Tyrosine Phosphorylation Of Hsc70 Located In The Cell Membrane May Regulate Methotrexate Transportation In Murine L1210 Leukemia Cells

Abstract Bhushan and co-workers previously described an alteration in an unidentified 66-68 kDa membrane-associated, tyrosine-phosphorylated, methotrexate (MTX)-binding protein in the L1210 murine leukemia cell line. This protein was absent, at least in the tyrosine phosphorylated form, in cisplatin-resistant, methotrexate cross-resistant L1210/DDP cells. The 66-68 kDa protein was isolated by affinity chromatography, first on MTX-agarose followed by purification using an anti-phosphotyrosine immunoaffinity column. Amino acid sequence analysis identified the protein as heat shock cognate protein 70 encoded by the HSPa8 gene. The HSPa8 gene was found to be transcribed in both sensitive and resistant cell lines by RT-PCR and the sequence of the HSPa8 gene was identical in both cell lines. Western blot analysis showed that HSC70 was also expressed in both cell lines. A binding pull-down assay with MTX-agarose beads, followed by western blotting for HSC70, showed HSC70 to be present in cell lysates and in the MTX-binding fraction. Fractionation of cell homogenates into membrane and cytosolic fractions showed that HSC70 was found in both fractions but was not detected in the MTX-binding assay with the membrane fraction from the L1210/DDP cells, suggesting HSC70 located in the membrane fraction may be controlling MTX transport in the sensitive cells. Comparing the tyrosine phosphorylation status between L1210/0 and L1210/DDP cells, a band around 70 kDa was observed in the membrane fraction of L1210/0 cell but was not present in the L1210/DDP cells. Additionally, immunoprecipitation with anti-phosphtyrosine (PY20) agarose beads and detection of HSC70 by western blot showed little to no PY-HSC70 in the membrane fraction from the L1210/0 cells. Thse results suggest that HSC70 is a putative MTX binding protein, and HSC70 associated with the cell membrane may regulate methotrexate uptake. Tyrosine phosphorylation may contribute to the function of HSC70 in the cross-resistance mechanism in the murine L1210 cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1720. doi:10.1158/1538-7445.AM2011-1720