Modifications of the expression of liver‐specific and non‐specific messenger RNAs during azo‐dye hepatocarcinogenesis

Abstract

The expression of specific and non-specific rat liver messenger RNAs has been studied during 3'-methyl-4-(dimethy1amino)azobenzene (3'-MeDAB) carcinogenesis, using cDNA probes complementary to inRNAs encoding aldolase A and B, L-type pyruvate kinase, albumin, a-foetoprotein, transferrin and an unidentified 2.7 x 103-base mRNA. mRNAs specific for undifferentiated cells, such as those encoding aldolase A and the unidentified 2.7 x 103-base species were re-expressed very early, being easily detectable at the 1st week of 3'-MeDAB treatment. They reached a maximum of expression at the 4th week. Simultaneously the levels of aldolase B and L-type pyruvate kinase mRNAs dramatically decreased as compared to controls, but remained responsive to induction by a high-carbohydrate diet.Albumin and transferrin mRNA levels were only slightly modified in the course of the carcinogenic diet. At the terminal stage of hepatocarcinogenesis, i. e. in malignant hepatoma cells, expression and inducibility of aldolase B and L-type pyruvate kinase mRNAs were similar to those in normal adult rats while mRNAs specific for undifferentiated or foetal stages were also synthesized.The very early changes in gene expression for aldolases A and B, L-type pyruvate kinase and the 2.7 x lo3-base mRNA species could indicate that carcinogenic diet modifies gene control mechanisms long before inducing hepatoma.I t is well established that rats fed a 3'-methyl-4-(dimethy1amino)azobenzene (3'-MeDAB)-containing diet for twenty weeks develop a well-differentiated (slow-growing) hepatoma [I]. Carcinogenesis is associated with very early histological and biochemical alterations : necrosis of hepatocytes with appearance of new cellular populations and, simultaneously, resurgence of foetal proteins and a decrease in the expression of adult specific markers as shown first by Schapira et al. for aldolase [2] and then later extended to other enzymes and proteins [3 -171. Aldolase B [7, 81 and L-type pyruvate kinase [9, I51 are characteristic of normal adult liver and are decreased in hepatoma cells, while the foetal-type isozymes, aldolases A and C, and M2-type pyruvate kinase increase [7,8,[10][11][12] 151. a-Foetoprotein, a foetal equivalent of serum albumin, is also re-expressed during hepatocarcinogenesis [ 5 , 6, 13, 141.The expression of the albumin and a-foetoprotein genes during liver development, regeneration and carcinogenesis has been widely investigated. a-Foetoprotein gene expression switches off at birth and conversely albumin gene expression switches on; phenotypic expression of a-foetoprotein is resumed in the adult rat during regeneration, exposure to chemical hepatocarcinogenes and in hepatoma [I 8 -241.During 3'-MeDAB hepatocarcinogenesis there is re-expression of a-foetoprotein but albumin mRNA is unchanged [24] decreased while aldolase A mRNA increased in carcinogentreated liver [25].However, the mechanisms involved in regulation of isozynie expression are not yet as clearly understood as are those leading to the resurgence of a-foetoprotein....

Structure and expression of mouse aldolase genes. Brain-specific aldolase C amino acid sequence is closely related to aldolase A

“…[31], who demonstrated the existence of two different forms of aldolase A mRNA in these tissues. More recently this was also confirmed by Lamas et al [33].…”

Regulation of genes for glycolytic enzymes in cultured rat hepatoma cell lines

“…We have previously demonstrated that the three L-type pyruvate kinase mRNA species are generated by the optional use of different polyadenylation sites and encode the same protein [46]. The ratio between the 3.2-kb and the 2.2-kb+2-kb species is tissue-specific [46] and ranges over 40-60% in normal liver and hepatomas [28]. This value is also found in the three cell lines studied here.…”

“…The same absence of response is observed with 10 mM fructose instead of glucose, in MHICI, Fao and Faza cell lines (not shown). This lack of response to carbohydrates is not due to a property originating from hepatoma cells, since Lamas et al have shown that, in vivo, hepatoma cells resulting from azo-dye carcinogenesis exhibited the same pattern of regulation under carbohydrate feeding as that found in normal hepatocytes [28]. In vivo the expression of the genes encoding pyruvate kinase L and the 5.4-kb mRNA species is very low, practically undetectable in fasting animals or in animals fed protein or lipid-rich diets and it increases by several orders of magnitude following carbohydrate refeeding [lo, 13,31,34).…”

“…The same absence of response is observed with 10 mM fructose instead of glucose, in MHICI, Fao and Faza cell lines (not shown). This lack of response to [28]. In vivo the expression of the genes encoding pyruvate kinase L and the 5.4-kb mRNA species is very low, practically undetectable in fasting animals or in animals fed protein or lipid-rich diets and it increases by several orders of magnitude following carbohydrate refeeding [lo, 13,31,34).…”