Maximal adamantyl-substituted retinoid-related molecule-induced apoptosis requires NF-κB noncanonical and canonical pathway activation.

NF-κB transcription factors have a critical role in regulating cell survival and apoptosis. We have previously shown that 4-(3-Cl-(1-adamantyl)-4-hydroxyphenyl)-3-chlorocinnamic acid (3-Cl-AHPC), an adamantyl-substituted retinoid molecule, induced apoptosis and required NF-κB activation in prostate and breast carcinoma cells. Here, we show that 3-Cl-AHPC activated both IκB kinase (IKK)α and IKKß with subsequent activation of the canonical and noncanonical NF-κB pathways in the human breast carcinoma and leukemia cell lines. 3-Cl-AHPC-mediated activation of the NF-κB canonical pathway occurred within 6 h, whereas maximal activation of the NF-κB noncanonical pathway required 48 h. Knockout of IKKα or IKKß expression in mouse embryonic fibroblast cells and knockdown of IKKα or IKKß in MDA-MB-468 cells resulted in the inhibition of 3-Cl-AHPC-mediated apoptosis, indicating that activation of canonical and noncanonical pathways are required for maximal 3-Cl-AHPC-mediated apoptosis. 3-Cl-AHPC activation of the noncanonical pathway was preceded by caspase-mediated decrease in the E3-ligase c-IAP1 with subsequent stabilization of NF-κB-inducing kinase (NIK) expression, increased binding of NIK by TRAF3, activation of IKKα, and the resultant increased levels of RelB and p52. Increased expression of c-IAP1 blocked 3-Cl-AHPC-mediated stabilization of NIK levels and 3-Cl-AHPC-mediated apoptosis. Cdc37 expression was required for activation of IKKα and IKKß by 3-Cl-AHPC. These findings suggest that NF-κB pathways have an important role in 3-Cl-AHPC-mediated apoptosis.

Regulation of Nur77 nuclear export by c-Jun N-terminal kinase and Akt.

Proapoptotic nuclear receptor family member Nur77 translocates from the nucleus to the mitochondria, where it interacts with Bcl-2 to trigger apoptosis. Nur77 translocation is induced by certain apoptotic stimuli, including the synthetic retinoid-related 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN)/CD437 class. In this study, we investigated the molecular mechanism by which AHPN/CD437 analog (E)-4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC) induces Nur77 nuclear export. Our results demonstrate that 3-Cl-AHPC effectively activated Jun N-terminal kinase (JNK), which phosphorylates Nur77. Inhibition of JNK activation by a JNK inhibitor suppressed 3-Cl-AHPC-induced Nur77 nuclear export and apoptosis. In addition, several JNK upstream activators, including the phorbol ester TPA, anisomycin and MAPK kinase kinase-1 (MEKK1), phosphorylated Nur77 and induced its nuclear export. However, Nur77 phosphorylation by JNK, although essential, was not sufficient for inducing Nur77 nuclear export. Induction of Nur77 nuclear export by MEKK1 required a prolonged MEKK1 activation and was attenuated by Akt activation. Expression of constitutively active Akt prevented MEKK1-induced Nur77 nuclear export. Conversely, transfection of dominant-negative Akt or treatment with a phosphatidylinositol 3-kinase (PI3-K) inhibitor accelerated MEKK1-induced Nur77 nuclear export. Furthermore, mutation of an Akt phosphorylation residue Ser351 in Nur77 abolished the effect of Akt or the PI3-K inhibitor. Together, our results demonstrate that both activation of JNK and inhibition of Akt play a role in translocation of Nur77 from the nucleus to the cytoplasm.

Synthetic retinoids and their nuclear receptors.

In addition to all-trans-retinoic acid and its 9 and 13-cis isomers, four synthetic retinoids are currently available to treat diseases of hyperproliferation, such as acne, psoriasis, and actinic keratosis, or cancers such as acute promelocytic leukemia, cutaneous T-cell lymphoma, and squamous or basal cell carcinoma. The retinoids extert their antiproliferative effects by interacting with their retinoic acid and retinoid X receptors that act as ligand-inducible transcription factors. These homologous receptors function either directly on retinoid response elements or indirectly by modifying the responses of other transcription factors. Their major domains for binding DNA and their ligands have been characterized by either nuclear magnetic resonance spectroscopy or X-ray crystallography. The identification and design of synthetic retinoids are overviewed, as are their selective interactions with specific retinoid receptor subtypes and their clinical effects against cancer. Emphasis is placed on the retinoid X receptors and their ligands.

