Differential expression of cell adhesion molecules in variants of K1735 melanoma cells differing in metastatic capacity

We have investigated the expression of 2 neural-cell adhesion molecules, NCAM and LI, in K1735-C116 and -MI melanoma cells which differ qualitatively in their metastatic potential, i.e., MI cells are metastatic whereas C116 cells are not. We have found that NCAM in C116 cells are expressed as 2 quantitatively major glycosylated polypeptides with Mr of 145,000 and 120,000 and a minor 190,000 Mr polypeptide, whereas MI cells expressed NCAM as 3 glycosylated polypeptides with MR of 200,000, 140,000 and 120,000. The amount of NCAM in MI cells constituted only 60% of the amount observed in C116 cells. In C116 cells, the 145,000 and 120,000 Mr NCAM polypeptides were sulphated whereas NCAM did not appear to be sulphated in MI cells. No phosphorylation of NCAM in the 2 cell lines was observed. LI was expressed as a phosphorylated glycoprotein with Mr of 210,000 in MI cells whereas no LI expression was observed in C116 cells. LI was not sulphated in MI cells.     

In vivo therapy of malignant melanoma by means of antagonists of alphav integrins

Integrin alphavbeta3 (vitronectin receptor) has been implicated in human malignant melanoma progression and angiogenesis as a receptor that provides survival signals. However, little is known about the therapeutic potential of antagonists of alphavbeta3. In this report, we characterize the activities of 2 antagonists of alphavbeta3 integrins: a human specific monoclonal antibody (MAb), 17E6, and a cyclic RGD peptide that blocked cell adhesion and induced detachment of previously substrate-attached cells in vitro. In vivo, alphavbeta3 antagonists behaved as anti-tumor drugs in a dose- and time-dependent manner. Moreover, different therapeutic treatments proved to be effective even in the therapy of established macroscopic tumor masses, thus supporting the use of these antagonists in clinical therapy. Using a panel of 6 human melanomas and 5 carcinomas, MAb 17E6 efficiently blocked the in vivo tumor growth of melanomas expressing alphavbeta3 as xenografts but did not affect the alphavbeta3-negative (although alphav integrin-positive) tumors. This demonstrated that alphavbeta3 is a pivotal integrin for the growth of human melanomas. Furthermore, since MAb 17E6 does not recognize murine alphavbeta3, the effect is due only to the direct anti-tumor activity and not to the well-known anti-angiogenic activity of alphav-integrin antagonists. Taken together, our results confirm the essential role of alphavbeta3 integrin in the growth of human malignant melanoma in vivo and provide strong evidence of the therapeutic potential of alphav-integrin antagonists for the treatment of such tumors.     

Expression of alphav integrins and vitronectin receptor identity in breast cancer cells.

In the present study we have used fluorocytometry and immunoprecipitation to characterize the expression of alphav-containing integrins in a panel of eight human breast cancer cell lines and one normal human mammary epithelial line. We show that the classical vitronectin receptor alphavbeta3 is expressed in only one cell line (MDA-MB-231), whereas alphavbeta5 is expressed on all breast cancer cell lines and alphavbeta1 is expressed on the majority. Using adherence assays to purified ligands in the presence and absence of function-blocking monoclonal antibodies, we have demonstrated that alphavbeta5 mediates adhesion to vitronectin in the majority of these cells. In one cell line, ZR75-1, alphavbeta1 contributes significantly to adhesion to immobilized vitronectin. The formation of focal adhesions containing the alphav and beta1 subunits on vitronectin is also demonstrated by indirect immunofluorescence.     

Development and progression of karyotypic variability in melanoma K1735 following X-irradiation

Chromosomal aberrations are often assumed to be deleterious to cells. However, we have found that many metastases are populated by cells with chromosomal recombinants induced by radiation of the original tumor population. The tumor, K-1735-M2, was already capable of metastasis so that the recombinant chromosomes were not necessary for this property of the tumor. Stable recombinants, like other aberrant forms, could be disadvantageous or, alternatively, could confer selective advantage to some tumor cells. We investigated these possibilities by irradiating the parental tumor line and examining the formation and persistence of chromosomal markers in cell culture and in s.c. tumors. The karyotype of the K-1735-M2 parental tumor is composed entirely of telocentric chromosomes, and recombinant forms are relatively easy to recognize. Unstable forms of chromosome damage were lost rapidly. The frequency of stable recombinants after two weeks in culture was higher than that in tumors growing in primary inoculation sites. In contrast, secondary (spontaneous metastatic) foci showed a far greater frequency of chromosomal markers, suggesting a positive association between markers and acquisition of properties benefiting growth and metastasis.     

