The multidrug resistance (MDR) phenotype, induced by the overexpression of several ABC transporters or by antiapoptotic mechanisms, has been identified as the major cause of drug resistance in the treatment of patients with acute myeloid leukemia (AML). In this study, we have shown that valproic acid (VPA) (a histone deacetylase inhibitor) can inhibit the proliferation of both P-glycoprotein (P-gp)-and MDR-associated protein 1 (MRP1)-positive and -negative cells. VPA also induced apoptosis of P-gp-positive cells. VPA induced apoptosis in K562 cells led to decrease in Flip (FLICE/caspase-8 inhibitory protein) expression with Flip cleavage, which could not be observed in HL60 cells. In HL60/MRP cell line, which proved to be resistant to apoptosis by VPA, we observed an abnormal expression of apoptotic regulatory proteins, overexpression of Bcl-2 and absence of Bax. Also, the Bcl-2 antagonist HA14-1 rapidly restored apoptosis in this cell line. Cotreatment with cytosine arabinoside induced very strong apoptosis in both K562/DOX and HL60/DNR cell lines. VPA also induced apoptosis in AML patient cells expressing P-gp and/or MRP1. Our findings show VPA as an interesting drug that should be tested in clinical trials for overcoming the MDR phenotype in AML patients.

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Multidrug resistance (MDR) is characterized by a decrease inof anthracyclines (N-acetyl-daunorubicin) or vinca-alkaloids the efficiency of chemotherapeutic agents correlated with the (vindoline), steroids (progesterone), and other lipophilic catexpression and activity of a membrane protein: the ionic molecules (amiodarone, dipyridamole, S9788 5 ) (for permeability-glycoprotein (Pgp 170). Clinically, detection of reviews see Refs 6 and 7). Some compounds including verapa-

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The conjunction of clinical features, cell morphology and immunological characteristics allows an accurate diagnosis in most cases of B cell chronic lymphoproliferative disorders (CLD). However, the diagnosis remains uncertain in a small percentage of cases, often referred as to unclassified B cell proliferation or atypical chronic lymphocytic leukemia (CLL). We have studied retrospectively the 192 cases of leukemic CLD seen in our institution over a 3-year period, for which both clinical and routine biological data at presentation were available. Forty cases (20%) did not fit into any of the well-identified categories according to the FAB criteria and remained unclassified. We assessed cyclin D1 expression in all of these cases and found that 10 of them expressed a high level of cyclin D1 protein. We compared the characteristics of these 10 cases with those of the 30 cyclin D1 negative CLD. Despite non-distinctive cytological and phenotypic features, the 10 cyclin D1 positive patients exhibited a strikingly uniform clinical presentation with elevated leukocytosis, massive spleen enlargement and no superficial lymphadenopathy. Their outcome was very poor with a median survival of 10 months, contrasting with the prolonged survival of the cyclin D1 negative patients. The cytological features of tumor cells from these 10 patients with cyclin D1 positive unclassified leukemic CLD were similar to those of the circulating lymphoid cells from 15 patients with histologically proven mantle cell lymphoma (MCL) and primary or secondary blood involvement. Therefore, cyclin D1 expression allowed identification among the unclassified CLD, a subset of aggressive disorders which represent a leukemic counterpart of MCL (mantle cell leukemia). We suggest that determination of cyclin D1 expression by any technique available should be systematically included when investigating atypical CLL.