Apoptosis induction in cancer cells by a novel analogue of 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid lacking retinoid receptor transcriptional activation activity.

The retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN) is reported to have anticancer activity in vivo. Induction of cell cycle arrest and apoptosis in cancer cell lines refractory to standard retinoids suggests a retinoid-independent mechanism of action for AHPN. Conformational studies suggested that binding of AHPN does not induce an unusual conformation in retinoic acid receptor (RAR) gamma. The 3-chloro AHPN analogue MM11453 inhibited the growth of both retinoid-resistant (HL-60R leukemia, MDA-MB-231 breast, and H292 lung) and retinoid-sensitive (MCF-7 breast, LNCaP prostate, and H460 lung) cancer cell lines by inducing apoptosis at similar concentrations. Before apoptosis, MM11453 induced transcription factor TR3 expression and loss of mitochondrial membrane potential characteristic of apoptosis. MM11453 lacked the ability to significantly activate RARs and retinoid X receptor alpha to initiate (TREpal)(2)-tk-CAT reporter transcription. These results, differential proteolysis-sensitivity assays, and glutathione S-transferase-pulldown experiments demonstrate that, unlike AHPN or the natural or standard synthetic retinoids, MM11453 does not behave as a RAR or retinoid X receptor alpha transcriptional agonist. These studies strongly suggest that AHPN exerts its cell cycle arrest and apoptotic activity by a signaling pathway independent of retinoid receptor activation.

Retinoic acid (RA) receptor transcriptional activation correlates with inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced ornithine decarboxylase (ODC) activity by retinoids: a potential role for trans-RA-induced ZBP-89 in ODC inhibition.

Evaluation of retinoic acid receptor (RAR) subtype-selective alpha and gamma agonists and antagonists and a retinoid X receptor (RXR) class-selective agonist for efficacy at inhibiting both induction of ornithine decarboxylase (ODC) by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse epidermis and rat tracheal epithelial cells and the appearance of papillomas in mouse epidermis treated in the 2-stage tumor initiation-promotion model indicated that (i) RXR class-selective transcriptional agonists, such as MM11246, were not involved in ODC inhibition; (ii) RAR-selective agonists that induce gene transcription from RA-responsive elements (RAREs) were active at low concentrations; (iii) RAR-selective antagonists that bind RARs and inhibit AP-1 activation on the collagenase promoter but do not activate RAREs to induce gene transcription were less effective inhibitors; and (iv) RARgamma-selective retinoid agonists were more effective inhibitors of TPA-induced ODC activity than RARalpha-selective agonists. These results suggest that RARE activation has a more important role in inhibition of ODC activity than RXR activation or AP-1 inhibition and that RARgamma-selective agonists would be the most useful inhibitors of epithelial cell proliferation induced by tumor promoters. The natural retinoid all-trans-RA induced expression of transcription factor ZBP-89, which represses activation of the GC box in the ODC promoter by the transcription factor Sp1.

Cytochrome c release and apoptosis induced by mitochondrial targeting of nuclear orphan receptor TR3.

TR3, an immediate-early response gene and an orphan member of the steroid-thyroid hormone-retinoid receptor superfamily of transcription factors, regulates apoptosis through an unknown mechanism. In response to apoptotic stimuli, TR3 translocates from the nucleus to mitochondria to induce cytochrome c release and apoptosis. Mitochondrial targeting of TR3, but not its DNA binding and transactivation, is essential for its proapoptotic effect. Our results reveal a mechanism by which a nuclear transcription factor translocates to mitochondria to initiate apoptosis.

Induction of apoptosis in ovarian carcinoma cells by AHPN/CD437 is mediated by retinoic acid receptors.

