Tumor‐associated Expression of a Serum Protein, Termed αX Protein (α‐Inhibitor III), and Its mRNA in Rat Liver

A cDNA clone bearing the mRNA sequence for rat αX protein (αX) was isolated from a cDNA library constructed from rat liver mRNA. The nucleotide sequence of αX protein cDNA showed 97% homology with that of the 3′ ‐proximal domain of α(1)‐inhibitor III cDNA. The amino acid sequence deduced from that of αX cDNA also exhibited high homology with the primary sequences of α(1)‐inhibitor III and α(2)‐macroglobulin. K231 ascites hepatoma cells were transplanted into male ACI rats, and the level of αX mRNA in the liver of the tumor‐bearing rats was determined by RNA blot hybridization with the cDNA probe. The serum concentration of αX decreased to about 30% of the control value with time after transplantation. The amount of αX mRNA in the liver of tumor‐bearing rats was proportional to the serum concentration of αX. The serum concentrations of transferrin and albumin in the tumor‐bearing rats also decreased to about 30 and 60% of the normal levels, respectively. However, the amounts of mRNAs for transferrin and albumin in the liver of tumor‐bearing rats did not decrease. These findings indicate that the mechanisms of tumor‐associated decrease in the concentrations of different serum proteins In tumor‐bearing rats may differ.