RARγ acts as a tumor suppressor in mouse keratinocytes

Abstract

All-trans retinoic acid (RA), the principal biologically active form of vitamin A, is essential for many developmental process as well as homeostasis in the adult. Many lines of evidence also suggest that RA, acting through the RA receptors (RARs), can also suppress growth of tumors of diverse origin. To assess directly the role of the RARs in a model of epidermal tumorigenesis, we investigated the incidence of tumor formation using keratinocytes lacking specific RAR types. Our data suggest that loss of RARc, but not RARa, predisposed keratinocytes to v-Ha-Rasinduced squamous cell carcinoma. We also found that ablation of RARc, but not RARa, abolished RA-induced cell cycle arrest and apoptosis in these keratinocytes. Reconstitution of receptor expression into RAR-null cells restored sensitivity to RA, and reversed the tumorigenic potential of receptor-deficient keratinocytes. These data strongly support a tumor suppressor effect for the RARs, in particular endogenous RARc, in murine keratinocytes.

A Unique Cytoplasmic Localization of Retinoic Acid Receptor-γ and Its Regulations

“…The evolutionary conservation of these receptor subtypes and isoforms and their distinct patterns of expression during development and in the adult organism suggest that each of them has discrete functions (3,4). RAR␥, but not RAR␣, plays a role in predisposing murine keratinocytes to Ras-induced tumorigenesis, in retinoic acid (RA)-induced cell cycle arrest and apoptosis in keratinocytes (5), and in the regulation of the balance between hematopoietic stem cell self-renewal and differentiation (6). At the level of transcriptional regulation, overexpression of RAR␥l inhibits transactivation of RA target genes by other RARs (7), and several specific RAR␥ target genes have recently been identified in F9 cells (8).…”

Dominant-negative Retinoic Acid Receptors Elicit Epidermal Defects through a Non-canonical Pathway

“…6D). The differential effect of RAR subtypes in terms of their ability to mediate gene activation has been reported in many cell types, including keratinocytes (31,42). This suggests that RA may be able to regulate ⌬N-p63 in the context of elevated levels of RAR␣.…”

“…A PstI-SmaI fragment from the RAR␣1 (G303E) mutant (designated dnRAR␣) was used to replace the same region from RAR␣1 (C88A), giving rise to a DNA binding-deficient variant of dnRAR␣, denoted dnRAR DBD . DNA binding of the RAR mutants was assessed by electrophoretic mobility shift assay using a canonical RARE as previously described (19,31 by transient transfection assays in P19 embryocarcinoma cells using a 3ϫ␤RARE reporter vector as previously detailed (19,31).…”

“…Monolayer cultures were subsequently treated with lysis buffer (Nonidet P-40, 10%; Tris (pH 8.0), 0.1 M; dithiothreitol, 1 mM), and luciferase activity was measured using an LB953 Autolumat luminometer (Berthold). Luciferase activity was normalized for ␤-galactosidase activity as described (19,31) and expressed as the mean Ϯ S.D. of independent triplicate transfections.…”

“…Cells were transfected using Lipofect-Ace (Invitrogen) as previously described (19,31). DNA used for transfections included ⌬N-p63 promoter-luc (36), 3ϫ␤RARE-luc (19,31), pLuc-CYP4A-Z (PPARE-luc) (37), and VDRE-lacZ (a gift from J.…”