Comparative analysis of immunocritical melanoma markers in the mouse melanoma cell lines B16, K1735 and S91-M3

The mouse melanoma cell lines B16, K1735 and Cloudman S91-M3 (and various sublines) are frequently used as melanoma models. Extensive comparative data of their immunological features are not available. In order to define the immunological profiles of these cell lines, relevant tumour markers were studied. S91-M3 melanoma cells constitutively expressed high levels of major histocompatibility complex (MHC) I, in contrast to K1735-M2 and B16-F1 cells. MHC II expression was restricted to B16-F1 cells following interferon-gamma treatment. Tyrosinase, tyrosinase-related protein-2 and gp100 were detected in B16-F1 and S91-M3 cells, but not in K1735-M2 cells. Constitutive surface expression and secretion of intercellular adhesion molecule-1 was found on S91-M3 cells. No substantial secretion of interleukin-10 could be detected. In contrast, low levels of latent transforming growth factor-beta were found in the cell supernatants of B16-F1 and K1735-M2 cells. The expression pattern of Fas, FasL and FLICE inhibitory protein was comparable in all three cell lines. Thus our findings indicate that each cell line presents a characteristic immunological profile, confirming that B16-F1 is an appropriate murine tumour model for tumours with low levels of MHC I but expressing melanoma-associated antigens. S91-M3 represents a complementary, more immunogenic model. In contrast, K1735-M2 does not seem to be an appropriate model for melanoma.     

Parallel expression of alphaIIbbeta3 and alphavbeta3 integrins in human melanoma cells upregulates bFGF expression and promotes their angiogenic phenotype

Previous studies indicated that transfection of the platelet integrin alphaIIbbeta3 into human melanoma cells expressing integrin alphavbeta3 promoted their in vivo (but not in vitro) growth and cell survival. To reveal the underlying pathomechanism, we have analyzed the angiogenic phenotype of alphaIIbbeta3 integrin-transduced human melanoma cells expressing integrin alphavbeta3. Upon heterotopic or orthotopic (intracutaneous) injections into SCID mice, the alphaIIbbeta3 integrin-overexpressing clones, ESL, ESH, 19L and 19H, grew more rapidly than the mock transfectant (alphavbeta3 expressing) clone, 3.1P. Morphometry demonstrated an increased intratumoral microvessel density in 19L and 19H tumors compared to 3.1P. Immunocytochemistry and flow cytometry indicated that vascular endothelial growth factor (VEGF) is constitutively expressed in the majority of the cells of both the mock and the alphaIIbbeta3 integrin-transfected clones. However, the mock transfectant clone, 3.1P, did not express basic fibroblast growth factor (bFGF) at protein level (<1%), unlike the alphaIIbbeta3 integrin-transfected clones, 19L and 19H, (33.9 and 84.1%, respectively). Quantitative PCR analysis of 6 related human melanoma clones with various levels of alphaIIbbeta3 integrin expressions revealed a correlation between the alphaIIb protein and bFGF mRNA expressions. Furthermore, cDNA microarray analysis of the 19H cells revealed 12 downregulated and 36 upregulated genes [among them 3 upregulated vasculogenic mimicry-genes (CD34, endothelin receptor B, Prostaglandin I-2 synthase)] when compared to 3.1P cells. The altered bFGF expression may be influenced by integrin-linked signaling, since bbeta3-endonexin is upregulated in alphaIIbbeta3-transfected cells and tyrosine kinase inhibitors downregulate bFGF both at mRNA and protein levels. We propose here that the illegitimate expression of alphaIIbbeta3 integrin in human melanoma cells already expressing alphavbeta3 integrin may alter their in vivo growth properties due to the modulation of their angiogenic phenotype.     

