The mechanisms of CTL-mediated tumor regression in vivo remain to be fully understood. If CTL do mediate tumor regression in vivo by direct cytotoxicity, this may occur via two major effector mechanisms involving the secretion of perforin/granzymes and/or engagement of Fas by Fas ligand (FasL) expressed by the activated CTL. Although the perforin pathway has been considered the dominant player, it is unclear whether Fas-mediated cytotoxicity is additionally required for optimal tumor rejection. Previously, we produced H-2Ld-restricted CTL reactive against the CMS4 sarcoma, which expresses a naturally occurring rejection Ag recognized by these CTL and harbors a cytokine (IFN-γ plus TNF)-inducible, Fas-responsive phenotype. The adoptive transfer of these CTL to syngeneic BALB/c mice with minimal (day 3 established) or extensive (day 10 established) experimental pulmonary metastases resulted in strong antitumor responses. Here we investigated whether a FasL-dependent CTL effector mechanism was important for optimal tumor regression in this adoptive immunotherapy model. The approach taken was to compare the therapeutic efficacy of wild-type to FasL-deficient (gld) CTL clones by adoptive transfer. In comparison with wild-type CTL, gld-CTL efficiently mediated tumor cytolysis and produced comparable amounts of IFN-γ, after tumor-specific stimulation, as in vitro assessments of Ag recognition. Moreover, gld-CTL mediated comparably potent antitumor effects in a minimal disease setting, but were significantly less effective under conditions of an extensive tumor burden. Overall, under conditions of extensive lung metastases, these data revealed for the first time an important role for a FasL-dependent CTL effector mechanism in optimal tumor regression.

-EJB 95 l670/1 I S 1 74T human colon carcinoma cells that contain approximately equal amounts of CAMP-dependent protein kinase (PKA) isozymes, PKA-I and PKA-11, were infecled with retroviral vectors coding for regulatory (R) and catalytic (C) subunits of human PKA. In cells overexpressing RIIcr, Rllij, and RTT, P (a RII,, mutant at the autophosphorylation site), PKA-JT levels increased whilc PKA-I levels decreased. PKA-I was almost completely eliminated in cells overexpressing RII,{ or RIT,,-P. In contrast, overexpression of either RI,, or C, had little or no effect on PKA isozyme levels. Although all infectants expressed high levels of PKA subunit inRNAs in accordance with gene introduction, the R subunit protein expression was reflected in PKA isozyme levels rather than in subunit mRNA levels. Only RII, infectants demonstrated marked growth inhibition in monolayer C U~~L I I .~, reduced thymidine incorporiition into DNA, and inability to grow in semisolid medium or in serum-free medium. Conversely, it11 other infectants displayed growth propcrties similar to uninfected parental cells. The growth-retardation properties of RIT, infectants were reflected i n their altered phenotypic appearances. Our findings that the mutant RIl,rP could not mimic the growth-inhibitory effect of RII, suggest thc functional importance of the autophosphurylation site in RII,,. Our results suggest a role for KIT,, in the suppression of neoplastic cell growth, and thus abnormal expression of R subunit isoforms of PKA may bc involved in neoplastic transformation.

In this study, we developed a mouse model of adoptive immunotherapy reflecting immune recognition of syngeneic tumor cells naturally expressing an endogenous rejection Ag. Specifically, in a pulmonary metastases model, we examined the potency and maintenance of an antitumor CD8+ CTL response in vivo, as well as its effectiveness against an “extensive” tumor burden. The approach taken was to first generate tumor-specific CTL from mice challenged with the CMS4 sarcoma coadministered with anti-CTLA4 mAb, which has been shown to facilitate the induction of Ag-specific T cell responses in vivo. An H-2Ld-restricted nonamer peptide, derived from an endogenous murine leukemia provirus was identified as a CMS4-reactive CTL epitope based upon the following: CTL cross-recognition of another syngeneic tumor cell line (CT26 colon carcinoma) previously characterized to express that gene product; sensitization of Ag-negative lymphoblasts or P815 targets with the peptide; and by cold target inhibition assays. In vivo, the adoptive transfer of CMS4-reactive CTL (≥1 × 106) resulted in nearly the complete regression of 3-day established lung metastases. Furthermore, mice that rejected CMS4 following a single adoptive transfer of CTL displayed antitumor activity to a rechallenge 45 days later, not only in the lung, but also at a s.c. distal site. Lastly, the adoptive transfer of CTL to mice harboring extensive pulmonary metastases (>150 nodules) led to a substantial reduction in tumor burden. Overall, these data suggest that the adoptive transfer of tumor-specific CTL may have therapeutic potential for malignancies that proliferate in or metastasize to the lung.

