PERSISTENT GROWTH OF BALB C MOUSE PLASMACYTOMA AND HUMAN MYELOMA CELL-LINES IN THE PRESENCE OF PHORBOL-MYRISTATE ACETATE IS ASSOCIATED WITHCONTINUED EXPRESSION OF LAP18 (STATHMIN)/

Lap18 is a highly conserved cytosolic protein that is expressed in div iding cells. Data from a number of studies show that a range of cell l ines and mitogen-stimulated normal cells cultured in PMA phosphorylate and subsequently down-regulate Lap18. This has been found to be assoc iated with growth arrest, although it is not clear that these events a re causally related. In the present study we confirm that the HL60 pro myelocytic leukemia and K562 erythroleukemia cell lines, when cultured with PMA, behave in this manner. This was not the case for any of fiv e mouse plasmacytoma cell lines and six lines derived from patients wi th multiple myeloma or plasma cell leukemia, All of these lines contai n Lap18, although the level of this protein in the mouse but not the h uman plasmacytoma cell-line cells is relatively low, All the neoplasti c plasma cell-line cells phosphorylate Lap18 on culture with PMA, but this does not induce growth arrest nor result in down-regulation of La p18 expression. Further experiments are required to test whether there is a mechanistic relationship between the continued growth of plasmac ytoma cell lines and their failure to down-regulate Lap18 on culture i n PMA.