Retinoylation (covalent modification by retinoic acid) of Rho-GDIβ in the human myeloid leukemia cell line HL60 and its functional significance

• Rho GDP dissociation inhibitor β (Rho-GDIβ) is modified by retinoic acid (RA).
• RA covalently binds to the Thr2 residue of Rho-GDIβ.
• RA increases Rho-GDIβ protein levels, but not its mRNA level or caspase-3 activity.
• RA treatment stabilizes Rho-GDIβ protein in cells.
• RA-binding to Rho-GDIβ may play a significant role in RA-induced differentiation.

Retinoic acid (RA) has a variety of biological effects in mammalian cells and tissues. It is well known that RA induces differentiation of human acute promyelocytic leukemia (APL) HL60 cells, fresh human APL cells, and clinical remission in patients with APL. Retinoylation (acylation of proteins by RA) is a possible pathway for RA action. However, an understanding of the role that retinoylation plays in the actions of RA is lacking. In the current study, several retinoylated proteins were detected in RA-treated HL60 fractions following Mono Q anion exchange chromatography and analysis using two-dimensional polyacrylamide gel electrophoresis. One of the retinoylated proteins was identified as Rho-GDIβ (28 kDa) by TOF-MS and co-migration with Rho-GDIβ (28 kDa). Truncated Rho-GDIβ (23 kDa, N ∆ 19), a product of cleavage by caspase-3, was not retinoylated. RA covalently bound to the Thr2 residue in Rho-GDIβ (5 kDa), which is the second product resulting from the cleavage of Rho-GDIβ (28 kDa) by caspase-3. RA treatment increased the level of Rho-GDIβ (28 kDa) and decreased the level of Rho-GDIβ (23 kDa). RA did not induce caspase-3 activity or Rho-GDIβ mRNA expression. It is likely that retinoylation of Rho-GDIβ increases its metabolic stability.