Novel role for the testis‐enriched HSPA2 protein in regulating epidermal keratinocyte differentiation

Abstract

HSPA2, a poorly characterized member of the HSPA (HSP70) chaperone family, is a testis-enriched protein involved in male germ cell differentiation. Previously, we revealed that HSPA2 is present in human stratified epithelia, including epidermis, however the contribution of this protein to epithelial biology remained unknown. Here, we show for the first time that HSPA2 is expressed in basal epidermal keratinocytes, albeit not in keratinocytes exhibiting features attributed to primitive undifferentiated progenitors, and participates in the keratinocyte differentiation process. We found that HSPA2 is dispensable for protection of HaCaT keratinocytes against heat shock-induced cytotoxicity. We also shown that lentiviral-mediated shRNA silencing of HSPA2 expression in HaCaT cells caused a set of phenotypic changes characteristic for keratinocytes committed to terminal differentiation such as reduced clonogenic potential, impaired adhesiveness and increased basal and confluency-induced expression of differentiation markers. Moreover, the fraction of undifferentiated cells that rapidly adhered to collagen IV was less numerous in HSPA2-deficient cells than in the control. In a 3D reconstructed human epidermis model, HSPA2 deficiency resulted in accelerated development of a filaggrin-positive layer. Collectively, our results clearly show a link between HSPA2 expression and maintenance of keratinocytes in an undifferentiated state in the basal layer of the epidermis. It seems that HSPA2 could retain keratinocytes from premature entry into the terminal differentiation process. Overall, HSPA2 appears to be necessary for controlling development of properly stratified epidermis and thus for maintenance of skin homeostasis.

HSPA2 Chaperone Contributes to the Maintenance of Epithelial Phenotype of Human Bronchial Epithelial Cells but Has Non-Essential Role in Supporting Malignant Features of Non-Small Cell Lung Carcinoma, MCF7, and HeLa Cancer Cells

“…We found that HBEC cells deficient in HSPA2 protein were less adhesive to fibronectin, laminin, and tenascin but not to collagens (I, II, and IV) or vitronectin ( Figure 4 c). These observations are in line with our previous findings showing that BEAS-2B cells deficient in HSPA2 were less adhesive to fibronectin but not to collagen IV [ 25 ]. Altogether, the results presented in Figure 4 allow us to conclude that HSPA2 in HBEC can play a role in modulating extracellular matrix (ECM) adhesiveness, but not motility.…”

“…Previously, we found that human HSPA2 is highly expressed in basal cells in various multilayered epithelia and in bronchial epithelium, which is a type of pseudostratified epithelium [ 21 ]. So far, the understanding of HSPA2 significance in normal somatic cells is limited to epidermal keratinocytes [ 25 ]. It is noteworthy that epidermal keratinocytes [ 25 ] and BEAS-2B cells (this paper) showed similar phenotypic changes after HSPA2 knockdown.…”

“…So far, the understanding of HSPA2 significance in normal somatic cells is limited to epidermal keratinocytes [ 25 ]. It is noteworthy that epidermal keratinocytes [ 25 ] and BEAS-2B cells (this paper) showed similar phenotypic changes after HSPA2 knockdown. Reduced clonogenic potential without changes in the cells’ proliferation rate seems paradoxical.…”

“…This shift might not include changes in migratory potential (of note, our unpublished results showed that HSPA2 knockdown did not affect keratinocytes’ motility either). In fact, we have found that HSPA2-deficient keratinocytes show accelerated differentiation in a reconstituted human epidermis model [ 25 ].…”

“…The assessment of endogenous HSPA2 in a physiologically relevant context confirmed its functional importance in somatic cells. The HSPA2 gene knockdown in epidermal keratinocytes revealed that its protein product was required to maintain their undifferentiated phenotype but not to provide protection against heat shock-induced toxicity [ 25 ].…”