Cancer cells are able to overproduce lactic acid aerobically, whereas normal cells undergo anaerobic glycolysis only when deprived of oxygen. Tumor aerobic glycolysis was recognized about seven decades ago; however, its molecular basis has remained elusive. The lactate dehydrogenase-A gene (LDH-A), whose product participates in normal anaerobic glycolysis and is frequently increased in human cancers, was identified as a c-Myc-responsive gene. Stably transfected Rat1a fibroblasts that overexpress LDH-

Expression of the lactate dehydrogenase A subunit (LDH-A) gene can be controlled by transcriptional as well as posttranscriptional mechanisms. In rat C6 glioma cells, LDH-A mRNA is stabilized by activation and synergistic interaction of protein kinases A and C. In the present study, we aimed to identify the sequence domain which determines and regulates mRNA stability/ instability by protein kinase A and focused our attention on the 3-untranslated region (3-UTR) of LDH-A mRNA. We have constructed various chimeric globin/ lactate dehydrogenase (ldh) genes linked to the c-fos promoter and stably transfected them into rat C6 glioma cells. After their transfection, we determined the halflife of transcribed chimeric globin/ldh mRNAs. The results showed that at least three sequence domains within the LDH- Analysis of the LDH 1 isoenzyme patterns in various cell types under a variety of physiologic conditions suggests complex regulatory mechanisms that determine specific isoenzyme expression (1-7). The LDH-A subunit, for instance, is subject to regulation by a number of different effector agents such as estrogen (3,8), epidermal growth factor (5), catecholamines (4, 9), phorbol ester (7), and c-Myc (10), which change the isoenzyme pattern almost exclusively in favor of the LDH-5 (A4) isoenzyme. The functional importance of these LDH isoenzyme shifts is generally attributed to a need for increased A subunitcontaining isoforms (such as LDH-4 or -5), which can derive more energy from the anaerobic pathway by reducing pyruvate to lactate. Investigations into the mechanism of LDH-A gene expression has identified two basic controls consisting of a transcription-regulatory cascade (4, 6, 11) and a mechanism that regulates the half-life of LDH-A mRNA (4, 12), both of which are major determinants of intracellular LDH-A mRNA levels.Messenger RNA turnover rates fluctuate over a wide range, and it is important to identify and characterize putative stability-regulating mRNA domains and their interacting factors that may be responsible for these functional effects. A great number of reports have demonstrated the existence of such domains and their trans-acting regulatory factors that are critical in determining the half-life of mRNA (13). Several of these studies indicate that the stability of some, but not all, mRNA is determined by specific cis-acting AU-rich domains located in the 3Ј-UTR. For example, a number of mRNAs such as cytokine, lymphokine and protooncogene mRNAs share a common sequence motif with a high content of A and U nucleotides in the 3Ј-UTR (14) and exhibit half-lives in the range of only a fraction of 1 h (15-18). In addition, attention has focused on modulation of mRNA stability in response to a variety of physiological signals. For instance, histone mRNA stability is regulated by the cell cycle (19) and intracellular iron levels control the stability of transferrin receptor mRNA (20,21). Moreover, manipulation of cells with several different effector agents can alter the steady-state level of mRNA during cell g...