Ligation of the CD44 adhesion molecule reverses blockage of differentiation in human acute myeloid leukemia.

Blockage in myeloid differentiation characterizes acute myeloid leukemia (AML); the stage of the blockage defines distinct AML subtypes (AML1/2 to AML5). Differentiation therapy in AML has recently raised interest because the survival of AML3 patients has been greatly improved using the differentiating agent retinoic acid. However, this molecule is ineffective in other AML subtypes. The CD44 surface antigen, on leukemic blasts from most AML patients, is involved in myeloid differentiation. Here, we report that ligation of CD44 with specific anti-CD44 monoclonal antibodies or with hyaluronan, its natural ligand, can reverse myeloid differentiation blockage in AML1/2 to AML5 subtypes. The differentiation of AML blasts was evidenced by the ability to produce oxidative bursts, the expression of lineage antigens and cytological modifications, all specific to normal differentiated myeloid cells. These results indicate new possibilities for the development of CD44-targeted differentiation therapy in the AML1/2 to AML5 subtypes.

Epigenetic patterns of the retinoic acid receptor beta2 promoter in retinoic acid-resistant thyroid cancer cells.

Treatment with retinoic acid (RA) is effective to restore radioactive iodine uptake in metastases of a small fraction of thyroid cancer patients. In order to find predictive markers of response, we took advantage of two thyroid cancer cell lines, FTC133 and FTC238, with low RA-receptor (RAR)beta expression but differing in their response to RA. We report that in both cell lines, RA signalling pathways are functional, as transactivation of an exogenous RARbeta2 promoter is effective in the presence of pharmacological concentrations of all-trans RA, and enhanced in RA-resistant FTC238 cells after ectopical expression of RARbeta, suggesting a defective endogenous RARbeta2 promoter in these cells. Further analyses show that whereas the RARbeta2 promoter is in an unmethylated permissive status in both FTC133 and FTC238 cells, it failed to be associated with acetylated forms of histones H3 or H4 in FTC238 cells upon RA treatment. Incubation with a histone deacetylase inhibitor, alone or in combination with RA, restored histone acetylation levels and reactivated RARbeta and differentiation marker Na+/I- symporter gene expression. Thus, histone modification patterns may explain RA-refractoriness in differentiated thyroid cancer patients and suggest a potential benefit of combined transcriptional and differentiation therapies.

All-trans retinoic acid modulates the retinoic acid receptor-alpha in promyelocytic cells.

We have recently demonstrated that all-trans retinoic acid (RA), the active metabolite of vitamin A, is an efficient alternative to chemotherapy in the treatment of acute promyelocytic leukemia (AML3). We have further shown that, in these AML3 cells, the gene of the retinoic acid receptor-alpha (RAR alpha) is translocated from chromosome 17 to chromosome 15, and fused to a new gene, PLM. This results in the expression of both normal and chimeric RAR alpha transcripts in AML3 cells. The PLM-RAR alpha protein may account for the impairment of differentiation and thus leukemogenesis, but not for the paradoxical efficacy of RA in these cells. In an attempt to elucidate RA's differentiative effect in AML3 patients, the present work examined the in vitro and in vivo modulation of the normal RAR alpha transcripts by all-trans RA in seven cases of AML3. In all samples, Northern blot analysis revealed a low expression of the two normal RAR alpha transcripts compared with other human myeloid leukemic cells. No modulation was observed after 4-8 d of in vivo therapy with all-trans RA 45 mg/m2 per d. In vitro incubation with all-trans RA, however, increased the level of expression of the normal RAR alpha transcripts in AML3 cells but not in other AML leukemic subtypes. This modulation of the two normal RAR alpha transcripts appeared to be an early and primary event of RA's differentiating effect. We therefore suggest that up-regulation of the normal RAR alpha gene expression by pharmacological concentrations of all-trans RA may restore the normal differentiation pathway in these cells.

Physical and functional interactions between cellular retinoic acid binding protein II and the retinoic acid-dependent nuclear complex.

