DIRECT INVOLVEMENT OF INTRACELLULAR CA2-HAMSTER LUNG FIBROBLASTS( TRANSPORT ATPASE IN THE DEVELOPMENT OF THAPSIGARGIN RESISTANCE BY CHINESE)

Thapsigargin (TG), a specific inhibitor of intracellular Ca2+ transpor t ATPases (SERCA), inhibits cell proliferation when added to culture m edia in the nanomolar concentration range, However, long term exposure to gradually increasing concentrations of TG induces resistance to TG inhibition in both the parental Chinese hamster lung fibroblast DC-3F and a subline derived from it via transfection and stable expression of a full-length cDNA encoding avian SERCA1 ATPase (DC-3F/Ca cells), T G resistance develops in parallel with selection of cells expressing h igher levels of the endogenous SERCA2 as well as of the exogenous tran sfected SERCA1 ATPase, whose Ca2+ transport function can be studied in situ by imaging techniques and following isolation in microsomal frac tions, Microsomes isolated from resistant cells contain two functional ly distinct populations of ATPases: a population that is inhibited by stoichiometric titration with TG, and a population displaying resistan ce to inhibition even when TG exceeds the enzyme stoichiometry, It is apparent that resistance to TG develops in parallel with (a) selection of cells expressing high levels of SERCA ATPases, and (b) selection o f an ATPase that is resistant to TG.