THE PHORBOL ESTER PHORBOL-MYRISTATE ACETATE INHIBITS HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 ENVELOPE-MEDIATED FUSION BY MODULATING AN ACCESSORY COMPONENT(S) IN CD4-EXPRESSING CELLS

The phorbol ester phorbol myristate acetate (PMA) strongly inhibits hu man immunodeficiency virus type 1 (HIV-1)-induced syncytium formation; it has been suggested that this inhibitory effect is due to the trans ient downmodulation of the surface-associated CD4 receptors by PMA (I. H. Chowdhury, Y. Koyanagi, S. Kobayashi, Y. Hamamoto, H. Yoshiyama, T . Yoshida, and N. Yamamoto, Virology 176:126-132, 1990). Surprisingly, PMA treatment of cells expressing truncated (A2.01.CD4.401) and hybri d (A2.01.CD4.CD8) CD4 molecules, which are not downmodulated (P. Bedin ger, A. Moriarty, R. C. von Borstel II, N. J. Donovan, K. S. Steimer, and D. R. Littman, Nature [London] 331:162-165, 1988), inhibited their fusion with CD4(-) (12E1) cells expressing vaccinia virus-encoded HIV -1 envelope glycoprotein (gp120-gp41) and with chronically HIV-1-infec ted H9 (MN, IIIB, or RF) cells. PMA pretreatment of T (12E1) and non-T (HeLa, U937.3, and Epstein-Barr virus-transformed B) cell lines expre ssing vaccinia virus-encoded CD4 also blocked fusion with 12E1 cells e xpressing vaccinia virus-encoded gp120-gp41. Interestingly, pretreatme nt of the gp120-gp41-expressing 12E1 cells with PMA did not alter thei r fusion with untreated CD4-expressing cells. Although the inhibitory effect of PMA was rapid and treatment for 1.5 h with 5 ng of PMA per m l was sufficient to reduce fusion by more than 50%, the recovery after treatment was slow and more than 40 h was needed before the cells reg ained half of their fusion potential. The inhibitory effect of PMA was blocked by staurosporine in a dose-dependent fashion, suggesting that it is mediated by protein kinase C. PMA treatment of A2.01.CD4.401 ce lls reduced the number of infected cells 6.7-fold, as estimated by a q uantitative analysis of the HIV-1 MN infection kinetics, probably by a ffecting the stage of virus entry into cells. CD26 surface expression was not significantly changed by PMA treatment. We conclude that PMA i nhibits the CD4-gp120-gp41-mediated fusion by modulating an accessory component(s), different from CD26, in the target CD4-expressing cells. These findings suggest a novel approach for identification of accesso ry molecules involved in fusion and may have implications for the deve lopment of antiviral agents.