Two sorts of proteins bind to, and mediate the developmental and homeostatic effects of, retinoic acid (RA): the RAR and RXR nuclear receptors, which act as ligand-dependent transcriptional regulators, and the cellular RA binding proteins (CRABPI and CRABPII). CRABPs are generally known to be implicated in the synthesis, degradation, and control of steady-state levels of RA, yet previous and recent data have indicated that they could play a role in the control of gene expression. Here we show for the first time that, both in vitro and in vivo, CRABPII is associated with RARalpha and RXRalpha in a ligand-independent manner in mammalian cells (HL-60, NB-4, and MCF-7). In the nucleus, this protein complex binds the RXR-RAR-specific response element of an RA target gene (RARE-DR5). Moreover, in the presence of retinoids that bind both the nuclear receptors and CRABPII, enhancement of transactivation by RXRalpha-RARalpha heterodimers is observed in the presence of CRABPII. Thus, CRABPII appears to be a novel transcriptional regulator involved in RA signaling.

Induction of retinoic acid-binding protein in normal and malignant human myeloid cells by retinoic acid in acute promyelocytic leukemia patients.

Retinoic acid has striking effects on development and cell differentiation. Its biological effect is a highly regulated process that is controlled by specific proteins. In the nucleus, different retinoic acid receptors have been identified and their genes cloned. In the cytosol, retinoid binding proteins, cellular retinoic acid-binding protein and cellular retinol-binding protein, have been correlated with normal and malignant tissue differentiation. Recently, differentiation therapy of acute promyelocytic leukemias (AML3 subtype) with all-trans-retinoic acid has been shown to be an efficient alternative to chemotherapy. The retinoic acid receptor alpha gene has been shown to be specifically rearranged in AML3 through the t(15;17) translocation. The molecular basis of the effect to reverse the leukemic phenotype of all-trans-retinoic acid is not yet elucidated. To further study retinoic acid efficacy in AML3 leukemia, retinoic acid-binding proteins were studied in the cytosol extracts of hematopoietic cells. No retinoic acid binding activity was detected in normal or malignant hematopoietic cells whether sensitive or not to retinoic acid. However, detectable binding to a cytosolic protein corresponding to cellular retinoic acid-binding protein (M(r) 15,000, Kd 3 nM) was observed in the bone marrow cells of AML3 patients undergoing all-trans-retinoic acid therapy. We suggest that both the induction and subsequent presence of cellular retinoic acid-binding protein may influence the therapeutic efficacy of retinoic acid and must be taken into account when studying its effect in acute promyelocytic patients.

Distinct sensitivity of neuroblastoma cells for retinoid receptor agonists: evidence for functional receptor heterodimers.

Retinoic acid (RA) plays a major role in embryogenesis of the nervous system and has been reported to induce differentiation in neuroblastoma cell lines. To identify RA signaling pathways involved in such differentiation processes, two RA-sensitive neuroblastoma cell lines (LA-N-5 and SH-SY5Y) were extensively studied. Northern blot experiments determined that of the three RAR mRNAs, only RARalpha was significantly expressed, with respectively weak or undetectable levels of RARgamma and RARbeta. RXRs (alpha and beta) receptors were weakly expressed. Western blotting analysis confirmed the constitutive expression of RARalpha and absence of RARbeta and weak levels of RXRalpha. Treatment with all-trans-RA up-regulated RARalpha and induced a drastic increase of RARbeta (both at the RNA and protein level). To further characterize the function of RARalpha, RARbeta and RXRalpha in NB cells, nuclear extracts from LA-N-5 cells were analysed by EMSA studies. Three specific retarded complexes were observed which were significantly decreased or shifted in the presence of monoclonal antibodies to RARalpha, RARbeta and RXRalpha. RA treatment dramatically induced a DR5-binding RXRalpha-RARbeta heterodimer. Treatment with combinations of RARalpha or RARbeta agonists with a RXRalpha agonist or with a RARalpha agonist alone, induced neurite-outgrowth supporting the probability that both RXRalpha-RARalpha or RXRalpha-RARbeta heterodimers are involved in RA-mediated differentiation of NB cells. The availability of novel synthetic RA-specific receptor ligands should provide the possibility of tissue specific therapeutic regimes.

In vitro all-trans retinoic acid (ATRA) sensitivity and cellular retinoic acid binding protein (CRABP) levels in relapse leukemic cells after remission induction by ATRA in acute promyelocytic leukemia.