Differential SP220K expression in renal carcinoma and oncocytoma cells

SP220K is a newly described serine proteinase which displays guanidinobenzoatase activity in its inactive form and gelatinolytic activity in its active form. SP220K expression was studied in 20 renal clear-cell carcinomas and in a series of renal oncocytomas, a rare benign tumor derived from the kidney tubule epithelium. We provide evidence that SP220K expression, as assessed by guanidinobenzoatase activity, gelatin zymography and Western blot immunodetection, was increased markedly in cancer basolateral membranes compared to kidney cortex controls, whereas no signal was detectable in basolateral membranes from the 5 renal oncocytomas studied. Cytoplasms of carcinoma cells were immunodetected consistently, whereas no expression was seen in oncocytic cells from any of the oncocytomas studied (12/12). Endothelial cells were immunodetected in all 3 tissue types. Our data favor a potential mechanistic relationship between expression of the matrix proteinase SP220K and invasive phenotype in kidney epithelium proliferative processes. Int. J. Cancer 72:752–757, 1997. © 1997 Wiley-Liss, Inc.     

Different deficiencies in the prevention of tumorigenic-low-metastatic murine K-1735b melanoma cells from producing metastases

To produce metastasis, malignant tumor cells must be able to complete a sequence of many steps that depend not only on tumor cell properties but also on ability of the tumor cells to interact effectively with host homeostatic mechanisms to avoid destruction. Therefore, it should be possible to isolate clonal populations non- or low metastatic. In a study of K-1735 clones introduced into normal syngeneic hosts, the reasons for the lack of or low ability of metastasis production did indeed differ among different clones. Some clones were identified that were low metastatic in syngeneic C3H/HeN mice because of antigenic characteristics. Others failed to give rise to metastases because they could not survive and grow once arrested in the lung parenchyma. These data suggested that the success of future studies dealing with genetic analysis of the metastatic phenotype could depend on the use of appropriate tumor cell populations.     

Transmigration of human ovarian adenocarcinoma cells through endothelial extracellular matrix involves alphav integrins and the participation of MMP2

The growth of ovarian carcinoma is dependent upon their vascularistion, but the interaction of ovarian cancer cells with the endothelium and their invasion through an endothelial environment remain poorly understood at the molecular level. To investigate adhesive events underlying this process with focusing on the role of alphav integrins and MT1MMP-MMP2 proteinases, we used in vitro models of cocultures of human ovarian adenocarcinoma cell lines (IGROV1 and SKOV3) with human umbilical vein endothelial cells (HUVECs). Immunostaining of HUVECs revealed the network organisation of fibrillar fibronectin (Fn) and pericellular vitronectin (Vn). During coculture, IGROV1 and SKOV3 cells gain access to subendothelial basement membrane of HUVECs and dislocated endothelial Fn without affecting endothelial Vn. Transmigration assays revealed that tumour cells invade Vn and, with an higher efficiency, Fn. Our data also highlighted that ovarian carcinoma cells migrated through the Fn-rich HUVEC-ECM. The expression of MMP2 and MT1-MMP was revealed in tumour cells within an endothelial environment. Furthermore, we found that cell migration through the endothelial ECM was almost totally dependent on alphav integrin function, whereas beta1 integrins were not solicited. In addition, inhibitors of MMP2 activity (alone or combined with anti-alphav integrin MAb) or TSRI265 (which blocks MMP2-alphavbeta3 association) were found to impede this process. Finally, alphav integrins, MT1-MMP and MMP2 were found in ovarian carcinoma cells within the 3-dimensional architecture of intraperitoneal tumour nodes collected from nude mice xenografted with IGROV1 or SKOV3 cell lines or within human tumour tissues. alphav integrins therefore appear as essential to the migration properties of human ovarian carcinoma cells, especially in an endothelial environment, with MMP2 participating to this process.     

Differential expression levels of Par-4 in melanoma

The pro-apoptotic prostate apoptosis response-4 gene product Par-4 sensitizes prostate cells to the induction of programmed cell death. In this study we examined Par-4 expression in human melanoma cell lines and melanoma metastases. The heterogeneous expression detected prompted us to investigate the biological relevance of Par-4 in a human melanoma xenotransplantation model. Overexpression of Par-4 by transfection decreased tumour development in xenotransplanted A375-C6 melanoma cells in SCID mice and correlated to an increase in tumour cell apoptosis. These data suggest that high expression of the pro-apoptotic protein Par-4 could qualify as a prognostic marker in human melanoma.     