Retinoids have great promise in the area of cancer therapy and chemoprevention. These natural and synthetic derivatives of vitamin A have been shown to play an important role in regulating cell differentiation and proliferation. While all-trans-retinoic acid (ATRA) has been demonstrated to inhibit the growth of several ovarian tumor cell lines, other ovarian carcinoma cell lines have been found to be resistant to retinoid dependent growth suppression. Interestingly, a novel synthetic retinoid, CD437 or AHPN, has been demonstrated to inhibit the growth of both ATRA-sensitive (CA-OV3) and ATRA-resistant (SK-OV3) ovarian tumor cell lines as well as to induce apoptosis. The overall goal of this research was to understand the mechanism by which AHPN/CD437 induces apoptosis in ovarian tumor cell lines. Since a number of studies have demonstrated the importance of nuclear receptors (RARs and RXRs) in mediating cellular responses to retinoids, we wished to determine the role of RARs in mediating the AHPN/CD437 response. We modulated RAR level and function by overexpressing either wild type RAR-gamma or a pan dominant negative mutant of all RAR subtypes called RAR-beta (R269Q), or through the use of an RAR-gamma antagonist, MM11253. We found that inhibition of RAR function reduced but did not eliminate induction of apoptosis in both CA-OV3 and SK-OV3 cells by AHPN/CD437. Likewise, overexpression of wild type RAR-gamma was found to increase apoptosis after treatment with AHPN/CD437. Our results suggest that in ovarian carcinomas, AHPN/CD437 induced apoptosis is mediated at least in part via an RAR pathway.

sp2-bridged diaryl retinoids: effects of bridge-region substitution on retinoid X receptor (RXR) selectivity.

RXR class selectivity and RXR transcriptional activation activity compared to those for the retinoic acid receptor subtypes were enhanced on the 4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylethenyl)be nzoic acid scaffold and its 3-methyl analogue by replacing their 1,1-ethenyl bridge by a 1,1-(2-methylpropenyl) or cyclopropylidenylmethylene group.

4-[3-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)phenyl]benzoic acid and heterocyclic-bridged analogues are novel retinoic acid receptor subtype and retinoid X receptor alpha agonists.

Aromatic retinoids having a meta-substituted aromatic ring bridge, such as 4-[3-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)phenyl]benzo ic acid and its 3,5-diaryl-substituted 4,5-dihydroisoxazole analogue, function as retinoid receptor panagonists by activating both retinoic acid and retinoid X receptors to induce gene transcription, and thereby provide novel scaffolds for retinoid drug development. Both classes of these ligand-inducible transcription factors are involved in mediating the inhibitory effects of retinoids on cancer cell growth.

Identification of a unique binding protein specific for a novel retinoid inducing cellular apoptosis.

The retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN, CD437) induces apoptosis in a variety of cell types, many of which are cancer cells that resist the antiproliferative and/or differentiating effects of retinoids. While the retinoids exert their effects by binding to the retinoic acid nuclear receptors (RARs) or retinoid X receptors (RXRs), AHPN (CD437) binds to another protein with different ligand specificity. In nuclear extracts from HL-60R cells the binding of AHPN (CD437) was only minimally competed by either retinoic acid (tRA)or 9-cis-retinoic acid (9-cis-RA), the natural ligands for the RARs and RXRs, respectively. Moreover, AHPN (CD437) was unable to compete with either tRA or 9-cis-RA for binding to endogenous retinoid receptors in nuclear extracts from the MDA-MB-468 breast carcinoma cell line. Size exclusion chromatography revealed AHPN binding to a 95 kDa protein(s) which is neither an RAR or RXR. Our results suggest that apoptosis induction by AHPN (CD437) may occur through interaction with another protein and is independent of the RAR/RXR-signaling pathways.

Interaction of GRASP, a protein encoded by a novel retinoic acid-induced gene, with members of the cytohesin family of guanine nucleotide exchange factors.

A novel, retinoic acid-induced gene, GRP1-associated scaffold protein (GRASP), was isolated from P19 embryonal carcinoma cells using a subtractive screening strategy. GRASP was found to be highly expressed in brain and exhibited lower levels of expression in lung, heart, embryo, kidney, and ovary. The predicted amino acid sequence of GRASP is characterized by several putative protein-protein interaction motifs, suggesting that GRASP may be a component of a larger protein complex in the cell. Although GRASP does not harbor a predicted membrane spanning domain(s), the protein was observed to be associated with the plasma membrane of transiently transfected mammalian cells. Yeast two-hybrid screening revealed that GRASP interacted strongly with the General Receptor for Phosphoinositides 1 (GRP1), a brefeldin A-insensitive guanine nucleotide exchange factor for the ADP-ribosylation factor family of proteins. GRASP. GRP1 interactions were also demonstrated in vitro and in mammalian cells in which GRASP was shown to enhance GRP1 association with the plasma membrane. Furthermore, GRASP colocalized with endogenous ADP-ribosylation factors at the plasma membrane in transfected cells, suggesting that GRASP may modulate signaling by this family of small GTPases.