The current treatment of acute promyelocytic leukemia (APL, also called AML3 subtype) is focused on differentiating agents such as the vitamin A derivative all-trans retinoic acid (ATRA). This agent is a novel and very promising therapy for this disease characterized cytogenetically by a translocation t(15;17)(q21;q22) involving the alpha retinoic acid receptor on chromosome 17 and the PML gene on chromosome 15. Clinical trials have demonstrated that ATRA followed by or combined with conventional chemotherapy may be more beneficial than chemotherapy for inducing complete remission. Unfortunately, ATRA as a single agent, does not appear able to maintain patients in remission (median 5 months), and when relapse occurs resistance to a second induction of ATRA therapy is observed in almost all cases. Recently our laboratory investigated whether specific features of the AML3 cells at relapse could explain the in vivo resistance observed. We have demonstrated that AML3 patients' cells (from four patients) at relapse show high levels of CRABP, a cytosolic retinoic acid binding protein and this protein was not detected prior to ATRA therapy. Relapse-AML3 cells (n = 12) showed reduced differentiation induction when compared with 'virgin'-AML3 cells. Results from this study suggest that CRABP could modulate ATRA cellular concentrations reaching the nucleus. This induced ATRA hypercatabolytic state should be monitored during consolidation therapy and at relapse by evaluating CRABP and RA metabolite levels, in order to detect ATRA resistance in patients with AML3.

In vitro all-trans retinoic acid (ATRA) sensitivity and cellular retinoic acid binding protein (CRABP) levels in relapse leukemic cells after remission induction by ATRA in acute promyelocytic leukemia.

The current treatment of acute promyelocytic leukemia (APL, also called AML3 subtype) is focused on differentiating agents such as the vitamin A derivative all-trans retinoic acid (ATRA). This agent is a novel and very promising therapy for this disease characterized cytogenetically by a translocation t(15;17)(q21;q22) involving the alpha retinoic acid receptor on chromosome 17 and the PML gene on chromosome 15. Clinical trials have demonstrated that ATRA followed by or combined with conventional chemotherapy may be more beneficial than chemotherapy for inducing complete remission. Unfortunately, ATRA as a single agent, does not appear able to maintain patients in remission (median 5 months), and when relapse occurs resistance to a second induction of ATRA therapy is observed in almost all cases. Recently our laboratory investigated whether specific features of the AML3 cells at relapse could explain the in vivo resistance observed. We have demonstrated that AML3 patients' cells (from four patients) at relapse show high levels of CRABP, a cytosolic retinoic acid binding protein and this protein was not detected prior to ATRA therapy. Relapse-AML3 cells (n = 12) showed reduced differentiation induction when compared with 'virgin'-AML3 cells. Results from this study suggest that CRABP could modulate ATRA cellular concentrations reaching the nucleus. This induced ATRA hypercatabolytic state should be monitored during consolidation therapy and at relapse by evaluating CRABP and RA metabolite levels, in order to detect ATRA resistance in patients with AML3.

Retinoic acid inhibits the expression of cytidine deaminase linked to the differentiation of the human leukemic cell line HL-60.

The activity of cytidine deaminase markedly increases during the differentiation of HL-60 cells induced by dimethylsulfoxide or 1,25-dihydroxy vitamin D3, but does not increase when the inducer is retinoic acid. Here it is demonstrated that retinoic acid inhibits the increase in cytidine deaminase activity elicited by the other two inducers. This inhibitory effect of retinoic acid (i) was not the result of a direct action on the enzymatic activity; (ii) was correlated with the differentiating effect of retinoic acid, as indicated by the similar time-course and dose-dependence of both effects, and by additional studies with various retinoids and with an HL-60 variant resistant to retinoic acid-induced differentiation; (iii) required the continued presence of the drug for more than 24 h, and could not be reversed after 48 h; (iv) was manifest, after a lag-time of 24 h, at whatever time retinoic acid was added during the 5 days of treatment of the cells with the differentiation inducers; and (v) was prevented by the addition of the protein synthesis inhibitor cycloheximide. These data indicate that retinoic acid negatively regulates the expression of cytidine deaminase in HL-60 cells, and suggest that this effect is mediated by a protein, the synthesis of which should be controlled by the nuclear receptor of retinoic acid.

Stimulatory effect of thyroid hormone on RA-induced granulocytic differentiation in leukemic cells.

Retinoic acid, vitamin D3, and dexamethasone are known inducers of myeloid leukemic cell differentiation. Recent evidence indicates that these drugs mediate their biological effects through binding to a nuclear receptor which belongs to the steroid/thyroid hormone receptor superfamily. This paper shows that the ligands of the other receptors of this family, estrogens, progesterone, androgens and thyroid hormone, do not induce leukemic cell differentiation. However, thyroid hormone potentiates, by one order of magnitude, the dose-response effect of retinoic acid in HL-60 cells.

