ALTERATIONS IN EXPRESSION OF P56(LCK) DURING MYELOID DIFFERENTIATION OF LSTRA CELLS

The src-related tyrosine kinase p56(lck) is overexpressed in the mouse leukemia cell line LSTRA. Although p56(lck) is thought to be a specific T-cell marker, we found that LSTRA cells can be induced to differentiate towards macrophages or granulocytes by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate or the cyclic nucleotide analogue, dibutyryl cAMP, respectively. Treatment of LSTRA cells with 12-O-tetradecanoylphorbol-1 3-acetate resulted in marked alterations in morphology including increased size, adherence, and spreading on culture dishes. These cells also ceased proliferating, accumulated in G(0)-G(1) and expressed nonspecific esterase activity. In contrast, although LSTRA cells treated with dibutyryl cAMP stopped growing and accumulated in G(0)-G(1), these cells expressed functionally active chemotactic peptide receptors and became irregular and granular in appearance. Differentiation of LSTRA cells was also found to be associated with altered expression of p56(lck) Thus, while 12-O-tetradecanoylphorbol-13-acetate treatment caused the cells to produce higher molecular weight forms of p56(lck), dibutyryl cAMP treatment resulted in increased expression of total p56(lck) mRNA as well as the more mature type II p56(lck) mRNA transcript. There were no major alterations in p56(lck) kinase activity in vitro following differentiation. Phospholipase C gamma and p21(ras)GAP, two putative substrates for the tyrosine kinase activity of p56(lck), were found to be constitutively phosphorylated on tyrosine in LSTRA cells. Tyrosine phosphorylation of these substrates was not altered following differentiation. These results indicate that LSTRA cells are relatively early precursors that have the capacity to develop along the myeloid differentiation pathway. Furthermore, expression of p56(lck) does not appear to predispose LSTRA cells to mature along a particular developmental pathway.