5-Aminolevulinic acid synthase (ALA synthase), the rate-controlling enzyme of hepatic heme biosynthesis, is feed-back repressed by heme. In the liver, chemicals such as barbiturates markedly induce ALA synthase, especially in the presence of partial defects of heme biosynthesis. The inducibility and regulation of ALA synthase have been investigated using a variety of models, including intact animals and liver cell culture systems. A widely used model that closely approximates what occurs in vivo and in humans is that of primary cultures of chick embryo liver cells (CELCs). However, CELCs have some limitations: the cells obtained are somewhat heterogeneous; isolation and culture must be repeated every week resulting in weekly variations; and cells are short-lived limiting the feasibility of time-course and transfection studies. The aim of this study was to determine if LMH cells, a chick hepatoma cell line, are a good model comparable to that of CELCs. In both cells similar patterns of response of, ALA synthase activities and mRNA levels, and of porphyrin accumulation were obtained following treatments known to affect heme biosynthesis. Similarly, heme repressed ALA synthase mRNA levels in both cell types and ALA synthase activities in LMH cells. We conclude that LMH cells are a useful model for the study of hepatic heme biosynthesis and regulation of ALA synthase.

Heme oxygenase-1 is an inducible enzyme that catalyzes heme degradation and has been proposed to play a role in protecting cells against oxidative stress-related injury. We investigated the induction of heme oxygenase-1 by the tumor promoter arsenite in a chicken hepatoma cell line, LMH. We identified a heme oxygenase-1 promoter-driven luciferase reporter construct that was highly and reproducibly expressed in response to sodium arsenite treatment. This construct was used to investigate the role of mitogen-activated protein (MAP) kinases in arsenite-mediated heme oxygenase-1 gene expression. In LMH cells, sodium arsenite, cadmium, and heat shock, but not heme, induced activity of the MAP kinases extracellular-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. To examine whether these MAP kinases were involved in mediating heme oxygenase-1 gene expression, we utilized constitutively activated and dominant negative components of the ERK, JNK, and p38 MAP kinase signaling pathways. Involvement of an AP-1 site in arsenite induction of heme oxygenase-1 gene expression was studied. We conclude that the MAP kinases ERK and p38 are involved in the induction of heme oxygenase-1, and that at least one AP-1 element (located -1576 base pairs upstream of the transcription start site) is involved in this response.