1,25-Dihydroxyvitamin D3 regulation of phorbol ester receptors in HL-60 leukemia cells

Calcitriol modulation of cardiac contractile performance via protein kinase C

Vitamin D(3) deficiency enhances cardiac contraction in experimental studies, yet paradoxically this deficiency is linked to congestive heart failure in humans. Activated vitamin D(3) (1alpha,25-dihydroxyvitamin D(3)) or calcitriol, decreases peak force and activates protein kinase C (PKC) in isolated perfused hearts. However, the direct influence of this hormone on adult cardiac myocyte contractile function is not well understood. Our aim is to investigate whether 1alpha,25-dihydroxyvitamin D(3) acutely modulates contractile function via PKC activation in adult rat cardiac myocytes. Sarcomere shortening and re-lengthening were measured in electrically stimulated myocytes isolated from adult rat hearts, and the vitamin D(3) response (10(-10) to 10(-7) M) was compared to shortening observed under basal conditions. Maximum changes in sarcomere shortening and relaxation were observed with 10(-9) M 1alpha,25-dihydroxyvitamin D(3). This dose decreased peak shortening, and accelerated contraction and relaxation rates within 5 min of administration, and changes in the Ca(2+) transient contributed to the peak shortening and relaxation effects. The PKC inhibitor, bis-indolylmaleimide (500 nM) largely blocked the acute influence of the most potent dose (10(-9) M) on contractile function. While peak shortening and shortening rate returned to baseline within 30 min, there was a sustained acceleration of relaxation that continued over 60 min. Phosphorylation of the Ca(2+) regulatory proteins, phospholamban, and cardiac troponin I correlated with the accelerated relaxation observed in response to acute application of 1alpha,25-dihydroxyvitamin D(3). Accelerated relaxation continued to be observed after chronic addition of 1alpha,25-dihydroxyvitamin D(3) (e.g. 2 days), yet this sustained increase in relaxation was not associated with increased phosphorylation of phospholamban or troponin I. These results provide evidence that 1alpha,25-dihydroxyvitamin D(3) directly modulates adult myocyte contractile function, and protein kinase C plays an important signaling role in the acute response. Phosphorylation of key Ca(2+) regulatory proteins by this kinase contributes to the enhanced relaxation observed in response to acute, but not chronic calcitriol.

Differential involvement of protein kinase C in human promyelocytic leukemia cell differentiation enhanced by artemisinin

Artemisinin, a sesquiterpene lactone endoperoxide that exists in several medicinal plants, is a well-known anti-malarial agent. In this report, we investigated the effect of artemisinin on cellular differentiation in the human promyelocytic leukemia HL-60 cell culture system. Artemisinin markedly increased the degree of HL-60 leukemia cell differentiation when simultaneously combined with low doses of 1 alpha,25-dihydoxyvitamin D(3) [1,25-(OH)(2)D(3)] or all-trans retinoic acid (all-trans RA). Artemisinin by itself had very weak effects on the differentiation of HL-60 cells. Cytofluorometric analysis and cell morphologic studies indicated that artemisinin potentiated 1,25-(OH)(2)D(3)-induced cell differentiation predominantly into monocytes and all-trans RA-induced cell differentiation into granulocytes, respectively. Extracellular-regulated kinase (ERK) inhibitors markedly inhibited HL-60 cell differentiation induced by artemisinin in combination with 1,25-(OH)(2)D(3) or all-trans RA, whereas phosphatidylinositol 3-kinase (PI3-K) inhibitors did not. Particularly, protein kinase C (PKC) inhibitors inhibited HL-60 cell differentiation induced by artemisinin in combination with 1,25-(OH)(2)D(3) but not with all-trans RA. Artemisinin enhanced PKC activity and protein level of PKC beta I isoform in only 1,25-(OH)(2)D(3)-treated HL-60 cells. Taken together, these results indicate that artemisinin strongly enhanced 1,25-(OH)(2)D(3)- and all-trans RA-induced cell differentiation in which PKC is differentially involved in arteminisin-mediated enhancement of leukemia cell differentiation.

Enhancement of 1 alpha,25-dihydroxyvitamin D(3)-induced differentiation of human leukaemia HL-60 cells into monocytes by parthenolide via inhibition of NF-kappa B activity

1. Transcription factors such as NF-kappa B provide powerful targets for drugs to use in the treatment of cancer. In this report parthenolide (PT), a sesquiterpene lactone of herbal remedies such as feverfew (Tanacetum parthenium) with NF-kappa B inhibitory activity, markedly increased the degree of human leukaemia HL-60 cell differentiation when simultaneously combined with 5 nM 1 alpha,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)). PT by itself did not induce HL-60 cell differentiation. 2. Cytofluorometric analysis indicated that PT stimulated 1,25-(OH)(2)D(3)-induced differentiation of HL-60 cells predominantly into monocytes. 3. Pretreatment of HL-60 cells with PT before the 1,25-(OH)(2)D(3) addition also potentiated the 1,25-(OH)(2)D(3)-induced HL-60 cell differentiation in both a dose- and a time-dependent manner, in which the enhanced levels of cell differentiation closely correlated with the inhibitory levels of NF-kappa B binding activity by PT. 4. In contrast, santonin, a sesquiterpene lactone without an inhibitory activity of NF-kappa B binding to the kappa B sites, did not enhance the 1,25-(OH)(2)D(3)-induced HL-60 cell differentiation. 5. In transfection experiments, PT enhanced 1,25-(OH)(2)D(3)-induced VDRE-dependent promoter activity. Furthermore, PT restored 1,25-(OH)(2)D(3)-induced VDRE-dependent promoter activity inhibited by TNF-alpha, an activator of NF-kappa B signalling pathway. 6. These results indicate that PT strongly potentiates the 1,25-(OH)(2)D(3)-induced HL-60 cell differentiation into monocytes via the inhibition of NF-kappa B activity and provide evidence that inhibition of NF-kappa B activation can be a pre-requisite to the efficient entry of promyelocytic leukaemia cells into a differentiation pathway.

