Three GADD45 isoforms contribute to hypertonic stress phenotype of murine renal inner medullary cells

Abstract

Mammalian renal inner medullary (IM) cells routinely face and resist hypertonic stress. Such stress causes DNA damage to which IM cells respond with cell cycle arrest. We report that three growth arrest and DNA damage-inducible 45 (GADD45) isoforms (GADD45α, GADDD45β, and GADD45γ) are induced by acute hypertonicity in murine IM cells. Maximum induction occurs 16–18 h after the onset of hypertonicity. GADD45γ is induced more strongly (7-fold) than GADD45β (3-fold) and GADD45α (2-fold). GADD45α and GADD45β protein induction is more pronounced and stable compared with the corresponding transcripts. Hypertonicity of various forms (NaCl, KCl, sorbitol, or mannitol) always induces GADD45 transcripts, whereas nonhypertonic hyperosmolality (urea) has no effect. Actinomycin D does not prevent hypertonic GADD45 induction, indicating that mRNA stabilization is the mechanism that mediates this induction. GADD45 induction patterns in IM cells exposed to 10 different stresses suggest isoform specificity, but similar functions, of individual isoforms during hypertonicity, heat shock, and heavy metal stress, when GADD45γ induction is strongest (17-fold). These data associate all known GADD45 isoforms with the hypertonicity phenotype of renal IM cells.