To identify novel marker molecules associated with chondro-osteogenic differentiation, we have set up a differential screening system based on a cDNA library subtraction in organ cultures of prenatal mouse mandibular condyles. Differential screening of a cDNA library constructed from in vitro cultured condyles allowed the isolation of a novel gene, named E25. Full-length E25 cDNA is predicted to encode a type II integral membrane protein of 263 amino acid residues. In situ hybridization experiments show that E25 is expressed in the outer perichondrial rim of the postnatal mandibular condyle, which contains the proliferating progenitor cells, but not in the deeper layers of the condyle containing the more differentiated chondroblasts and chondrocytes. Other cartilagenous tissues and their perichondrium were negative. Strong in situ hybridization signals were also detected on bone trabeculae of mature bone in tooth germs and in hair follicles. Northern blot analysis showed strong expression in osteogenic tissues, such as neonatal mouse calvaria, paws, tail, and in skin. This expression profile suggests that E25 could be a useful marker for chondro-osteogenic differentiation. Homology searches of DNA databanks showed that E25 belongs to a novel multigene family, containing three members both in man and mouse. The mouse E25 gene locus (Itm2) was mapped to the X chromosome.

Summary The antiproliferative effects of 5-azacytidine (acaCyd) and 5-aza-2'-deoxycytidine (azadCyd) were studied in murine B16 melanoma and a series of B16 melanoma derived mutant strains with selective resistances to the respective drugs. The in vitro cytotoxicities of azaCyd and azadCyd on B16 wild type, expressed in terms of IC50 values, were found to be 5 1iM and 0.2,uM, respectively. The in vitro cytotoxity of both drugs was dependent on the duration of exposure. Uridine and cytidine were able to reverse the in vitro cytotoxicity of azaCyd, but not of azadCyd. Conversely, 2'-deoxycytidine was able to reverse the cytotoxic effect of azadCyd but not of azaCyd. Thymidine and 2'-deoxyuridine had no detectable effects on the in vitro cytotoxicity of either azaCyd or azadCyd. B16 melanoma mutant strains that were selected for resistance to azaCyd showed no cross-resistance to azadCyd, cytosine arabinoside or the fluorinated pyrimidine analogues FUrd, FCyd, FdUrd and FdCyd. Mutant strains that were selected for resistance to azadCyd showed no cross-resistance to azaCyd or fluorinated pyrimidine analogs, but only to cytosine arabinoside. The combined data suggest that azaCyd and azadCyd follow different routes of intracellular metabolic activation and exert their cytotoxic activity via different intracellular targets.

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