Chemotherapy and TRAIL-mediated colon cancer cell death: the roles of p53, TRAIL receptors, and c-FLIP.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has recently attracted attention as a potential therapeutic agent in the treatment of cancer. We assessed the roles of p53, TRAIL receptors, and cellular Fas-associated death domain-like interleukin-1beta-converting enzyme inhibitory protein (c-FLIP) in regulating the cytotoxic effects of recombinant TRAIL (rTRAIL) alone and in combination with chemotherapy [5-fluorouracil (5-FU), oxaliplatin, and irinotecan] in a panel of colon cancer cell lines. Using clonogenic survival and flow cytometric analyses, we showed that chemotherapy sensitized p53 wild-type, mutant, and null cell lines to TRAIL-mediated apoptosis. Although chemotherapy treatment did not modulate mRNA or cell surface expression of the TRAIL receptors death receptor 4, death receptor 5, decoy receptor 1, or decoy receptor 2, it was found to down-regulate expression of the caspase-8 inhibitor, c-FLIP. Stable overexpression of the long c-FLIP splice form but not the short form was found to inhibit chemotherapy/rTRAIL-induced apoptosis. Furthermore, siRNA-mediated down-regulation of c-FLIP, particularly the long form, was found to sensitize colon cancer cells to rTRAIL-induced apoptosis. In addition, treatment of a 5-FU-resistant cell line with 5-FU down-regulated c-FLIP expression and sensitized the chemotherapy-resistant cell line to rTRAIL. We conclude that TRAIL-targeted therapies may be used to enhance conventional chemotherapy regimens in colon cancer regardless of tumor p53 status. Furthermore, inhibition of c-FLIP may be a vital accessory strategy for the optimal use of TRAIL-targeted therapies.

In situ expression of mononuclear cell markers and interleukin-2 receptor in renal allograft biopsies of acute rejection and borderline cases

The correct diagnosis of renal allograft rejection may be difficult using only clinical and/or histopathological criteria. Immunological assays should be considered in order to evaluate the phenotype of inflammatory infiltrate in renal allograft biopsies. Immunohistochemical studies were performed to detect mononuclear cells, CD4 and CD8 T lymphocytes, B lymphocytes, macrophages, null cells, and positive cells for interleukin-2 receptors. A total of 41 allograft biopsies classified into three groups were studied: acute cellular rejection (28 biopsies/22 patients), borderline (7 biopsies/5 patients) and control (6 biopsies/6 patients). In the rejection group (RG), increased cellularity was found mainly at the tubulo-interstitial level. Expression of CD8 positive cells was higher in RG when compared to borderline (BG) and control (CG) groups, respectively (0.9 vs. 0.0 vs. 0.35 cells/mm2; p < 0.001). Expression of macrophages was not statistically significant among the three groups (RG = 0.6 vs. BG = 0.2 vs. CG = 0.0 cells/mm2; p < 0.02). In the BG, CD4 + cells predominated (BG = 0.2 vs. RG = 0.05 vs. CG = 0.0 cells/mm2; p < 0.05). Clinically these patients were treated as cases of acute rejection. The numbers and different types of infiltrating cells did not correlate with patient's clinical outcome. Copyright © Informa Healthcare.

Spindle cell oncocytoma of the adenohypophysis: report of a case with a 16-year follow-up

Spindle cell oncocytoma (SCO) is a recently described, rare neoplasm of the anterior pituitary. Clinically and radiologically simulating a non-functioning macroadenoma, its eponymous fusiform cells display a non-epithelial phenotype with conspicuous cytoplasmic accumulation of mitochondria. We report a case of SCO retrospectively identified in a biopsy specimen 16 years after transsphenoidal operation of a 48-year-old woman. Presenting symptoms were adynamia and transient decrease of visual acuity. Neuroimaging showed an isointense, enhancing, sellar-centered mass 1.8 cm in diameter without evidence of invasive growth. No postoperative adjuvant therapy was administered. The patient was left with panhypopituitarism, yet no recurrence was seen during follow-up. Initially diagnosed as a null cell adenoma of oncocytic type, repeat immunohistochemistry showed the characteristic coexpression of S100 protein, vimentin, and epithelial membrane antigen. Oncocytic granula stained intensely with antimitochondrial antibody 113-1, and were negative with the lysosomal marker CD68. Anterior pituitary hormones tested negative, and there was no evidence of neuroendocrine differentiation using antibodies to synaptophysin and chromogranin. Few cells stained for glial fibrillary acidic protein (GFAP). SCO has been proposed to represent a neoplasm of folliculo-stellate cells (FSCs). While the dynamic properties of the latter are incompletely characterized, and indeed no specific marker allows for their identification, overlapping features of SCO with look alikes, in particular pituicytoma, point to FSCs being a potential adult stem cell. The favorable outcome of the present case further argues for SCO to be considered a low-grade neoplasm. Moderate tumor size, lack of invasiveness, and low proliferation rate are likely predictors of benign behavior.

