Differential expression of steroid receptors, hsp27, and pS2 in a series of drug resistant human breast tumor cell lines derived following exposure to antitumor drugs or to fractionated X-irradiation

Heat shock protein 27 is a potential indicator for response to YangZheng XiaoJi and chemotherapy agents in cancer cells

Heat shock protein 27 (HSP27) is a member of the heat shock protein family which has been linked to tumour progression and, most interestingly, to chemotherapy resistance in cancer patients. The present study examined the potential interplay between HSP27 and YangZheng XiaoJi, a traditional Chinese medicine used in cancer treatment. A range of cell lines from different tumour types including pancreatic, lung, gastric, colorectal, breast, prostate and ovarian cancer (both wild-type and resistant) were used. Levels and activation of HSP27 and its potential associated signalling pathways were evaluated by protein array and western blotting. Knockdown of HSP27 in cancer cells was achieved using siRNA. Localisation and co-localisation of HSP27 and other proteins were carried out by immunofluorescence. Cell growth and migration were evaluated in their response to a range of chemotherapeutic agents. The present study first identified, by way of protein array, that YangZheng XiaoJi was able to inhibit the phosphorylation of HSP27 protein in cancer cells. We further demonstrated that HSP27, which is co-localised with caspase-9, can be blocked from localising in focal adhesions and co-localising with caspase-9 by YangZheng XiaoJi. The study also demonstrated that YangZheng XiaoJi was able to sensitise cancer cells including those cells that were resistant to chemotherapy, to chemotherapeutic agents. Finally, knocking down HSP27 markedly reduced the migration of cancer cells and increased the sensitivity of cancer cells to the inhibitory effect on cellular migration by YangZheng XiaoJi. YangZheng XiaoJi can act as an agent in first sensitising cancer cells to chemotherapy and secondly to overcome, to some degree, chemoresistance when used in an appropriate fashion in patients who have active HSP27.

Changes in protein expression after neoadjuvant use of aromatase inhibitors in primary breast cancer: a proteomic approach to search for potential biomarkers to predict response or resistance

OBJECTIVE

Aromatase inhibitors (AI) have been established as a useful hormonal therapy in hormone receptor-expressing breast carcinoma. However, changes in tumor protein expression after exposure to AIs are not necessarily well understood. These changes may provide insight into how breast carcinomas respond or develop into a state of resistance towards AIs, and lead to the discovery of potential biomarkers to predict treatment responses.

METHODS

Post-menopausal breast cancer patients were recruited to receive 3 months treatment with neoadjuvant AI. Carcinoma tissues were collected before and after the use of AIs and protein expression profiles were compared using two-dimensional gel electrophoresis. Protein spots with different levels of expression were identified using mass spectrometry.

RESULTS

A total of 14 matched pairs of tumor tissues were collected. Both up-regulated and down-regulated proteins were selected and identified as follows: heat shock protein 70 protein 2; Cyclin M3, alpha 1 antichymotrypsin precursor; carbonic anhydrase I, cancer antigen 1; SOBP protein; Rho GDP dissociation inhibitor alpha; glyoxalase I with benzyl glutathione inhibitor; lipid-free human apolipoprotein A-I and RAB4A member RAS oncogene family. Among these proteins, heat shock protein 70 demonstrated the most significant positive correlation with clinical response of the patients.

CONCLUSION

After neoadjuvant use of AI, heat shock protein 70 demonstrated the most consistent phenotypic consistency in both up-regulated and down-regulated protein expression levels among the 14 studied pairs of tumor tissue. Other proteins worthwhile exploring were also identified in this study. These proteins could serve as potential predictors for AI response.

Increased expression of HSP27 linked to vincristine resistance in human gastric cancer cell line

PURPOSE

To understand the mechanisms of multidrug resistance (MDR) in vincristine-resistant human gastric cancer cell line SGC7901/VCR.

METHODS

Comparative proteomics involving two-dimensional gel electrophoresis (2-DE) and ESI-Q-TOF Mass Spectrometry (MS) was performed on total proteins extracts from vincristine-resistant SGC7901/VCR and its parental cell line SGC7901. Then the association of heat shock protein 27 (HSP27), one of the highly expressed proteins in SGC7901/VCR, with MDR was analyzed using antisense oligonucleotides (ASOs) inhibition. To further elucidate the biological functions executed by HSP27 in SGC7901/VCR, we investigated a comprehensive interactome map of HSP27 by coimmunoprecipitation (IP) coupled with MS.

