HAMLET - A Protein Folding Variant with Tumoricidal Activity

HAMLET (Humana-lactalbumin made lethal to tumor cells) is a protein-lipid complex that induces apoptosis in tumor cells, but leaves fully differentiated cells unaffected. The lecture will summarize the information on the molecular characteristics of the complex, the tumor cell biology and the in vivo effects of HAMLET in patients and tumor models.

a) Structure. HAMLET is formed when a-lactalbumin unfolds by releasing the strongly bound Ca2+ ion. The unfolded protein exposes a new fatty acid binding site that fits oleic acid (C18:1,9 cis) (1). The unfolded protein or the fatty acid alone do not trigger cell death in the same manner as the HAMLET complex. The results illustrate how protein-folding variants may be beneficial, and how their formation in peripheral tissues may depend on the folding change and the availability of the lipid cofactor. The acid pH in the stomach unfolds a-lactalbumin and releases oleic acid suggesting that HAMLET might be formed in vivo, and be beneficial to the breast fed child.

b) Mechanism of action. HAMLET kills >40 different lymphoma and carcinoma cell lines in vitro. The broad anti-tumor activity is due to a new mechanism of cell death. The dying cells show apoptosis like features, but tumor cell death is independent of caspases and the p53 or bcl-2 genotype of the cells. The sensitivity of tumor cells reflects the massive uptake of HAMLET. The complex enters the cytoplasm of tumor cells, and translocates to the nuclei, where it accumulates. In the cytoplasm, HAMLET targets proteasomes and mitochondria. In the nuclei, HAMLET binds strongly to histones and disrupts the chromatin. Microarray studies have shown marked response differences between tymor cells and healthy cells, and have identified poteitial effector mechanisms of cell death which are being explored.  HAMLET thus activates fundamental death response pathways that remain active in tumor cells (2).

c) Effects  on tumors in vivo. HAMLET limits the progression of human glioblastomas in a rat xenograft model (3). In a placebo-controlled study, HAMLET was shown to be therapeutic in skin papilloma patients (4). Studies in bladder cancer patients have shown that HAMLET triggers a rapid death response in tumor cells (5). HAMLET thus shows great promise as a new anti-cancer agent.

In summary, HAMLET is an interesting tool to understand conserved cell death mechanisms in tumor cells, and a new tool in tumor therapy.

1)    Svensson et al, PNAS, 2000.
2)    Svanborg, Advances in Cancer Research. 2002
3)    Fischer et al, Cancer Res. 2003.
4)    Gustafsson, et al, N Engl J Med, 2004.
5)   Mossberg, et al, submitted