The activation domain of the enhancer binding protein p45NF-E2 interacts with TAFII130 and mediates long-range activation of the α- and β-globin gene loci in an erythroid cell line

We have used the interaction between the erythroid-specific enhancer in hypersensitivity site 2 of the human β-globin locus control region and the globin gene promoters as a paradigm to examine the mechanisms governing promoter/enhancer interactions in this locus. We have demonstrated that enhancer-dependent activation of the globin promoters is dependent on the presence of both a TATA box in the proximal promoter and the binding site for the erythroid-specific heteromeric transcription factor NF-E2 in the enhancer. Mutational analysis of the transcriptionally active component of NF-E2, p45NF-E2, localizes the critical region for this function to a proline-rich transcriptional activation domain in the NH2-terminal 80 amino acids of the protein. In contrast to the wild-type protein, expression of p45 NF-E2 lacking this activation domain in an NF-E2 null cell line fails to support enhancer-dependent transcription in transient assays. More significantly, the mutated protein also fails to reactivate expression of the endogenous β- or α-globin loci in this cell line. Protein-protein interaction studies reveal that this domain of p45 NF-E2 binds specifically to a component of the transcription initiation complex, TATA binding protein associated factor TAFII130. These findings suggest one potential mechanism for direct recruitment of distal regulatory regions of the globin loci to the individual promoters.

Physiological levels of PTEN control the size of the cellular Ins(1,3,4,5,6)P5 pool

To understand how a signaling molecule's activities are regulated, we need insight into the processes controlling the dynamic balance between its synthesis and degradation. For the Ins(1,3,4,5,6)P5 signal, this information is woefully inadequate. For example, the only known cytosolic enzyme with the capacity to degrade Ins(1,3,4,5,6)P5 is the tumour-suppressor PTEN [J.J. Caffrey, T. Darden, M.R. Wenk, S.B. Shears, FEBS Lett. 499 (2001) 6 ], but the biological relevance has been questioned by others [E.A. Orchiston, D. Bennett, N.R. Leslie, R.G. Clarke, L. Winward, C.P. Downes, S.T. Safrany, J. Biol. Chem. 279 (2004) 1116 ]. The current study emphasizes the role of physiological levels of PTEN in Ins(1,3,4,5,6)P5 homeostasis. We employed two cell models. First, we used a human U87MG glioblastoma PTEN-null cell line that hosts an ecdysone-inducible PTEN expression system. Second, the human H1299 bronchial cell line, in which PTEN is hypomorphic due to promoter methylation, has been stably transfected with physiologically relevant levels of PTEN. In both models, a novel consequence of PTEN expression was to increase Ins(1,3,4,5,6)P5 pool size by 30-40% (p<0.01); this response was wortmannin-insensitive and, therefore, independent of the PtdIns 3-kinase pathway. In U87MG cells, induction of the G129R catalytically inactive PTEN mutant did not affect Ins(1,3,4,5,6)P(5) levels. PTEN induction did not alter the expression of enzymes participating in Ins(1,3,4,5,6)P5 synthesis. Another effect of PTEN expression in U87MG cells was to decrease InsP6 levels by 13% (p<0.02). The InsP6-phosphatase, MIPP, may be responsible for the latter effect; we show that recombinant human MIPP dephosphorylates InsP6 to D/L-Ins(1,2,4,5,6)P5, levels of which increased 60% (p<0.05) following PTEN expression in U87MG cells. Overall, our data add higher inositol phosphates to the list of important cellular regulators [Y. Huang, R.P. Wernyj, D.D. Norton, P. Precht, M.C. Seminario, R.L. Wange, Oncogene, 24 (2005) 3819 ] the levels of which are modulated by expression of the highly pleiotropic PTEN protein.

