Involvement of protein kinase C-ε in signal transduction of thrombopoietin in enhancement of interleukin-3-dependent proliferation of primitive hematopoietic progenitors

We studied the effect of thrombopoietin (TPO) on interleukin-3 (IL-3)-dependent bone marrow cell colony formation of mice to clarify the role of protein kinase C (PKC) in the signal transduction of TPO for the proliferation of primitive hematopoietic progenitors. TPO alone hardly yielded colonies. However, TPO in combination with IL-3 increased colony numbers synergistically from 2- to 4-fold, compared with those supported by IL-3 alone. Serial observation of colony development showed that TPO may hasten the appearance of colonies by shortening the dormant period (G_o) of primitive progenitors. Immunocytochemical studies on PKC isoforms in progenitor cells stimulated with TPO have revealed that the expression pattern of PKC-ε is changed, but not that of PKC-α, -β, -γ, -δ or -ζ. Selective PKC inhibitors, such as calphostin C and GF 109203X, and PKC-ε-specific translocation inhibitor peptide abrogated the enhancing effect of TPO on IL-3-dependent colony formation and the changes in the intracellular expression pattern of PKC-ε. These data taken together suggest that TPO has a direct effect on primitive progenitors and enhances IL-3-dependent colony formation, at least partly through the activation of PKC-ε.