Apolipoprotein B mRNA and lipoprotein secretion are increased in McArdle RH-7777 cells by expression of betaine-homocysteine S-methyltransferase

The cDNA encoding rat betaine-homocysteine S-methyltransferase (BHMT) was i solated through production of monoclonal antibodies against protein fractio ns enriched with apolipoprotein B (apo B)-mRNA-editing complexes. BHMT mRNA was expressed predominantly in liver, and also in kidney, but not in small intestine. In stable McArdle RH-7777 (McA) cell lines expressing differing levels of BHMT, the editing efficiency of apo B mRNA was unchanged. Evalua tion of apo B-mRNA expression revealed that steady-state levels were increa sed significantly and in parallel with BHMT protein expression. The highest levels of BHMT mRNA and BHMT enzyme activity expressed in stably transfect ed McA cells were comparable with those found in rat hepatocytes. In contra st to the changes in apo B-mRNA abundance, levels of other apolipoprotein-e ncoding mRNAs and several liver-specific and ubiquitously expressed mRNAs w ere unchanged by BHMT expression. In the cell line expressing the highest l evel of BHMT, apo B-containing lipoprotein secretion was increased, indicat ing utilization of increased endogenous message. Results suggest that apo B -mRNA abundance in McA cells is related to the expression of BHMT, an enzym e important in homocysteine metabolism.