The Role of Thymidylate Synthase Induction in Modulating p53-regulated Gene Expression in Response to 5-Fluorouracil and Antifolates

Thymidylate synthase (TS) is a critical target for chemotherapeutic agents such as 5-fluorouracil (5-FU) and antifolates such as tomudex (TDX),multitargeted antifolate, and ZD9331. Using the MCF-7 breast cancer line, we have developed p53 wild-type (M7TS90) and null (M7TS90-E6) isogenic lines with inducible TS expression (approximately 6-fold induction compared with control after 48 h). In the M7TS90 line, inducible TS expression resulted in a moderate approximately 3-fold increase in 5-FU IC-50(72 h) dose and a dramatic >20-fold increase in the IC-50(72 h) doses of TDX, multitargeted antifolate, and ZD9331. S-phase cell cycle arrest and apoptosis induced by the antifolates were abrogated by TS induction. In contrast, cell cycle arrest and apoptosis induced by 5-FU was unaffected by TS expression levels. Inactivation of p53 significantly increased resistance to 5-FU and the antifolates with IC-50(72 h) doses for 5-FU and TDX of >100 and >10 microM, respectively, in the M7TS90-E6 cell line. Furthermore, p53 inactivation completely abrogated the cell cycle arrest and apoptosis induced by 5-FU. The antifolates induced S-phase arrest in the p53 null cell line; however, the induction of apoptosis by these agents was significantly reduced compared with p53 wild-type cells. Both inducible TS expression and the addition of exogenous thymidine (10 microM) blocked p53 and p21 induction by the antifolates but not by 5-FU in the M7TS90 cell line. Similarly, inducible TS expression and exogenous thymidine abrogated antifolate but not 5-FU-mediated up-regulation of Fas/CD95 in M7TS90 cells. Our results indicate that in M7TS90 cells, inducible TS expression modulates p53 and p53 target gene expression in response to TS-targeted antifolate therapies but not to 5-FU.

Effect of p53 Status and STAT1 on Chemotherapy-Induced, Fas-Mediated Apoptosis in Colorectal Cancer

We investigated the role of p53 and the signal transducer and activator of transcription 1 (STAT1) in regulating Fas-mediated apoptosis in response to chemotherapies used to treat colorectal cancer. We found that 5-fluorouracil (5-FU) and oxaliplatin only sensitized p53 wild-type (WT) colorectal cancer cell lines to Fas-mediated apoptosis. In contrast, irinotecan (CPT-11) and tomudex sensitized p53 WT, mutant, and null cells to Fas-mediated cell death. Furthermore, CPT-11 and tomudex, but not 5-FU or oxaliplatin, up-regulated Fas cell surface expression in a p53-independent manner. In addition, increased Fas cell surface expression in p53 mutant and null cell lines in response to CPT-11 and tomudex was accompanied by only a slight increase in total Fas mRNA and protein expression, suggesting that these agents trigger p53-independent trafficking of Fas to the plasma membrane. Treatment with CPT-11 or tomudex induced STAT1 phosphorylation (Ser727) in the p53-null HCT116 cell line but not the p53 WT cell line. Furthermore, STAT1-targeted small interfering RNA (siRNA) inhibited up-regulation of Fas cell surface expression in response to CPT-11 and tomudex in these cells. However, we found no evidence of altered Fas gene expression following siRNA-mediated down-regulation of STAT1 in drug-treated cells. This suggests that STAT1 regulates expression of gene(s) involved in cell surface trafficking of Fas in response to CPT-11 or tomudex. We conclude that CPT-11 and tomudex may be more effective than 5-FU and oxaliplatin in the treatment of p53 mutant colorectal cancer tumors by sensitizing them to Fas-mediated apoptosis in a STAT1-dependent manner.

Infrequent mutations of Archipelago (hAGO, hCDC4, Fbw7) in primary ovarian cancer

Objective: Archipelago (AGO, also known as hCdc4, Fbw7, or Sel-10) is an F-box containing component of the SCF complex implicated in the ubiquitination and proteolysis of cyclin E and c-Myc, and found to be mutated in 16% of endometrial carcinomas. We have previously reported somatic mutations in AGO in 3/10 ovarian cancer cell lines, but the frequency of such mutations in primary ovarian cancer is unknown.

Methods: The coding sequence of AGO was analyzed in 95 primary sporadic ovarian tumors and 16 cases of familial ovarian cancer, and correlated with levels of cyclin E and c-Myc protein expression. Constructs encoding mutations in AGO were transfected into an AGO-null cell line to directly test their ability to regulate cyclin E and c-Myc levels.