RESULTS

In this study, HSP27 was identified as a protein showing increased expression in SGC7901/VCR. The suppression of HSP27 expression by HSP27 ASOs could enhance vincristine and adriamycin chemosensitivity in SGC7901/VCR. Identified 25 HSP27-interacting proteins by IP coupled with MS could be classified into eight categories based on their functions: cytoskeleton organization, chaperones, metabolic enzymes, proteins relative to signal transduction, ribosomal proteins, DNA repair proteins, proteins involved in transcription and translation, and RNA processing, which correspond to the reported functions of HSP27 with MDR.

CONCLUSION

These data clearly link HSP27 and multidrug resistance mechanisms in gastric cancer.

Topoisomerase IIalpha and APE/ref-1 are associated with pathologic response to primary anthracycline-based chemotherapy for breast cancer

The aim of this study was to evaluate the role of several biological and histological markers (topoisomerase IIalpha, MIB-1, E2F, apoptotic index, APE/ref-1, p53, Her-2/neu, estrogen and porgesterone receptors, and histological grading) as predictors of pathologic response after anthracycline-based chemotherapy for breast cancer. A series of 50 consecutive breast cancer patients receiving anthracycline-based primary chemotherapy were retrospectively studied. Biological markers were assessed by immunohistochemistry (and by TUNEL assay for apoptotic index) in pre-treatment core biopsies and post-treatment surgical samples. The expression of topoisomerase IIalpha, E2F, MIB-1, estrogen and progesterone receptors decreased, while APE/ref-1 staining increased after treatment. Higher topoisomerase IIalpha (P=0.007) and lower APE/ref-1 (P=0.04) expression were associated with better pathologic response.

Paclitaxel inhibits expression of heat shock protein 27 in ovarian and uterine cancer cells

The purpose of the study was to determine whether paclitaxel inhibits the expression of heat shock protein 27 (HSP27) in two gynecologic cancer cell lines compared with other antineoplastic agents having different cytotoxic mechanisms. BG-1 ovarian cancer cells and HeLa uterine cancer cells were treated with a tubulin depolymerization inhibitor (paclitaxel), a topoisomerase-II inhibitor (etoposide), and two tubulin polymerization inhibitors (colcemid and vincristine). Cell kills were evaluated by counting the number of cells. Propidium iodide staining and flow cytometric analysis were applied for the determination of cell-cycle perturbation. HSP27 was stained by the indirect immunofluorescence technique and analyzed with a flow cytometer. In both BG-1 and HeLa cells, growth arrest and G2 / M accumulation were dependent on the dose of each cytotoxic agent. There were positive correlations between HSP27 overexpression and growth arrest and G2 / M accumulation when the cell lines were treated with etoposide, colcemid, or vincristine, but not with paclitaxel. Paclitaxel completely inhibited the expression of HSP27. The results of this study indicated that paclitaxel may possess unique mechanisms able to overcome drug resistance by inhibiting HSP27 expression.

Clinicopathological study of the expression of hsp27, pS2, cathepsin D and metallothionein in primary invasive breast cancer

Expression of the hormone-related proteins hsp27, pS2, and also of cathepsin D (CD) and metallothionein (MT) was studied by immunohistochemistry and analyzed against clinical data in breast cancer. Archived material of paraffin-embedded breast carcinoma tissues from a cohort of 134 patients with primary invasive breast cancer was used. Hsp27 and pS2 (>10% of tumor cells stained) were found to be expressed in 63.6% and 37.6% of cases, respectively, and were correlated negatively with grading (P=0.006 and 0.01) and positively with estrogen receptors (ER) (P=0.04 and 0.04). pS2 expression was correlated with lymph node status (P=0.02), tumor size (P=0.01), progesterone receptor (PR) content (P=0.02), hsp27 (P=0.015) and bcl-2 protein (P=0.001). An inverse relationship between pS2 expression and the expression of p53 protein (P=0.005) and proliferation-associated index MIB1 (P<0.0001) was noted. Stromal cathepsin D was positively correlated with tumor grade (P=0.01), PCNA (P=0.007), MIB1 (P=0.001) and p53 (P=0.01), and negatively with ER (P=0.04) and bcl-2 (P<0.0001). MT was correlated positively with stromal CD (P=0.007) and inversely with PgR (P=0.04). Univariate analysis showed CD expression to be a positive prognostic factor for survival (P=0.035), with borderline significance, while MT was more strongly positive (P=0.01). However, none of the proteins studied was found to be related to disease outcome in univariate analysis. Our data show that hsp27, pS2 and stromal CD expression may reflect tumor differentiation and the functional status of ER in breast cancer, but stromal CD and tumor MT expression were the only factors found that may be of limited prognostic value.

