The enigmatic protein kinase Cδ:: complex roles in cell proliferatin and survival

Protein kinase C delta (PKC delta) has been implicated both as a tumor suppressor and a positive regulator of cell cycle progression. PKC delta has also been reported to positively and negatively regulate apoptotic programs. This has led to conflicting hypotheses on the role of PKC delta in the control of cell proliferation and survival. Surprisingly, PKC delta mice develop normally and are fertile, indicating that PKC delta is not critical for normal cell proliferation during development. However, PKC delta may play important roles in neoplastic cell proliferation. In this review, we have summarized the apparent multifunctional properties of this enigmatic protein with regard to its role in the regulation of cell cycle progression and cell survival. It is proposed that PKC delta has both tumor suppressor and proliferation capabilities that can be recruited as a backup kinase for both gatekeeper tumor suppression and as an activator of the Ras/Raf/MEK/MAP kinase signaling pathway in cell proliferation.