A specific defect of prosolin phosphorylation in T cell leukemic lymphoblasts is associated with impaired down-regulation of DNA synthesis.

Prosolin is a major cytosolic phosphoprotein of proliferating normal PBL. Treatment of growing PBL with phorbol ester (12-O-tetradecanoylphorbol-13-acetate (TPA)) or calcium ionophore (A23187) for 1 h caused phosphorylation of prosolin with the production of up to four prominent phosphorylated forms differing in degree of phosphorylation and/or two-dimensional electrophoretic mobility (peptides B to E). Formation of these phosphopeptides coincided with rapid down-regulation of DNA synthesis. A23187 was particularly effective in inducing phosphorylation of the more highly phosphorylated peptides D and E, suggesting the existence of a (Ca2+)-activated mechanism in their phosphorylation. The T cell leukemia cell lines Jurkat, HuT-78, CCRF-CEM, and Molt-4 showed reduced to absent ability to phosphorylate prosolin peptides rapidly in response to A23187 and also showed diminished down-regulation of DNA synthesis. In leukemic cells treated with both TPA and A23187, peptides B and C were rapidly phosphorylated, but the phosphorylation of peptides D and E seen in normal PBL remained deficient. The T cell leukemic cells appear to have intact a TPA-activated mechanism for phosphorylating prosolin peptides B and C, but share an impairment of a specific Ca2(+)-activated mechanism, possibly a Ca2(+)-dependent protein kinase, required for phosphorylation of prosolin phosphopeptides D and E. The degree of rapid down-regulation of DNA synthesis was correlated with degree of phosphorylation of peptide E in PBL and in three of four T cell leukemic cell lines. Thus, rapid phosphorylation of prosolin may mediate responses to TPA and A23187 in normal proliferating PBL, including down-regulation of DNA synthesis. A deficiency of this pathway in leukemic T cells may impede their response to physiologic growth regulatory signals utilizing this pathway and contribute to unrestrained cell growth.