and permeant diacylglycerol 1,2-dioctanoyl-sn-glycerol was correlated to growth arrest induced by the protein kinase C (PKC) activators. We now investigate the possible identity of this protein with the estrogen-regulated "24-kDa" protein shown as related to the mammalian heat shock protein 27 (Fuqua, S. A. W., Blum-Salingaros, M., and McGuire, W. L. (1989) Cancer Res 49, 4126-4129). 32P-Labeled 28-kDa protein from TPA-treated MCF-7 cells was immunoprecipitated with a 24-kDa-specific monoclonal antibody. Immunoblots from cell extracts fractionated by two-dimensional isoelectric focusing/SDS-polyacrylamide gel electrophoresis demonstrated that TPA induced the conversion of a 28-kDa isoform "a" (pI 6.7) to a more acidic isoform "b" (pI 6.2). Two-dimensional gel analysis of [3H]leucine-labeled MCF-7 cell extracts demonstrated that conversely to TPA, which induced only phosphorylation of 28-kDa protein, heat shock induced both synthesis (increase of isoform a) and phosphorylation (conversion of isoforms a to b) of the protein. 32P labeling of MCF-7 cells allowed demonstration of the presence of an extra phosphoisoform "c" (pI 5.9) upon TPA as well as heat shock treatment. When cells were pretreated with the bisindolylmaleimide GF109203X, a selective inhibitor of PKC, the heat shock-induced phosphorylation was unchanged, while the TPA effect was almost abolished, suggesting that the heat shock-activated protein kinase was very likely different from PKC. However, peptide mapping of the 28-kDa phosphoprotein suggested identical sites of phosphorylation upon TPA and heat shock stimulation. Partial amino acid sequencing of the 28-kDa protein revealed identity with both the 24-kDa protein and the mammalian HSP27. The fact that estrogens and PKC, respectively, regulate expression and phosphorylation of this 24/28-kDa protein strongly argues for its key role in MCF-7 cell proliferation and differentiation.
Highlights
We have previously reported the presence of a 28- phoprotein suggested identical sitesof phosphorylation kDaprotein in human mammary adenocarcinoma upon TPA and heat shock stimulation
L. (1989)Cancer Res 49, 4126-4129). 32P-Labeled28-kDa protein Heat shock proteins (HSPs)’ consist of a number of highly from TPA-treated MCF-7 cells was immunoprecipi- conserved proteins that aresynthesized by all pro- and eucartated with 2a4-kDa-specific monoclonal antibody
Two-dimensional gel analy- [2,3].High molecular weight HSPs including HSPSO, HSP70, sis of [SH]leucine-labeledMCF-7 cell extracts demon- and HSP6O have been studied with the greatest details and strated that to TPA, which induced only phosphorylation of 28-kDa protein, heasht ock induced both synthesis andphosphorylation of the protein. 32P labeling of MCF-7 cells allowed demonstration of the presence of an extraphosphoisoform “c” (PI 5.9) upon TPA as well as heat shock treatment
Summary
Peptide Mapping of 28-kDa Protein Isoforrm-One-dimensional peptide mapping was carried out according to Cleveland et al [19]. TPA, protease V8 (from Staphylococcusabreus), diphenylcarbamyl IEF/SDS-PAGE, the respective phosphoisoforms b and c of 28-kDa chloride-treated trypsin, keyhole limpet hemocyanin, and Tween 20 protein were excised from the gels and directly loaded on 4.5-15%. + 28-kDa isoform a, isolatedfrom two-dimensional IEF/SDS-PAGE, was digested in the gel matrix with porcine trypsin, and the resulting peptides were separated on a narrow bore C18 Altex column (25 X isoform a (PI = 6.7) to isoform b (PI = 6.2), but not of the 28-kDa protein Two-dimensional IEF/SDS-PAGEAnalysis of fH]Leucine- strated by thedisappearance of this PKC-specific protein and :i2P-Labeled 28-kDa Protein upon TPA or Heat Shock phosphorylation in cells treated with this compound.
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