Identification of receptor-selective retinoids that are potent inhibitors of the growth of human head and neck squamous cell carcinoma cells.

Retinoids modulate the growth and differentiation of cancer cells presumably by activating gene transcription via the nuclear retinoic acid receptor (RAR) alpha, beta, and gamma and retinoid X receptor (RXR) alpha, beta, and gamma. We analyzed the effects of 38 RAR-selective and RXR-selective retinoids on the proliferation of 10 human head and neck squamous cell carcinoma (HNSCC) cell lines. All of these cell lines expressed constitutively all of the receptor subtypes except RARbeta, which was detected in only two of them. Most of the RAR-selective retinoids inhibited the growth of HNSCC cells to varying degrees, whereas the RXR-selective retinoids showed very weak or no inhibitory effects. Three RAR antagonists suppressed growth inhibition by RAR-selective agonists, as well as by RAR/RXR panagonists such as 9-cis-retinoic acid. Combinations of RXR-selective and RAR-selective retinoids exhibited additive growth-inhibitory effects. Furthermore, we found that CD437, the most potent growth-inhibitory retinoid induced apoptosis and up-regulated the expression of several apoptosis-related genes in HNSCC cells. These results indicate that: (a) retinoid receptors are involved in the growth-inhibitory effects of retinoids; (b) RXR-RAR heterodimers rather than RXR-RXR homodimer are the major mediators of growth inhibition by retinoids in HNSCC cells; and (c) induction of apoptosis can account for one mechanism by which retinoids such as CD437 inhibit the growth of HNSCC cells. Finally, these studies identified several synthetic retinoids, which are much more effective than the natural RAs and can be good candidates for chemoprevention and therapy of head and neck cancers.

S-phase arrest and apoptosis induced in normal mammary epithelial cells by a novel retinoid.

The addition of all-trans-retinoic acid has been found to mediate a G1 cell cycle phase arrest but not apoptosis in normal mammary epithelial cells. We have now found that addition of the novel retinoid 6-[3-(1-adamantyl)]-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437), which appears to function through a pathway independent of retinoic acid nuclear receptors, results in an S-phase arrest that is preceded by a 4-fold elevation in the levels of the cyclin-cyclin dependent kinase (cdk) inhibitor p21WAF1/CIP1. Failure to inhibit E2F-1 activation of genes through its phosphorylation by the cyclin cdk2 kinase has been shown to result in S-phase arrest and apoptosis in a number of cell types. Although exposure of the normal mammary cells to CD437 does not result in modulation of cyclin A or cdk2 levels, an increase in E2F-1 levels and a marked inhibition of cyclin A/cdk2 kinase activity are observed. Exposure to CD437 results in enhanced E2F-1 binding to its DNA consensus sequences and transcriptional activity during S phase. We hypothesize that this enhanced E2F-1 transcriptional activity results in S-phase arrest and subsequent apoptosis that has been observed in other systems.

Modulation of retinoic acid receptor function alters the growth inhibitory response of oral SCC cells to retinoids.

Retinoids have been shown to inhibit the growth of many human tumor cells including breast, ovarian and squamous cell carcinoma (SCC). While the exact mechanism of retinoid mediated growth suppression is not known, a role for the retinoic acid receptors (RARs) and retinoid X receptors (RXRs) has been established in both the breast and ovarian tumor cell models. We set out to determine if modulation of RAR/RXR function would alter the retinoid sensitivity of oral SCC cells. We found that the growth of SCC cells was significantly inhibited by treatment with either all-trans-retinoic acid (trans-RA) or the synthetic, conformationally restricted RARgamma selective retinoids MM11254 and MM11389. In order to demonstrate a role for RAR/RXR function in this process, stable oral SCC cell clones constitutively overexpressing the dominant negative mutant RARbeta2 (R269Q) were prepared and shown to exhibit reduced RAR/RXR transcriptional transactivation activity. We found that oral SCC cells exhibiting reduced RAR/RXR function became resistant to growth inhibition by all-trans-RA, MM11254 and MM11389. Likewise, treatment of oral SCC cells with the RARgamma antagonist MM11253 was found to block the ability of MM11254 and MM11389 to inhibit SCC cell growth. Thus, modulation of RAR function through the use of RAR-gamma selective agonists, an RAR-gamma selective antagonist or a pan-RAR dominant negative mutant significantly alters the growth inhibitory response of oral SCC cells to retinoids.