Induction of human promyelocytic leukemia HL-60 cell differentiation into monocytes by silibinin: involvement of protein kinase C 1 1Abbreviations: 1,25-(OH)2D3, 1α,25-dihydroxyvitamin D3; ERK, extracellular signal-regulated kinase; mAb, monoclonal antibody; MAPK, mitogen-activated protein kinase; NBT, nitroblue tetrazolium; and PKC, protein kinase C

The effect of silibinin, an active component of Silybum marianum, on cellular differentiation was investigated in the human promyelocytic leukemia HL-60 cell culture system. Treatment of HL-60 cells with silibinin inhibited cellular proliferation and induced cellular differentiation in a dose-dependent manner. Cytofluorometric analysis and morphologic studies indicated that silibinin induced differentiation of HL-60 cells predominantly into monocytes. Importantly, strongly synergistic induction of differentiation into monocytes was observed when silibinin was combined with 5 nM 1alpha,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)], a well-known differentiation inducer of HL-60 cells into the monocytic lineage. Silibinin enhanced protein kinase C (PKC) activity and increased protein levels of both PKCalpha and PKCbeta in 1,25-(OH)(2)D(3)-treated HL-60 cells. PKC and extracellular signal-regulated kinase (ERK) inhibitors significantly inhibited HL-60 cell differentiation induced by silibinin alone or in combination with 1,25-(OH)(2)D(3), indicating that PKC and ERK may be involved in silibinin-induced HL-60 cell differentiation.

Capsaicin potentiates 1,25-dihydoxyvitamin D3- and all-trans retinoic acid-induced differentiation of human promyelocytic leukemia HL-60 cells

Human promyelocytic leukemia HL-60 cells are differentiated into monocytic or granulocytic lineage when treated with 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] or all-trans retinoic acid, respectively. In this study, the effect of capsaicin, an active component of the red pepper of the genus Capsocum, on cell differentiation was investigated in a HL-60 cell culture system. Treatment of HL-60 cells with 5-30 microg/ml capsaicin for 72 h inhibited cell proliferation and induced a small increase in cell differentiation. Interestingly, synergistic induction of HL-60 cell differentiation was observed when capsaicin was combined with either 5 nM 1,25-(OH)2D3 or 50 nM all-trans retinoic acid. Flow cytometric analysis indicated that combinations of 1,25-(OH)2D3 and capsaicin stimulated differentiation predominantly to monocytes whereas combinations of all-trans retinoic acid and capsaicin stimulated differentiation predominantly to granulocytes. Capsaicin enhanced protein kinase C activity in 1,25-(OH)2D3- and all-trans retinoic acid-treated HL-60 cells. In addition, inhibitors for protein kinase C [bisindolylmaleimide (GF-109203X), chelerythrine, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7)] and an inhibitor for extracellular signal-regulated kinase [2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD-098059)] significantly inhibited HL-60 cell differentiation induced by capsaicin in combination with either 1,25-(OH)2D3 or all-trans retinoic acid. These results indicate that capsaicin potentiates 1,25-(OH)2D3- or all-trans retinoic acid-induced HL-60 cell differentiation and that both protein kinase C and extracellular signal-regulated kinase are involved in the cell differentiation synergistically enhanced by capsaicin.

1,25-Dihydroxyvitamin D3-induced calcium efflux from calvaria is mediated by protein kinase C

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is an important regulator of bone metabolism involved in both formation and resorption. Traditionally it was assumed that vitamin D receptors are intracellular. Recent data indicate that vitamin D may also act through a membrane receptor, specifically raising intracellular calcium and inositol 1,4,5 triphosphate. The present study was undertaken to explore further the mechanism(s) of vitamin D-induced bone resorption in cultured bone. 1,25(OH)2D3 induced a dose-dependent increase of calcium efflux from cultured bone. This increase was completely obliterated by inhibition of protein kinase C (PKC) with either staurosporine or calphostin C. In cultured rat calvariae, 1,25(OH)2D3 also induced a dose-dependent translocation of PKC from cytosol to membrane. The activation of PKC by 1, 25(OH)2D3 occurred following a 30-s incubation, peaked at 1 minute, and disappeared by 5 minutes. 1,25(OH)2D3 did not increase cAMP production in similarly cultured calvaria. These results suggest that the action of 1,25(OH)2D3 on calcium flux from cultured bone is mediated, in part, via activation of PKC.

c-myc intron element-binding proteins are required for 1, 25-dihydroxyvitamin D3 regulation of c-myc during HL-60 cell differentiation and the involvement of HOXB4

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) suppresses c-myc expression during differentiation of HL-60 cells along the monocytic pathway by blocking transcriptional elongation at the first exon/intron border of the c-myc gene. In the present study, the physiological relevance of three putative regulatory protein binding sites found within a 280-base pair region in intron 1 of the c-myc gene was explored. HL-60 promyelocytic leukemia cells were transiently transfected with three different c-myc promoter constructs cloned upstream of a chloramphenicol acetyltransferase (CAT) reporter gene. With the wild-type c-myc promoter construct (pMPCAT), which contains MIE1, MIE2, and MIE3 binding sites, 1,25-(OH)2D3 was able to decrease CAT activity by 45.4 +/- 7.9% (mean +/- S.E., n = 8). The ability of 1, 25-(OH)2D3 to inhibit CAT activity was significantly decreased to 18. 5 +/- 4.3% (59.3% reversal, p < 0.02) when examined with a MIE1 deletion construct (pMPCAT-MIE1). Moreover, 1,25-(OH)2D3 was completely ineffective at suppressing CAT activity in cells transfected with pMPCAT-287, a construct without MIE1, MIE2, and MIE3 binding sites (-6.5 +/- 10.9%, p < 0.002). MIE1- and MIE2-binding proteins induced by 1,25-(OH)2D3 had similar gel shift mobilities, while MIE3-binding proteins migrated differently. Furthermore, chelerythrine chloride, a selective protein kinase C (PKC) inhibitor, and a PKCbeta antisense oligonucleotide completely blocked the binding of nuclear proteins induced by 1,25-(OH)2D3 to MIE1, MIE2, and MIE3. A 1,25-(OH)2D3-inducible MIE1-binding protein was identified to be HOXB4. HOXB4 levels were significantly increased in response to 1,25-(OH)2D3. Taken together, these results indicate that HOXB4 is one of the nuclear phosphoproteins involved in c-myc transcription elongation block during HL-60 cell differentiation by 1,25-(OH)2D3.

