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Abstract
Activation of PKC is intimately involved in mitogenic signalling in a wide range of cell types [Nishizuka, 1984; Rozengurt, 1986; Blumberg, 1988] but also leads to the induction of differentiation and concomitant inhibition of proliferation in other cell types, including myelomonocytes [Vandenbark et al., 1984]. In this thesis, the role of PKC activation in proliferative responses of a murine macrophage tumour, M5076, was investigated. Phorbol esters and mezerein potently inhibited growth of this cell line whereas the diacylglycerols (DGs), OAG (1-oleoyl 2-acetyl glycerol) and DiC8 (1,2-dioctanoyl glycerol) did not affect prolferation of growing cells. Through the induction of reversible growth arrest by serum deprivation, it was shown that OAG was capable of acting as a mitogen for M5076 cells. All of the agents were effective PKC activators in these cells and thus PKC activation can be associated with either inhibition or stimulation of growth in the one cell line; this, to my knowledge, is the first demonstration of a mitogenic ability of a PKC activator on a myelomonocytic cell line and also of opposite proliferative responses occurring in a single cell line following PKC activation.
Phorbol esters produce long term activation of PKC and down-regulation of this enzyme whereas DGs are metabolised rapidly and thus do not down-regulate PKC levels [Murray et al. 1987]. PKC down-regulation may be a deterministic event for phorbol ester-induced differentiation of myelomonocytic cells and the inability of DGs to decrease PKC levels could account for their inability to induce differentiation in the same cells [Kreutter et al., 1985; Yamamoto et al., 1985; Morin et al., 1987]. Results presented herein support this assertion; phorbol esters down regulate PKC in M5076 cells whereas a single application of DGs does not. Ebeling et al. [1985] reported that repeated application of DiC8 was capable of inducing differentiation of HL-60 myelomonocytes and I have extended this finding by showing that frequent and repeated application of DiC8 or OAG inhibited the proliferation of M5076 cells and that this effect was accompanied by down-regulation of PKC levels. Thus, PKC down-regulation appears to regulate the anti-proliferative response in the M5076 line and, additionally, it was shown that levels of PKC in these cells correlated with, and appeared to determine proliferative rates.