Decreased glycolytic metabolism accelerates apoptosis in response to 2-acetyl furanonaphthoquinone in K1735 melanoma irrespective of bcl-2 overexpression

Solid tumors are often placed under stress conditions, such as glucose starvation which may result in topoisomerase II drug resistance. In this study, we investigated whether glucose deprivation or substitution by fructose regulates tumor cell apoptosis induced by 2-acetyl furanonaphthoquinone (FNQ). We now show that FNQ exerts much greater antitumor activity than either 7-methoxy 2-ethyl FNQ or 2-ethyl FNQ. Whereas 0.8 microM FNQ induces apoptosis after 16 hours in glucose-supplemented conditions irrespective of bcl-2 overexpression in K1735 melanoma, 0.5 microM FNQ is also effective within 12 hours in low glucose or in fructose-supplemented medium. Under the latter conditions, apoptosis-associated PARP cleavage and cytosolic cytochrome C are increased, together with induction and partial translocation to mitochondria of phosphorylated Jun-N-terminal kinase and massive upregulation of mitochondrial Mn superoxide dismutase. We propose that mitochondrial colocalization of these activities is important in this synergistic anti-tumor effect of FNQ and glucose depletion. Since glucose limitation slows proliferation and decreases efficacy of some genotoxic drugs that trigger apoptosis in rapidly dividing cells, we propose evaluating FNQ as a novel therapeutic anti-cancer adjuvant against slowly proliferating tumors.     

Differential expression of melanoma associated antigens in acral lentiginous melanoma and in nodular melanoma lesions.

The reactivity in an avidin-biotin complex immunoperoxidase reaction with a large panel of anti-human melanoma associated antigen (MAA) and anti-HLA monoclonal antibodies of 24 primary and 11 metastatic acral lentiginous melanoma (ALM) lesions was compared to that of 12 primary and 12 metastatic nodular melanoma (NM) lesions. The expression of the membrane bound vitronectin receptor, Mr 110,000 MAA, Mr 97,000 MAA, and intercellular adhesion molecule-1 was significantly lower in both primary and metastatic ALM lesions than in their NM counterparts. Furthermore, primary ALM lesions displayed a significantly lower expression than primary NM lesions of the membrane bound high molecular weight melanoma associated antigen (HMW-MAA), Mr 110,000 MAA, Mr 100,000 MAA, 9-O-acetyl-GD3, GD2-GD3, and GD2, of the cytoplasmic monoclonal antibody 465.12 defined MAA and of transferrin receptor and of HLA-DQ and DP antigens; ALM metastases expressed a significantly lower level of carcinoembryonic antigen-MAA than NM metastases. These antigenic differences do not reflect an antigenic paucity of ALM cells, since ALM lesions express a higher level of T4-tyrosinase than NM lesions and a level of HLA Class I antigens similar to that of NM lesions. In view of the use of HMW-MAA, Mr 97,000 MAA, and GD3 in immunoscintigraphy and/or in immunotherapy, it is noteworthy that the three antigens are expressed in a similar high percentage of ALM metastases and of primary and metastatic NM lesions, while the HMW-MAA is expressed in a markedly lower percentage of primary ALM lesions than Mr 97,000 MAA and GD3. However, the degree of heterogeneity of HMW-MAA within a positive primary ALM lesion, as measured by the percentage of stained melanoma cells, is lower than that of Mr 97,000 MAA and GD3. The expression of the antigens investigated in ALM and NM lesions was not correlated with the presence of lymphocyte infiltrates, melanin content of melanoma cells, and epithelioid and spindle type of melanoma cells in the lesions. On the other hand, the survival of patients with ALM was inversely correlated with the expression of intercellular adhesion molecule 1 or HMW-MAA in their primary lesions. A potential role of HMW-MAA in the course of the disease is suggested by its significantly higher expression in metastatic than in primary ALM lesions.(ABSTRACT TRUNCATED AT 400 WORDS)     

Rapamycin inhibits lung metastasis of B16 melanoma cells through down‐regulating alphav integrin expression and up‐regulating apoptosis signaling

Currently available data indicate the potential application of rapamycin and its analogues in the clinic as anticancer therapeutic agents through inhibiting tumor cell growth and tumor angiogenesis. However, whether rapamycin can directly suppress tumor metastasis remains unclear. In the present study, we demonstrated that rapamycin treatment results in reduced formation of metastatic nodules in the lung by B16 cells. This is due to two mechanisms. First, the expression of αv integrin is down‐regulated by rapamycin treatment, and subsequently, the phosphorylation of focal adhesion kinase (FAK) is reduced. Second, rapamycin promotes apoptosis by up‐regulating the proapoptotic molecules Bid and Bax and down‐regulating Bcl‐xL. Blocking the apoptosis pathway by pan‐caspase inhibitor zVAD partially reversed the suppression of rapamycin in B16 metastasis. Interestingly, rapamycin up‐regulates Bax and Bid in B16 cells via the S6K1 pathway and down‐regulates the expression of αv integrin via other pathway(s). In addition, our data showed that autophagy was not involved in the mechanisms of rapamycin‐mediated metastasis suppression. Our findings demonstrate a potential anti‐metastatic effect of rapamycin via down‐regulating αv integrin expression and up‐regulating apoptosis signaling, suggesting that rapamycin might be worthy of clinical evaluation as an antimetastatic agent. (Cancer Sci 2009)     

