Induction of metallothionein expression during monocyte to melanoma-associated macrophage differentiation

Tumor-associated macrophages (TAMs) play a critical role in melanoma growth and metastasis. Infiltration of TAMs correlates with the poor prognosis of melanoma. TAMs are differentiated from monocytes in response to the tumor microenvironment cue. However, the mechanism how TAMs adapt to the tumor microenvironment after differentiation from monocytes is not fully understood. In addition, specific identification of TAMs in melanoma is difficult because the expression of the most commonly used macrophage marker, CD68, is also expressed in melanoma cells. In an earlier study, we found by gene microarray analysis that seven members of the metallothionein (MTs) family were upregulated in melanoma-conditioned medium induced macrophages (MCIM-Mφ). MTs have been implicated in zinc metabolism and inflammation. In the present study, we confirmed that expression of metallothionein is induced in M-CSF differentiated macrophages (M-CSF/Mφ) and MCIM-Mφ at both the mRNA and protein levels using real-time PCR, immunofluorescence, and western blot analysis. Furthermore, we demonstrated the presence of metallothionein in melanoma tissues in vivo and that metallothionein was co-localized with TAMs markers, CD68 and CD163. Finally, we demonstrated the induction of the zinc importer gene Zip8 both in M-CSF/Mφ and MCIM-Mφ. Our study identifies metallothionein as a novel marker for TAMs and suggests that metallothionein might play important roles in macrophage adaptation and function in the tumor microenvironment.