Antisense oligonucleotides targeted against protein kinase Cbeta and CbetaII block 1,25-(OH)2D3-induced differentiation

It is now recognized that protein kinase C (PKC) plays a critical role in 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) promotion of HL-60 cell differentiation. In this study, the effects of phosphorothioate antisense oligonucleotides directed against PKCalpha, PKCbeta, PKCbetaI, and PKCbetaII on HL-60 promyelocyte cell differentiation and proliferation were examined. Cellular differentiation was determined by nonspecific esterase activity, nitro blue tetrazolium reduction, and CD14 surface antigen expression. Differentiation promoted by 1,25-(OH)2D3 (20 nM for 48 h) was inhibited similarly in cells treated with PKCbeta antisense (30 microM) 24 h prior to or at the same time as hormone treatment (86 +/- 9% inhibition; n = 4 versus 82 +/- 8% inhibition; n = 4 (mean +/- S.E.), respectively). In contrast, cells treated with PKCbeta antisense 24 h after 1, 25-(OH)2D3 were unaffected and fully differentiated. PKCalpha antisense did not block 1,25-(OH)2D3 promotion of HL-60 cell differentiation. Next, the ability of PKCbetaI- and PKCbetaII-specific antisense oligonucleotides to block 1,25-(OH)2D3 promotion of cell differentiation was examined. PKCbetaII antisense (30 microM) completely blocked CD14 expression induced by 1, 25-(OH)2D3, whereas PKCbetaI antisense had little effect. Interestingly, PKCbetaII antisense blocked differentiation by 87 +/- 7% (n = 2, mean +/- S.D.) but had no effect on 1,25-(OH)2D3 inhibition of cellular proliferation. These results indicate that the effects of 1,25-(OH)2D3 on HL-60 cell differentiation and proliferation can be dissociated by blocking PKCbetaII expression.

Involvement of Src in the vitamin D signaling in human keratinocytes

1,25-dihydroxyvitamin D3 (VD) is a modulator of growth and differentiation of many cell types, including keratinocytes. We have recently shown in cultured keratinocytes that VD induces tyrosine phosphorylation of proteins involved in signal transduction, such as Shc. In an attempt to identify VD-responsive tyrosine kinases, we studied the effects of VD on the activity of the nonreceptor tyrosine kinase Src. Although VD did not stimulate Src activity in keratinocytes cultured in standard media containing 0.15 mM calcium, preincubation of the cells with 1.8 mM Ca2+ caused a rapid activation of Src in response to VD (10(-8)-10(-7) M). Elevation of calcium concentration alone caused an increase in Src activity as well, but the peak of Src activity was delayed (60 min vs. 15 min) and approximately 2-fold lower in comparison with VD-treated cells. VD treatment also induced tyrosine dephosphorylation of Src and a formation of an Src-Shc-Grb2 complex. Taken together, these findings imply that Src is involved in VD signaling in keratinocytes.

Promotion of HL-60 cell differentiation by 1,25-dihydroxyvitamin D3 regulation of protein kinase C levels and activity

The hormone 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] promotes differentiation of a number of cell types including HL-60 promyelocytic leukemia cells. It is now established that protein kinase Cbeta (PKCbeta) plays a critical role in HL-60 cell maturation to a monocyte/macrophage phenotype. In the present study, we investigated the importance of PKCbeta levels and activation in 1,25-(OH)2D3-mediated differentiation of HL-60 cells. Cell differentiation promoted by 1,25-(OH)2D3 at 48 hr was 39 +/- 3% (mean +/- SEM) nitroblue tetrazolium (NBT) positive and at 72 hr it was 35 +/- 2% NBT positive and 70% CD14 positive. Thus, promotion of cell differentiation by 20 nM 1,25-(OH)2D3 treatment was maximal at 48-72 hr. When PKCbeta levels and cell differentiation were assayed at 72 hr, treatment with 20 nM 1,25-(OH)2D3 for the initial 6 hr increased PKCbeta levels by 175% but had little effect on cell differentiation (7 +/- 2% NBT positive; 11% CD14 positive). The effect of ionomycin, a calcium ionophore, on PKCbeta levels and cell differentiation also was examined. Alone, 5 microM ionomycin promoted few cells (3% CD14 positive) to differentiate. In contrast, cells treated with 5 microM ionomycin for 66 hr after a 6-hr pretreatment with 20 nM 1,25-(OH)2D3 resulted in 34 +/- 5% NBT positive cells and 73% CD14 positive cells. Quantitatively, this induction of differentiation was identical to that observed in cultures continuously treated with 1,25-(OH)2D3 (35 +/- 2% NBT positive; 70% CD14 positive). Therefore, ionomycin seemed to replace the requirement for the continuous presence of 1,25-(OH)2D3. Chelerythrine chloride (3 microM), a specific PKC inhibitor, blocked differentiation promoted by 1,25-(OH)2D3 alone (82 +/- 2% inhibition) or in sequence with ionomycin (86 +/- 3% inhibition). Taken together, our data show that the capacity of 1,25-(OH)2D3 to both increase PKCbeta levels and activate PKC is utilized to promote HL-60 cell differentiation. These data further suggest that 1,25-(OH)2D3 has a genomic action to increase PKCbeta levels and also a nongenomic action requiring its continuous presence to promote HL-60 cell differentiation.

DIF-1, putative morphogen of D. discoideum, suppresses cell growth and promotes retinoic acid-induced cell differentiation in HL-60

Differentiation-inducing factor-1 (DIF-1) is a putative morphogen that induces stalk cell formation in the cellular slime mold Dictyostelium discoideum. In this study, we have examined the effects of DIF-1 on the human leukemia HL-60 cells. DIF-1 at 10-40 microM suppressed cell growth in a dose-dependent manner, and approximately 50% growth inhibition was attained with 15-20 microM DIF-1. FACS analysis of cell-cycle phase distribution using propidium iodide revealed that many cells were accumulated in the G1 phase after treatment with 15-20 microM DIF-1. These concentrations of DIF-1 also raised [Ca2+]i in a dose-dependent manner irrespective of the presence of extracellular Ca2+, indicating that DIF-1 elicited Ca2+-release from some intracellular Ca2+ store(s). Most importantly, relatively low concentrations of DIF-1 (1-5 microM) were found to promote retinoic acid-induced cell differentiation. The present results indicate that DIF-1 may be a useful tool for the analysis of myeloid cell differentiation and have therapeutic potential in the treatment of human myeloid leukemia.