Heat shock protein expression in human tumours grown in severe combined immunodeficient mice

The constitutional expression of heat shock proteins (HSP) 27, 70 and 90 in human breast, colon and ovarian cancer cells transplanted into severe combined immunodeficient (SCID) mice was evaluated. In addition their induced expression under chemotherapeutic stress was analyzed. The oestrogen receptor positive breast cancer cell lines (MCF-7, T47D) demonstrated an increased level of HSP 27 and 70 expression compared with oestrogen receptor negative cell lines (BT20, HBL100). After 5-fluorouracil application for 4 days, HSP 27 and 70 expression was increased in HT29 colon tumours. Hence, the human/SCID mouse model is well suited to evaluate the constitutional and induced expression of human HSPs under various experimental conditions.

Endogenous tumour necrosis factor regulates heat-inducible heat shock protein 72 synthesis

Endogenous tumour necrosis factor (enTNF) acts as a resistant factor against cytotoxicity of heat by induction of manganous superoxide dismutase (MnSOD), thereby scavenging reactive oxygen free radicals. On the other hand, it is also well known that heat shock proteins (HSPs), which are induced by heat-stress, behave as cytoprotecting factor against this stress. However, the relationship of these two resistant factors is not yet elucidated. In the present study we would therefore propose the possibility that enTNF enhances HSP72 expression. Heat-sensitive L-M (mouse tomourigenic fibroblast) cells, which normally do not express enTNF, were transfected with a nonsecretory-type human TNF expression vector to produce enTNF. Stable transfectants showed resistance to heat treatment and an increase of HSP72 expression. Conversely, when HeLa (human uterine cervical cancer) cells, which normally produce an appreciable amount of enTNF, were transfected with an antisense TNF mRNA expression vector to inhibit enTNF synthesis, their heat sensitivity was enhanced and HSP72 expression was reduced by half. In conclusion, these findings indicate that enTNF regulates heat-inducible HSP72 synthesis.

Reactivity of monoclonal antibody FC-2.15 against drug resistant breast cancer cells. Additive cytotoxicity of adriamycin and taxol with FC-2.15

Monoclonal antibody (MAb) FC-2.15 recognizes Lewis x antigen (Le(x)-Ag) expressed on the cell surface of most human breast cancer cells. FC-2.15 displays important human complement (C')-mediated cytotoxicity (CMC) against its target cells. In this study the reactivity of FC-2.15 against drug resistant-breast cancer cells was investigated, as well as the possibility to combine the antitumor activities of this MAb with adriamycin (Adr) or taxol. Since resistant clones with altered expression of tumor-associated antigens usually emerge after chemotherapy, the expression of Le(x)-Ag was analyzed in Adr(R) MCF-7 breast cancer cells (Adr resistant subline) and in tumor samples from nine patients with locally advanced breast carcinoma who were treated with FEC chemotherapy. A flow cytometry assay showed that most of Adr(R) MCF-7 cells, as well as wild type (WT) cells, expressed Le(x)-Ag; however, the Le(x) epitope is probably bound to different backbones in these cells. When the cytotoxic ability of FC-2.15 against WT and Adr(R) MCF-7 cells was compared, it was found that a 90 min treatment with FC-2.15 plus C' induced similar CMC against both cell lines. An important cytolysis was obtained at 5 microg/ml FC-2.15, reaching a plateau at 25 microg/ml, at which cell population was diminished to 21.1% for WT and 27.9 for Adr(R) MCF-7 cells. Regarding human tumors, Le(x)-Ag expression was evaluated in samples obtained before and in most cases after chemotherapy, and it could be observed that: 1) before treatment, tumor samples from all patients analyzed (responders and non-responders to chemotherapy) were FC-2.15-positive; 2) the presence of Le(x)-Ag was not modified after treatment. The combined action of Adr or taxol with FC-2.15 was then evaluated. WT and Adr(R) MCF-7 cells were cultured with Adr or taxol followed by an incubation with different FC-2.15 concentrations plus C'. When the effect of Adr alone was determined, ID50 were 1 x 10(-7) M for WT and 4.2 x 10(-5) M for Adr(R) MCF-7 cells. The cytotoxic ability of taxol alone was also tested, and ID50 were 6.4 x 10(-9) M for WT and 3.1 x 10(-6) M for Adr(R) MCF-7 cells. When FC-2.15 was added to Adr or taxol, the cytotoxicity of the drug-FC-2.15 combined treatment was always higher than the isolated effects, showing additive cytotoxicity at the different concentrations tested and with both cell lines. Our results suggest that FC-2.15 may be a useful agent against breast tumor cells which survive chemotherapy with Adr or taxol.