Differential uptake of all-trans retinoic acid by acute promyelocytic leukemic cells: evidence for its role in retinoic acid efficacy.

All-trans retinoic acid (ATRA) has been demonstrated to be an efficient alternative to chemotherapy in the treatment of acute promyelocytic leukemia (APL or AML3). Complete remission is obtained by inducing granulocytic differentiation of the leukemic cells. To date, the exact mechanism through which ATRA exerts its differentiating effect is not known. The present investigation was initiated to characterize ATRA intracellular concentrations achieved in human myeloid leukemic cells in relation to their different sensitivity to ATRA differentiating effect. During the first 24 h of incubation, a significant decrease of ATRA in the culture medium and a marked increase in the intracellular concentrations were observed. Maximal uptake by the leukemic cells was reached within minutes, with levels between 20 and 260 pmol/10(6) cells (median = 100). Interestingly, a correlation between ATRA-induced differentiation and the intracellular ATRA concentration achieved was observed. In fact, patients with intracellular levels below 60 pmol/10(6) cells defined slow uptakers, never exceeded 40% differentiated cells at day 3. On the other hand, cells with 2-4-fold higher concentration (100-250 pmol/10(6) cells) achieved 100% differentiated cells at day 3. This report suggests that intracellular ATRA concentration is a key pharmacological parameter that should be taken into account to gain further insights into ATRA sensitivity in APL patients.

In vitro treatment with retinoids or the topoisomerase inhibitor, VP-16, evidences different functional apoptotic pathways in acute promyelocytic leukemic cells.

Understanding the mechanisms inherent to malignant cell eradication is a major determinant for cancer therapy. Recent data have demonstrated that apoptosis may be one of the mechanisms through which both cytotoxic and differentiating drugs may eliminate malignant cells. Treatment of acute promyelocytic leukemia (APL) by all-trans retinoic acid (ATRA) is the first model of differentiation therapy allowing achievement of more than 90% complete remission (CR). However, disease-free survival (DFS) is short if patients are not subsequently treated with chemotherapy. In order to address the question of APL cells' elimination during ATRA therapy, we studied phenotypic and molecular features of 14 APL cases relative to cell survival in primary culture in the presence or absence of ATRA. Compared to other acute myeloid leukemia (AML) subtypes, APL cells in short-term suspension culture present a better survival rate (P < 0.001). After incubation with ATRA, cell survival was not altered and was correlated with a concomitant absence of apoptosis, despite a significant decrease of the BcL-2 protein in APL differentiated cells. Indeed, after 6 days of culture, only 3 +/- 0.5% of APL cells exhibit morphological features of apoptosis after ATRA treatment compared to 30 +/- 5% in HL-60-treated cells. Treatment of APL cells with 9-cis RA, 13-cis RA or analogs of RAR alpha or RXR alpha also failed to induce apoptosis. Treatment of either APL or ATRA-differentiated APL cells with 40 microM etoposide resulted in DNA fragmentation and morphological changes characteristic of apoptosis in 23 +/- 5% cells after only 20 h of treatment and 68 +2- 3% after 48 h suggesting that other pathways of apoptosis are still functional in APL cells. Though these in vitro data cannot fully represent the mechanism of cell death and cell elimination in vivo, they clearly indicate that ATRA alone may not induce leukemic clone eradication by apoptosis correlating with the persistence of minimal residual disease and constant relapse after CR obtained with ATRA alone.

All-trans retinoic acid rapidly decreases cathepsin G synthesis and mRNA expression in acute promyelocytic leukemia.

The cells from patients with acute promyelocytic leukemia (AML M3) undergo terminal differentiation when treated with all-trans retinoic acid (ATRA). We have analyzed the expression of the mRNA for cathepsin G, a promyelocyte stage-specific transcript, in the leukemia and in retinoic acid responsive cell lines. We showed that the transcript is perpetually synthesized in patients' cells and that it rapidly disappears when the cells are treated with ATRA. In ATRA-sensitive (HL-60, NB4) cell lines and an ATRA-resistant (HL-60R) cell line we have shown that this process is dependent on proteins synthesized during the first 6h of ATRA-triggered differentiation and may involve both pre- and post-transcriptional mechanisms. A corresponding decrease in cathepsin G protein synthesis then follows. These findings indicate that the maturation arrest in AML M3 results in cells that may constitutively continue to produce proteins whose production is temporally confined during normal hemopoiesis. This would explain the elevated plasma-free serine protease activity we have demonstrated in this disease, and has implications for both the coagulopathy and the 'retinoic acid syndrome' in AML M3.

