TLR4 as a negative regulator of keratinocyte proliferation

Abstract

TLR4 is an innate immune receptor with expression in human skin, keratinocytes as well as squamous cell carcinoma (SCC) of the skin. In the present study we investigate the role of TLR4 as a negative regulator of keratinocyte proliferation. We present here that the expression of TLR4 increased with the differentiation of cultured keratinocytes in a passage-dependent manner or under calcium-rich conditions. Moreover, the down-regulation of TLR4 by specific knockdown increased the proliferation of HaCaT keratinocytes in vitro. In addition, subcutaneously injected HaCaT keratinocytes with shTLR4 formed growing tumors in nude mice. In contrast, we observed lower proliferation and increased migration in vitro of the SCC13 cell line stably overexpressing TLR4 in comparison to SCC13 TLR4 negative cells. In vivo, SCC13 TLR4-overexpressing tumors showed delayed growth in comparison to TLR4 negative tumors. The overexpression of TLR4 in SCC13 tumor cells was followed by phosphorylation of ERK1/2 and JNK and increased expression of ATF3. In gene expression arrays, the overexpression of TLR4 in tumor cells correlated with gene expression of ATF-3, IL-6, CDH13, CXCL-1 and TFPI. In summary, TLR4 negatively regulates the proliferation of keratinocytes and its overexpression reduces tumor growth of SCC cells.

TLR4 in skin cancer: From molecular mechanisms to clinical interventions

“…In addition, our analysis indicates that downstream intermediates of TLR4, such as MyD88 and NF‐kB also increase in protein expression during the progression to NMSC . However, a recent report noted a different pattern of epidermal staining, in which the TLR4 expression increased as cells differentiated away from the basal layers of the skin with no differential staining detectable between normal skin and cutaneous SCCs . These opposing immunohistochemical data may be attributable to different antibodies recognizing TLR4 isoforms, and highlight the need for further expression analysis of TLR4 in the skin.…”

“…31 However, a recent report noted a different pattern of epidermal staining, in which the TLR4 expression increased as cells differentiated away from the basal layers of the skin with no differential staining detectable between normal skin and cutaneous SCCs. 44 These opposing immunohistochemical data may be attributable to different antibodies recognizing TLR4 isoforms, and highlight the need for further expression analysis of TLR4 in the skin. Strikingly, the same report examined the role of TLR4 in cutaneous SCC (without the inclusion of UV exposure), providing evidence that genetic TLR4 antagonism in HaCaT keratinocytes results in increased proliferation, and that overexpression of TLR4 in cutaneous SCCderived cells reduced xenograft growth.…”

“…Strikingly, the same report examined the role of TLR4 in cutaneous SCC (without the inclusion of UV exposure), providing evidence that genetic TLR4 antagonism in HaCaT keratinocytes results in increased proliferation, and that overexpression of TLR4 in cutaneous SCCderived cells reduced xenograft growth. 44 This highlights the need for further mechanistic studies and improved analytical tools assessing the activity of TLR4 in cutaneous SCC. 44 In addition to its role as a key regulator of inflammatory cellular signaling, the importance of TLR4 in the development of NMSC has been attributed to a variety of other molecular mechanisms.…”

“…44 This highlights the need for further mechanistic studies and improved analytical tools assessing the activity of TLR4 in cutaneous SCC. 44 In addition to its role as a key regulator of inflammatory cellular signaling, the importance of TLR4 in the development of NMSC has been attributed to a variety of other molecular mechanisms. TLR4 is integral for UV-induced photoimmunosuppression, as well as DNA repair of dendritic and epidermal cells after UV exposure.…”

A single-step genome wide association study on Body Size Traits using imputation-based whole-genome sequence data in Yorkshire pigs

“…According to bioinformatics analysis, CDH13 near SCC6:5671575 could be used as a candidate gene affecting body length. CDH13 is a unique cadherin [44] that regulates cell adhesion, signal transduction and cell growth [29], and plays an important role in the formation of tissues and organs [22]. The ingestion and transfer of Ca will affect the bone development of body for a long time [27], therefore, CDH13…”