The role of dietary heme in the cellular processes contributing to colorectal cancer progression

Abstract

In 2015, the International Agency for Research on Cancer (IARC) evaluated the consumption of red meat as probably carcinogenic to humans (Group 2A), but the mechanisms still remain unclear. [1] It has been shown that red meat contains higher amount of heme iron, which is a prosthetic group in hemoglobin, compared to white meat and therefore has been seen as probably associated with the formation of colorectal cancer (CRC). [2] The mutagenic and genotoxic potential of heme iron, along with its ability to induce reactive oxygen species (ROS), has already been verified; however, its role as a component of red meat in promoting tumor progression— particularly in relation to the epithelial-mesenchymal transition (EMT)—remains unclear.[3].[4] To assess how dietary heme iron affects the mechanisms of colorectal cancer progression—including invasion, migration, growth in low-attachment conditions (GILA), and single-cell colony formation—we conducted a series of biological assays in human colon epithelial cells (HCEC) and human colon cancer cells (HCT116).

The cytotoxic effects of heme iron and the well-established EMT inducer TNFa were evaluated in the mentioned cell lines using the resazurin reduction assay (RRA). To assess cell migration, manual scratches were introduced into confluent cell monolayers (Wound healing assay) followed by treatment with varying concentrations of heme or TNF_α. The ability for anchorage-independent cell growth was tested by GILA assay. The protein expression of GILA and wound healing assay was analyzed via Western Blotting. The capacity to form colonies from single cells was determined by colony formation assay in HCT116 cells.

Notably, heme treatment resulted in a significant, dose-dependent reduction in the migratory capacity compared to the TNF_α positive control in both cell lines. Nevertheless, heme-treated cells showed no significant changes in the protein expression of EMT-marker proteins, obtained via Western Blotting. Western-Blot analysis of the harvested cells from the GILA assay, treated with heme, showed a significant upregulation of HO-1 while decrease in EMT-marker protein SLUG in HCT116 cells. However, the colony formation assay using single HCT116 cells treated with heme revealed a slight dose-dependent reduction in colony-forming capacity, comparable to the effects observed with TNFa treatment.

In summary, dietary heme from red meat seems to reduce the EMT-related properties in human colon (cancer) cells. In the future the cellular mechanisms like heme-induced HO-1 expression will be focused in the context of tumor progression.