Induction of heat shock protein 72 synthesis by endogenous tumor necrosis factor via enhancement of the heat shock element-binding activity of heat shock factor 1

Endogenous tumor necrosis factor (enTNF) acts as a resistance factor against cytotoxicity caused by heat by inducing manganous superoxide dismutase (MnSOD), thereby scavenging reactive oxygen free radicals. On the other hand, it is also well known that heat shock proteins (HSP) which are induced by heat stress behave as cytoprotective factor against this stress. However, the relationship of these two resistance factors is not elucidated yet. In the present study, we therefore proposed the possibility that enTNF enhances HSP72 expression. Heat-sensitive L-M (mouse tumorigenic fibroblast) cells, which normally do not express enTNF, were transfected with a nonsecretory-type human TNF-alpha expression vector to produce enTNF. Stable transfectants showed resistance to heat treatment and an increase of HSP72 expression. Conversely, when HeLa (human uterine cervical cancer) cells, which normally produce an appreciable amount of enTNF, were transfected with an antisense TNF-alpha mRNA expression vector to inhibit enTNF synthesis, their heat sensitivity was enhanced and HSP72 expression was reduced by half. Although enTNF caused no difference in the level of heat shock factor (HSF) 1 in these cells, enTNF expression correlated well with the binding activity of HSF-1 to a 32P-labeled synthetic oligonucleotide containing the human heat shock element (HSE). These results indicate that enTNF participates not only in intrinsic resistance against heat via induction of MnSOD but also via enhancement of the HSE-binding activity of HSF 1 followed by augmentation of HSP72 expression.

Expression and intracellular localization of heat shock proteins in multidrug resistance of a cisplatin resistant human ovarian cancer cell line

TYK-R10 is a cisplatin resistant human ovarian carcinoma cell line and showed a cross resistance to various anti-cancer drugs including adriamycin (ADR), vincristine (VCR) and etoposide, despite a lack of multidrug phenotype. Under normal conditions, various heat shock proteins (HSPs) were expressed in TYK-R10 but not in parental line (TYK-nu). Non-lethal short-term heat shock treatment induced a high tolerance for cisplatin and VCR in TYK-R10 and ADR, and VCR in TYK-nu. This treatment induced and/or enhanced the expression of various types of HSPs in various intracellular localizations in both TYK-R10 and TYK-nu, with minor differences. These findings indicate that combined expression and intracellular localization of HSPs may play an important role in drug resistance of TYK-R10.

Identification of a distinctive P-glycoprotein-mediated resistance phenotype in human ovarian carcinoma cells after their in vitro exposure to fractionated X-irradiation

BACKGROUND

Clinical drug resistance is recognized in patients previously treated with radiotherapy and after chemotherapy. In vitro exposure of mammalian tumor cells to fractionated X-irradiation also resulted in the expression of drug resistance. Analysis of the resistance phenotype of irradiated Chinese hamster ovary sublines revealed P-glycoprotein overexpression, without any concomitant increase in P-glycoprotein messenger RNA, under posttranslational regulation. This study aimed to determine whether this distinctive resistance phenotype could also be identified in irradiated human tumor cells.

METHODS

Irradiated sublines established from two human ovarian tumor cell lines, SK-OV-3 and JA-T, which showed resistance to vincristine and to etoposide, were studied. Protein and RNA expression were quantitated by Western and Northern blotting or RNase protection assays. P-glycoprotein turnover was measured after immunoprecipitation of metabolically labelled cells.

RESULTS

Significant P-glycoprotein overexpression was detected using the C219 and C494 monoclonal antibodies in the two irradiated human ovarian tumor sublines. No concomitant increase in P-glycoprotein messenger RNA was detectable in the SK-OV-3/DXR10 subline, contrasting with the increased message characteristic of vincristine-selected SKVCR sublines. In addition, turnover of P-glycoprotein was significantly reduced in these DXR10 cells when compared with that measured in a vincristine-selected subline. These irradiated sublines showed reduced levels of epidermal growth factor receptors and unchanged levels of topoisomerase II, but they overexpressed c-erbB2 marginally and heat shock protein 27 significantly. These latter elevations in protein levels, however, were associated with concomitant increases in their respective messenger RNAs, implicating regulation at the transcriptional level.