1,25-dihydroxyvitamin D3 stimulates the assembly of adherens junctions in keratinocytes: involvement of protein kinase C

Signaling via intercellular junctions plays an important role in the regulation of growth and differentiation of epithelial cells. Loss of cell-cell contacts has been implicated in carcinogenesis, tumor progression, and metastasis. Here, we investigated whether 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] was able to stimulate the assembly of adherens junctions and/or desmosomes in cultured human keratinocytes. After 4-day incubation, 1,25-(OH)2D3 caused assembly of adherens junctions, but not desmosomes. The adherens junctions were identified upon known ultrastructural criteria and evidence of the translocation of specific junctional proteins (E-cadherin, P-cadherin, alpha-catenin, and vinculin) to the cell-cell borders. The presence of alpha-catenin and vinculin at cell-cell borders indicated that the adherens junctions were functional. This was further supported by showing that anti E-cadherin antibody inhibited the 1,25-(OH)2D3-induced keratinocyte stratification. A relation between protein kinase C and adherens junction regulation was noticed. 1,25-(OH)2D3-dependent formation of junctions was blocked by the inhibitors of protein kinase C, bisindolylmaleimide and 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H-7), and treatment of keratinocytes with 1,25-(OH)2D3 caused a rapid activation of protein kinase C and its translocation to the membranes. Formation of intercellular contacts may be an important mechanism of 1,25-(OH)2D3 action in hyperproliferative and neoplastic diseases.

Biphasic effect of 1,25-dihydroxyvitamin D3 on primary mouse epidermal keratinocyte proliferation

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] has been proposed as a physiologic regulator of keratinocyte growth and differentiation. Utilizing a proliferative serum-free culture system, we have found that a physiologic (picomolar) concentrations this hormone stimulated proliferation of primary mouse epidermal keratinocytes; at higher (nanomolar to micromolar) doses, growth was inhibited by 1,25(OH)2D3. We investigated the nature of the signal transduction mechanism underlying the response to 1,25(OH)2D3 and observed little or no effect of either low or high concentrations of the hormone on cytosolic calcium levels or Fos expression. Furthermore, the protein kinase C inhibitor, Ro 31-7549, had very little effect on the growth inhibition induced by a high dose (1 microM) of 1,25(OH)2D3. This lack of rapid signal transduction events was consistent with the inability of a short (4-hour) exposure to 1,25(OH)2D3 to initiate a complete growth-inhibitory response as measured using [3H]thymidine incorporation. Our results indicate that physiologic concentrations of 1,25(OH)2D3 are required for optimal keratinocyte growth. Furthermore, we found no evidence of rapid effects of 1,25(OH)2D3 and suggest that in mouse epidermal keratinocytes, the response to this hormone is mediated by a slow transduction pathway, such as that activated by the intracellular 1,25(OH)2D3 receptor (VDR).

Increased PKC activity in cultured human keratinocytes and fibroblasts after treatment with 1 alpha, 25-dihydroxyvitamin D3

1 alpha,25-Dihydroxyvitamin D3 (10(-12) M to 10(-8) M) caused a dose dependent increase in PKC activity in the solubilized membrane fractions of cultured human keratinocytes and in the cytosolic fractions of cultured human fibroblasts. Maximum activity was induced by 1 alpha,25-dihydroxyvitamin D3 at 24 h. Sphingosine, which is believed to inhibit PKC mediated biological responses, blunted 1 alpha,25(OH)2D3's inducement of PKC activity in both keratinocytes and fibroblasts. Identical hormone treatment of vitamin D receptor deficient fibroblasts did not increase PKC activity. Treatment of keratinocytes and fibroblasts with 1 beta,25-dihydroxyvitamin D3, which is believed to be ineffective in inducing genomic responses, did not induce PKC activity.

Regulation of protein kinase C and role in cancer biology

Protein kinase C (PKC) is a family of closely related lipid-dependent and diacyglycerol-activated isoenzymes known to play an important role in the signal transduction pathways involved in hormone release, mitogenesis and tumor promotion. Reversible activation of PKC by the second messengers diacylglycerol and calcium is an established model for the short term regulation of PKC in the immediate events of signal transduction. PKC can also be modulated long term by changes in the levels of activators or inhibitors for a prolonged period or by changes in the levels of functional PKC isoenzymes in the cell during development or in response to hormones and/or differentiation factors. Indeed, studies have indicated that the sustained activation or inhibition of PKC activity in vivo may play a critical role in regulation of long term cellular events such as proliferation, differentiation and tumorigenesis. In addition, these regulatory events are important in colon cancer, where a decrease in PKC activators and activity suggests PKC acts as an anti-oncogene, in breast cancer, where an increase in PKC activity suggests an oncogenic role for PKC, and in multidrug resistance (MDR) and metastasis where an increase in PKC activity correlates with increased resistance and metastatic potential. These studies highlight the importance and significance of regulation of PKC activity in vivo.

Protein kinase C is involved in 24-hydroxylase gene expression induced by 1,25(OH)2D3 in rat intestinal epithelial cells

Effects of protein kinase C (PKC) inhibitor and activator on 1,25(OH)2D3-induced gene expression were examined in rat intestinal epithelial cells, IEC-6 cells. A potent PKC inhibitor, H-7 (20 microM), completely abated 1,25(OH)2D3-induced 24-hydroxylase gene expression at 3 and 6 h. The effect of H-7 was dose dependent with IC50 around 5 microM. Other protein kinase inhibitors, HA-1004 and H-89 (20 microM), had no effects. Furthermore, the activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) potentiated the effect of 1,25(OH)2D3 by 1 h. TPA appeared to exert its effect at a transcriptional step, since mRNA stability was not affected by TPA treatment. At 3 h after the treatment of the cells with H-7 and TPA, vitamin D receptor (VDR) contents estimated by 3H-1,25(OH)2D3 binding capacity were 72.4 and 63.2% of vehicle-treated cells without significant changes of binding affinities, suggesting that the effect of H-7 and TPA was not the result of changes in VDR content or its binding affinity. In conclusion, PKC is involved in 1,25(OH)2D3-induced 24-hydroxylase gene expression in IEC-6 cells between 1,25(OH)2D3-VDR binding and VDR-induced gene transactivation.