Modulation of IL-8, IL-1 beta, and G-CSF secretion by all-trans retinoic acid in acute promyelocytic leukemia.

Acute promyelocytic leukemia (APL) is a homogeneous subgroup of acute myeloid leukemias (AML) characterized by the presence of the t(15;17) translocation and the resulting PML/RAR alpha fusion proteins. To date APL is the only AML which is sufficiently sensitive to all-trans retinoic acid (ATRA) differentiating effect. We have recently reported that APL express and secrete hematopoietic growth factors (HGF) such as IL-1 beta, TNF alpha, and IL-6. In vivo ATRA alone allows achievement of complete remission in APL patients. One of ATRA therapy's drawbacks is the increase of peripheral blast cells often associated with the ATRA leukocyte activation syndrome. To determine if this specific side-effect was linked to an increase of HGF release by APL cells, we studied the modulation of cytokine production by APL cells, we studied the modulation of cytokine production by APL samples (n = 12) before and after incubation with ATRA. ATRA failed to modulate TNF alpha, IL-6 or GM-CSF secretion levels; however, IL-8 levels decreased in 11 cases, and in four cases up-regulation of IL-1 beta and G-CSF protein expression was observed. These modulations were found to be linked to ATRA sensitivity as ATRA failed to modulate cytokine production in non-APL cells (n = 8). Interestingly, the increase of IL-1 beta and G-CSF production in the presence of ATRA was highly correlated to an increase in APL cell count in vitro and in vivo hyperleukocytosis, resulting in fatal outcome. IL-1 beta, TNF alpha, IL-6, and IL-8 are known to be implicated in leukocyte activation. The results of this study suggest that ATRA-induced hyperleukocytosis and ATRA leukocyte activation syndrome in APL may be inherent to the secretion of specific hematopoietic growth factors by the APL cells.

Quantitation of minimal residual disease in acute promyelocytic leukemia patients with t(15;17) translocation using real-time RT-PCR.

We took advantage of a recently developed system allowing performance of real-time quantitation of polymerase chain reaction to develop a quantitative method of measurement of PML-RARalpha transcripts which are hallmarks of acute promyelocytic leukemia (APL) with t(15;17) translocation. Indeed, although quantitation of minimal residual disease has proved to be useful in predicting clinical outcome in other leukemias such as chronic myeloid leukemia or acute lymphoblastic leukemia, no quantitative data have been provided in the case of APL. We present here a method for quantitation of the most frequent subtypes of t(15;17) transcripts (namely bcr1 and bcr3). One specific forward primer is used for each subtype in order to keep amplicon length under 200 bp. The expression of PML-RARalpha transcripts is normalized using the housekeeping porphobilinogen deaminase (PBGD) gene. This technique allows detection of 10 copies of PML-RARalpha or PBGD plasmids, and quantitation was efficient up to 100 copies. One t(15;17)-positive NB4 cell could be detected among 106 HL60 cells, although quantitation was efficient up to one cell among 105. Repeatability and reproducibility of the method were satisfying as intra- and inter-assay variation coefficients were not higher than 15%. The efficiency of the method was finally tested in patient samples, showing a decrease of the PML-RARalpha copy number during therapy, and an increase at the time of relapse.

Induction of high-affinity GM-CSF receptors during all-trans retinoic acid treatment of acute promyelocytic leukemia.

Differentiation of normal myeloid cells is accompanied by the increase of high-affinity GM-CSF receptors necessary for progenitor proliferation/differentiation and mature neutrophil function. All-trans retinoic acid (ATRA) induces terminal differentiation of acute promyelocytic leukemia cells (AML3 subtype). We report in this study that AML3 cells, like other AML subtypes, harbor high-affinity GM-CSF R (n = 138.3 +/- 69.3 sites/cell, Kd = 76.9 +/- 68.8 pM). In all cases, incubation with ATRA induces either an increase in the number of affinity of GM-CSF R (n = 212.7 +/- 116.2 sites/cell, Kd = 43.2 +/- 22.5 pM). The data presented show that modulation of GM-CSF receptors cells is correlated to the degree of ATRA-induced granulocytic differentiation but not to increased cell growth.

