Cloning, overexpression, purification, and modeling of a lectin (Rhinocelectin) with antiproliferative activity from Tiger Milk Mushroom, Lignosus rhinocerus.

A lectin gene from the Tiger Milk Mushroom Lignosus rhinocerus TM02® was successfully cloned and expressed via vector pET28a in Escherichia coli BL21(DE3). The recombinant lectin, Rhinocelectin, with a predicted molecular mass of 22.8 kDa, was overexpressed in water-soluble form without signal peptide and purified via native affinity chromatography Ni-NTA agarose. Blast protein analysis indicated the lectin to be homologous to jacalin-related plant lectin. In its native form, Rhinocelectin exists as a homo-tetramer predicted with four chains of identical proteins consisting of 11 beta-sheet structures with only one alpha-helix structure. The antiproliferative activity of the Rhinocelectin against human cancer cell lines was concentration dependent and selective. The IC₅₀ values against triple negative breast cancer cell lines MDA-MB-231 and breast cancer MCF-7 are 36.52 ± 13.55 μg mL^-1 and 53.11 ± 22.30 μg mL^-1 , respectively. Rhinocelectin is only mildly cytotoxic against the corresponding human nontumorigenic breast cell line 184B5 with IC₅₀ value at 142.19 ± 36.34 μg mL^-1 . The IC₅₀ against human lung cancer cell line A549 cells is 46.14 ± 7.42 μg mL^-1 while against nontumorigenic lung cell line NL20 is 41.33 ± 7.43 μg mL^-1 . The standard anticancer drug, Doxorubicin exhibited IC₅₀ values mostly below 1 μg mL^-1 for the cell lines tested. Flow cytometry analysis showed the treated breast cancer cells were arrested at G0/G1 phase and apoptosis induced. Rhinocelectin agglutinated rat and rabbit erythrocytes at a minimal concentration of 3.125 μg mL^-1 and 6.250 μg mL^-1 , respectively.