Interactions between retinoic acid and protein kinase C in induction of melanoma differentiation

Retinoic acid treatment of B16 mouse melanoma cells induces a differentiated phenotype. This is accompanied by a decrease in monolayer growth rate, loss of the ability to form colonies in soft agarose, increased production of melanin and other melanocyte-specific markers. In addition, retinoic acid treatment of these cells decreases their tumorigenicity when injected subcutaneously into mice. Our laboratory has found that an early biochemical change after the addition of retinoic acid is a large increase in PKC. PKC is an enzyme whose activity is activated by diacylglycerol and calcium and has been shown to be an important mediator of substances that stimulate growth or differentiation. Since PKC is a multi-gene family, it was important for us to determine which isotype(s) was expressed in B16 cells and which type was induced by retinoic acid. We found that only PKC-alpha is expressed in these cells, and this is the form that is induced by retinoic acid. The retinoic acid-induced increased in PKC-alpha is found at both the RNA and protein level. The mechanism of induction is not yet clear since there is only a small increase in the transcription rate and no change in the stability of the mRNA for PKC-alpha in treated cells. In addition, the induction of PKC by retinoic acid can be blocked by inhibitors of protein synthesis, suggesting that the induction requires the synthesis of new protein(s). In order to determine the role of increased PKC-alpha in the retinoic acid-induced differentiation, we transfected full-length PKC-alpha cDNA in mammalian expression vectors into B16 cells. Two clones that stably overexpressed PKC-alpha to different levels were isolated. The phenotype of these clones resembled WT cells treated with retinoic acid, i.e. they had longer doubling times, decreased ability to form colonies in soft agar, increased melanin production, and decreased tumorigenicity in mice. Recent data suggest a role for the RAR-beta in mediating the effect of retinoic acid on PKC induction. B16 cells express a very low amount of RAR-beta mRNA. The level is increased drastically by retinoic acid treatment without any requirement for protein synthesis. When B16 cells were transfected with and overexpressed RAR-beta, they also expressed more PKC-alpha mRNA and protein, and the induction of PKC by retinoic acid was not blocked by protein synthesis inhibitors. In summary, these finding suggest a key role for PKC-alpha in the pathway by which retinoic acid induces B16 mouse melanoma differentiation.

1,25-Dihydroxyvitamin D3 and macrophage colony-stimulating factor-1 synergistically phosphorylate talin

Macrophage colony stimulating factor (CSF-1) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are potent inducers of macrophage differentiation. Both appear to modulate protein phosphorylation, at least in part, through protein kinase C (PKC) raising the question as to whether they concurrently impact on macrophage-like cells. In this regard, we utilized the CSF-1 dependent murine macrophage-like line BAC 1.25F5. CSF-1 treatment of these cells for 30 min leads to particular phosphorylation of a 165 kDa protein, the putative CSF-1 receptor, and a 210 kDa moiety. 1,25(OH)2D3 exposure for 24 h prior to addition of CSF-1 enhances phosphorylation of the 165 kDa species and, especially, the 210 kDa protein. Phosphorylation of the latter protein is 1,25(OH)2D3 dose- and time-dependent and the molecule is specifically immunoprecipitated with a rabbit polyclonal anti-talin antibody. Experiments with okadaic acid show that the enhanced phosphorylation of talin does not result from serine phosphatase inhibition. CSF-1 and 1,25(OH)2D3, alone or in combination, do not increase talin protein expression. The tyrosine kinase inhibitor, genestein, blocks 1,25(OH)2D3/CSF-1 induced phosphorylation of the putative CSF-1 receptor but has no effect on talin phosphorylation which occurs exclusively on serine. In contrast to genestein, staurosporin, an inhibitor of PKC, inhibits phosphorylation of talin. Moreover, exposure of 1,25(OH)2D3 pretreated cells to phorbol 12-myristate 13-acetate (PMA) in place of CSF-1 also prompts talin phosphorylation. Finally, 1,25(OH)2D3 enhances 3[H]PDBu binding, indicating that the steroid increases PMA receptor capacity. Thus, CSF-1 and 1,25(OH)2D3 act synergistically via PKC to phosphorylate talin, a cytoskeletal-associated protein.

Phorbol ester markedly increases the sensitivity of intestinal epithelial cells to 1,25-dihydroxyvitamin D3

We have used a clonal intestinal epithelial cell line (IEC-18) to study the mechanism of action of 1,25-dihydroxyvitamin D3 (1,25(OH)2D) in vitro. 1,25(OH)2D(10(-7) M) elevated by over 10-fold the mRNA levels for the cytochrome P450 component (P450cc24) of the 1,25(OH)2D-24-hydroxylase. Increased P450cc24 mRNA levels were detectable at 6 h and peaked at 36 h. Below a concentration of 10(-7) M, 1,25(OH)2D had almost no effect. However, addition of phorbol ester for 2 h made the intestine responsive to 1,25(OH)2D concentrations as low as 10(-9) M.

Evidence for the participation of protein kinase C and 3',5'-cyclic AMP-dependent protein kinase in the stimulation of muscle cell proliferation by 1,25-dihydroxy-vitamin D3