CONCLUSIONS

Exposure of human ovarian tumor cells to fractionated X-irradiation in vitro resulted in the expression of a distinctive multiple drug resistance phenotype unusually involving posttranslational regulation of P-glycoprotein. Monitoring tumor biopsies for P-glycoprotein-associated drug resistance in patients treated with radiotherapy should evaluate protein levels rather than, or as well as, MDR1 mRNA expression.

A 27 kDa heat shock protein that has anomalous prognostic powers in early and advanced breast cancer

This paper describes a prospective immunohistochemical analysis of 27 kDa heat shock protein (HSP27) in 361 patients with primary breast cancer in relation to disease-free survival (DFS) and survival from first relapse (SR). Oestradiol (ER) and progesterone (PR) receptors were also quantitated and related to the HSP27 data. While ER positively predicted a good outcome for both DFS and SR, HSP27 positivity predicted a prolonged SR but short DFS. The association between HSP27 and DFS only attained statistical significance in node-negative patients. Subgroup analysis reinforced the complementary relationship of HSP27 and ER for SR and opposing influences for DFS. In both node-negative and node-positive women, ER+ HSP27- patients had a longer DFS than ER- HSP27+ counterparts. There was no relationship between HSP27 and overall survival. HSP27 staining was highly correlated with ER but not PR, patient age, tumour size or menstrual status. There was a marginal correlation (P = 0.04) with histological grade with well-differentiated tumours having the highest HSP27. Cox multivariate regression analysis of the contribution of HSP27 in the presence of data on ER, PR, stage, nodal status and histological grade indicated that HSP27 was not of independent prognostic importance for DFS or overall survival and was only of borderline significance for OS (P < 0.07). However, in the absence of ER and PR data, HSP27 staining is an effective way of getting the same prognostic information. HSP27 staining appears to correlate with different biological features in early and advanced breast, high HSP27 being linked with short DFS in node-negative patients but with prolonged survival from first recurrence.

Modified multiple drug resistance phenotype of Chinese hamster ovary cells selected with X-rays and vincristine versus X-rays only

Exposure of Chinese hamster ovary (CHO) cells to fractionated X-irradiation [ten fractions of 9 Gray (Gy)] resulted in the expression of a multiple drug resistance phenotype which was distinct from that of drug-selected cells in two features: (i) resistance to vinca alkaloids and epipodophyllotoxins but sensitivity to anthracyclines was retained; (ii) overexpression of P-glycoprotein (Pgp) but regulated by post-translational stability rather than by any elevation in Pgp mRNA (Hill et al., 1990). It was also reported that when these cells (designated DXR-10) were subsequently exposed to another ten fractions of 9 Gy (20 x 9 Gy in total), no further increases in drug resistance or in the extent of Pgp expression were observed. To examine this apparent plateauing of the drug resistance phenotype following X-ray pretreatment, DXR-10 cells were instead treated with ten pulsed vincristine exposures. The resultant cell line, designated DXR-10/VCR-10, proved to be more resistant to vincristine, implying that the effect of further drug selection was additive to that of X-ray pretreatment. In addition, these cells showed resistance to doxorubicin and increased Pgp expression which was matched by a concomitant elevation in Pgp mRNA. These findings appear to confirm that Pgp expression is differentially regulated in tumour cells showing drug resistance after drug as opposed to X-ray selection.

Differential cytotoxic effects of docetaxel in a range of mammalian tumor cell lines and certain drug resistant sublines in vitro