Fas, an important death receptor-mediated signaling pathway, has been shown to be down-regulated during human colon tumorigenesis; however, how alterations in Fas expression influence the metastatic process remains unresolved. In mouse models, loss of Fas function was found to be both necessary and sufficient for tumor progression. In this study, we investigated the link between functional Fas status and malignant phenotype using a matched pair of naturally occurring primary (Fas-sensitive) and metastatic (Fas-resistant) human colon carcinoma cell lines in both in vitro and in vivo (xenograft) settings. Metastatic sublines were produced in vitro from the primary tumor cell line by functional elimination of Fas-responsive cells. Conversely, sublines derived from the primary tumor in vivo at distal metastatic sites were Fas-resistant. In contrast, simply disrupting the Fas pathway by molecular-based strategies in the Fas-sensitive primary tumor failed to achieve the same metastatic outcome. Interestingly, both in vitro- and in vivo-produced sublines resembled the naturally occurring metastatic population, based on functional and morphologic studies and genome-scale gene expression profiling. Overall, using this human colon carcinoma model, we: 1) showed that loss of Fas function was linked to, but alone was insufficient for, acquisition of a detectable metastatic phenotype; 2) demonstrated that metastatic subpopulations pre-existed within the heterogeneous primary tumor, and that anti-Fas interactions served as a selective pressure for their outgrowth; and 3) identified a large set of differentially expressed genes distinguishing the primary from metastatic malignant phenotypes. Thus, Fas-based interactions may represent a novel mechanism for the biologic or immunologic selection of certain types of Fas-resistant neoplastic clones with enhanced metastatic ability.

Expression of the tropomyosin-1 isoform was enhanced by cDNA transfer in non-transformed murine 3T3 fibroblasts and also in v-Ki-ras transformed fibroblasts in which native tropomyosin-1 expression had been reduced and tropomyosin-2 synthesis virtually eliminated by action of the oncogene. The level of synthesis of insert-derived tropomyosin-1 was similar in normal and transformed transductants (3 -5 times normal levels). The high level of insert-derived tropomyosin-1 expression resulted in a considerable increase in tropomyosin-1 utilization in the cytoskeleton of transformed cells, but this expression still did not reach normal levels, suggesting an oncogenerelated inhibition of tropomyosin utilization. A large proportion of newly synthesized native tropomyosin-1 in normal, unmodified fibroblasts appeared in homodimers which, upon prolonged incubation, were largely converted to the heterodimers. Excess tropomyosin-1 derived from the inserted cDNA also appeared largely as the homodimer in both normal and transformed cells. This homodimer was utilized effectively in the formation of cytoskeletal structures but was partially converted to heterodimer by chain exchange. Under steady-state conditions, approximately 33 % of the cytoskeletal tropomyosin-1 -containing dimers were homodimers, compared to approximately 10% in normal fibroblasts. The results show that the increased amount of tropomyosin-1 homodimer entering the cytoskeleton under conditions of tropomyosin-1 excess, results in an atypical microfilament composition. The effect of this excess of tropomyosin-1 homodimers on stability or function of microfilament fibers remains to be determined. The results also confirm that the mechanisms of rapid homodimer formation with conversion to heterodimers by chain exchange, known from in vitro studies, also occur in vivo.