Results: Mutations were present in only 2 of 95 sporadic cases: a premature stop within the WD domain (471 Ter) and a missense change near the F-box (S245T). Both primary tumor specimens containing these mutations showed high levels of cyclin E and c-Myc, but reconstitution of an AGO-null cell line with constructs encoding these mutations showed 471 Ter to be inactive in regulating endogenous cyclin E and c-Myc levels, while the S245T mutant was indistinguishable from wild-type. No germ-line mutations were found in familial cases of ovarian cancer.

Conclusion: Somatic AGO mutations are infrequent in primary ovarian cancers and are unlikely to contribute to familial ovarian cancer. Reconstitution experiments, rather than measuring tumor levels of cyclin E and c-Myc, provide an effective approach to determine the functional significance of AGO mutations identified in human cancers.

Regulation of human dUTPase gene expression and p53-mediated transcriptional repression in response to oxaliplatin-induced DNA damage

Deoxyuridine triphosphate nucleotidohydrolase (dUTPase) catalyzes the hydrolysis of dUTP to dUMP and PPi. Although dUTP is a normal intermediate in DNA synthesis, its accumulation and misincorporation into DNA is lethal. Importantly, uracil misincorporation is a mechanism of cytotoxicity induced by fluoropyrimidine chemotherapeutic agents including 5-fluorouracil (5-FU) and elevated expression of dUTPase is negatively correlated with clinical response to 5-FU-therapy. In this study we performed the first functional characterization of the dUTPase promoter and demonstrate a role for E2F-1 and Sp1 in driving dUTPase expression. We establish a direct role for both mutant and wild-type forms of p53 in modulating dUTPase promoter activity. Treatment of HCT116 p53(+/+) cells with the DNA-damaging agent oxaliplatin induced a p53-dependent transcriptional downregulation of dUTPase not observed in the isogenic null cell line. Oxaliplatin treatment induced enrichment of p53 at the dUTPase promoter with a concomitant reduction in Sp1. The suppression of dUTPase by oxaliplatin promoted increased levels of dUTP that was enhanced by subsequent addition of fluoropyrimidines. The novel observation that oxaliplatin downregulates dUTPase expression may provide a mechanistic basis contributing to the synergy observed between 5-FU and oxaliplatin in the clinic. Furthermore, these studies provide the first evidence of a direct transcriptional link between the essential enzyme dUTPase and the tumor suppressor p53.

Leiomyomatoid angiomatous neuroendocrine tumor (LANT) of the pituitary: a distinctive biphasic neoplasm with primitive secretory phenotype and smooth muscle-rich stroma

We describe a hitherto undocumented variant of dimorphic pituitary neoplasm composed of an admixture of neurosecretory cells and profuse leiomyomatous stroma around intratumoral vessels. Radiologically perceived as a macroadenoma of 3.8 cm in diameter, this pituitary mass developed in an otherwise healthy 43-year-old female. At the term of a yearlong history of amenorrhea and progressive bitemporal visual loss, subtotal resection was performed via transsphenoidal microsurgery. Discounting mild hyperprolactinemia, there was no evidence of excess hormone production. Histologically, solid sheets, nests and cords of epithelial-looking, yet cytokeratin-negative cells were seen growing in a richly vascularized stroma of spindle cells. While strong immunoreactivity for NCAM, Synaptophysin and Chromogranin-A was detected in the former, the latter showed both morphological and immunophenotypic hallmarks of smooth muscle, being positive for vimentin, muscle actin and smooth muscle actin. Architectural patterns varied from monomorphous stroma-dominant zones through biphasic neuroendocrine-leiomyomatous areas, to pseudopapillary fronds along vascular cores. Only endothelia were labeled with CD34. Staining for S100 protein and GFAP, characteristics of sustentacular cells, as well as bcl-2 and c-kit was absent. Except for alpha-subunit, anterior pituitary hormones tested negative in tumor cells, as did a panel of peripheral endocrine markers, including serotonin, somatostatin, calcitonin, parathormone and vasoactive intestinal polypeptide. Mitotic activity was absent and the MIB-1 labeling index low (1-2%). While assignment of this lesion to any established neoplastic entity is not forthcoming, we propose it is being considered as a low-grade neuroendocrine tumor possibly related to null cell adenoma.