The in vitro cytotoxic effects of docetaxel (Taxotere; RP56976, NSC688503) proved both time and concentration dependent. Amongst thirteen human cell lines from various tumor types, exposure to increasing concentrations of docetaxel over 24 hrs resulted in a plateau-shaped dose response curve, suggesting that increased cell kill becomes more dependent on increased exposure duration than on concentration. IC50 concentrations (reducing survival by 50%) ranged from 0.13-3.3 ng/ml, with three neuroblastoma lines proving most sensitive and three breast and two colon carcinoma lines showing least sensitivity. There was significant cross-resistance to docetaxel in the classic multidrug resistant (MDR) Chinese hamster ovarian (CHO) CHRC5 line and the human lymphoblastoid CCRF-CEMVLB1000 line, as well as in two vincristine (VCR)-selected MDR MCF-7 sublines. All four of these MDR sublines overexpress P-glycoprotein (Pgp), as did a 6-fold docetaxel-selected resistant CHO subline. As an apparent corollary, in two human teratoma lines selected for etoposide resistance and showing some cross-resistance to VCR and in two CHO sublines expressing low levels of VCR resistance, yet all proving Pgp positive, no docetaxel cross-resistance was identified. Verapamil modulated docetaxel resistance only in sublines expressing resistance to the drug and overexpressing Pgp. Four other human tumor sublines selected for resistance to 5-fluorouracil, cisplatin or teniposide, showed a lack of cross-resistance to docetaxel. Furthermore, cross-resistance to docetaxel was not apparant in four epipodophyllotoxin-selected resistant sublines with alterations in topoisomerase II, indicating its effectiveness against tumor cells expressing the topoisomerase II-related MDR phenotype. Our observation that docetaxel cross-resistance was not automatically expressed by classic MDR tumour cells appears of interest and of potential clinical relevance.

Relationship of HSP27 and oestrogen receptor in hormone sensitive and insensitive cell lines

A 27 kDa heat shock (HSP27) has been analysed by immunoassay and immunoblotting in oestradiol sensitive and insensitive cells. Oestradiol growth responsive MCF7 and T47D human breast cancer cells and growth unresponsive variants derived therefrom have unaltered levels of HSP27 as well as retaining their oestradiol receptor phenotype. MCF7 cells induced to become doxorubicin resistant in culture lose both HSP27 and oestradiol receptor. Thus, in these three pairs of cells, HSP27 content parallels oestradiol receptor (ER). Analysis of a range of ER positive and negative human cell lines supports the positive relationship between HSP27 and ER. This included six ER positive and two ER negative breast tumour lines, one ER positive and one ER negative endometrial tumour cell line and seven ER negative human lines from other sites. One ER negative osteosarcoma line (HTB96) had appreciable levels of HSP27 that were unaffected after stable transfunction with an ER cDNA. Heat shock increases HSP27 levels in some but not all cell lines tested, the effect being inversely proportional to the basal (37 degrees C) content. In a mouse mammary tumour cell line, loss of androgen sensitivity was accompanied by loss of HSP27. Loss of HSP27 occurred in MCF7 cells made drug resistant to Novatrone, vincristine and etoposide as well as doxorubicin; no detectable change was seen in cells made resistant by 5 fluorouracil or X-irradiation. In ER positive ZR75 human breast tumour cells and in both ER negative and positive variants of the HTB96 human osteosarcoma line, the intracellular distribution of HSP27 was analysed. Over 96% of the HSP27 was in the cytosol fraction and the distribution was unaffected by incubation with oestradiol. HSP27 has been discussed in the literature under three different names p29, p24 and HSP27. The data presented in this paper are reviewed in the context of the previous data. It is concluded that there is a good but not absolute correlation between the presence of ER and high amounts of HSP27 but that low amounts of HSP27 are present in many ER negative cells. The correlations between HSP27 and drug resistance are more complex.

Differing patterns of cross-resistance resulting from exposures to specific antitumour drugs or to radiation in vitro

This article reviews the patterns of cross-resistance identified in various P-glycoprotein-mediated and non-P-glycoprotein-mediated drug resistant mammalian tumour cell lines. The differing patterns of cross-resistance and the variable levels of resistance expressed are summarised and discussed. Although the mechanism by which P-glycoprotein can recognise and transport a large group of structurally-unrelated substrates remains to be defined, the recent evidence indicating that membrane associated domains participate in substrate recognition and binding is summarised, and other possible explanations for these variable cross-resistance patterns are considered. Amongst the non-P-glycoprotein-overexpressing multidrug resistant cell lines, two subsets are clearly identifiable, one lacking and the other expressing cross-resistance to the Vinca alkaloids. Resistance mechanisms implicated in these various sublines and possible explanations for their differing levels and patterns of cross-resistance are summarised. Clinical resistance is identified in patients following treatment not only with antitumour drugs, but also after radiotherapy. Experimental data providing a biological basis for this observation are summarised. A distinctive multiple drug resistance phenotype has been identified in tumour cells following exposure in vitro to fractionated X-irradiation characterised by: the expression of resistance to the Vinca alkaloids and the epipodophyllotoxins but not the anthracyclines and overexpression of P-glycoprotein which is post-translationally regulated, but without any concomitant overexpression of P-glycoprotein mRNA. Finally, the possible clinical relevance of these variable patterns of cross-resistance to the antitumour drugs commonly used in the clinic is considered.