Treatment with 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) (1-12 h, 10(-10) M) stimulates DNA synthesis in proliferating myoblasts, with an early response at 2-4 h of treatment followed by a maximal effect at 10 h. To investigate the mechanism involved in the mitogenic action of the hormone we studied the possible activation of intracellular messengers by 1,25(OH)2D3. The initial phase of stimulation of [3H]thymidine incorporation into DNA by the sterol was mimicked by the protein kinase C activator tetradecanoylphorbol acetate (TPA) in a manner which was dose dependent and specific as the inactive analog 4 alpha-phorbol was without effect. Maximal responses to TPA (100 nM) were obtained at 4 h. Staurosporine, a protein kinase C inhibitor, blocked the effect of 1,25(OH)2D3 on myoblast proliferation at 4 h. In addition, a fast (1-5 min) elevation of diacylglycerol levels and membrane-associated protein kinase C activity was observed in response to 1,25(OH)2D3. The adenylate cyclase activator forskolin (20 microM) and dibutyryl-cAMP (50 microM) increased DNA synthesis reproducing the second 1,25(OH)2D3-dependent stimulatory phase at 10 h. Inhibitors of protein kinase A blocked the increase in muscle cell DNA synthesis induced by 1,25(OH)2D3 at 10 h. Significant increases in cyclic AMP levels were detected in myoblasts treated with the sterol for 1-10 h. The calcium channel antagonist nifedipine (5-10 microM) abolished both the effects of 4-h treatment with 1,25(OH)2D3 or TPA and 10-h treatment with 1,25(OH)2D3 or dibutyryl-cAMP. Similar to the calcium channel agonist Bay K8644, 1,25(OH)2D3 stimulated myoblast 45Ca uptake and its effects were blocked by nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

1,25-Dihydroxycholecalciferol modulates 3H-thymidine incorporation in FRTL5 cells

1,25-dihydroxycholecalciferol (1,25(OH)2D3) possesses proliferation and differentiation modulating effects in many cell types in vitro. We studied the effect of 1,25(OH)2D3 on 3H-thymidine incorporation in FRTL5 cells, a cultured rat thyroid follicular cell line. 1,25(OH)2D3 alone at 10(-11) and 10(-9) M exerted no effect on 3H-thymidine incorporation. However, at 10(-7) M, 1,25(OH)2D3 slightly enhanced 3H-thymidine incorporation. In the presence of 5% calf serum, 1,25(OH)2D3 increased 3H-thymidine incorporation induced by calf serum in a dose-dependent manner. 1,25(OH)2D3 also enhanced 3H-thymidine incorporation induced by PMA, an extrinsic stimulator of protein kinase C, without directly affecting PMA-induced protein kinase C translocation. In contrast to the stimulatory effects of 1,25(OH)2D3 on the calf serum and PMA-induced 3H-thymidine incorporation, 1,25(OH)2D3 inhibited the increase in 3H-thymidine incorporation induced by TSH in a dose-dependent manner. This effect of 1,25(OH)2D3 on TSH-induced 3H-thymidine incorporation may be, in part, due to post-cAMP pathways since 1,25(OH)2D3 also inhibited the increase in 3H-thymidine incorporation induced by Bu2cAMP without affecting the TSH-induced increase in cAMP. The stimulatory effect of insulin on 3H-thymidine incorporation, a cAMP-independent process, was also inhibited by 1,25(OH)2D3. We conclude that 1,25(OH)2D3 affects 3H-thymidine incorporation in FRTL5 cells raising the possibility of a physiologic role for 1,25(OH)2D3 in the growth and function of thyroid follicular cells.

Differential expression of PKC isoforms and PC12 cell differentiation

Recent reports indicate that the protein kinase inhibitor H7 is capable of inducing both morphological and functional differentiation of a number of neural cell types. This investigation demonstrates that H7 potentiates the neurogenic properties of nerve growth factor (NGF) in PC12 cells with a concomitant change in the accumulation of the beta II-protein kinase C (beta IIPKC) isoform protein without changes in either alpha or gamma. However, NGF alone stimulates a coordinate increase in all three isoforms. The assay of acetylcholine esterase as a functional marker of neuronal differentiation demonstrates that H7 alone is not capable of stimulating morphological or functional differentiation in PC12 cells. H7 synergizes with NGF through a PKC-dependent pathway and by differential expression of PKC subtypes. The expression of the PKC transcripts for alpha, beta II, and gamma all undergo simultaneous yet differential changes in their patterns of expression during treatment with H7 and/or NGF. These data suggest that isoform switching is regulated primarily at the protein level. Last, these findings suggest that expression of PKC isoforms is tightly coupled with neuronal differentiation and may play a role in the maintenance of the differentiated state.

1,25(OH)2D3 increases membrane associated protein kinase C in MDBK cells

To determine whether 1,25-dihydroxycholecalciferol [1,25(OH)2D3] affects protein kinase C (PKC) activity in kidney, as has been demonstrated in HL-60 cells we measured 1,25(OH)2D3 binding, PKC activity and PKC immunoreactivity in Madin Darby bovine kidney (MDBK) cells, a normal renal epithelial cell line derived from bovine kidney. Our data demonstrate that MDBK cells exhibit specific high affinity binding for 1,25(OH)2D3, indicating the presence of the vitamin D receptor (VDR). Treatment of MDBK cells with 1,25(OH)2D3 for 24 h increased membrane PKC activity and immunoreactivity. The effect of 1,25(OH)2D3 was dose-dependent, with a peak effect observed at 10(-7)M 1,25(OH)2D3. The 1,25(OH)2D3 induced increase in membrane PKC was paralleled by a comparable decrease in cytosolic PKC activity and amount. Although time course studies were consistent with a VDR mediated effect of 1,25(OH)2D3 on PKC protein synthesis, total PKC activity was not increased by 1,25(OH)2D3, suggesting an effect on PKC translocation or localization. These results suggest that 1,25(OH)2D3 modulates PKC mediated events in kidney, a classic target for this steroid hormone.

In vivo effect of 1,25-dihydroxyvitamin D3 on phagocyte function in hemodialysis patients

1,25-dihydroxyvitamin D3 [1,25(OH)2D3] has been shown to modulate the immune function of monocytes and macrophages. Patients with end-stage renal disease (ESRD) on chronic hemodialysis treatment usually present a deficiency of this active form of vitamin D3. The aim of this study was to investigate the effect of 1,25(OH)2D3 replacement therapy on phagocytosis, bactericidal capacity, and oxidative metabolism of peripheral blood polymorphonuclear leukocytes (PMNL) and monocytes (MN) in chronic hemodialysis patients. Phagocyte function tests were performed before and after four weeks of an oral replacement therapy with 0.5 micrograms/day of 1,25(OH)2D3 (Rocaltrol). The superoxide (O2-) generation of monocytes, measured by cytochrome c reduction and lucigenin-enhanced chemiluminescence (CL) from patients receiving hemodialysis treatment was significantly diminished compared to healthy controls. After the replacement therapy with 1,25(OH)2D3 the O2- production showed a significant improvement, resulting in an increased cytochrome c reduction and lucigenin-CL response. The bactericidal capacity of MN was also impaired and exhibited a significant enhancement of their killing activity after the administration of 1,25(OH)2D3. On the other hand, the luminol-enhanced CL, which reflects the myeloperoxidase-dependent oxidative metabolism, and the phagocytic ability of MN was not affected by the hormone. The function of polymorphonuclear leukocytes (PMNL) from hemodialysis patients showed no impairment in the state of 1,25(OH)2D3 deficiency and the replacement of the hormone did not enhance their function. These results suggest that the deficiency of 1,25(OH)2D3 in patients with ESRD on chronic hemodialysis treatment may be responsible for an impaired monocyte function, which could be improved by an in vivo replacement of the hormone.

