The A/B domain of truncated retinoic acid receptors can block differentiation and promote features of malignancy

Keratinocyte differentiation induced by calcium, phorbol ester or interferon-gamma elicits distinct changes in the retinoid signalling pathways

BACKGROUND

Retinoids influence keratinocyte proliferation and differentiation via binding to nuclear retinoic acid receptors (RARalpha, -gamma) and retinoid X receptor alpha (RXRalpha). The effect of keratinocyte differentiation on expression of nuclear retinoid receptors and on the conversion of retinol into retinoic acid has not been examined earlier in depth.

OBJECTIVES

Our aim was to examine the expression of retinoid receptors and a retinoid-regulated gene CRABPII, as well as the metabolism of exogenous [(3)H]retinol in cultured human keratinocytes induced to differentiate by exposure to either calcium, phorbol 12-myristate 13-acetate (PMA), or interferon-gamma (IFNgamma).

METHODS

Normal human keratinocytes were cultured and exposed to differentiation-inducing agents. The mRNA and protein expression of retinoid receptors were examined using real-time PCR and Western blot. [(3)H]Retinol uptake and metabolism was monitored by HPLC with on-line radioactivity detection.

RESULTS

In calcium-exposed cells, increased expression of RARgamma and RXRalpha, enhanced metabolism of [(3)H]retinol to 3,4-didehydro-RA (ddRA), and an induction of CRABPII mRNA and protein was noted. In contrast, treatment with PMA and IFNgamma reduced the RARgamma and RXRalpha protein expression (preventable by the proteasome inhibitor MG132), increased the accumulation of [(3)H]RA and/or [(3)H]ddRA in the cells, and changed the CRABPII transcription.

CONCLUSIONS

Retinoid signalling is profoundly altered upon differentiation of keratinocytes and the effects depend on how cellular differentiation is initiated.

Isolation and functional analysis of a keratinocyte-derived, ligand-regulated nuclear receptor comodulator

Keratinocyte (KC) gene expression is regulated by members of the nuclear receptor (NR) superfamily including retinoic acid receptors, retinoid X receptors (RAR and RXR, respectively), and peroxisome proliferator activated receptors (PPAR). In addition to ligand, NR transcriptional activity is controlled by interaction with proteins, collectively known as coregulators, which function as corepressors or coactivators. To improve our understanding of coregulators expressed in epidermis, we screened a KC cDNA library for PPARalpha-interacting proteins. The screen yielded previously unknown proteins including one we named COPR1, for comodulator of PPAR and RXR. COPR1 and its longer variant COPR2 target the AF-2 domains of NR but exhibit quantitative differences in their functional interactions with RAR, RXRalpha and PPAR. They decrease but do not completely repress the activity of RXRalpha and PPARalpha because of a proline-acid-rich autonomous activation domain. An NR box motif contributes to but is not solely responsible for functional and physical association with RXRalpha. The activation domain, their relatively small size (COPR1, 26.9 kDa; COPR2, 32.4 kDa), and strict dependence on AF-2 for interaction distinguish COPR1 and COPR2 from the SMRT/NCoR type of corepressor and may represent a means of control that dampens rather than completely represses NR-mediated gene expression.

Enzyme association with PPARγ: evidence of a new role for 15-lipoxygenase type 2

Fatty acids have historically important structural roles in contributing to epidermal barrier function and therefore cutaneous health. Their metabolism to bioactive compounds is often up-regulated in response to cutaneous toxins thus providing them with functional roles. Some metabolites of arachidonic acid, such as 15S-hydroxyeicosatetraenoic acid (HETE), also serve functional roles as direct ligands for peroxisome proliferator activated receptors (PPARs). 15S-HETE, produced by 15-lipoxygenase type 2 (15-LOX-2), is an endogenous ligand for PPARgamma. This report demonstrates epidermal keratinocyte expression of both 15-LOX-2 and PPARgamma and provides evidence for a relationship beyond that of ligand-producer and -user, namely in vivo association of the two proteins at the molecular level making the enzyme a candidate nuclear receptor coregulator. Such close physical approximation of the 15S-HETE-producing enzyme and PPARgamma could potentiate the receptor response to a short-lived ligand. 15-LOX-2 may exemplify a class of enzymatically active nuclear receptor coactivator proteins distinct from those previously described but sharing their ability to promote expression from nuclear receptor-regulated promoters.

Deletion of RAR carboxyl terminus reveals promoter- and receptor-specific AF-1 effects

Retinoic acid receptors (RARs) are transcription factors with both amino-terminal ligand-independent and carboxyl-terminal ligand-dependent activation functions (AF-1 and AF-2, respectively). RAR-dependent gene activation in keratinocytes was investigated via expression of varied RARalpha and RARgamma carboxyl terminal truncation mutants lacking the AF-2 domain. Overexpression of the AF-1 domain of RARalpha or RARgamma was sufficient to decrease transcriptional activation of retinoid-dependent genes in keratinocytes. Conversely, expression of the same constructs was associated with an increase in expression of endogenous and synthetic reporter genes otherwise negatively regulated by RARs. These effects on transcription driven by some but not all retinoid-sensitive promoters tested could be alleviated by mutation of a serine phosphorylation site in the A/B domain. These results further support the promoter-specificity previously attributed to the RAR AF-1 region and functionally define a particular amino acid residue likely to contribute to the regulation of RARs and other proteins in the transcription complex.