Low-grade dermal angiosarcoma of the breast following radiotherapy.

Dermal angiosarcoma is a rare complication of radiotherapy. It is characterized by an aggressive nature and is distinct from other forms of angiosarcoma in that it does not always present with chronic lymphedema. A few case reports have appeared of cutaneous angiosarcoma arising in women previously treated with breast-conserving treatment and radiation for breast cancer. A review of the literature reveals that these lesions are difficult to diagnose because of their rarity and their highly variable and benign appearance, which may mimic radiation-induced changes in the skin. The majority of the literature reports describe high-grade lesions; only one case of low-grade angiosarcoma was described. We now report a second case of low-grade angiosarcoma arising 10 years after segmental mastectomy, axillary lymph node dissection, and radiation treatment for infiltrating ductal carcinoma. The prompt diagnosis of dermal angiosarcoma is strongly dependent upon a high index of suspicion. Biopsy should be considered in patients who present with new skin lesions after radiation treatment for breast cancer.

Sphingomyelin inhibits the lecithin-cholesterol acyltransferase reaction with reconstituted high density lipoproteins by decreasing enzyme binding.

Lecithin-cholesterol acyltransferase (LCAT) catalyzes the formation of cholesterol esters on high density lipoproteins (HDL) and plays a critical role in reverse cholesterol transport. Sphingomyelin, an important constituent of HDL, may regulate the activity of LCAT at any of the key steps of the enzymatic reaction: binding of LCAT to the interface, activation by apo A-I, or inhibition at the catalytic site. In order to clarify the role of sphingomyelin in the regulation of the LCAT reaction and its effects on the structure of apolipoprotein A-I, we prepared reconstituted HDL (rHDL) containing egg phosphatidylcholine, cholesterol, apolipoprotein A-I, and up to 22 mol % sphingomyelin. Because the interfacial properties of substrate particles can dramatically affect LCAT binding and kinetics, we also prepared and analyzed proteoliposome substrates having the same components as the rHDL, except for a 4-fold higher ratio of phospholipid to apolipoprotein A-I. The reaction kinetics of LCAT with the rHDL particles revealed no significant change in the apparent Vmax but showed a concentration-dependent increase in slope of the reciprocal plots and in the apparent Km values with sphingomyelin content. The dissociation constant (Kd) for LCAT with these particles increased linearly with sphingomyelin content up to 22 mol %, changing in parallel with the apparent Km values. No structural changes of apolipoprotein A-I were detected in the particles with increasing content of sphingomyelin, but fluorescence results with lipophilic probes revealed that significant changes in the acyl chain, backbone, and head group regions of the lipid bilayer of the particles are introduced by the addition of sphingomyelin. On the other hand, the proteoliposome substrates also had increased Kdvalues for LCAT at high sphingomyelin contents but compared with the rHDL particles had a 6-10-fold lower affinity for LCAT binding and exhibited kinetics consistent with competitive inhibition by sphingomyelin at the active site. These results show conclusively that the dominant mechanism for the inhibition of LCAT activity with rHDL particles by sphingomyelin is the impaired binding of the enzyme to the interface. The results also underscore the significant differences in the enzyme reaction kinetics with different substrate particles.

Binding of lecithin:cholesterol acyltransferase to reconstituted high density lipoproteins is affected by their lipid but not apolipoprotein composition.

The reaction of lecithin:cholesterol acyltransferase (LCAT) with high density lipoproteins (HDL) is of critical importance in reverse cholesterol transport. We studied the relationship between LCAT binding and HDL composition using two assays to determine LCAT binding affinity for discoidal reconstituted HDL (rHDL). We prepared rHDL with egg phosphatidylcholine (egg-PC), cholesterol, and either apolipoprotein (apo) A-I or apoA-II. The rHDL, identical in lipid content, had sizes of 96 and 100 A, respectively. Binding constants (Kd) determined by the activity-inhibition method were 2.2 +/- 0.3 x 10(-7) and 1.1 +/- 0.3 x 10(-16) M, whereas those determined by a solid-phase method were 3.3 +/- 0.9 x 10(-7) and 3.7 +/- 0.9 x 10(-7) M for apoA-I and apoA-II rHDL, respectively. The stoichiometry was 1 LCAT bound/rHDL. The appVmax/appKm for apoA-I was 80-fold higher than for apoA-II rHDL. The large difference between LCAT-binding constants and enzymatic activity measurements for the two particles suggests that LCAT binding and activation by apolipoproteins are independent events. To determine the effects of phospholipid headgroup on LCAT binding affinity, we tested rHDL containing up to 16 mol % of egg phosphatidylethanolamine (egg-PE), egg phosphatidic acid (egg-PA), or bovine phosphatidylserine (PS). Kd was unchanged for PS rHDL, but increased 6-fold with increasing PE content. AppVmax increased with PE content, but decreased with PS or PA relative to egg-PC controls. AppKm increased in PE rHDL, but remained unchanged in rHDL with PA or PS. Fluorescence characterization of the lipid domains of rHDL shows small differences in the polarity of the headgroup region of PE rHDL. Thus, LCAT binding is influenced by the lipid, but not the protein composition of rHDL. AppVmax values reflect active site preferences, while appKm values reflect interfacial binding affinity.