Role of putrescine in interleukin 1 beta production in human histiocytic lymphoma cell line U937

Treatment with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and incubation with lipopolysaccharide (LPS) induces interleukin 1 beta (IL-1 beta) production in the histiocytic lymphoma cell line U937. Here we investigated the effect of treatment with both TPA and 1 alpha, 25-dihydroxyvitamin D3 (1,25(OH)2D3) on LPS-induced IL-1 beta production in U937 cells. To clarify the mechanism of IL-1 beta production, the possible role of polyamines in this process was examined. Combined treatment with TPA and 1,25(OH)2D3 for 72 h followed by incubation with LPS for 24 h caused synergistic induction of both IL-1 beta release and mRNA expression. On the other hand, TPA increased the numbers of vitamin D3 receptors, which may be one mechanism of this synergistic induction. Ornithine decarboxylase (ODC), a rate-limiting enzyme for polyamine biosynthesis, was also induced by these compounds biphasically: the first peak of ODC activity was observed at 4 h of the incubation with the two compounds and the second peak was at 4 h after the addition of LPS. To find whether these peaks were related to IL-1 beta production, DL-alpha-difluoromethylornithine (DFMO), a specific irreversible inhibitor of ODC, was added together with TPA and 1,25(OH)2D3. DFMO decreased the cellular levels of putrescine and spermidine and suppressed IL-1 beta release and IL-1 beta mRNA expression by 65%. Exogenous putrescine, but not spermidine, abrogated these kinds of inhibition. Similar results were obtained with DFMO and the polyamines during the differentiation of the cells up to the monocyte or macrophage stage. These results thus suggest that changes in either of these intracellular polyamines, especially putrescine, help to regulate the differentiation of U937 cells, resulting in partial control of the regulation of IL-1 beta production.

Calcitriol-mediated modulation of urokinase-type plasminogen activator and plasminogen activator inhibitor-2

Calcitriol-induced differentiation of U937 mononuclear phagocytes is known to have divergent effects on the synthesis of urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitor-2 (PAI-2). In this study, we sought to determine whether calcitriol affects the expression of these proteins by modulating intermediate signal transduction involving intracellular calcium and protein kinase C (PKC). U937 cells were stimulated with calcitriol (50 nM) for 6-72 hr, inducing a transient increase in specific binding of [3H]phorbol dibutyrate ([3H]PDBu), seen only after 24 hr. Staurosporine (2 nM), a PKC inhibitor, had no effect on calcitriol-induced secretion of plasminogen activator (PA) activity. However, staurosporine significantly (P less than 0.05) inhibited the ability of calcitriol to enhance phorbol myristate acetate (PMA)-induced secretion of PA inhibitor activity, indicating that this priming effect of calcitriol requires expression of PKC. The calcium ionophore A23187 (0.1 microM) induced a modest increase in secreted PA inhibitor activity, in contrast to the secretion of PA activity which is consistently seen in response to calcitriol. Northern blot analysis demonstrated that A23187 induced an increase in PAI-2 mRNA and a marked reduction in uPA mRNA, while calcitriol induced opposite changes in both mRNA species. We conclude that calcitriol modulates uPA and PAI-2 expression by multiple mechanisms that are both PKC dependent and PKC independent. Our studies also demonstrated that increased intracellular calcium alters the synthesis of both uPA and PAI-2 in a manner which favors expression of PA inhibitor activity.

Dephosphorylation of specific proteins in HL-60 cells by 1 alpha, 25-dihydroxyvitamin D3: possible involvement of cAMP-dependent protein kinase

Changes of phosphoprotein patterns in HL-60 cells were studied during short exposures to 1 alpha, 25-dihydroxyvitamin D3 [1,25(OH)2D3]. One hundred nanometers 1,25(OH)2D3 dephosphorylated at least three proteins in 6 h: phosphoproteins with molecular weights of 82 kD (pp82), 33 kD (pp33), and 31 kD (pp31). Phosphorylation of pp33 and pp31 was also suppressed by 1 mM dbcAMP, and dephosphorylation of the two protein by 1,25(OH)2D3 was inhibited by 8 microM H-8, an inhibitor of cAMP-dependent protein kinase (PKA). Furthermore, 8 microM H-8 inhibited dephosphorylation of the two proteins when it was added with 1,25(OH)2D3. On the other hand, 10 nM TPA gave no significant change to these two phosphoproteins. These results suggest the possibility that PKA is involved in the early stages of 1,25(OH)2D3-induced HL-60 cell differentiation through specific protein dephosphorylation.

Oxysterol activation of arachidonic acid release and prostaglandin E2 biosynthesis in NRK 49F cells is partially dependent on protein kinase C activity [corrected]

We previously demonstrated that the oxysterol potentiation of arachidonic acid release and prostaglandin biosynthesis induced by foetal calf serum activation of normal rat kidney (NRK) cells (fibroblastic clone 49F) was not related to a direct effect of oxysterols on cell free Ca2+ level. Since both Ca2+ variations and protein kinase C are involved in arachidonic acid release in some models, we looked for a possible modulation by protein kinase C in the oxysterol effect on arachidonic acid release. We show that when the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA), a protein kinase C activator, was added to the culture medium, the oxysterol effect on arachidonic acid release and prostaglandin synthesis clearly increased. Moreover, the effect of TPA was dose-dependent and TPA EC50 (4 x 10(-9) M) was unchanged in the presence of the oxysterol. Preincubation of cells with TPA for 24 h prevented the arachidonic acid release induced by TPA alone, whereas the oxysterol effect was decreased but not abolished. In the absence of serum, TPA and ionomycin added together induced the same noticeable (arachidonic acid) release and PGE2 synthesis as serum alone. Nevertheless, the potentiating effect of cholest-5-ene-3 beta, 25-diol was much higher when serum itself was used to activate NRK cells than it was in the present serum-mimicking experimental conditions. Thus, the presence of growth factors is probably required to obtain a full oxysterol effect. We conclude that the oxysterol effect was synergistic with, but not fully dependent on, protein kinase C and Ca2+ ion fluxes, therefore oxysterols could affect earlier events triggered by serum growth factor binding to their cell membrane receptors.

