Lgr6 is a stem cell marker in mouse skin squamous cell carcinoma

Abstract

The G-protein-coupled receptors Lgr4/5/6 are Wnt signalling mediators, but their functions in squamous carcinomas (SCCs) are unclear. Using lineage tracing in Lgr5-EGFP-CreERT2- and Lgr6-EGFP-CreERT2- Rosa26/Tomato reporter mice, we demonstrate that Lgr6, but not Lgr5, acts as an epithelial stem cell marker in vivo in SCCs. We identify, by single molecule in situ hybridisation and cell sorting, rare Lgr6-positive cells in immortalised keratinocytes, and show that their frequency increases in advanced SCCs. Lgr6 expression is enriched in cells with stem cell characteristics, and Lgr6 downregulation in vivo causes increased epidermal proliferation, with expanded lineage tracing from Lgr6+ epidermal stem cells. Surprisingly, Lgr6 germline knockout mice are predisposed to SCC development, by a mechanism that includes compensatory upregulation of Lgr5. These data provide a model for human patients with germline loss of function mutations in WNT pathway genes RSPO1 or LGR4, who show increased susceptibility to squamous tumour development.

Pathogenesis of Skin Carcinomas and a Stem Cell as Focal Origin

“…Our group could not detect any appreciable presence of Lgr6+ cells in tumors and only some sporadic remnants of progeny deep into the differentiated compartments, i.e., no indication that Lgr6+ cells were TICs or drove tumor growth ( 63 ). In contrast, Huang et al ( 62 ) reported the presence of Lgr6+ cells in chemically induced tumors in a different mouse strain than we used, and with a different protocol for lineage tracing. However, these Lgr6+ cells did not exclusively reside in the germinative compartment of the tumors, showed a lack of expression of K14 (marker of germinative basal cells) and some even appeared flattened out in terminal differentiation.…”

“…Recently, a new class of proliferating stem cells (either Lgr5+ or Lgr6+) was studied as possible TICs in chemical and UV carcinogenesis; this was done by “lineage tracing” to identify the progeny of these stem cells in tumors. Lgr5+ cells and progeny were not detected in either chemically or UV-induced tumors ( 61 , 62 ). Our group could not detect any appreciable presence of Lgr6+ cells in tumors and only some sporadic remnants of progeny deep into the differentiated compartments, i.e., no indication that Lgr6+ cells were TICs or drove tumor growth ( 63 ).…”

“…Apparently Lgr6 was no longer a marker of stem cells in these tumors (reminiscent of what we found with Mts24/Plet1). Intriguingly, Huang et al ( 62 ) concluded from experiments with Lgr6 knockout mice that Lgr6 in normal epidermis functioned as a tumor suppressor. Interestingly in this respect, we found that Lgr6+ cells and progeny were lost from IF epidermis under chronic UV exposure long before the occurrence of SCCs; in contrast, a TPA regimen caused a clear expansion of progeny in the IF epidermis ( 63 ).…”

Cancer: the dark side of wound healing

“…LGR6 is a marker associated with stem/progenitor cells within the IFE or HF niche . Unsurprisingly, LGR6 was also found to be a specific CSC marker in mouse SCC models . The contribution of injury‐induced HFSC migration to cancer is demonstrated in two contiguous articles published in 2011 on transgenic BCC mouse models.…”

High expression of CD34 and α6-integrin contributes to the cancer-initiating cell behaviour in ultraviolet-induced mouse skin squamous cell carcinoma

“…Most of the identified cell surface markers suggest that cancer-initiating cells originate from embryonic or adult stem cells via the accumulation of epigenetic and genetic alterations 12 . Recently, both Lgr5 and Lgr6 were reported to be epithelial stem cell markers in SCC 13 , 14 .…”