The importance of vitamin A in nutrition.

Preformed vitamin A (all-trans-retinol and its esters) and provitamin A (beta-carotene) are essential dietary nutrients that provide a source of retinol. Both retinyl esters and beta-carotene are metabolized to retinol. The retinol-binding proteins on binding retinol provide a means for solubilizing retinol for delivery to target tissues and for regulating retinol plasma concentrations. Oxidation of retinol provides retinal, which is essential for vision, and retinoic acid, a transcription factor ligand that has important roles in regulating genes involved in cell morphogenesis, differentiation, and proliferation. The observations that vitamin A can produce cell and tissue changes similar to those found during neoplastic transformation and that vitamin supplementation can reverse this process indicated a potential role for vitamin A in cancer prevention. Thus far, correlative epidemiological studies on vitamin A use and cancer prevention have produced mixed results, as this review indicates. Apparently, in populations deficient in vitamin A (caused by an inadequate diet or tobacco use), supplementation programs appear to be effective in reducing cancer incidence. In groups already having sufficient dietary or supplemental vitamin A, cancer prevention by added vitamin A may not be particularly effective. The most likely reason for the low efficacy in the latter groups is that feedback mechanisms that increase retinol storage in the liver limit retinol plasma levels; whereas, supplementation at higher doses causes toxicity. In addition to serving as a metabolic source of retinol, beta-carotene, along with other dietary carotenoids, function as antioxidants that can prevent carcinogenesis by decreasing the levels of the free-radicals that cause DNA damage.

Post-transcriptional regulation of the DNA damage-inducible gadd45 gene in human breast carcinoma cells exposed to a novel retinoid CD437.

The biologically active synthetic retinoid CD437 (6-[3-adamantyl-4-hydroxyphenyl]-2-naphthalene, AHPN) and different human breast carcinoma (HBC) cell lines were used to examine the possible mechanism(s) of gadd45 induction. Northern blot analysis of mRNA isolated from MCF-7, MDA-MB-468 and MDA-MB-231 HBC cell lines demonstrated a progressive increase in the 1.4 kb gadd45 transcript after exposure to 1 microM CD437. Western blot analysis showed increased gadd45 protein levels in MDA-MB-468 HBC cells following exposure to CD437. CD437 increased gadd45 mRNA levels by approximately 20-fold in MDA-MB-468 cells, however, the transcriptional activity was increased approximately 2-3-fold as demonstrated by the human gadd45 promoter-luciferase reporter construct and nuclear run-off assays. Sublines of MDA-MB-468 HBC cells expressing stably integrated GADD45 cDNA fragments were obtained and CD437-dependent induction of GADD45 analyzed. We report that approximately 300 nt located in the 5"-untranslated region (5"-UTR) of gadd45 mRNA are involved in the CD437-dependent 4-fold enhanced stability of gadd45 transcripts. MDA-MB-468 cells were stably transfected with either a plasmid having a CMV promoter-driven rabbit beta-globin gene or plasmids having a CMV promoter-driven chimeric gadd45 5"-UTR-rabbit beta-globin gene, where the entire gadd45 5"-UTR (from +1 to +298) or a 45 bp subfragment of the gadd45 5"-UTR (from +10 to +55) was positioned at the 5"-end of the rabbit beta-globin gene. CD437 was found to up-regulate expression of both the chimeric gadd45 -rabbit beta-globin transcripts, suggesting that cis element(s) involved in the CD437-dependent enhanced stability of gadd45 mRNA are contained in the 45 nt of the 5"-UTR of the gadd45 mRNA.

Synergistic inhibition of prostate cancer cell lines by a 19-nor hexafluoride vitamin D3 analogue and anti-activator protein 1 retinoid.

