QUANTITATIVE-ANALYSIS OF THE EXPRESSION AND REGULATION OF AN ACTIVATION-REGULATED PHOSPHOPROTEIN (ONCOPROTEIN 18) IN NORMAL AND NEOPLASTIC-CELLS

Activation of protein kinase C results in phosphorylation of a 19-kDa protein termed 19K. Isolation and sequence analysis of a cDNA encoding the 19K protein revealed that this protein has been studied in other systems under different names. The name oncoprotein 18 (Op18) has been proposed on the basis of a postulated up-regulation in neoplastic cel ls. In the present report we adopt the designation Op18 for the 19K pr otein, and quantify this phosphoprotein in a series of leukemia/lympho ma cell lines, a panel of non-transformed cells and some terminally di fferentiated cell types. For this purpose we have developed reagents a llowing quantitative Western-blot analysis, and quantification of Op18 on the single cell level by flow cytometric analysis. The data demons trates a pronounced up-regulation of the Op18 protein in most leukemia /lymphoma cell lines. The HPB-ALL cell line provided the most extreme case and expressed 7 x 10(6) Op18 molecules/cell, which compares with 0.65 x 10(6) Op18 molecules/cell in non-transformed lymphoblastoid cel ls. The expression of Op18 appears to be restricted to cell types with proliferative potential, but it is clear from our results that up-reg ulation of Op18 is uncoupled from cellular proliferation. Moreover, by employing an Epstein-Barr virus based shuttle vector, we expressed Op 18 cDNA in lymphoblastoid cells. This resulted in a three to fourfold up-regulation of Op18 that did not have any detectable consequences fo r cell-surface phenotype or cell size. However, increased expression o f Op18 resulted in a partial inhibition of cell proliferation. Taken a ltogether, the results suggest that up-regulated Op18 levels in leukem ia/lymphoma cells are strongly associated with, but not a direct cause of tumour progression.