Immunotherapy for murine K1735 melanoma: combinatorial use of recombinant adenovirus expressing CD40L and other immunomodulators

We have constructed and tested five recombinant adenoviruses (Ads) that express a variety of immunomodulators, including CD40 ligand (CD40L), a potent costimulator of several components of the immune system. We demonstrate that CD40L expressed from Ad in K1735 mouse melanoma cells leads to a strong reduction in tumorigenicity and to efficient protective immunity in a vaccination setting. Subsequently, using a therapeutic approach, we found that local, intratumoral coinjection of CD40L- and IL-2-expressing Ads was superior to any other agents tested and resulted in an at least 1.9-fold increase in mean survival time, in contrast to systemic application of recombinant CD40L or GM-CSF proteins, which had no significant effects. When using vaccination as a therapeutic approach, the combinations of CD40L plus IL-2 or GM-CSF plus IL-2 from Ad gave rise to an extended (2.8-fold) increase in mean survival time. A detailed analysis of immune cells present within regressing tumors indicated that mainly CD4(+) and CD8(+) T cells, and to a lesser extent dendritic cells, infiltrated the tumor mass, but not NK cells, macrophages, or granulocytes. These results propose that a combination of CD40L plus IL-2 has an improved efficacy over the use of single agents when applied for direct in situ therapy or vaccination therapy.     

miRNA在恶性黑色素瘤中的差异性表达及临床意义

miRNA即微小RNA,是指一类自身具有典型茎环结构且在进化上高度保守的非编码RNA,其无法编码蛋白质,但却被证实是基因表达调节机制中非常重要的成员之一。在细胞的完整生长周期中发挥重要的调节作用。研究表明miRNA约半数都位于癌基因所在的区域,且不同miRNA的表达水平差异性较大,表明miRNA的表达情况与肿瘤的发生具有一定的相关性,明确miRNA在肿瘤发生、发展中的作用,在肿瘤的早期诊断以及确定与之相对应的miRNA靶向治疗方面均具有重要意义。本文就近年来恶性黑色素瘤相关性miRNA及其靶基因的研究进行综述,旨在阐述几种不同类型miRNA在恶性黑色素瘤发生、发展过程中的差异性表达及临床意义,为恶性黑色素瘤的miRNA靶向治疗提供理论基础。     

Differential expression of angioregulatory matricellular proteins in posterior uveal melanoma

Metastases from uveal melanoma, the most common primary malignant eye tumour in adults, develop solely via their vascular bed due to the absence of intraocular lymphatics. The present study investigated the expression in this tumour of three matricellular proteins--Secreted Protein Acidic and Rich in Cysteine (SPARC), thrombospondin 1 (TSP1) and thrombospondin 2 (TSP2)--with putative contrasting roles in the regulation of angiogenesis. Immunohistochemical analysis of the three proteins was carried out in paraffin-embedded specimens from 27 posterior uveal melanomas and was corroborated with Western blot analysis of fresh-frozen samples from seven of the tumours. SPARC immunoreactivity was detected in all specimens and defined two categories of tumour: SPARC-rich (21 of 27 specimens) and SPARC-patchy (six of 27 specimens) uveal melanomas. SPARC-rich tumours had a significantly higher proportion of specimen area occupied by blood vessels (P=0.04) and showed a positive association with the presence of epithelioid-type tumoral cells (P=0.101). TSP1 was not detected by either of the methods in any of the tumours analysed. Some immunopositivity for TSP2 was detected in tumour cells in approximately 40% of specimens, but was not associated with survival, tumour vascularity or any other histopathological indices of survival. The pattern of expression of these matricellular proteins in uveal melanoma is consistent with a cooperative mechanism for establishing an enhanced environment favourable to angiogenesis. Interventions inducing TSP1 expression and/or inhibiting SPARC expression may be candidates for therapies directed towards the inhibition of angiogenesis in posterior uveal melanoma.     