Induction of differentiation of HL-60 cells by protein kinase C inhibitor, K252a

To clarify the role of protein kinase C and protein kinase A in cell proliferation and differentiation, the effects of K252a and its derivatives (K252b, KT5720), which have different inhibitory activity to these protein kinases, on the proliferation and differentiation of HL-60 cells were investigated. The proliferation and DNA synthesis of the HL-60 cells were inhibited by K252a in a dose dependent manner. However, K252b and KT5720 which are more specific inhibitors of protein kinase C or protein kinase A, respectively, had no observable effect on cell proliferation. K252a (40nM) enhanced the differentiation of HL-60 cells induced by 1,25(OH)2D3, retinoic acid and DMSO. K252b and KT5720 did not affect 1,25(OH)2D3-induced differentiation. K252a significantly inhibited the differentiation induced by PMA. These results demonstrate that K252a but not its derivatives can function as an antitumor drug and enhancer of the differentiation induced by various inducers.

Calcium is essential in the fusion of mouse alveolar macrophages induced by 1 alpha,25-dihydroxyvitamin D3

We have reported that the active form of vitamin D3, 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3], directly induces activation and fusion of mouse alveolar macrophages (Abe et al., 1983, 1984). The activated state appeared to be a prerequisite to the fusion of macrophages. Macrophages began to fuse 36 hr after adding 1 alpha,25(OH)2D3; the fusion rate attained a maximum of 70-80% at 72 hr. During the course of further investigating the mechanisms of fusion induced by the vitamin, we found that the calcium ion is closely involved in the fusion process of macrophages induced by 1 alpha,25(OH)2D3. When alveolar macrophages were cultured with 1 alpha,25(OH)2D3 in medium with graded concentrations (0.13-1.85 mM) of calcium, the fusion rate went down in parallel with the decrease of medium calcium. Neither calcium ionophore A23187 nor 12-O-tetradecanoylphorbol-13-acetate (TPA) induced fusion of freshly isolated macrophages, but the two compounds greatly promoted fusion of the macrophages pretreated for 18 hr with 1 alpha,25(OH)2D3. The vitamin effect for the first 18 hr was similar, irrespective of the medium calcium concentration. In contrast, millimolar amounts of calcium were essential in the subsequent period of incubation(18-72 hr) for inducing fusion. The activation of macrophages measured by the induction of cytotoxicity and the enhancement of glucose consumption by 1 alpha,25(OH)2D3 occurred similarly, irrespective of the medium calcium concentration. These results clearly indicate that the fusion process of alveolar macrophages induced by 1 alpha,25(OH)2D3 can be divided into two phases: 1) the calcium-independent priming phase (0-18 hr) and 2) the calcium-dependent progression phase (18-72 hr). 1 alpha,25(OH)2D3 is necessary only in the priming phase; A23187 and TPA can be substituted for 1 alpha,25(OH)2D3 in the progression phase.

Effects of protein kinase inhibitors 1(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) and N-[2-guanidinoethyl]-5-isoquinolinesulfonamide hydrochloride (HA1004) on calcitriol-induced differentiation of HL-60 cells

HL-60 promyelocytic leukemia cells were induced to differentiate by 1,25-dihydroxyvitamin D3 (calcitriol) into mature monocytes. Differentiation was assessed by nitro blue tetrazolium dye reduction, nonspecific esterase activity, and DNA synthesis. Terminal differentiation of cultures induced by calcitriol (10 nM) was inhibited by 80% when cells were treated simultaneously with protein kinase inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) (32 microM) and N-[2-guanidinoethyl]-5-isoquinolinesulfonamide hydrochloride (HA1004) (320 microM). The IC50 for inhibition of calcitriol-induced differentiation was approximately 15 microM for H-7 and 170 microM for HA1004. The IC50 values for H-7 and HA1004 antagonism of calcitriol-induced differentiation are quantitatively and relatively correlated to their known action to inhibit protein kinase C activity. Treatment of cells with concentrations of 0-32 microM H-7 or 0-320 microM HA1004 alone did not affect cell growth, differentiation, or trypan blue exclusion. However, higher concentrations of H7 (greater than 32 microM) and HA1004 (greater than 320 microM) were found to be cytotoxic. The data presented suggest that calcitriol-induced differentiation is antagonized by inhibitors of protein kinase and are consistent with the hypothesis that kinase C activity is required for HL-60 cell differentiation.

Effect of prostaglandin E2 on gamma-interferon and 1,25(OH)2D3 vitamin D3-induced c-myc reduction during HL-60 cell differentiation

The effect of prostaglandin E2 (PGE2) on the reduction of c-myc expression during the differentiation of the human leukemic cell line, HL-60, was examined. PGE2, a potent inducer of intracellular cyclic AMP (cAMP) in HL-60 cells, augmented monocyte-associated cell surface antigens induced by human gamma-interferon (IFN-gamma) or 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) in these cells. The elevation of intracellular cAMP was induced dose-dependently by PGE2, but not by IFN-gamma or 1,25(OH)2D3. Changes were also seen in functional differentiation, such as, the increase of phagocytic capability and superoxide generation. PGE2 also enhanced the reduction of c-myc expression and the down-regulation of transferrin receptor by IFN-gamma or 1,25(OH)2D3, whereas PGE2 alone did not induce these phenotypic changes. These data suggest that IFN-gamma and 1,25(OH)2D3 reduce c-myc expression of HL-60 cells by a mechanism other than the augmentation of intracellular cAMP.