The retinoic acid receptor alpha gene is rearranged in retinoic acid-sensitive promyelocytic leukemias.

All-trans retinoic acid (RA), the active metabolite of vitamin A, has recently been demonstrated to be an efficient alternative to chemotherapy in the treatment of acute promyelocytic leukemia (M3 subtype of the French-American-British cytological classification). Complete remission is obtained by inducing terminal granulocytic differentiation of the leukemic cells. To elucidate whether the effect of retinoic acid on the differentiation of M3 leukemic cells was related to any specific characteristics of its receptor, we analyzed the structure and expression of retinoic acid receptor (RAR) genes in 16 M3 patients. Abnormal RAR alpha transcripts were detected in 13 cases. In nine patients, the genomic DNA was analyzed by Southern blotting and evidence for a rearranged RAR alpha gene was found generated in four cases. Normal RAR transcripts and germline restriction fragments were found in samples from normal or other leukemic cells, suggesting that this alteration of the RAR alpha gene is specifically seen in M3 leukemias. These results suggest that alteration of the retinoic acid receptor alpha may be implicated in M3 leukemogenesis.

Poor response to all-trans retinoic acid therapy in a t(11;17) PLZF/RAR alpha patient.

All-trans retinoic acid (ATRA) is a potent inducer of differentiation and cell death in malignant cells. Its effect is known to be mediated through binding to specific nuclear (RARs and RXRs) or cytoplasmic (CRABP) proteins. ATRA is strikingly effective in acute promyelocytic leukemia (the AML3 subtype) inducing a high incidence of complete remissions. Paradoxically, most AML3 cells harbor an abnormal retinoic acid receptor (PML/RAR alpha) resulting from the t(15;17) translocation. Though few AML3 patients do not respond to ATRA therapy, individualization of these cases is of practical importance. Recently the RAR alpha gene has been demonstrated to be involved in a novel fusion transcript (PLZF/RAR alpha) through a t(11;17) translocation. We describe here the second case of such a patient with a t(11;17)-PLZF/RAR alpha leukemic clone. Southern analysis revealed that the breakpoint in the RAR alpha gene was within the second intron (as for PML/RAR alpha) and the intron separating the second and third zinc finger of the PLZF gene. In vitro, the leukemic cells did not show increased NBT reduction or loss of self-renewal after incubation with ATRA. After therapy with ATRA, only partial remission was obtained. These results suggest that the t(11;17) (PLZF/RAR alpha) case of this study was less responsive to ATRA therapy than t(15;17) (PML/RAR alpha) cases and raises the question of the definition of this novel AML subtype.

Hydroxyurea suicide study of the kinetic heterogeneity of colony forming cells in human bone marrow.

The kinetics of granulomonocyte colony forming cells have been studied in unfractionated normal human bone marrow by hydroxyurea suicide and in cells separated by velocity sedimentation. Sequential studies revealed two subpopulations of colony forming cells, having different sizes and different multiplication potentialities. There are large cells with a high suicide rate which develop small granulocyte and monocyte colonies during the first week of culture in semi-solid agar. Smaller cells develop larger colonies of granulocytes, monocytes and eosinophils between 2 and 3 weeks of culture. Only granulocyte progenitors have a substantial suicide rate in this small cell population. This population is also less responsive to stimulation than is the large cell class, which is a more highly differentiated progeny. The role of these different kinetics of colony forming cells is discussed in the context of the heterogeneity of the in vitro differentiation of neutrophil, monocyte and eosinophil lines.

The novel co-activator CRABPII binds to RARalpha and RXRalpha via two nuclear receptor interacting domains and does not require the AF-2 'core'.

We identify the RARalpha, RXRalpha and CRABPII domains required for the physical interaction of these proteins. On RARalpha and RXRalpha, the sequences correspond to the DEF and DE domains, respectively, but the interaction with CRABPII does not require the AF-2AD 'core'. On CRABPII, two interacting domains are identified (NRID1 and NRID2), one of which contains the only enhancement transactivation domain of CRABPII. The interaction is ligand-independent and does not require the ligand-binding domain of CRABPII. These results further stress that interaction of CRABPII with the nuclear receptors defines a novel level of transcriptional control.