Folliculo-stellate cells of "true dendritic" type are involved in the inflammatory microenvironment of tumor immunosurveillance of pituitary adenomas

Folliculo-stellate cells are a nonendocrine, sustentacular-like complementary population of the anterior pituitary. They currently are considered as functionally and phenotypically heterogeneous, with one subpopulation of folliculo-stellate cells possibly representing resident adenohypophyseal macrophages. We took advantage of a limited T-cell mediated inflammatory reaction selectively involving tumor tissue in three cases of pituitary adenoma (2 prolactin cell adenomas, and 1 null cell adenoma) to test the hypothesis whether some folliculo-stellate cells within inflammatory foci would also assume monocytic/dendritic properties. Immunohistochemical double labeling for S-100 protein and the class II major histocompatibility antigen HLA-DR indeed showed several arborized cells to coexpress both epitopes. These were distributed both amidst adenomatous acini and along intratumoral vessels, and were morphologically undistinguishable from conventional folliculo-stellate cells. On the other hand, markers of follicular dendritic cells (CD21) and Langerhans' cells (CD1a) tested negative. Furthermore, no S-100/HLA-DR coexpressing folliculo-stellate cells were seen in either peritumoral parenchyma of the cases in point nor in control pituitary adenomas lacking inflammatory reaction. These findings suggest that a subset of folliculo-stellate cells may be induced by an appropriate local inflammatory microenvironment to assume a dendritic cell-like immunophenotype recognizable by their coexpression of S-100 protein and HLA-DR. By analogy with HLA-DR expressing cells in well-established extrapituitary inflammatory constellations, we speculate that folliculo-stellate cells with such immunophenotype may actually perform professional antigen presentation. A distinctly uncommon finding in pituitary adenomas, lymphocytic infiltrates may therefore be read as a manifestation of tumoral immunosurveillance.

Peptide receptor radionuclide therapy in a patient with disabling non-functioning pituitary adenoma

Non-functioning pituitary adenoma (NFPA) with higher proliferation index (WHO II) are often a therapeutical challenge. Low somatostatin receptor expression in these tumors usually prevents a treatment with somatostatin analogs. In 1996, a 55-year-old patient was referred due to right-sided headache. A pituitary macroadenoma with infiltration into the right cavernous sinus was diagnosed. There was no visual field deficit and the clinical and biochemical work up was consistent with a NFPA. The patient underwent transsphenoidal surgery. Residual adenoma remained in the right cavernous sinus. Histologically, a null-cell adenoma with a high proliferation index was documented (MIB-1: 11.6 %, WHO II). Somatostatin receptor autoradiography was performed in the surgical specimen showing a homogenous expression of sst2 receptors. Radiosurgery was completed with stable disease for 8 years. In 2004, the patient was diagnosed with an incomplete palsy of the right oculomotorius nerve and a significant increase in the volume of the adenoma in the right cavernous sinus. After a positive Octreoscan(®) the patient consented to an experimental therapy approach using Lutetium DOTATOC (3 × 200 mCi). The palsy of the oculomotorius nerve improved and remained stable until today (March 2013), the follow-up MRI scans demonstrated stable disease. This is the first case of a patient with a NFPA (WHO II) in whom PRRT successfully improved the local complications of the tumor for more than 8 years after ineffective surgery and gamma knife therapy. The determination of sst2 in vitro using autoradiography and in vivo by Octreoscan was instrumental to administer this therapy in a challenging situation.

Analysis of endogenous retinoic acid receptor expression and function in murine melanoma cells

There have been numerous reports over the past several years on the ability of vitamin A analogs (retinoids) to modulate cell proliferation, malignant transformation, morphogenesis, and differentiation in a wide variety of cell types and organisms. Two families of nuclear retinoid-inducible, trans-acting, transcription-enhancing receptors that bear strong DNA sequence homology to thyroid and steroid hormone receptors have recently been discovered. The retinoic acid receptors (RARs) and retinoid X receptors (RXRs) each have at least three types designated $\alpha,$ $\beta,$ and $\gamma,$ which are encoded by separate genes and expressed in a tissue and cell type-specific manner. We have been interested in the mechanism by which retinoids inhibit tumor cell proliferation and induce differentiation. As a model system we have employed several murine melanoma cell lines (S91-C2, K1735P, and B16-F1), which are sensitive to the growth-inhibitory and differentiation-inducing effects of RA, as well as a RA-resistant subclone of one of the cell lines (S91-C154), in order to study the role of the nuclear RARs in these effects. The initial phase of this project consisted of the characterization of the expression pattern of the three known RAR and RXR types in the murine melanoma cell lines in order to determine whether any differences exist which may elucidate a role for any of the receptors in RA-induced growth inhibition and differentiation. The novel finding was made that the RAR-$\beta$ gene is rapidly induced from undetectable levels by RA treatment at the mRNA and protein level, and that the induction of RAR-$\beta$ by other biologically active retinoids correlated with their ability to inhibit the growth of the highly RA-sensitive S91-C2 cell line. This suggests a role for RAR-$\beta$ in the growth inhibiting effect of retinoids. The second phase of this project involves the stable expression of RAR-$\beta$ in the S91-C2 cells and the RAR-$\beta$ receptor-null cell line, K1735P. These studies have indicated an inverse correlation between RAR-$\beta$ expression and proliferation rate. ^