Prosolin is a major cytosolic protein (Mr 18400, isoelectric point 5.9) first reported in HL-60 promyelocytic leukemia cells. It is rapidly phosphorylated (15 to 30 min) in response to TPA treatment as an early event in a sequence that leads to cessation of cell proliferation and to differentiation of promyelocytes into monocytes. In our study we examined the expression of prosolin in human peripheral lymphocytes and investigated the effects of TPA treatment on prosolin phosphorylation and on lymphocyte proliferation. Prosolin was not expressed in resting PBL but was induced after 24 to 36 h of PHA stimulation, simultaneously with induction of DNA synthesis. In rapidly proliferating (IL-2 dependent) PBL prosolin was a major cytosolic component, comprising 0.5% of total cytosolic protein, of which approximately 28% was phosphorylated. Expression of prosolin decreased again when either mitogen-induced or IL-2-dependent proliferation diminished during extended periods in culture. Thus, expression of prosolin is correlated with periods when PBL are cycling through S-phase. TPA treatment of IL-2-dependent PBL at the peak of their growth caused phosphorylation of about two-thirds of preexisting unphosphorylated prosolin within 1 h. This was accompanied by cessation of cell proliferation, as indicated by measurements of TdR incorporation. Although TPA has well known mitogenic effects in lymphocytes during initial activation, this result shows that it exerts an antiproliferative effect in rapidly dividing PBL. It is suggested that increased phosphorylation of prosolin may be an initiating event in the antiproliferative response to TPA, which would occur only in proliferating lymphocytes expressing prosolin.

Prosolin is a major cytosolic phosphoprotein of proliferating normal PBL. Treatment of growing PBL with phorbol ester (12-O-tetradecanoylphorbol-13-acetate (TPA)) or calcium ionophore (A23187) for 1 h caused phosphorylation of prosolin with the production of up to four prominent phosphorylated forms differing in degree of phosphorylation and/or two-dimensional electrophoretic mobility (peptides B to E). Formation of these phosphopeptides coincided with rapid down-regulation of DNA synthesis. A23187 was particularly effective in inducing phosphorylation of the more highly phosphorylated peptides D and E, suggesting the existence of a (Ca2+)-activated mechanism in their phosphorylation. The T cell leukemia cell lines Jurkat, HuT-78, CCRF-CEM, and Molt-4 showed reduced to absent ability to phosphorylate prosolin peptides rapidly in response to A23187 and also showed diminished down-regulation of DNA synthesis. In leukemic cells treated with both TPA and A23187, peptides B and C were rapidly phosphorylated, but the phosphorylation of peptides D and E seen in normal PBL remained deficient. The T cell leukemic cells appear to have intact a TPA-activated mechanism for phosphorylating prosolin peptides B and C, but share an impairment of a specific Ca2(+)-activated mechanism, possibly a Ca2(+)-dependent protein kinase, required for phosphorylation of prosolin phosphopeptides D and E. The degree of rapid down-regulation of DNA synthesis was correlated with degree of phosphorylation of peptide E in PBL and in three of four T cell leukemic cell lines. Thus, rapid phosphorylation of prosolin may mediate responses to TPA and A23187 in normal proliferating PBL, including down-regulation of DNA synthesis. A deficiency of this pathway in leukemic T cells may impede their response to physiologic growth regulatory signals utilizing this pathway and contribute to unrestrained cell growth.

Prosolin is a major cytosolic phosphoprotein expressed prominently in rapidly proliferating human peripheral lymphocytes but produced at very low levels in resting (G0) PBL. It undergoes rapid phosphorylation upon treatment of growing cells with tumor-producing phorbol esters (TPA) and this phosphorylation event is correlated with a rapid down-regulation of DNA synthesis. In the present report we have studied various agents that, like TPA, act as partial or complete mitogens for G0 PBL and have determined their effect on phosphorylation of prosolin and on DNA synthesis in rapidly proliferating (IL-2-dependent) human PBL. Agents that activate the TCR (OKT3 and PHA), as well as agents that by-pass the receptor but activate biochemical pathways associated with TCR activation (TPA and Ca2(+)-ionophore), all produced rapid phosphorylation of prosolin and prompt down-regulation of DNA synthesis. Four phosphorylated forms of prosolin were produced, indicating activation of a complex phosphorylation pathway. Down-regulation of DNA synthesis did not lead to cell death or to permanent arrest, but was reversed after 24 to 48 h, and was not associated with any reduction in overall protein synthesis. Agents that bind to determinants closely connected to the TCR but without activating it (OKT4 and OKT8) had no effect on either prosolin phosphorylation or DNA synthesis. The results indicate that prosolin is an early target of the protein kinase activities induced by activation of the TCR in proliferating PBL, and suggest that its phosphorylation mediates the TCR signal, transmitting it into a biochemical pathway leading specifically to down-regulation of DNA synthesis. In G0 PBL, in which the negligible expression of prosolin precludes significant production of phosphorylated species, this inhibitory pathway is effectively blocked.