The secosteroid hormones, all-trans- and 9-cis-retinoic acid and vitamin D3, have demonstrated significant capacity to control proliferation in vitro of many solid tumour cell lines. Cooperative synergistic effects by these two ligands have been reported, and it is, therefore, possible that greater therapeutic effects could be achieved if these compounds were administered together. The role of retinoid-dependent anti-activator protein 1 (anti-AP-1) effects in controlling cancer cell proliferation appears significant. We have utilized an anti-AP-1 retinoid [2-(4,4-dimethyl-3,4-dihydro-2H-1 benzopyran-6-yl)carbonyl-2-(4-carboxyphenyl)-1,3,-dithiane; SR11238], which does not transactivate through a retinoic acid response element (RARE), and a potent vitamin D3 analogue [1alpha,25(OH)2-16-ene-23-yne-26,27-F6-19-nor-D3, code name LH] together at low, physiologically safer doses against a panel of prostate cancer cell lines that represent progressively more transformed phenotypes. The LNCaP (least transformed) and PC-3 (intermediately transformed) cell lines were synergistically inhibited in their clonal growth by the combination of LH and SR11238, whereas SR11238 alone was essentially inactive. DU-145 cells (most transformed) were completely insensitive to these analogues. LNCaP cells, but neither PC-3 nor DU-145, underwent apoptosis in the presence of LH and SR11238. Transactivation of the human osteocalcin vitamin D response element (VDRE) by LH was not enhanced in the presence of SR11238, although the expression of E-cadherin in these cells was additively up-regulated in the presence of both compounds. These data suggest the anti-AP-1 retinoid and the vitamin D3 analogue may naturally act synergistically to control cell proliferation, a process that is interrupted during transformation, and that this combination may form the basis for treatment of some androgen-independent prostate cancer.

Antioxidant activity of michellamine alkaloids.

The alkaloids michellamines A, B, and C are natural products isolated from a Central African tropical plant Ancistrocladus korupensis. We have investigated the radical scavenging ability of these compounds. The alkaloids inhibited the azo-induced oxidation of beta-phycoerythrin with IC50 values in the 0.5- to 0.8-microM range. Michellamine B also protected rat liver mitochondria against lipid peroxidation induced by adenosine diphosphate and Fe2+. The alkaloids were more potent antioxidants in these assays than several compounds being considered clinically as chemoprevention agents.

Kinetic and thermodynamic analysis of 9-cis-retinoic acid binding to retinoid X receptor alpha.

The interaction of retinoid X receptor alpha with 9-cis-retinoic acid was studied using stopped-flow fluorescence spectroscopy. Transient kinetic analyses of this interaction suggest a two-step binding mechanism involving a rapid, enthalpically driven pre-equilibrium followed by a slower, entropically driven reaction that may arise from a conformational change within the ligand binding domain of the receptor. The assignment of this kinetic mechanism was supported by agreement between the overall equilibrium constant, Kov, derived from kinetic studies with that determined by equilibrium fluorescence titrations. Although these analyses do not preclude ligand-induced alteration in the oligomerization state of the receptor in solution, the simplest model that can be applied to these data involves the stoichiometric interaction of 9-cis-retinoic acid with retinoid X receptor alpha monomers.

Michellamine alkaloids inhibit protein kinase C.

Michellamines A, B, and C have shown antiviral activity against HIV-1 and HIV-2 in cell culture. They act in a complex manner by at least two reported antiviral mechanisms, inhibition of HIV reverse transcriptase and inhibition of HIV-induced cellular fusion. On the basis of their structural similarity to other protein kinase C (PKC) inhibitors, we have investigated another possible mechanism-inhibition of PKC. The michellamines were found to inhibit rat brain PKC with IC50 values in the 15-35 microM range. Michellamine B was a noncompetitive PKC inhibitor with respect to ATP with a Ki value of 4-6 microM, whereas mixed-type inhibition was observed when the peptide concentration was varied. Michellamine B inhibited the kinase domain of PKC similarly. These results indicate that the michellamines bind to the PKC kinase domain and not its regulatory domain. Molecular modeling showed that all three michellamines can bind in the active site cleft of the PKC kinase domain, to block both the ATP and the peptide substrate subsites.