Functional activation of p53 induces apoptosis in the absence of MDM2

The p53 tumor suppressor gene product is negatively regulated by the product of its downstream target, mdm2. The mdm2 oncogene abrogates p53 transactivation function. Amplification of mdm2 occurs in 36% of human sarcomas, which often retain p53 in wild type form, suggesting that overexpression of mdm2 in tumors results in p53 inactivation. Thus, the relationship of p53 to mdm2 is important in tumorigenesis. The deletion of mdm2 in the mouse results in embryonic lethality by 5.5 days post coitum. Embryonic lethality of the mdm2 null embryos was overcome by simultaneous loss of the p53 tumor suppressor, which substantiates the importance of the negative regulatory function of MDM2 on p53 function in vivo. These data suggest that the loss of MDM2 function allowed the constitutively active p53 protein to induce either a complete G1 arrest or the p53-dependent apoptotic pathway, resulting in the death of the mdm2−/− embryos.^ The present study examines the hypothesis that the absence of mdm2 induces apoptosis due to p53 activation. Viability of the p53−/−mdm2−/− mice has allowed establishment of mouse embryo fibroblasts (MEFs) and a detailed examination of the properties of these cells. To introduce p53 into this system, and essentially recreate a mdm2 null cell, a temperature sensitive p53 (tsp53) point mutant (A135V) was used, which exhibits a nonfunctional, mutant conformation at 39°C and wild type, functional conformation at 32°C. Infected pools of p53−/− and p53−/−mdm2−/− MEFs with the tsp53 gene were established and single-cell clonal populations expressing tsp53 were selected. Shifting the cells from 39°C to 32°C caused p53−/−mdm2 −/− lines expressing tsp53 to undergo up to 80% apoptosis, which did not occur in the p53−/− lines expressing tsp53 nor the parental lines lacking p53 expression. Furthermore, the amount of p53 present in the clonal population determined the extent of apoptosis. Tsp53 is transcriptionally active in this system, however, it discriminates among different target promoters and does not induce the apoptosis effector targets bax or Fas/Apo1. ^ In summary, this study indicates that the presence or absence of mdm2 is the determining factor for the ability of p53 to trigger apoptosis in this system. The loss of mdm2 promotes p53-dependent apoptosis in MEFs in a cell cycle and dose-dependent manner. p53 is differentially phosphorylated in the presence and absence of mdm2, but does not induce the apoptosis effectors, bax or Fas/ Apo1. ^

A Developmentally Regulated Kinesin-related Motor Protein from Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum is an attractive system for studying the roles of microtubule-based motility in cell development and differentiation. In this work, we report the first molecular characterization of kinesin-related proteins (KRPs) in Dictyostelium. A PCR-based strategy was used to isolate DNA fragments encoding six KRPs, several of which are induced during the developmental program that is initiated by starvation. The complete sequence of one such developmentally regulated KRP (designated K7) was determined and found to be a novel member of the kinesin superfamily. The motor domain of K7 is most similar to that of conventional kinesin, but unlike conventional kinesin, K7 is not predicted to have an extensive α-helical coiled-coil domain. The nonmotor domain is unusual and is rich in Asn, Gln, and Thr residues; similar sequences are found in other developmentally regulated genes in Dictyostelium. K7, expressed in Escherichia coli, supports plus end–directed microtubule motility in vitro at a speed of 0.14 μm/s, indicating that it is a bona fide motor protein. The K7 motor is found only in developing cells and reaches a peak level of expression between 12 and 16 h after starvation. By immunofluorescence microscopy, K7 localizes to a membranous perinuclear structure. To examine K7 function, we prepared a null cell line but found that these cells show no gross developmental abnormalities. However, when cultivated in the presence of wild-type cells, the K7-null cells are mostly absent from the prestalk zone of the slug. This result suggests that in a population composed largely of wild-type cells, the absence of the K7 motor protein interferes either with the ability of the cells to localize to the prestalk zone or to differentiate into prestalk cells.