Studies of
ceramide metabolism and function in a wide range of biological processes have revealed a role for this
lipid in regulating key cellular responses. Our research on the role of
sphingolipids in HIV entry has led to the hypothesis that modulation of
ceramide levels in target cells affects their susceptibility to
HIV infection by rearranging
HIV receptors. Cellular
ceramide levels were modulated by application of pharmacological agents such as
N-(4-hydroxyphenyl)retinamide (4-HPR, fenretinide), by treatment with
sphingomyelinase (Smase), or by exogenous addition of long-chain
ceramide, and determined after metabolic incorporation of [3H]
sphingosine. Infectivity assays were performed by using a HeLa-derived
indicator cell line, TZM-bl, CD4+ lymphocytes, and monocytes. We observed a dose-dependent inhibition by
4-HPR of
infection of TZM-bl cells by a broad range of HIV-1 isolates at low micromolar concentrations with an IC50 of <1 microM for most isolates tested. Nearly complete inhibition was seen at 5 microM, a dose that enhanced
ceramide levels by 50-100%, yet was nontoxic to the cells. Treating cells with other pharmacological agents that enhanced
ceramide levels, with Smase, or exogenous addition of long-chain
ceramide also resulted in inhibition of HIV-1
infection. Enhancing
ceramide levels in CD4+ lymphocytes and in monocyte-derived macrophages with
4-HPR or Smase significantly reduced infectivity without toxicity. The minimal toxicity of normal cells exposed to
4-HPR should make the
drug exceedingly suitable as an anti-HIV therapeutic.