Growth regulation by peroxisome proliferators: opposing activities in early and late G1

Peroxisome proliferators (PPs) are a diverse group of nongenotoxic rodent liver carcinogens. One potential mechanism for the carcinogenicity of PPs is epigenetic modulation of growth-regulatory signal transduction pathways. We investigated the effects of PPs on growth-regulatory gene expression and cell proliferation in immortalized mouse liver cells, comparing PPs with other growth regulators and tumor promoters of known activity. The PPs Wy-14643, mono-ethylhexyl phthalate, clofibrate, and ciprofibrate ethyl-ester were found to be potent inducers of immediate-early gene expression (including c-fos, c-jun, junB, egr-1, NUP475, and to a lesser extent fosB, JE, and KC, with maximal expression seen 1 h after treatment of serum-deprived quiescent cells. The gene induction was potently inhibited by protein kinase inhibitor H7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride] but not by H8 [N-¿2-(methyl-amino)ethyl¿-5-isoquinolinesulfonamide dihydrochloride], indomethacin, or nordihydroguaiaretic acid. Compared with other growth regulators, the profile of PP-induced gene expression was most similar to that induced by arachidonic acid and eicosatetraynoic acid. The induction of immediate-early gene expression by PPs was followed by enhanced progression into S phase (DNA synthesis) when quiescent cells were treated with the PPs for only 1 h, washed, and then incubated without PPs. However, no stimulation of DNA synthesis was seen when the PPs were continually present. Furthermore, the PPs inhibited serum-induced DNA synthesis, even when they were added 6 h after serum stimulation (in late G1). Dehydroepiandrosterone-sulfate, a unique PP in being a steroid, had no detectable effect on immediate-early gene expression, did not stimulate DNA synthesis when applied for only 1 h, but did inhibit serum-induced DNA synthesis. Thapsigargin and A23187 mimicked this mitoinhibitory activity of PPs, suggesting that calcium mobilization by PPs might be involved. Our results demonstrate that PPs can modulate cell proliferation either by a stimulatory activity that functions in early G1, associated with activation of immediate-early gene expression, or by an inhibitory activity that functions in late G1; both activities could potentially play a role in tumor promotion by PPs.