Erythropoietin receptor expression in human melanoma cells

Erythropoietin is well known for its role in the control of erythropoiesis, where it acts by binding to its cognate receptor (EpoR) on the surface of erythroid progenitor cells. Here we present the novel finding that the EpoR is also expressed in cells of the melanocytic lineage. It is expressed in transformed cell lines established from normal melanocytes and also in established human melanoma cell lines derived from melanoma metastases, but not in normal primary human melanocytes. The analysis of individual subclones isolated from spontaneously transformed melanocytes revealed that approximately 50% of all the clones examined expressed the EpoR. Further analysis of the individual growth characteristics of EpoR-positive and EpoR-negative clones indicated that, under standard cell culture conditions, expression of the receptor did not affect cell growth. Expression of this receptor is consequently most likely driven by an event that is associated with, but not absolutely required for, the transformed phenotype. While the definite function of this receptor in melanoma cells is still unknown and additional studies are required, our findings support the hypothesis that the EpoR may serve as a progression marker for human melanoma. This observation might be useful in the early diagnosis of melanoma.     

Integrin chains beta1 and alphav as prognostic factors in human metastatic melanoma

The expression pattern of integrin-type cell adhesion receptors is often changed during malignant transformation. In the present work, we studied the prognostic significance of beta1 and alphav integrin chains for survival of patients with metastatic melanoma. The expression levels of beta1 integrin were also compared with those of Bcl-2, an anti-apoptotic protein, the presence of which is associated with treatment response and survival in melanoma. The expression of beta1 and alphav integrins in 68 melanoma metastases obtained from 55 patients treated with combined chemoimmunotherapy was studied by immunohistochemistry using anti-beta1 and anti-alphav antibodies. The patients were divided into two groups (using a cut-off point of >/= 81%) for beta1 integrin expression levels and into three categories (negative/low, median, high) for alphav integrin expression levels. All tumours were positive for beta1 integrin, and the tumours (n = 6) which had the highest alphav score were also strongly positive for beta1 (94%; P = 0.0055). Patients (n = 43) with 80% or less beta1 integrin-positive tumour cells in their samples had a median disease-free survival (DFS) of 17.0 months, and patients (n = 12) with 81% or more beta1 integrin-positive tumour cells had a DFS of only 5.7 months (P = 0.0001). Patients (n = 32) with low alphav integrin expression levels had shorter DFS (median 12.3 months; P = 0.0146) than patients (n = 20) with median expression levels (median 16.7 months; P = 0.0146). However, three patients who had a very strong alphav expression in their tumours had a median DFS of only 1.8 months (P = 0.0146). Median level expression of beta1 integrin was associated with the presence of Bcl-2 in tumour cells (P = 0.0033). Our results suggest that beta1 and alphav integrin chains are independently expressed in metastatic melanoma and may have an effect on the metastatic potential of melanoma cells.     

Differential induction of 12-O-tetradecanoylphorbol-13-acetate sequence gene expression in murine melanocytes and melanoma cells.

We previously showed that growth of the nontumorigenic, immortal murine melanocyte line Mel-ab correlates with the depletion of protein kinase C (PKC), whereas quiescence is associated with elevated levels of this enzyme (Brooks G, et al., Cancer Res 51: 3281-3288, 1991). Here we report responses that occur in these cells downstream of PKC activation or downregulation. We examined induction of 12-O-tetradecanoylphorbol-13-acetate (TPA)-inducible sequence (TIS) gene expression in Mel-ab melanocytes and in their transformed counterparts, B16 melanoma cells. Exposure of quiescent Mel-ab cells to the PKC-activating phorbol esters TPA or sapintoxin A at 81 nM for 2 h increased levels of mRNA for six of seven TIS genes examined (twofold to 80-fold increase in steady-state RNA levels for TIS 1, 7, 8, 11, 21, and 28 (c-fos); TIS 10 expression was not affected). No induction of TIS gene expression was observed either in growing Mel-ab cells maintained in 324 nM phorbol 12,13-dibutyrate or in B16 cells previously unexposed to phorbol esters, in which normal PKC levels were endogenously depressed. The cAMP-elevating agents choleratoxin (10 nM) and dibutyryl cyclic AMP (2.5 mM) increased levels of TIS mRNA (with the exception of TIS 10) in both proliferating Mel-ab and B16 cells, suggesting that downregulation of the PKC pathway is specific and not a consequence of a general inhibition of all signalling pathways.